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|---|---|---|---|---|---|---|
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝¹ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝¹ : Primrec Prod.snd
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n✝ : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L : Primrec fun a => a.1.1.1
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1.1)).1
n : Primrec fun a => a.1.1.2
cf : Primrec fun a => a.2.1
cg : Primrec fun a => a.2.2.1
| Primrec fun p =>
(fun a x => do
let y ← Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.2.1) a.1.1.2
some (Nat.pair x y))
p.1 p.2
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
|
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝¹ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝¹ : Primrec Prod.snd
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n✝ : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L : Primrec fun a => a.1.1.1
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1.1)).1
n : Primrec fun a => a.1.1.2
cf : Primrec fun a => a.2.1
cg : Primrec fun a => a.2.2.1
| Primrec fun p =>
(fun a x => do
let y ← Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.2.1) a.1.1.2
some (Nat.pair x y))
p.1 p.2
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
|
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝¹ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝¹ : Primrec Prod.snd
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n✝ : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L : Primrec fun a => a.1.1.1
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1.1)).1
n : Primrec fun a => a.1.1.2
cf : Primrec fun a => a.2.1
cg : Primrec fun a => a.2.2.1
| Primrec fun p =>
(fun a x => do
let y ← Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.2.1) a.1.1.2
some (Nat.pair x y))
p.1 p.2
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
|
congr
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
case f
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝¹ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝¹ : Primrec Prod.snd
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n✝ : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L : Primrec fun a => a.1.1.1
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1.1)).1
n : Primrec fun a => a.1.1.2
cf : Primrec fun a => a.2.1
cg : Primrec fun a => a.2.2.1
| fun p =>
(fun a x => do
let y ← Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.2.1) a.1.1.2
some (Nat.pair x y))
p.1 p.2
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
|
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
case f
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝¹ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝¹ : Primrec Prod.snd
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n✝ : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L : Primrec fun a => a.1.1.1
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1.1)).1
n : Primrec fun a => a.1.1.2
cf : Primrec fun a => a.2.1
cg : Primrec fun a => a.2.2.1
| fun p =>
(fun a x => do
let y ← Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.2.1) a.1.1.2
some (Nat.pair x y))
p.1 p.2
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
·
|
ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
·
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
case f
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝¹ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝¹ : Primrec Prod.snd
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n✝ : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L : Primrec fun a => a.1.1.1
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1.1)).1
n : Primrec fun a => a.1.1.2
cf : Primrec fun a => a.2.1
cg : Primrec fun a => a.2.2.1
| fun p =>
(fun a x => do
let y ← Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.2.1) a.1.1.2
some (Nat.pair x y))
p.1 p.2
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
·
|
ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
·
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
case f
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝¹ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝¹ : Primrec Prod.snd
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n✝ : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L : Primrec fun a => a.1.1.1
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1.1)).1
n : Primrec fun a => a.1.1.2
cf : Primrec fun a => a.2.1
cg : Primrec fun a => a.2.2.1
| fun p =>
(fun a x => do
let y ← Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.2.1) a.1.1.2
some (Nat.pair x y))
p.1 p.2
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
·
|
ext p
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
·
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
case f.h
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝¹ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝¹ : Primrec Prod.snd
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n✝ : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L : Primrec fun a => a.1.1.1
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1.1)).1
n : Primrec fun a => a.1.1.2
cf : Primrec fun a => a.2.1
cg : Primrec fun a => a.2.2.1
p : (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ
| (fun a x => do
let y ← Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.2.1) a.1.1.2
some (Nat.pair x y))
p.1 p.2
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
|
dsimp only []
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
case f.h
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝¹ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝¹ : Primrec Prod.snd
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n✝ : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L : Primrec fun a => a.1.1.1
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1.1)).1
n : Primrec fun a => a.1.1.2
cf : Primrec fun a => a.2.1
cg : Primrec fun a => a.2.2.1
p : (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ
| do
let y ← Nat.Partrec.Code.lup p.1.1.1.1 ((ofNat (ℕ × Code) (List.length p.1.1.1.1)).1, p.1.2.2.1) p.1.1.1.2
some (Nat.pair p.2 y)
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
|
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
case hpr
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝¹ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝¹ : Primrec Prod.snd
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n✝ : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L : Primrec fun a => a.1.1.1
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1.1)).1
n : Primrec fun a => a.1.1.2
cf : Primrec fun a => a.2.1
cg : Primrec fun a => a.2.2.1
⊢ Primrec fun p =>
Option.map (fun y => Nat.pair p.2 y)
(Nat.Partrec.Code.lup p.1.1.1.1 ((ofNat (ℕ × Code) (List.length p.1.1.1.1)).1, p.1.2.2.1) p.1.1.1.2)
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
|
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
case hpr
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝¹ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝¹ : Primrec Prod.snd
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n✝ : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L : Primrec fun a => a.1.1.1
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1.1)).1
n : Primrec fun a => a.1.1.2
cf : Primrec fun a => a.2.1
cg : Primrec fun a => a.2.2.1
⊢ Primrec₂ fun p y => Nat.pair p.2 y
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
|
unfold Primrec₂
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
case hpr
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝¹ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝¹ : Primrec Prod.snd
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n✝ : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L : Primrec fun a => a.1.1.1
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1.1)).1
n : Primrec fun a => a.1.1.2
cf : Primrec fun a => a.2.1
cg : Primrec fun a => a.2.2.1
⊢ Primrec fun p => (fun p y => Nat.pair p.2 y) p.1 p.2
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
|
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
case hco
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝ : Primrec Prod.snd
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
⊢ Primrec fun a => do
let x ← Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.2.1) a.1.1.2
Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.1) x
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
·
|
have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
·
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
case hco
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝ : Primrec Prod.snd
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L : Primrec fun a => a.1.1.1
⊢ Primrec fun a => do
let x ← Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.2.1) a.1.1.2
Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.1) x
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
|
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
case hco
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝¹ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝ : Primrec Prod.snd
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L : Primrec fun a => a.1.1.1
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1.1)).1
⊢ Primrec fun a => do
let x ← Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.2.1) a.1.1.2
Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.1) x
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
|
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
case hco
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝¹ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝¹ : Primrec Prod.snd
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n✝ : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L : Primrec fun a => a.1.1.1
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1.1)).1
n : Primrec fun a => a.1.1.2
⊢ Primrec fun a => do
let x ← Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.2.1) a.1.1.2
Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.1) x
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
|
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
case hco
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝¹ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝¹ : Primrec Prod.snd
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n✝ : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L : Primrec fun a => a.1.1.1
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1.1)).1
n : Primrec fun a => a.1.1.2
cf : Primrec fun a => a.2.1
⊢ Primrec fun a => do
let x ← Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.2.1) a.1.1.2
Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.1) x
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
|
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
case hco
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝¹ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝¹ : Primrec Prod.snd
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n✝ : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L : Primrec fun a => a.1.1.1
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1.1)).1
n : Primrec fun a => a.1.1.2
cf : Primrec fun a => a.2.1
cg : Primrec fun a => a.2.2.1
⊢ Primrec fun a => do
let x ← Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.2.1) a.1.1.2
Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.1) x
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
|
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
case hco
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝¹ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝¹ : Primrec Prod.snd
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n✝ : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L : Primrec fun a => a.1.1.1
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1.1)).1
n : Primrec fun a => a.1.1.2
cf : Primrec fun a => a.2.1
cg : Primrec fun a => a.2.2.1
⊢ Primrec₂ fun a x => Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.1) x
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
|
unfold Primrec₂
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
case hco
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝¹ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝¹ : Primrec Prod.snd
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n✝ : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L : Primrec fun a => a.1.1.1
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1.1)).1
n : Primrec fun a => a.1.1.2
cf : Primrec fun a => a.2.1
cg : Primrec fun a => a.2.2.1
⊢ Primrec fun p =>
(fun a x => Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.1) x) p.1 p.2
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
|
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
case hco
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝¹ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝¹ : Primrec Prod.snd
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n✝ : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L : Primrec fun a => a.1.1.1
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1.1)).1
n : Primrec fun a => a.1.1.2
cf : Primrec fun a => a.2.1
cg : Primrec fun a => a.2.2.1
h :
Primrec fun a =>
Nat.Partrec.Code.lup (a.1.1.1.1, ((ofNat (ℕ × Code) (List.length a.1.1.1.1)).1, a.1.2.1), a.2).1
(a.1.1.1.1, ((ofNat (ℕ × Code) (List.length a.1.1.1.1)).1, a.1.2.1), a.2).2.1
(a.1.1.1.1, ((ofNat (ℕ × Code) (List.length a.1.1.1.1)).1, a.1.2.1), a.2).2.2
⊢ Primrec fun p =>
(fun a x => Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.1) x) p.1 p.2
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
|
exact h
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
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case hpc
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝ : Primrec Prod.snd
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
⊢ Primrec fun a =>
let z := (unpair a.1.1.2).1;
Nat.casesOn (unpair a.1.1.2).2 (Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.1) z)
fun y => do
let i ← Nat.Partrec.Code.lup a.1.1.1 (a.1.2, (ofNat (ℕ × Code) (List.length a.1.1.1)).2) (Nat.pair z y)
Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.2.1) (Nat.pair z (Nat.pair y i))
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/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
·
|
have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
·
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
case hpc
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝ : Primrec Prod.snd
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L : Primrec fun a => a.1.1.1
⊢ Primrec fun a =>
let z := (unpair a.1.1.2).1;
Nat.casesOn (unpair a.1.1.2).2 (Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.1) z)
fun y => do
let i ← Nat.Partrec.Code.lup a.1.1.1 (a.1.2, (ofNat (ℕ × Code) (List.length a.1.1.1)).2) (Nat.pair z y)
Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.2.1) (Nat.pair z (Nat.pair y i))
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
|
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
case hpc
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝¹ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝ : Primrec Prod.snd
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L : Primrec fun a => a.1.1.1
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1.1)).1
⊢ Primrec fun a =>
let z := (unpair a.1.1.2).1;
Nat.casesOn (unpair a.1.1.2).2 (Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.1) z)
fun y => do
let i ← Nat.Partrec.Code.lup a.1.1.1 (a.1.2, (ofNat (ℕ × Code) (List.length a.1.1.1)).2) (Nat.pair z y)
Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.2.1) (Nat.pair z (Nat.pair y i))
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
|
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
case hpc
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝¹ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝¹ : Primrec Prod.snd
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n✝ : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L : Primrec fun a => a.1.1.1
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1.1)).1
n : Primrec fun a => a.1.1.2
⊢ Primrec fun a =>
let z := (unpair a.1.1.2).1;
Nat.casesOn (unpair a.1.1.2).2 (Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.1) z)
fun y => do
let i ← Nat.Partrec.Code.lup a.1.1.1 (a.1.2, (ofNat (ℕ × Code) (List.length a.1.1.1)).2) (Nat.pair z y)
Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.2.1) (Nat.pair z (Nat.pair y i))
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
|
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
case hpc
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝¹ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝¹ : Primrec Prod.snd
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n✝ : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L : Primrec fun a => a.1.1.1
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1.1)).1
n : Primrec fun a => a.1.1.2
cf : Primrec fun a => a.2.1
⊢ Primrec fun a =>
let z := (unpair a.1.1.2).1;
Nat.casesOn (unpair a.1.1.2).2 (Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.1) z)
fun y => do
let i ← Nat.Partrec.Code.lup a.1.1.1 (a.1.2, (ofNat (ℕ × Code) (List.length a.1.1.1)).2) (Nat.pair z y)
Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.2.1) (Nat.pair z (Nat.pair y i))
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
|
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
case hpc
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝¹ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝¹ : Primrec Prod.snd
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n✝ : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L : Primrec fun a => a.1.1.1
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1.1)).1
n : Primrec fun a => a.1.1.2
cf : Primrec fun a => a.2.1
cg : Primrec fun a => a.2.2.1
⊢ Primrec fun a =>
let z := (unpair a.1.1.2).1;
Nat.casesOn (unpair a.1.1.2).2 (Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.1) z)
fun y => do
let i ← Nat.Partrec.Code.lup a.1.1.1 (a.1.2, (ofNat (ℕ × Code) (List.length a.1.1.1)).2) (Nat.pair z y)
Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.2.1) (Nat.pair z (Nat.pair y i))
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
|
have z := Primrec.fst.comp (Primrec.unpair.comp n)
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
case hpc
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝¹ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝¹ : Primrec Prod.snd
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n✝ : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L : Primrec fun a => a.1.1.1
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1.1)).1
n : Primrec fun a => a.1.1.2
cf : Primrec fun a => a.2.1
cg : Primrec fun a => a.2.2.1
z : Primrec fun a => (unpair a.1.1.2).1
⊢ Primrec fun a =>
let z := (unpair a.1.1.2).1;
Nat.casesOn (unpair a.1.1.2).2 (Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.1) z)
fun y => do
let i ← Nat.Partrec.Code.lup a.1.1.1 (a.1.2, (ofNat (ℕ × Code) (List.length a.1.1.1)).2) (Nat.pair z y)
Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.2.1) (Nat.pair z (Nat.pair y i))
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
|
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
case hpc
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝¹ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝¹ : Primrec Prod.snd
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n✝ : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L : Primrec fun a => a.1.1.1
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1.1)).1
n : Primrec fun a => a.1.1.2
cf : Primrec fun a => a.2.1
cg : Primrec fun a => a.2.2.1
z : Primrec fun a => (unpair a.1.1.2).1
⊢ Primrec fun p =>
(fun a n =>
(fun y => do
let i ←
Nat.Partrec.Code.lup a.1.1.1 (a.1.2, (ofNat (ℕ × Code) (List.length a.1.1.1)).2)
(Nat.pair (unpair a.1.1.2).1 y)
Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.2.1)
(Nat.pair (unpair a.1.1.2).1 (Nat.pair y i)))
n)
p.1 p.2
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
|
have L := L.comp (Primrec.fst (β := ℕ))
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
case hpc
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝¹ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝¹ : Primrec Prod.snd
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n✝ : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L✝ : Primrec fun a => a.1.1.1
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1.1)).1
n : Primrec fun a => a.1.1.2
cf : Primrec fun a => a.2.1
cg : Primrec fun a => a.2.2.1
z : Primrec fun a => (unpair a.1.1.2).1
L : Primrec fun a => a.1.1.1.1
⊢ Primrec fun p =>
(fun a n =>
(fun y => do
let i ←
Nat.Partrec.Code.lup a.1.1.1 (a.1.2, (ofNat (ℕ × Code) (List.length a.1.1.1)).2)
(Nat.pair (unpair a.1.1.2).1 y)
Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.2.1)
(Nat.pair (unpair a.1.1.2).1 (Nat.pair y i)))
n)
p.1 p.2
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
|
have z := z.comp (Primrec.fst (β := ℕ))
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
case hpc
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝¹ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝¹ : Primrec Prod.snd
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n✝ : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L✝ : Primrec fun a => a.1.1.1
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1.1)).1
n : Primrec fun a => a.1.1.2
cf : Primrec fun a => a.2.1
cg : Primrec fun a => a.2.2.1
z✝ : Primrec fun a => (unpair a.1.1.2).1
L : Primrec fun a => a.1.1.1.1
z : Primrec fun a => (unpair a.1.1.1.2).1
⊢ Primrec fun p =>
(fun a n =>
(fun y => do
let i ←
Nat.Partrec.Code.lup a.1.1.1 (a.1.2, (ofNat (ℕ × Code) (List.length a.1.1.1)).2)
(Nat.pair (unpair a.1.1.2).1 y)
Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.2.1)
(Nat.pair (unpair a.1.1.2).1 (Nat.pair y i)))
n)
p.1 p.2
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
|
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
case hpc
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝¹ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝¹ : Primrec Prod.snd
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n✝ : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L✝ : Primrec fun a => a.1.1.1
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1.1)).1
n : Primrec fun a => a.1.1.2
cf : Primrec fun a => a.2.1
cg : Primrec fun a => a.2.2.1
z✝ : Primrec fun a => (unpair a.1.1.2).1
L : Primrec fun a => a.1.1.1.1
z : Primrec fun a => (unpair a.1.1.1.2).1
y : Primrec Prod.snd
⊢ Primrec fun p =>
(fun a n =>
(fun y => do
let i ←
Nat.Partrec.Code.lup a.1.1.1 (a.1.2, (ofNat (ℕ × Code) (List.length a.1.1.1)).2)
(Nat.pair (unpair a.1.1.2).1 y)
Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.2.1)
(Nat.pair (unpair a.1.1.2).1 (Nat.pair y i)))
n)
p.1 p.2
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
|
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
case hpc
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝¹ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝¹ : Primrec Prod.snd
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n✝ : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L✝ : Primrec fun a => a.1.1.1
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1.1)).1
n : Primrec fun a => a.1.1.2
cf : Primrec fun a => a.2.1
cg : Primrec fun a => a.2.2.1
z✝ : Primrec fun a => (unpair a.1.1.2).1
L : Primrec fun a => a.1.1.1.1
z : Primrec fun a => (unpair a.1.1.1.2).1
y : Primrec Prod.snd
h₁ :
Primrec fun a =>
Nat.Partrec.Code.lup
(a.1.1.1.1, (a.1.1.2, (ofNat (ℕ × Code) (List.length a.1.1.1.1)).2), Nat.pair (unpair a.1.1.1.2).1 a.2).1
(a.1.1.1.1, (a.1.1.2, (ofNat (ℕ × Code) (List.length a.1.1.1.1)).2), Nat.pair (unpair a.1.1.1.2).1 a.2).2.1
(a.1.1.1.1, (a.1.1.2, (ofNat (ℕ × Code) (List.length a.1.1.1.1)).2), Nat.pair (unpair a.1.1.1.2).1 a.2).2.2
⊢ Primrec fun p =>
(fun a n =>
(fun y => do
let i ←
Nat.Partrec.Code.lup a.1.1.1 (a.1.2, (ofNat (ℕ × Code) (List.length a.1.1.1)).2)
(Nat.pair (unpair a.1.1.2).1 y)
Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.2.1)
(Nat.pair (unpair a.1.1.2).1 (Nat.pair y i)))
n)
p.1 p.2
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
|
refine' Primrec.option_bind h₁ (_ : Primrec _)
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
case hpc
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝¹ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝¹ : Primrec Prod.snd
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n✝ : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L✝ : Primrec fun a => a.1.1.1
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1.1)).1
n : Primrec fun a => a.1.1.2
cf : Primrec fun a => a.2.1
cg : Primrec fun a => a.2.2.1
z✝ : Primrec fun a => (unpair a.1.1.2).1
L : Primrec fun a => a.1.1.1.1
z : Primrec fun a => (unpair a.1.1.1.2).1
y : Primrec Prod.snd
h₁ :
Primrec fun a =>
Nat.Partrec.Code.lup
(a.1.1.1.1, (a.1.1.2, (ofNat (ℕ × Code) (List.length a.1.1.1.1)).2), Nat.pair (unpair a.1.1.1.2).1 a.2).1
(a.1.1.1.1, (a.1.1.2, (ofNat (ℕ × Code) (List.length a.1.1.1.1)).2), Nat.pair (unpair a.1.1.1.2).1 a.2).2.1
(a.1.1.1.1, (a.1.1.2, (ofNat (ℕ × Code) (List.length a.1.1.1.1)).2), Nat.pair (unpair a.1.1.1.2).1 a.2).2.2
⊢ Primrec fun p =>
(fun p i =>
Nat.Partrec.Code.lup p.1.1.1.1 ((ofNat (ℕ × Code) (List.length p.1.1.1.1)).1, p.1.2.2.1)
(Nat.pair (unpair p.1.1.1.2).1 (Nat.pair p.2 i)))
p.1 p.2
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
|
have z := z.comp (Primrec.fst (β := ℕ))
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
case hpc
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝¹ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝¹ : Primrec Prod.snd
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n✝ : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L✝ : Primrec fun a => a.1.1.1
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1.1)).1
n : Primrec fun a => a.1.1.2
cf : Primrec fun a => a.2.1
cg : Primrec fun a => a.2.2.1
z✝¹ : Primrec fun a => (unpair a.1.1.2).1
L : Primrec fun a => a.1.1.1.1
z✝ : Primrec fun a => (unpair a.1.1.1.2).1
y : Primrec Prod.snd
h₁ :
Primrec fun a =>
Nat.Partrec.Code.lup
(a.1.1.1.1, (a.1.1.2, (ofNat (ℕ × Code) (List.length a.1.1.1.1)).2), Nat.pair (unpair a.1.1.1.2).1 a.2).1
(a.1.1.1.1, (a.1.1.2, (ofNat (ℕ × Code) (List.length a.1.1.1.1)).2), Nat.pair (unpair a.1.1.1.2).1 a.2).2.1
(a.1.1.1.1, (a.1.1.2, (ofNat (ℕ × Code) (List.length a.1.1.1.1)).2), Nat.pair (unpair a.1.1.1.2).1 a.2).2.2
z : Primrec fun a => (unpair a.1.1.1.1.2).1
⊢ Primrec fun p =>
(fun p i =>
Nat.Partrec.Code.lup p.1.1.1.1 ((ofNat (ℕ × Code) (List.length p.1.1.1.1)).1, p.1.2.2.1)
(Nat.pair (unpair p.1.1.1.2).1 (Nat.pair p.2 i)))
p.1 p.2
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
|
have y := y.comp (Primrec.fst (β := ℕ))
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
case hpc
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝¹ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝¹ : Primrec Prod.snd
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n✝ : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L✝ : Primrec fun a => a.1.1.1
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1.1)).1
n : Primrec fun a => a.1.1.2
cf : Primrec fun a => a.2.1
cg : Primrec fun a => a.2.2.1
z✝¹ : Primrec fun a => (unpair a.1.1.2).1
L : Primrec fun a => a.1.1.1.1
z✝ : Primrec fun a => (unpair a.1.1.1.2).1
y✝ : Primrec Prod.snd
h₁ :
Primrec fun a =>
Nat.Partrec.Code.lup
(a.1.1.1.1, (a.1.1.2, (ofNat (ℕ × Code) (List.length a.1.1.1.1)).2), Nat.pair (unpair a.1.1.1.2).1 a.2).1
(a.1.1.1.1, (a.1.1.2, (ofNat (ℕ × Code) (List.length a.1.1.1.1)).2), Nat.pair (unpair a.1.1.1.2).1 a.2).2.1
(a.1.1.1.1, (a.1.1.2, (ofNat (ℕ × Code) (List.length a.1.1.1.1)).2), Nat.pair (unpair a.1.1.1.2).1 a.2).2.2
z : Primrec fun a => (unpair a.1.1.1.1.2).1
y : Primrec fun a => a.1.2
⊢ Primrec fun p =>
(fun p i =>
Nat.Partrec.Code.lup p.1.1.1.1 ((ofNat (ℕ × Code) (List.length p.1.1.1.1)).1, p.1.2.2.1)
(Nat.pair (unpair p.1.1.1.2).1 (Nat.pair p.2 i)))
p.1 p.2
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
|
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
case hpc
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝¹ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝¹ : Primrec Prod.snd
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n✝ : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L✝ : Primrec fun a => a.1.1.1
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1.1)).1
n : Primrec fun a => a.1.1.2
cf : Primrec fun a => a.2.1
cg : Primrec fun a => a.2.2.1
z✝¹ : Primrec fun a => (unpair a.1.1.2).1
L : Primrec fun a => a.1.1.1.1
z✝ : Primrec fun a => (unpair a.1.1.1.2).1
y✝ : Primrec Prod.snd
h₁ :
Primrec fun a =>
Nat.Partrec.Code.lup
(a.1.1.1.1, (a.1.1.2, (ofNat (ℕ × Code) (List.length a.1.1.1.1)).2), Nat.pair (unpair a.1.1.1.2).1 a.2).1
(a.1.1.1.1, (a.1.1.2, (ofNat (ℕ × Code) (List.length a.1.1.1.1)).2), Nat.pair (unpair a.1.1.1.2).1 a.2).2.1
(a.1.1.1.1, (a.1.1.2, (ofNat (ℕ × Code) (List.length a.1.1.1.1)).2), Nat.pair (unpair a.1.1.1.2).1 a.2).2.2
z : Primrec fun a => (unpair a.1.1.1.1.2).1
y : Primrec fun a => a.1.2
i : Primrec Prod.snd
⊢ Primrec fun p =>
(fun p i =>
Nat.Partrec.Code.lup p.1.1.1.1 ((ofNat (ℕ × Code) (List.length p.1.1.1.1)).1, p.1.2.2.1)
(Nat.pair (unpair p.1.1.1.2).1 (Nat.pair p.2 i)))
p.1 p.2
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
|
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
case hpc
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝¹ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝¹ : Primrec Prod.snd
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n✝ : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L✝ : Primrec fun a => a.1.1.1
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1.1)).1
n : Primrec fun a => a.1.1.2
cf : Primrec fun a => a.2.1
cg : Primrec fun a => a.2.2.1
z✝¹ : Primrec fun a => (unpair a.1.1.2).1
L : Primrec fun a => a.1.1.1.1
z✝ : Primrec fun a => (unpair a.1.1.1.2).1
y✝ : Primrec Prod.snd
h₁ :
Primrec fun a =>
Nat.Partrec.Code.lup
(a.1.1.1.1, (a.1.1.2, (ofNat (ℕ × Code) (List.length a.1.1.1.1)).2), Nat.pair (unpair a.1.1.1.2).1 a.2).1
(a.1.1.1.1, (a.1.1.2, (ofNat (ℕ × Code) (List.length a.1.1.1.1)).2), Nat.pair (unpair a.1.1.1.2).1 a.2).2.1
(a.1.1.1.1, (a.1.1.2, (ofNat (ℕ × Code) (List.length a.1.1.1.1)).2), Nat.pair (unpair a.1.1.1.2).1 a.2).2.2
z : Primrec fun a => (unpair a.1.1.1.1.2).1
y : Primrec fun a => a.1.2
i : Primrec Prod.snd
h₂ :
Primrec fun a =>
Nat.Partrec.Code.lup
(a.1.1.1.1.1, ((ofNat (ℕ × Code) (List.length a.1.1.1.1.1)).1, a.1.1.2.2.1),
Nat.pair (unpair a.1.1.1.1.2).1 (Nat.pair a.1.2 a.2)).1
(a.1.1.1.1.1, ((ofNat (ℕ × Code) (List.length a.1.1.1.1.1)).1, a.1.1.2.2.1),
Nat.pair (unpair a.1.1.1.1.2).1 (Nat.pair a.1.2 a.2)).2.1
(a.1.1.1.1.1, ((ofNat (ℕ × Code) (List.length a.1.1.1.1.1)).1, a.1.1.2.2.1),
Nat.pair (unpair a.1.1.1.1.2).1 (Nat.pair a.1.2 a.2)).2.2
⊢ Primrec fun p =>
(fun p i =>
Nat.Partrec.Code.lup p.1.1.1.1 ((ofNat (ℕ × Code) (List.length p.1.1.1.1)).1, p.1.2.2.1)
(Nat.pair (unpair p.1.1.1.2).1 (Nat.pair p.2 i)))
p.1 p.2
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
|
exact h₂
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
case hrf
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝ : Primrec Prod.snd
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
⊢ Primrec fun a =>
let z := (unpair a.1.1.2).1;
let m := (unpair a.1.1.2).2;
do
let x ← Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.1) (Nat.pair z m)
Nat.casesOn x (some m) fun x =>
Nat.Partrec.Code.lup a.1.1.1 (a.1.2, (ofNat (ℕ × Code) (List.length a.1.1.1)).2) (Nat.pair z (m + 1))
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
·
|
have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
·
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
case hrf
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝ : Primrec Prod.snd
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L : Primrec fun a => a.1.1.1
⊢ Primrec fun a =>
let z := (unpair a.1.1.2).1;
let m := (unpair a.1.1.2).2;
do
let x ← Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.1) (Nat.pair z m)
Nat.casesOn x (some m) fun x =>
Nat.Partrec.Code.lup a.1.1.1 (a.1.2, (ofNat (ℕ × Code) (List.length a.1.1.1)).2) (Nat.pair z (m + 1))
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
|
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
case hrf
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝¹ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝ : Primrec Prod.snd
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L : Primrec fun a => a.1.1.1
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1.1)).1
⊢ Primrec fun a =>
let z := (unpair a.1.1.2).1;
let m := (unpair a.1.1.2).2;
do
let x ← Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.1) (Nat.pair z m)
Nat.casesOn x (some m) fun x =>
Nat.Partrec.Code.lup a.1.1.1 (a.1.2, (ofNat (ℕ × Code) (List.length a.1.1.1)).2) (Nat.pair z (m + 1))
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
|
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
case hrf
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝¹ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝¹ : Primrec Prod.snd
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n✝ : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L : Primrec fun a => a.1.1.1
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1.1)).1
n : Primrec fun a => a.1.1.2
⊢ Primrec fun a =>
let z := (unpair a.1.1.2).1;
let m := (unpair a.1.1.2).2;
do
let x ← Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.1) (Nat.pair z m)
Nat.casesOn x (some m) fun x =>
Nat.Partrec.Code.lup a.1.1.1 (a.1.2, (ofNat (ℕ × Code) (List.length a.1.1.1)).2) (Nat.pair z (m + 1))
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
|
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
case hrf
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝¹ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝¹ : Primrec Prod.snd
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n✝ : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L : Primrec fun a => a.1.1.1
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1.1)).1
n : Primrec fun a => a.1.1.2
cf : Primrec fun a => a.2.1
⊢ Primrec fun a =>
let z := (unpair a.1.1.2).1;
let m := (unpair a.1.1.2).2;
do
let x ← Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.1) (Nat.pair z m)
Nat.casesOn x (some m) fun x =>
Nat.Partrec.Code.lup a.1.1.1 (a.1.2, (ofNat (ℕ × Code) (List.length a.1.1.1)).2) (Nat.pair z (m + 1))
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
|
have z := Primrec.fst.comp (Primrec.unpair.comp n)
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
case hrf
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝¹ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝¹ : Primrec Prod.snd
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n✝ : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L : Primrec fun a => a.1.1.1
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1.1)).1
n : Primrec fun a => a.1.1.2
cf : Primrec fun a => a.2.1
z : Primrec fun a => (unpair a.1.1.2).1
⊢ Primrec fun a =>
let z := (unpair a.1.1.2).1;
let m := (unpair a.1.1.2).2;
do
let x ← Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.1) (Nat.pair z m)
Nat.casesOn x (some m) fun x =>
Nat.Partrec.Code.lup a.1.1.1 (a.1.2, (ofNat (ℕ × Code) (List.length a.1.1.1)).2) (Nat.pair z (m + 1))
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
|
have m := Primrec.snd.comp (Primrec.unpair.comp n)
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
case hrf
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝¹ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝¹ : Primrec Prod.snd
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n✝ : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L : Primrec fun a => a.1.1.1
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1.1)).1
n : Primrec fun a => a.1.1.2
cf : Primrec fun a => a.2.1
z : Primrec fun a => (unpair a.1.1.2).1
m : Primrec fun a => (unpair a.1.1.2).2
⊢ Primrec fun a =>
let z := (unpair a.1.1.2).1;
let m := (unpair a.1.1.2).2;
do
let x ← Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.1) (Nat.pair z m)
Nat.casesOn x (some m) fun x =>
Nat.Partrec.Code.lup a.1.1.1 (a.1.2, (ofNat (ℕ × Code) (List.length a.1.1.1)).2) (Nat.pair z (m + 1))
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
|
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
case hrf
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝¹ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝¹ : Primrec Prod.snd
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n✝ : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L : Primrec fun a => a.1.1.1
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1.1)).1
n : Primrec fun a => a.1.1.2
cf : Primrec fun a => a.2.1
z : Primrec fun a => (unpair a.1.1.2).1
m : Primrec fun a => (unpair a.1.1.2).2
h₁ :
Primrec fun a =>
Nat.Partrec.Code.lup
(a.1.1.1, ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.1), Nat.pair (unpair a.1.1.2).1 (unpair a.1.1.2).2).1
(a.1.1.1, ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.1), Nat.pair (unpair a.1.1.2).1 (unpair a.1.1.2).2).2.1
(a.1.1.1, ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.1), Nat.pair (unpair a.1.1.2).1 (unpair a.1.1.2).2).2.2
⊢ Primrec fun a =>
let z := (unpair a.1.1.2).1;
let m := (unpair a.1.1.2).2;
do
let x ← Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.1) (Nat.pair z m)
Nat.casesOn x (some m) fun x =>
Nat.Partrec.Code.lup a.1.1.1 (a.1.2, (ofNat (ℕ × Code) (List.length a.1.1.1)).2) (Nat.pair z (m + 1))
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
|
refine' Primrec.option_bind h₁ (_ : Primrec _)
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
case hrf
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝¹ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝¹ : Primrec Prod.snd
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n✝ : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L : Primrec fun a => a.1.1.1
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1.1)).1
n : Primrec fun a => a.1.1.2
cf : Primrec fun a => a.2.1
z : Primrec fun a => (unpair a.1.1.2).1
m : Primrec fun a => (unpair a.1.1.2).2
h₁ :
Primrec fun a =>
Nat.Partrec.Code.lup
(a.1.1.1, ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.1), Nat.pair (unpair a.1.1.2).1 (unpair a.1.1.2).2).1
(a.1.1.1, ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.1), Nat.pair (unpair a.1.1.2).1 (unpair a.1.1.2).2).2.1
(a.1.1.1, ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.1), Nat.pair (unpair a.1.1.2).1 (unpair a.1.1.2).2).2.2
⊢ Primrec fun p =>
(fun a x =>
Nat.casesOn x (some (unpair a.1.1.2).2) fun x =>
Nat.Partrec.Code.lup a.1.1.1 (a.1.2, (ofNat (ℕ × Code) (List.length a.1.1.1)).2)
(Nat.pair (unpair a.1.1.2).1 ((unpair a.1.1.2).2 + 1)))
p.1 p.2
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
|
have m := m.comp (Primrec.fst (β := ℕ))
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
case hrf
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝¹ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝¹ : Primrec Prod.snd
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n✝ : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L : Primrec fun a => a.1.1.1
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1.1)).1
n : Primrec fun a => a.1.1.2
cf : Primrec fun a => a.2.1
z : Primrec fun a => (unpair a.1.1.2).1
m✝ : Primrec fun a => (unpair a.1.1.2).2
h₁ :
Primrec fun a =>
Nat.Partrec.Code.lup
(a.1.1.1, ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.1), Nat.pair (unpair a.1.1.2).1 (unpair a.1.1.2).2).1
(a.1.1.1, ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.1), Nat.pair (unpair a.1.1.2).1 (unpair a.1.1.2).2).2.1
(a.1.1.1, ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.1), Nat.pair (unpair a.1.1.2).1 (unpair a.1.1.2).2).2.2
m : Primrec fun a => (unpair a.1.1.1.2).2
⊢ Primrec fun p =>
(fun a x =>
Nat.casesOn x (some (unpair a.1.1.2).2) fun x =>
Nat.Partrec.Code.lup a.1.1.1 (a.1.2, (ofNat (ℕ × Code) (List.length a.1.1.1)).2)
(Nat.pair (unpair a.1.1.2).1 ((unpair a.1.1.2).2 + 1)))
p.1 p.2
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
|
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
case hrf
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝¹ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝¹ : Primrec Prod.snd
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n✝ : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L : Primrec fun a => a.1.1.1
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1.1)).1
n : Primrec fun a => a.1.1.2
cf : Primrec fun a => a.2.1
z : Primrec fun a => (unpair a.1.1.2).1
m✝ : Primrec fun a => (unpair a.1.1.2).2
h₁ :
Primrec fun a =>
Nat.Partrec.Code.lup
(a.1.1.1, ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.1), Nat.pair (unpair a.1.1.2).1 (unpair a.1.1.2).2).1
(a.1.1.1, ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.1), Nat.pair (unpair a.1.1.2).1 (unpair a.1.1.2).2).2.1
(a.1.1.1, ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.1), Nat.pair (unpair a.1.1.2).1 (unpair a.1.1.2).2).2.2
m : Primrec fun a => (unpair a.1.1.1.2).2
⊢ Primrec₂ fun p n =>
(fun x =>
Nat.Partrec.Code.lup p.1.1.1.1 (p.1.1.2, (ofNat (ℕ × Code) (List.length p.1.1.1.1)).2)
(Nat.pair (unpair p.1.1.1.2).1 ((unpair p.1.1.1.2).2 + 1)))
n
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
|
unfold Primrec₂
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
case hrf
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝¹ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝¹ : Primrec Prod.snd
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n✝ : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
L : Primrec fun a => a.1.1.1
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1.1)).1
n : Primrec fun a => a.1.1.2
cf : Primrec fun a => a.2.1
z : Primrec fun a => (unpair a.1.1.2).1
m✝ : Primrec fun a => (unpair a.1.1.2).2
h₁ :
Primrec fun a =>
Nat.Partrec.Code.lup
(a.1.1.1, ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.1), Nat.pair (unpair a.1.1.2).1 (unpair a.1.1.2).2).1
(a.1.1.1, ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.1), Nat.pair (unpair a.1.1.2).1 (unpair a.1.1.2).2).2.1
(a.1.1.1, ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.1), Nat.pair (unpair a.1.1.2).1 (unpair a.1.1.2).2).2.2
m : Primrec fun a => (unpair a.1.1.1.2).2
⊢ Primrec fun p =>
(fun p n =>
(fun x =>
Nat.Partrec.Code.lup p.1.1.1.1 (p.1.1.2, (ofNat (ℕ × Code) (List.length p.1.1.1.1)).2)
(Nat.pair (unpair p.1.1.1.2).1 ((unpair p.1.1.1.2).2 + 1)))
n)
p.1 p.2
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
|
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
|
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
|
Mathlib.Computability.PartrecCode.976_0.A3c3Aev6SyIRjCJ
|
private theorem hG : Primrec G
|
Mathlib_Computability_PartrecCode
|
k : ℕ
c : Code
n : ℕ
⊢ (Option.bind (Option.map (evaln k c) (List.get? (List.range k) n)) fun b => b) = evaln k c n
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
|
by_cases kn : n < k
|
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
|
Mathlib.Computability.PartrecCode.1078_0.A3c3Aev6SyIRjCJ
|
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n
|
Mathlib_Computability_PartrecCode
|
case pos
k : ℕ
c : Code
n : ℕ
kn : n < k
⊢ (Option.bind (Option.map (evaln k c) (List.get? (List.range k) n)) fun b => b) = evaln k c n
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
·
|
simp [List.get?_range kn]
|
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
·
|
Mathlib.Computability.PartrecCode.1078_0.A3c3Aev6SyIRjCJ
|
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n
|
Mathlib_Computability_PartrecCode
|
case neg
k : ℕ
c : Code
n : ℕ
kn : ¬n < k
⊢ (Option.bind (Option.map (evaln k c) (List.get? (List.range k) n)) fun b => b) = evaln k c n
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
·
|
rw [List.get?_len_le]
|
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
·
|
Mathlib.Computability.PartrecCode.1078_0.A3c3Aev6SyIRjCJ
|
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n
|
Mathlib_Computability_PartrecCode
|
case neg
k : ℕ
c : Code
n : ℕ
kn : ¬n < k
⊢ (Option.bind (Option.map (evaln k c) Option.none) fun b => b) = evaln k c n
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
·
|
cases e : evaln k c n
|
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
·
|
Mathlib.Computability.PartrecCode.1078_0.A3c3Aev6SyIRjCJ
|
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n
|
Mathlib_Computability_PartrecCode
|
case neg.none
k : ℕ
c : Code
n : ℕ
kn : ¬n < k
e : evaln k c n = Option.none
⊢ (Option.bind (Option.map (evaln k c) Option.none) fun b => b) = Option.none
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
·
|
rfl
|
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
·
|
Mathlib.Computability.PartrecCode.1078_0.A3c3Aev6SyIRjCJ
|
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n
|
Mathlib_Computability_PartrecCode
|
case neg.some
k : ℕ
c : Code
n : ℕ
kn : ¬n < k
val✝ : ℕ
e : evaln k c n = some val✝
⊢ (Option.bind (Option.map (evaln k c) Option.none) fun b => b) = some val✝
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
|
exact kn.elim (evaln_bound e)
|
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
|
Mathlib.Computability.PartrecCode.1078_0.A3c3Aev6SyIRjCJ
|
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n
|
Mathlib_Computability_PartrecCode
|
case neg
k : ℕ
c : Code
n : ℕ
kn : ¬n < k
⊢ List.length (List.range k) ≤ n
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
exact kn.elim (evaln_bound e)
|
simpa using kn
|
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
exact kn.elim (evaln_bound e)
|
Mathlib.Computability.PartrecCode.1078_0.A3c3Aev6SyIRjCJ
|
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n
|
Mathlib_Computability_PartrecCode
|
x✝ : Unit
p : ℕ
⊢ Nat.Partrec.Code.G
(x✝,
List.map
(fun n =>
let a := ofNat (ℕ × Code) n;
List.map (evaln a.1 a.2) (List.range a.1))
(List.range p)).2 =
some
(let a := ofNat (ℕ × Code) p;
List.map (evaln a.1 a.2) (List.range a.1))
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
exact kn.elim (evaln_bound e)
simpa using kn
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
|
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
|
Mathlib.Computability.PartrecCode.1088_0.A3c3Aev6SyIRjCJ
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2
|
Mathlib_Computability_PartrecCode
|
x✝ : Unit
p : ℕ
⊢ List.map
(fun n =>
Nat.rec Option.none
(fun n_1 n_ih =>
rec (some 0) (some (Nat.succ n)) (some (unpair n).1) (some (unpair n).2)
(fun cf cg x x => do
let x ←
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range p))
((unpair p).1, cf) n
let y ←
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range p))
((unpair p).1, cg) n
some (Nat.pair x y))
(fun cf cg x x => do
let x ←
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range p))
((unpair p).1, cg) n
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range p))
((unpair p).1, cf) x)
(fun cf cg x x =>
Nat.rec
(Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range p))
((unpair p).1, cf) (unpair n).1)
(fun n_2 n_ih => do
let i ←
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range p))
(n_1, ofNat Code (unpair p).2) (Nat.pair (unpair n).1 n_2)
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range p))
((unpair p).1, cg) (Nat.pair (unpair n).1 (Nat.pair n_2 i)))
(unpair n).2)
(fun cf x => do
let x ←
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range p))
((unpair p).1, cf) n
Nat.rec (some (unpair n).2)
(fun n_2 n_ih =>
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range p))
(n_1, ofNat Code (unpair p).2) (Nat.pair (unpair n).1 ((unpair n).2 + 1)))
x)
(ofNat Code (unpair p).2))
(unpair p).1)
(List.range (unpair p).1) =
List.map (evaln (unpair p).1 (ofNat Code (unpair p).2)) (List.range (unpair p).1)
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
exact kn.elim (evaln_bound e)
simpa using kn
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
|
refine List.map_congr fun n => ?_
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
|
Mathlib.Computability.PartrecCode.1088_0.A3c3Aev6SyIRjCJ
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2
|
Mathlib_Computability_PartrecCode
|
x✝ : Unit
p n : ℕ
⊢ n ∈ List.range (unpair p).1 →
Nat.rec Option.none
(fun n_1 n_ih =>
rec (some 0) (some (Nat.succ n)) (some (unpair n).1) (some (unpair n).2)
(fun cf cg x x => do
let x ←
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range p))
((unpair p).1, cf) n
let y ←
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range p))
((unpair p).1, cg) n
some (Nat.pair x y))
(fun cf cg x x => do
let x ←
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range p))
((unpair p).1, cg) n
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range p))
((unpair p).1, cf) x)
(fun cf cg x x =>
Nat.rec
(Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range p))
((unpair p).1, cf) (unpair n).1)
(fun n_2 n_ih => do
let i ←
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range p))
(n_1, ofNat Code (unpair p).2) (Nat.pair (unpair n).1 n_2)
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range p))
((unpair p).1, cg) (Nat.pair (unpair n).1 (Nat.pair n_2 i)))
(unpair n).2)
(fun cf x => do
let x ←
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range p))
((unpair p).1, cf) n
Nat.rec (some (unpair n).2)
(fun n_2 n_ih =>
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range p))
(n_1, ofNat Code (unpair p).2) (Nat.pair (unpair n).1 ((unpair n).2 + 1)))
x)
(ofNat Code (unpair p).2))
(unpair p).1 =
evaln (unpair p).1 (ofNat Code (unpair p).2) n
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
exact kn.elim (evaln_bound e)
simpa using kn
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
|
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
|
Mathlib.Computability.PartrecCode.1088_0.A3c3Aev6SyIRjCJ
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2
|
Mathlib_Computability_PartrecCode
|
x✝ : Unit
p n : ℕ
⊢ List.range p = List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
exact kn.elim (evaln_bound e)
simpa using kn
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
|
simp
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
|
Mathlib.Computability.PartrecCode.1088_0.A3c3Aev6SyIRjCJ
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2
|
Mathlib_Computability_PartrecCode
|
x✝ : Unit
p n : ℕ
this : List.range p = List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))
⊢ n ∈ List.range (unpair p).1 →
Nat.rec Option.none
(fun n_1 n_ih =>
rec (some 0) (some (Nat.succ n)) (some (unpair n).1) (some (unpair n).2)
(fun cf cg x x => do
let x ←
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range p))
((unpair p).1, cf) n
let y ←
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range p))
((unpair p).1, cg) n
some (Nat.pair x y))
(fun cf cg x x => do
let x ←
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range p))
((unpair p).1, cg) n
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range p))
((unpair p).1, cf) x)
(fun cf cg x x =>
Nat.rec
(Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range p))
((unpair p).1, cf) (unpair n).1)
(fun n_2 n_ih => do
let i ←
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range p))
(n_1, ofNat Code (unpair p).2) (Nat.pair (unpair n).1 n_2)
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range p))
((unpair p).1, cg) (Nat.pair (unpair n).1 (Nat.pair n_2 i)))
(unpair n).2)
(fun cf x => do
let x ←
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range p))
((unpair p).1, cf) n
Nat.rec (some (unpair n).2)
(fun n_2 n_ih =>
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range p))
(n_1, ofNat Code (unpair p).2) (Nat.pair (unpair n).1 ((unpair n).2 + 1)))
x)
(ofNat Code (unpair p).2))
(unpair p).1 =
evaln (unpair p).1 (ofNat Code (unpair p).2) n
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
exact kn.elim (evaln_bound e)
simpa using kn
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
|
rw [this]
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
|
Mathlib.Computability.PartrecCode.1088_0.A3c3Aev6SyIRjCJ
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2
|
Mathlib_Computability_PartrecCode
|
x✝ : Unit
p n : ℕ
this : List.range p = List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))
⊢ n ∈ List.range (unpair p).1 →
Nat.rec Option.none
(fun n_1 n_ih =>
rec (some 0) (some (Nat.succ n)) (some (unpair n).1) (some (unpair n).2)
(fun cf cg x x => do
let x ←
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))))
((unpair p).1, cf) n
let y ←
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))))
((unpair p).1, cg) n
some (Nat.pair x y))
(fun cf cg x x => do
let x ←
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))))
((unpair p).1, cg) n
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))))
((unpair p).1, cf) x)
(fun cf cg x x =>
Nat.rec
(Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))))
((unpair p).1, cf) (unpair n).1)
(fun n_2 n_ih => do
let i ←
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))))
(n_1, ofNat Code (unpair p).2) (Nat.pair (unpair n).1 n_2)
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))))
((unpair p).1, cg) (Nat.pair (unpair n).1 (Nat.pair n_2 i)))
(unpair n).2)
(fun cf x => do
let x ←
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))))
((unpair p).1, cf) n
Nat.rec (some (unpair n).2)
(fun n_2 n_ih =>
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))))
(n_1, ofNat Code (unpair p).2) (Nat.pair (unpair n).1 ((unpair n).2 + 1)))
x)
(ofNat Code (unpair p).2))
(unpair p).1 =
evaln (unpair p).1 (ofNat Code (unpair p).2) n
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
exact kn.elim (evaln_bound e)
simpa using kn
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
|
generalize p.unpair.1 = k
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
|
Mathlib.Computability.PartrecCode.1088_0.A3c3Aev6SyIRjCJ
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2
|
Mathlib_Computability_PartrecCode
|
x✝ : Unit
p n : ℕ
this : List.range p = List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))
k : ℕ
⊢ n ∈ List.range k →
Nat.rec Option.none
(fun n_1 n_ih =>
rec (some 0) (some (Nat.succ n)) (some (unpair n).1) (some (unpair n).2)
(fun cf cg x x => do
let x ←
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode (ofNat Code (unpair p).2)))))
(k, cf) n
let y ←
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode (ofNat Code (unpair p).2)))))
(k, cg) n
some (Nat.pair x y))
(fun cf cg x x => do
let x ←
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode (ofNat Code (unpair p).2)))))
(k, cg) n
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode (ofNat Code (unpair p).2)))))
(k, cf) x)
(fun cf cg x x =>
Nat.rec
(Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode (ofNat Code (unpair p).2)))))
(k, cf) (unpair n).1)
(fun n_2 n_ih => do
let i ←
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode (ofNat Code (unpair p).2)))))
(n_1, ofNat Code (unpair p).2) (Nat.pair (unpair n).1 n_2)
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode (ofNat Code (unpair p).2)))))
(k, cg) (Nat.pair (unpair n).1 (Nat.pair n_2 i)))
(unpair n).2)
(fun cf x => do
let x ←
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode (ofNat Code (unpair p).2)))))
(k, cf) n
Nat.rec (some (unpair n).2)
(fun n_2 n_ih =>
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode (ofNat Code (unpair p).2)))))
(n_1, ofNat Code (unpair p).2) (Nat.pair (unpair n).1 ((unpair n).2 + 1)))
x)
(ofNat Code (unpair p).2))
k =
evaln k (ofNat Code (unpair p).2) n
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
exact kn.elim (evaln_bound e)
simpa using kn
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
|
generalize ofNat Code p.unpair.2 = c
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
|
Mathlib.Computability.PartrecCode.1088_0.A3c3Aev6SyIRjCJ
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2
|
Mathlib_Computability_PartrecCode
|
x✝ : Unit
p n : ℕ
this : List.range p = List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))
k : ℕ
c : Code
⊢ n ∈ List.range k →
Nat.rec Option.none
(fun n_1 n_ih =>
rec (some 0) (some (Nat.succ n)) (some (unpair n).1) (some (unpair n).2)
(fun cf cg x x => do
let x ←
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode c))))
(k, cf) n
let y ←
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode c))))
(k, cg) n
some (Nat.pair x y))
(fun cf cg x x => do
let x ←
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode c))))
(k, cg) n
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode c))))
(k, cf) x)
(fun cf cg x x =>
Nat.rec
(Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode c))))
(k, cf) (unpair n).1)
(fun n_2 n_ih => do
let i ←
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode c))))
(n_1, c) (Nat.pair (unpair n).1 n_2)
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode c))))
(k, cg) (Nat.pair (unpair n).1 (Nat.pair n_2 i)))
(unpair n).2)
(fun cf x => do
let x ←
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode c))))
(k, cf) n
Nat.rec (some (unpair n).2)
(fun n_2 n_ih =>
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode c))))
(n_1, c) (Nat.pair (unpair n).1 ((unpair n).2 + 1)))
x)
c)
k =
evaln k c n
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
exact kn.elim (evaln_bound e)
simpa using kn
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
|
intro nk
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
|
Mathlib.Computability.PartrecCode.1088_0.A3c3Aev6SyIRjCJ
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2
|
Mathlib_Computability_PartrecCode
|
x✝ : Unit
p n : ℕ
this : List.range p = List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))
k : ℕ
c : Code
nk : n ∈ List.range k
⊢ Nat.rec Option.none
(fun n_1 n_ih =>
rec (some 0) (some (Nat.succ n)) (some (unpair n).1) (some (unpair n).2)
(fun cf cg x x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode c))))
(k, cf) n
let y ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode c))))
(k, cg) n
some (Nat.pair x y))
(fun cf cg x x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode c))))
(k, cg) n
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode c))))
(k, cf) x)
(fun cf cg x x =>
Nat.rec
(Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode c))))
(k, cf) (unpair n).1)
(fun n_2 n_ih => do
let i ←
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode c))))
(n_1, c) (Nat.pair (unpair n).1 n_2)
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode c))))
(k, cg) (Nat.pair (unpair n).1 (Nat.pair n_2 i)))
(unpair n).2)
(fun cf x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode c))))
(k, cf) n
Nat.rec (some (unpair n).2)
(fun n_2 n_ih =>
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode c))))
(n_1, c) (Nat.pair (unpair n).1 ((unpair n).2 + 1)))
x)
c)
k =
evaln k c n
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
exact kn.elim (evaln_bound e)
simpa using kn
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
|
cases' k with k'
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
|
Mathlib.Computability.PartrecCode.1088_0.A3c3Aev6SyIRjCJ
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2
|
Mathlib_Computability_PartrecCode
|
case zero
x✝ : Unit
p n : ℕ
this : List.range p = List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))
c : Code
nk : n ∈ List.range Nat.zero
⊢ Nat.rec Option.none
(fun n_1 n_ih =>
rec (some 0) (some (Nat.succ n)) (some (unpair n).1) (some (unpair n).2)
(fun cf cg x x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair Nat.zero (encode c))))
(Nat.zero, cf) n
let y ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair Nat.zero (encode c))))
(Nat.zero, cg) n
some (Nat.pair x y))
(fun cf cg x x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair Nat.zero (encode c))))
(Nat.zero, cg) n
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair Nat.zero (encode c))))
(Nat.zero, cf) x)
(fun cf cg x x =>
Nat.rec
(Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair Nat.zero (encode c))))
(Nat.zero, cf) (unpair n).1)
(fun n_2 n_ih => do
let i ←
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair Nat.zero (encode c))))
(n_1, c) (Nat.pair (unpair n).1 n_2)
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair Nat.zero (encode c))))
(Nat.zero, cg) (Nat.pair (unpair n).1 (Nat.pair n_2 i)))
(unpair n).2)
(fun cf x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair Nat.zero (encode c))))
(Nat.zero, cf) n
Nat.rec (some (unpair n).2)
(fun n_2 n_ih =>
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair Nat.zero (encode c))))
(n_1, c) (Nat.pair (unpair n).1 ((unpair n).2 + 1)))
x)
c)
Nat.zero =
evaln Nat.zero c n
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
exact kn.elim (evaln_bound e)
simpa using kn
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
·
|
simp [evaln]
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
·
|
Mathlib.Computability.PartrecCode.1088_0.A3c3Aev6SyIRjCJ
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2
|
Mathlib_Computability_PartrecCode
|
case succ
x✝ : Unit
p n : ℕ
this : List.range p = List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))
c : Code
k' : ℕ
nk : n ∈ List.range (Nat.succ k')
⊢ Nat.rec Option.none
(fun n_1 n_ih =>
rec (some 0) (some (Nat.succ n)) (some (unpair n).1) (some (unpair n).2)
(fun cf cg x x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (Nat.succ k') (encode c))))
(Nat.succ k', cf) n
let y ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (Nat.succ k') (encode c))))
(Nat.succ k', cg) n
some (Nat.pair x y))
(fun cf cg x x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (Nat.succ k') (encode c))))
(Nat.succ k', cg) n
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (Nat.succ k') (encode c))))
(Nat.succ k', cf) x)
(fun cf cg x x =>
Nat.rec
(Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (Nat.succ k') (encode c))))
(Nat.succ k', cf) (unpair n).1)
(fun n_2 n_ih => do
let i ←
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (Nat.succ k') (encode c))))
(n_1, c) (Nat.pair (unpair n).1 n_2)
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (Nat.succ k') (encode c))))
(Nat.succ k', cg) (Nat.pair (unpair n).1 (Nat.pair n_2 i)))
(unpair n).2)
(fun cf x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (Nat.succ k') (encode c))))
(Nat.succ k', cf) n
Nat.rec (some (unpair n).2)
(fun n_2 n_ih =>
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (Nat.succ k') (encode c))))
(n_1, c) (Nat.pair (unpair n).1 ((unpair n).2 + 1)))
x)
c)
(Nat.succ k') =
evaln (Nat.succ k') c n
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
exact kn.elim (evaln_bound e)
simpa using kn
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
|
let k := k' + 1
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
|
Mathlib.Computability.PartrecCode.1088_0.A3c3Aev6SyIRjCJ
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2
|
Mathlib_Computability_PartrecCode
|
case succ
x✝ : Unit
p n : ℕ
this : List.range p = List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))
c : Code
k' : ℕ
nk : n ∈ List.range (Nat.succ k')
k : ℕ := k' + 1
⊢ Nat.rec Option.none
(fun n_1 n_ih =>
rec (some 0) (some (Nat.succ n)) (some (unpair n).1) (some (unpair n).2)
(fun cf cg x x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (Nat.succ k') (encode c))))
(Nat.succ k', cf) n
let y ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (Nat.succ k') (encode c))))
(Nat.succ k', cg) n
some (Nat.pair x y))
(fun cf cg x x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (Nat.succ k') (encode c))))
(Nat.succ k', cg) n
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (Nat.succ k') (encode c))))
(Nat.succ k', cf) x)
(fun cf cg x x =>
Nat.rec
(Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (Nat.succ k') (encode c))))
(Nat.succ k', cf) (unpair n).1)
(fun n_2 n_ih => do
let i ←
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (Nat.succ k') (encode c))))
(n_1, c) (Nat.pair (unpair n).1 n_2)
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (Nat.succ k') (encode c))))
(Nat.succ k', cg) (Nat.pair (unpair n).1 (Nat.pair n_2 i)))
(unpair n).2)
(fun cf x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (Nat.succ k') (encode c))))
(Nat.succ k', cf) n
Nat.rec (some (unpair n).2)
(fun n_2 n_ih =>
Nat.Partrec.Code.lup
(List.map
(fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (Nat.succ k') (encode c))))
(n_1, c) (Nat.pair (unpair n).1 ((unpair n).2 + 1)))
x)
c)
(Nat.succ k') =
evaln (Nat.succ k') c n
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
exact kn.elim (evaln_bound e)
simpa using kn
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
|
simp only [show k'.succ = k from rfl]
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
|
Mathlib.Computability.PartrecCode.1088_0.A3c3Aev6SyIRjCJ
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2
|
Mathlib_Computability_PartrecCode
|
case succ
x✝ : Unit
p n : ℕ
this : List.range p = List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))
c : Code
k' : ℕ
nk : n ∈ List.range (Nat.succ k')
k : ℕ := k' + 1
⊢ rec (some 0) (some (Nat.succ n)) (some (unpair n).1) (some (unpair n).2)
(fun cf cg x x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode c))))
(k' + 1, cf) n
let y ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode c))))
(k' + 1, cg) n
some (Nat.pair x y))
(fun cf cg x x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode c))))
(k' + 1, cg) n
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode c))))
(k' + 1, cf) x)
(fun cf cg x x =>
Nat.rec
(Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode c))))
(k' + 1, cf) (unpair n).1)
(fun n_1 n_ih => do
let i ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode c))))
(k', c) (Nat.pair (unpair n).1 n_1)
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode c))))
(k' + 1, cg) (Nat.pair (unpair n).1 (Nat.pair n_1 i)))
(unpair n).2)
(fun cf x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode c))))
(k' + 1, cf) n
Nat.rec (some (unpair n).2)
(fun n_1 n_ih =>
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode c))))
(k', c) (Nat.pair (unpair n).1 ((unpair n).2 + 1)))
x)
c =
evaln (k' + 1) c n
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
exact kn.elim (evaln_bound e)
simpa using kn
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
|
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
|
Mathlib.Computability.PartrecCode.1088_0.A3c3Aev6SyIRjCJ
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2
|
Mathlib_Computability_PartrecCode
|
case succ
x✝ : Unit
p n : ℕ
this : List.range p = List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))
c : Code
k' : ℕ
nk : n ∈ List.range (Nat.succ k')
k : ℕ := k' + 1
⊢ rec (some 0) (some (Nat.succ n)) (some (unpair n).1) (some (unpair n).2)
(fun cf cg x x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode c))))
(k' + 1, cf) n
let y ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode c))))
(k' + 1, cg) n
some (Nat.pair x y))
(fun cf cg x x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode c))))
(k' + 1, cg) n
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode c))))
(k' + 1, cf) x)
(fun cf cg x x =>
Nat.rec
(Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode c))))
(k' + 1, cf) (unpair n).1)
(fun n_1 n_ih => do
let i ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode c))))
(k', c) (Nat.pair (unpair n).1 n_1)
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode c))))
(k' + 1, cg) (Nat.pair (unpair n).1 (Nat.pair n_1 i)))
(unpair n).2)
(fun cf x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode c))))
(k' + 1, cf) n
Nat.rec (some (unpair n).2)
(fun n_1 n_ih =>
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode c))))
(k', c) (Nat.pair (unpair n).1 ((unpair n).2 + 1)))
x)
c =
evaln (k' + 1) c n
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
exact kn.elim (evaln_bound e)
simpa using kn
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says
|
simp only [List.mem_range, lt_succ_iff] at nk
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says
|
Mathlib.Computability.PartrecCode.1088_0.A3c3Aev6SyIRjCJ
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2
|
Mathlib_Computability_PartrecCode
|
case succ
x✝ : Unit
p n : ℕ
this : List.range p = List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))
c : Code
k' : ℕ
k : ℕ := k' + 1
nk : n ≤ k'
⊢ rec (some 0) (some (Nat.succ n)) (some (unpair n).1) (some (unpair n).2)
(fun cf cg x x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode c))))
(k' + 1, cf) n
let y ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode c))))
(k' + 1, cg) n
some (Nat.pair x y))
(fun cf cg x x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode c))))
(k' + 1, cg) n
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode c))))
(k' + 1, cf) x)
(fun cf cg x x =>
Nat.rec
(Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode c))))
(k' + 1, cf) (unpair n).1)
(fun n_1 n_ih => do
let i ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode c))))
(k', c) (Nat.pair (unpair n).1 n_1)
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode c))))
(k' + 1, cg) (Nat.pair (unpair n).1 (Nat.pair n_1 i)))
(unpair n).2)
(fun cf x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode c))))
(k' + 1, cf) n
Nat.rec (some (unpair n).2)
(fun n_1 n_ih =>
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode c))))
(k', c) (Nat.pair (unpair n).1 ((unpair n).2 + 1)))
x)
c =
evaln (k' + 1) c n
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
exact kn.elim (evaln_bound e)
simpa using kn
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
|
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
|
Mathlib.Computability.PartrecCode.1088_0.A3c3Aev6SyIRjCJ
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2
|
Mathlib_Computability_PartrecCode
|
x✝ : Unit
p n : ℕ
this : List.range p = List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))
c : Code
k' : ℕ
k : ℕ := k' + 1
nk : n ≤ k'
⊢ ∀ {k' : ℕ} {c' : Code} {n : ℕ},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode c))))
(k', c') n =
evaln k' c' n
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
exact kn.elim (evaln_bound e)
simpa using kn
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
|
intro k₁ c₁ n₁ hl
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
|
Mathlib.Computability.PartrecCode.1088_0.A3c3Aev6SyIRjCJ
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2
|
Mathlib_Computability_PartrecCode
|
x✝ : Unit
p n : ℕ
this : List.range p = List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))
c : Code
k' : ℕ
k : ℕ := k' + 1
nk : n ≤ k'
k₁ : ℕ
c₁ : Code
n₁ : ℕ
hl : Nat.pair k₁ (encode c₁) < Nat.pair k (encode c)
⊢ Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode c))))
(k₁, c₁) n₁ =
evaln k₁ c₁ n₁
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
exact kn.elim (evaln_bound e)
simpa using kn
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
|
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
|
Mathlib.Computability.PartrecCode.1088_0.A3c3Aev6SyIRjCJ
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2
|
Mathlib_Computability_PartrecCode
|
case succ
x✝ : Unit
p n : ℕ
this : List.range p = List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))
c : Code
k' : ℕ
k : ℕ := k' + 1
nk : n ≤ k'
hg :
∀ {k' : ℕ} {c' : Code} {n : ℕ},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode c))))
(k', c') n =
evaln k' c' n
⊢ rec (some 0) (some (Nat.succ n)) (some (unpair n).1) (some (unpair n).2)
(fun cf cg x x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode c))))
(k' + 1, cf) n
let y ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode c))))
(k' + 1, cg) n
some (Nat.pair x y))
(fun cf cg x x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode c))))
(k' + 1, cg) n
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode c))))
(k' + 1, cf) x)
(fun cf cg x x =>
Nat.rec
(Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode c))))
(k' + 1, cf) (unpair n).1)
(fun n_1 n_ih => do
let i ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode c))))
(k', c) (Nat.pair (unpair n).1 n_1)
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode c))))
(k' + 1, cg) (Nat.pair (unpair n).1 (Nat.pair n_1 i)))
(unpair n).2)
(fun cf x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode c))))
(k' + 1, cf) n
Nat.rec (some (unpair n).2)
(fun n_1 n_ih =>
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode c))))
(k', c) (Nat.pair (unpair n).1 ((unpair n).2 + 1)))
x)
c =
evaln (k' + 1) c n
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
exact kn.elim (evaln_bound e)
simpa using kn
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
|
cases' c with cf cg cf cg cf cg cf
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
|
Mathlib.Computability.PartrecCode.1088_0.A3c3Aev6SyIRjCJ
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2
|
Mathlib_Computability_PartrecCode
|
case succ.zero
x✝ : Unit
p n : ℕ
this : List.range p = List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))
k' : ℕ
k : ℕ := k' + 1
nk : n ≤ k'
hg :
∀ {k' : ℕ} {c' : Code} {n : ℕ},
Nat.pair k' (encode c') < Nat.pair k (encode zero) →
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode zero))))
(k', c') n =
evaln k' c' n
⊢ rec (some 0) (some (Nat.succ n)) (some (unpair n).1) (some (unpair n).2)
(fun cf cg x x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode zero))))
(k' + 1, cf) n
let y ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode zero))))
(k' + 1, cg) n
some (Nat.pair x y))
(fun cf cg x x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode zero))))
(k' + 1, cg) n
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode zero))))
(k' + 1, cf) x)
(fun cf cg x x =>
Nat.rec
(Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode zero))))
(k' + 1, cf) (unpair n).1)
(fun n_1 n_ih => do
let i ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode zero))))
(k', zero) (Nat.pair (unpair n).1 n_1)
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode zero))))
(k' + 1, cg) (Nat.pair (unpair n).1 (Nat.pair n_1 i)))
(unpair n).2)
(fun cf x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode zero))))
(k' + 1, cf) n
Nat.rec (some (unpair n).2)
(fun n_1 n_ih =>
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode zero))))
(k', zero) (Nat.pair (unpair n).1 ((unpair n).2 + 1)))
x)
zero =
evaln (k' + 1) zero n
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
exact kn.elim (evaln_bound e)
simpa using kn
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
|
simp [evaln, nk, Bind.bind, Functor.map, Seq.seq, pure]
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
|
Mathlib.Computability.PartrecCode.1088_0.A3c3Aev6SyIRjCJ
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2
|
Mathlib_Computability_PartrecCode
|
case succ.succ
x✝ : Unit
p n : ℕ
this : List.range p = List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))
k' : ℕ
k : ℕ := k' + 1
nk : n ≤ k'
hg :
∀ {k' : ℕ} {c' : Code} {n : ℕ},
Nat.pair k' (encode c') < Nat.pair k (encode succ) →
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode succ))))
(k', c') n =
evaln k' c' n
⊢ rec (some 0) (some (Nat.succ n)) (some (unpair n).1) (some (unpair n).2)
(fun cf cg x x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode succ))))
(k' + 1, cf) n
let y ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode succ))))
(k' + 1, cg) n
some (Nat.pair x y))
(fun cf cg x x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode succ))))
(k' + 1, cg) n
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode succ))))
(k' + 1, cf) x)
(fun cf cg x x =>
Nat.rec
(Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode succ))))
(k' + 1, cf) (unpair n).1)
(fun n_1 n_ih => do
let i ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode succ))))
(k', succ) (Nat.pair (unpair n).1 n_1)
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode succ))))
(k' + 1, cg) (Nat.pair (unpair n).1 (Nat.pair n_1 i)))
(unpair n).2)
(fun cf x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode succ))))
(k' + 1, cf) n
Nat.rec (some (unpair n).2)
(fun n_1 n_ih =>
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode succ))))
(k', succ) (Nat.pair (unpair n).1 ((unpair n).2 + 1)))
x)
succ =
evaln (k' + 1) succ n
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
exact kn.elim (evaln_bound e)
simpa using kn
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
|
simp [evaln, nk, Bind.bind, Functor.map, Seq.seq, pure]
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
|
Mathlib.Computability.PartrecCode.1088_0.A3c3Aev6SyIRjCJ
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2
|
Mathlib_Computability_PartrecCode
|
case succ.left
x✝ : Unit
p n : ℕ
this : List.range p = List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))
k' : ℕ
k : ℕ := k' + 1
nk : n ≤ k'
hg :
∀ {k' : ℕ} {c' : Code} {n : ℕ},
Nat.pair k' (encode c') < Nat.pair k (encode left) →
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode left))))
(k', c') n =
evaln k' c' n
⊢ rec (some 0) (some (Nat.succ n)) (some (unpair n).1) (some (unpair n).2)
(fun cf cg x x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode left))))
(k' + 1, cf) n
let y ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode left))))
(k' + 1, cg) n
some (Nat.pair x y))
(fun cf cg x x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode left))))
(k' + 1, cg) n
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode left))))
(k' + 1, cf) x)
(fun cf cg x x =>
Nat.rec
(Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode left))))
(k' + 1, cf) (unpair n).1)
(fun n_1 n_ih => do
let i ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode left))))
(k', left) (Nat.pair (unpair n).1 n_1)
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode left))))
(k' + 1, cg) (Nat.pair (unpair n).1 (Nat.pair n_1 i)))
(unpair n).2)
(fun cf x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode left))))
(k' + 1, cf) n
Nat.rec (some (unpair n).2)
(fun n_1 n_ih =>
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode left))))
(k', left) (Nat.pair (unpair n).1 ((unpair n).2 + 1)))
x)
left =
evaln (k' + 1) left n
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
exact kn.elim (evaln_bound e)
simpa using kn
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
|
simp [evaln, nk, Bind.bind, Functor.map, Seq.seq, pure]
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
|
Mathlib.Computability.PartrecCode.1088_0.A3c3Aev6SyIRjCJ
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2
|
Mathlib_Computability_PartrecCode
|
case succ.right
x✝ : Unit
p n : ℕ
this : List.range p = List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))
k' : ℕ
k : ℕ := k' + 1
nk : n ≤ k'
hg :
∀ {k' : ℕ} {c' : Code} {n : ℕ},
Nat.pair k' (encode c') < Nat.pair k (encode right) →
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode right))))
(k', c') n =
evaln k' c' n
⊢ rec (some 0) (some (Nat.succ n)) (some (unpair n).1) (some (unpair n).2)
(fun cf cg x x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode right))))
(k' + 1, cf) n
let y ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode right))))
(k' + 1, cg) n
some (Nat.pair x y))
(fun cf cg x x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode right))))
(k' + 1, cg) n
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode right))))
(k' + 1, cf) x)
(fun cf cg x x =>
Nat.rec
(Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode right))))
(k' + 1, cf) (unpair n).1)
(fun n_1 n_ih => do
let i ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode right))))
(k', right) (Nat.pair (unpair n).1 n_1)
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode right))))
(k' + 1, cg) (Nat.pair (unpair n).1 (Nat.pair n_1 i)))
(unpair n).2)
(fun cf x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode right))))
(k' + 1, cf) n
Nat.rec (some (unpair n).2)
(fun n_1 n_ih =>
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode right))))
(k', right) (Nat.pair (unpair n).1 ((unpair n).2 + 1)))
x)
right =
evaln (k' + 1) right n
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
exact kn.elim (evaln_bound e)
simpa using kn
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
|
simp [evaln, nk, Bind.bind, Functor.map, Seq.seq, pure]
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
|
Mathlib.Computability.PartrecCode.1088_0.A3c3Aev6SyIRjCJ
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2
|
Mathlib_Computability_PartrecCode
|
case succ.pair
x✝ : Unit
p n : ℕ
this : List.range p = List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))
k' : ℕ
k : ℕ := k' + 1
nk : n ≤ k'
cf cg : Code
hg :
∀ {k' : ℕ} {c' : Code} {n : ℕ},
Nat.pair k' (encode c') < Nat.pair k (encode (pair cf cg)) →
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode (pair cf cg)))))
(k', c') n =
evaln k' c' n
⊢ rec (some 0) (some (Nat.succ n)) (some (unpair n).1) (some (unpair n).2)
(fun cf_1 cg_1 x x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (pair cf cg)))))
(k' + 1, cf_1) n
let y ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (pair cf cg)))))
(k' + 1, cg_1) n
some (Nat.pair x y))
(fun cf_1 cg_1 x x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (pair cf cg)))))
(k' + 1, cg_1) n
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (pair cf cg)))))
(k' + 1, cf_1) x)
(fun cf_1 cg_1 x x =>
Nat.rec
(Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (pair cf cg)))))
(k' + 1, cf_1) (unpair n).1)
(fun n_1 n_ih => do
let i ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (pair cf cg)))))
(k', pair cf cg) (Nat.pair (unpair n).1 n_1)
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (pair cf cg)))))
(k' + 1, cg_1) (Nat.pair (unpair n).1 (Nat.pair n_1 i)))
(unpair n).2)
(fun cf_1 x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (pair cf cg)))))
(k' + 1, cf_1) n
Nat.rec (some (unpair n).2)
(fun n_1 n_ih =>
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (pair cf cg)))))
(k', pair cf cg) (Nat.pair (unpair n).1 ((unpair n).2 + 1)))
x)
(pair cf cg) =
evaln (k' + 1) (pair cf cg) n
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
exact kn.elim (evaln_bound e)
simpa using kn
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
|
simp [evaln, nk, Bind.bind, Functor.map, Seq.seq, pure]
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
|
Mathlib.Computability.PartrecCode.1088_0.A3c3Aev6SyIRjCJ
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2
|
Mathlib_Computability_PartrecCode
|
case succ.comp
x✝ : Unit
p n : ℕ
this : List.range p = List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))
k' : ℕ
k : ℕ := k' + 1
nk : n ≤ k'
cf cg : Code
hg :
∀ {k' : ℕ} {c' : Code} {n : ℕ},
Nat.pair k' (encode c') < Nat.pair k (encode (comp cf cg)) →
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode (comp cf cg)))))
(k', c') n =
evaln k' c' n
⊢ rec (some 0) (some (Nat.succ n)) (some (unpair n).1) (some (unpair n).2)
(fun cf_1 cg_1 x x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (comp cf cg)))))
(k' + 1, cf_1) n
let y ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (comp cf cg)))))
(k' + 1, cg_1) n
some (Nat.pair x y))
(fun cf_1 cg_1 x x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (comp cf cg)))))
(k' + 1, cg_1) n
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (comp cf cg)))))
(k' + 1, cf_1) x)
(fun cf_1 cg_1 x x =>
Nat.rec
(Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (comp cf cg)))))
(k' + 1, cf_1) (unpair n).1)
(fun n_1 n_ih => do
let i ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (comp cf cg)))))
(k', comp cf cg) (Nat.pair (unpair n).1 n_1)
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (comp cf cg)))))
(k' + 1, cg_1) (Nat.pair (unpair n).1 (Nat.pair n_1 i)))
(unpair n).2)
(fun cf_1 x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (comp cf cg)))))
(k' + 1, cf_1) n
Nat.rec (some (unpair n).2)
(fun n_1 n_ih =>
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (comp cf cg)))))
(k', comp cf cg) (Nat.pair (unpair n).1 ((unpair n).2 + 1)))
x)
(comp cf cg) =
evaln (k' + 1) (comp cf cg) n
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
exact kn.elim (evaln_bound e)
simpa using kn
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
|
simp [evaln, nk, Bind.bind, Functor.map, Seq.seq, pure]
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
|
Mathlib.Computability.PartrecCode.1088_0.A3c3Aev6SyIRjCJ
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2
|
Mathlib_Computability_PartrecCode
|
case succ.prec
x✝ : Unit
p n : ℕ
this : List.range p = List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))
k' : ℕ
k : ℕ := k' + 1
nk : n ≤ k'
cf cg : Code
hg :
∀ {k' : ℕ} {c' : Code} {n : ℕ},
Nat.pair k' (encode c') < Nat.pair k (encode (prec cf cg)) →
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode (prec cf cg)))))
(k', c') n =
evaln k' c' n
⊢ rec (some 0) (some (Nat.succ n)) (some (unpair n).1) (some (unpair n).2)
(fun cf_1 cg_1 x x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (prec cf cg)))))
(k' + 1, cf_1) n
let y ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (prec cf cg)))))
(k' + 1, cg_1) n
some (Nat.pair x y))
(fun cf_1 cg_1 x x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (prec cf cg)))))
(k' + 1, cg_1) n
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (prec cf cg)))))
(k' + 1, cf_1) x)
(fun cf_1 cg_1 x x =>
Nat.rec
(Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (prec cf cg)))))
(k' + 1, cf_1) (unpair n).1)
(fun n_1 n_ih => do
let i ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (prec cf cg)))))
(k', prec cf cg) (Nat.pair (unpair n).1 n_1)
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (prec cf cg)))))
(k' + 1, cg_1) (Nat.pair (unpair n).1 (Nat.pair n_1 i)))
(unpair n).2)
(fun cf_1 x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (prec cf cg)))))
(k' + 1, cf_1) n
Nat.rec (some (unpair n).2)
(fun n_1 n_ih =>
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (prec cf cg)))))
(k', prec cf cg) (Nat.pair (unpair n).1 ((unpair n).2 + 1)))
x)
(prec cf cg) =
evaln (k' + 1) (prec cf cg) n
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
exact kn.elim (evaln_bound e)
simpa using kn
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
|
simp [evaln, nk, Bind.bind, Functor.map, Seq.seq, pure]
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
|
Mathlib.Computability.PartrecCode.1088_0.A3c3Aev6SyIRjCJ
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2
|
Mathlib_Computability_PartrecCode
|
case succ.rfind'
x✝ : Unit
p n : ℕ
this : List.range p = List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))
k' : ℕ
k : ℕ := k' + 1
nk : n ≤ k'
cf : Code
hg :
∀ {k' : ℕ} {c' : Code} {n : ℕ},
Nat.pair k' (encode c') < Nat.pair k (encode (rfind' cf)) →
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode (rfind' cf)))))
(k', c') n =
evaln k' c' n
⊢ rec (some 0) (some (Nat.succ n)) (some (unpair n).1) (some (unpair n).2)
(fun cf_1 cg x x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (rfind' cf)))))
(k' + 1, cf_1) n
let y ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (rfind' cf)))))
(k' + 1, cg) n
some (Nat.pair x y))
(fun cf_1 cg x x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (rfind' cf)))))
(k' + 1, cg) n
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (rfind' cf)))))
(k' + 1, cf_1) x)
(fun cf_1 cg x x =>
Nat.rec
(Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (rfind' cf)))))
(k' + 1, cf_1) (unpair n).1)
(fun n_1 n_ih => do
let i ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (rfind' cf)))))
(k', rfind' cf) (Nat.pair (unpair n).1 n_1)
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (rfind' cf)))))
(k' + 1, cg) (Nat.pair (unpair n).1 (Nat.pair n_1 i)))
(unpair n).2)
(fun cf_1 x => do
let x ←
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (rfind' cf)))))
(k' + 1, cf_1) n
Nat.rec (some (unpair n).2)
(fun n_1 n_ih =>
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (rfind' cf)))))
(k', rfind' cf) (Nat.pair (unpair n).1 ((unpair n).2 + 1)))
x)
(rfind' cf) =
evaln (k' + 1) (rfind' cf) n
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
exact kn.elim (evaln_bound e)
simpa using kn
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
|
simp [evaln, nk, Bind.bind, Functor.map, Seq.seq, pure]
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
|
Mathlib.Computability.PartrecCode.1088_0.A3c3Aev6SyIRjCJ
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2
|
Mathlib_Computability_PartrecCode
|
case succ.pair
x✝ : Unit
p n : ℕ
this : List.range p = List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))
k' : ℕ
k : ℕ := k' + 1
nk : n ≤ k'
cf cg : Code
hg :
∀ {k' : ℕ} {c' : Code} {n : ℕ},
Nat.pair k' (encode c') < Nat.pair k (encode (pair cf cg)) →
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode (pair cf cg)))))
(k', c') n =
evaln k' c' n
⊢ (Option.bind
(Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (pair cf cg)))))
(k' + 1, cf) n)
fun x =>
Option.bind
(Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (pair cf cg)))))
(k' + 1, cg) n)
fun y => some (Nat.pair x y)) =
Option.bind (Option.map Nat.pair (evaln (k' + 1) cf n)) fun y => Option.map y (evaln (k' + 1) cg n)
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
exact kn.elim (evaln_bound e)
simpa using kn
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
simp [evaln, nk, Bind.bind, Functor.map, Seq.seq, pure]
·
|
cases' encode_lt_pair cf cg with lf lg
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
simp [evaln, nk, Bind.bind, Functor.map, Seq.seq, pure]
·
|
Mathlib.Computability.PartrecCode.1088_0.A3c3Aev6SyIRjCJ
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2
|
Mathlib_Computability_PartrecCode
|
case succ.pair.intro
x✝ : Unit
p n : ℕ
this : List.range p = List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))
k' : ℕ
k : ℕ := k' + 1
nk : n ≤ k'
cf cg : Code
hg :
∀ {k' : ℕ} {c' : Code} {n : ℕ},
Nat.pair k' (encode c') < Nat.pair k (encode (pair cf cg)) →
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode (pair cf cg)))))
(k', c') n =
evaln k' c' n
lf : encode cf < encode (pair cf cg)
lg : encode cg < encode (pair cf cg)
⊢ (Option.bind
(Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (pair cf cg)))))
(k' + 1, cf) n)
fun x =>
Option.bind
(Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (pair cf cg)))))
(k' + 1, cg) n)
fun y => some (Nat.pair x y)) =
Option.bind (Option.map Nat.pair (evaln (k' + 1) cf n)) fun y => Option.map y (evaln (k' + 1) cg n)
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
exact kn.elim (evaln_bound e)
simpa using kn
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
simp [evaln, nk, Bind.bind, Functor.map, Seq.seq, pure]
· cases' encode_lt_pair cf cg with lf lg
|
rw [hg (Nat.pair_lt_pair_right _ lf), hg (Nat.pair_lt_pair_right _ lg)]
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
simp [evaln, nk, Bind.bind, Functor.map, Seq.seq, pure]
· cases' encode_lt_pair cf cg with lf lg
|
Mathlib.Computability.PartrecCode.1088_0.A3c3Aev6SyIRjCJ
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2
|
Mathlib_Computability_PartrecCode
|
case succ.pair.intro
x✝ : Unit
p n : ℕ
this : List.range p = List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))
k' : ℕ
k : ℕ := k' + 1
nk : n ≤ k'
cf cg : Code
hg :
∀ {k' : ℕ} {c' : Code} {n : ℕ},
Nat.pair k' (encode c') < Nat.pair k (encode (pair cf cg)) →
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode (pair cf cg)))))
(k', c') n =
evaln k' c' n
lf : encode cf < encode (pair cf cg)
lg : encode cg < encode (pair cf cg)
⊢ (Option.bind (evaln k cf n) fun x => Option.bind (evaln k cg n) fun y => some (Nat.pair x y)) =
Option.bind (Option.map Nat.pair (evaln (k' + 1) cf n)) fun y => Option.map y (evaln (k' + 1) cg n)
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
exact kn.elim (evaln_bound e)
simpa using kn
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
simp [evaln, nk, Bind.bind, Functor.map, Seq.seq, pure]
· cases' encode_lt_pair cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lf), hg (Nat.pair_lt_pair_right _ lg)]
|
cases evaln k cf n
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
simp [evaln, nk, Bind.bind, Functor.map, Seq.seq, pure]
· cases' encode_lt_pair cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lf), hg (Nat.pair_lt_pair_right _ lg)]
|
Mathlib.Computability.PartrecCode.1088_0.A3c3Aev6SyIRjCJ
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2
|
Mathlib_Computability_PartrecCode
|
case succ.pair.intro.none
x✝ : Unit
p n : ℕ
this : List.range p = List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))
k' : ℕ
k : ℕ := k' + 1
nk : n ≤ k'
cf cg : Code
hg :
∀ {k' : ℕ} {c' : Code} {n : ℕ},
Nat.pair k' (encode c') < Nat.pair k (encode (pair cf cg)) →
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode (pair cf cg)))))
(k', c') n =
evaln k' c' n
lf : encode cf < encode (pair cf cg)
lg : encode cg < encode (pair cf cg)
⊢ (Option.bind Option.none fun x => Option.bind (evaln k cg n) fun y => some (Nat.pair x y)) =
Option.bind (Option.map Nat.pair Option.none) fun y => Option.map y (evaln (k' + 1) cg n)
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
exact kn.elim (evaln_bound e)
simpa using kn
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
simp [evaln, nk, Bind.bind, Functor.map, Seq.seq, pure]
· cases' encode_lt_pair cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lf), hg (Nat.pair_lt_pair_right _ lg)]
cases evaln k cf n
·
|
rfl
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
simp [evaln, nk, Bind.bind, Functor.map, Seq.seq, pure]
· cases' encode_lt_pair cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lf), hg (Nat.pair_lt_pair_right _ lg)]
cases evaln k cf n
·
|
Mathlib.Computability.PartrecCode.1088_0.A3c3Aev6SyIRjCJ
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2
|
Mathlib_Computability_PartrecCode
|
case succ.pair.intro.some
x✝ : Unit
p n : ℕ
this : List.range p = List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))
k' : ℕ
k : ℕ := k' + 1
nk : n ≤ k'
cf cg : Code
hg :
∀ {k' : ℕ} {c' : Code} {n : ℕ},
Nat.pair k' (encode c') < Nat.pair k (encode (pair cf cg)) →
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode (pair cf cg)))))
(k', c') n =
evaln k' c' n
lf : encode cf < encode (pair cf cg)
lg : encode cg < encode (pair cf cg)
val✝ : ℕ
⊢ (Option.bind (some val✝) fun x => Option.bind (evaln k cg n) fun y => some (Nat.pair x y)) =
Option.bind (Option.map Nat.pair (some val✝)) fun y => Option.map y (evaln (k' + 1) cg n)
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
exact kn.elim (evaln_bound e)
simpa using kn
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
simp [evaln, nk, Bind.bind, Functor.map, Seq.seq, pure]
· cases' encode_lt_pair cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lf), hg (Nat.pair_lt_pair_right _ lg)]
cases evaln k cf n
· rfl
|
cases evaln k cg n
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
simp [evaln, nk, Bind.bind, Functor.map, Seq.seq, pure]
· cases' encode_lt_pair cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lf), hg (Nat.pair_lt_pair_right _ lg)]
cases evaln k cf n
· rfl
|
Mathlib.Computability.PartrecCode.1088_0.A3c3Aev6SyIRjCJ
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2
|
Mathlib_Computability_PartrecCode
|
case succ.pair.intro.some.none
x✝ : Unit
p n : ℕ
this : List.range p = List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))
k' : ℕ
k : ℕ := k' + 1
nk : n ≤ k'
cf cg : Code
hg :
∀ {k' : ℕ} {c' : Code} {n : ℕ},
Nat.pair k' (encode c') < Nat.pair k (encode (pair cf cg)) →
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode (pair cf cg)))))
(k', c') n =
evaln k' c' n
lf : encode cf < encode (pair cf cg)
lg : encode cg < encode (pair cf cg)
val✝ : ℕ
⊢ (Option.bind (some val✝) fun x => Option.bind Option.none fun y => some (Nat.pair x y)) =
Option.bind (Option.map Nat.pair (some val✝)) fun y => Option.map y Option.none
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
exact kn.elim (evaln_bound e)
simpa using kn
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
simp [evaln, nk, Bind.bind, Functor.map, Seq.seq, pure]
· cases' encode_lt_pair cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lf), hg (Nat.pair_lt_pair_right _ lg)]
cases evaln k cf n
· rfl
cases evaln k cg n <;>
|
rfl
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
simp [evaln, nk, Bind.bind, Functor.map, Seq.seq, pure]
· cases' encode_lt_pair cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lf), hg (Nat.pair_lt_pair_right _ lg)]
cases evaln k cf n
· rfl
cases evaln k cg n <;>
|
Mathlib.Computability.PartrecCode.1088_0.A3c3Aev6SyIRjCJ
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2
|
Mathlib_Computability_PartrecCode
|
case succ.pair.intro.some.some
x✝ : Unit
p n : ℕ
this : List.range p = List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))
k' : ℕ
k : ℕ := k' + 1
nk : n ≤ k'
cf cg : Code
hg :
∀ {k' : ℕ} {c' : Code} {n : ℕ},
Nat.pair k' (encode c') < Nat.pair k (encode (pair cf cg)) →
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode (pair cf cg)))))
(k', c') n =
evaln k' c' n
lf : encode cf < encode (pair cf cg)
lg : encode cg < encode (pair cf cg)
val✝¹ val✝ : ℕ
⊢ (Option.bind (some val✝¹) fun x => Option.bind (some val✝) fun y => some (Nat.pair x y)) =
Option.bind (Option.map Nat.pair (some val✝¹)) fun y => Option.map y (some val✝)
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
exact kn.elim (evaln_bound e)
simpa using kn
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
simp [evaln, nk, Bind.bind, Functor.map, Seq.seq, pure]
· cases' encode_lt_pair cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lf), hg (Nat.pair_lt_pair_right _ lg)]
cases evaln k cf n
· rfl
cases evaln k cg n <;>
|
rfl
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
simp [evaln, nk, Bind.bind, Functor.map, Seq.seq, pure]
· cases' encode_lt_pair cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lf), hg (Nat.pair_lt_pair_right _ lg)]
cases evaln k cf n
· rfl
cases evaln k cg n <;>
|
Mathlib.Computability.PartrecCode.1088_0.A3c3Aev6SyIRjCJ
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2
|
Mathlib_Computability_PartrecCode
|
case succ.comp
x✝ : Unit
p n : ℕ
this : List.range p = List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))
k' : ℕ
k : ℕ := k' + 1
nk : n ≤ k'
cf cg : Code
hg :
∀ {k' : ℕ} {c' : Code} {n : ℕ},
Nat.pair k' (encode c') < Nat.pair k (encode (comp cf cg)) →
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode (comp cf cg)))))
(k', c') n =
evaln k' c' n
⊢ (Option.bind
(Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (comp cf cg)))))
(k' + 1, cg) n)
fun x =>
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (comp cf cg)))))
(k' + 1, cf) x) =
Option.bind (evaln (k' + 1) cg n) fun x => evaln (k' + 1) cf x
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
exact kn.elim (evaln_bound e)
simpa using kn
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
simp [evaln, nk, Bind.bind, Functor.map, Seq.seq, pure]
· cases' encode_lt_pair cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lf), hg (Nat.pair_lt_pair_right _ lg)]
cases evaln k cf n
· rfl
cases evaln k cg n <;> rfl
·
|
cases' encode_lt_comp cf cg with lf lg
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
simp [evaln, nk, Bind.bind, Functor.map, Seq.seq, pure]
· cases' encode_lt_pair cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lf), hg (Nat.pair_lt_pair_right _ lg)]
cases evaln k cf n
· rfl
cases evaln k cg n <;> rfl
·
|
Mathlib.Computability.PartrecCode.1088_0.A3c3Aev6SyIRjCJ
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2
|
Mathlib_Computability_PartrecCode
|
case succ.comp.intro
x✝ : Unit
p n : ℕ
this : List.range p = List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))
k' : ℕ
k : ℕ := k' + 1
nk : n ≤ k'
cf cg : Code
hg :
∀ {k' : ℕ} {c' : Code} {n : ℕ},
Nat.pair k' (encode c') < Nat.pair k (encode (comp cf cg)) →
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode (comp cf cg)))))
(k', c') n =
evaln k' c' n
lf : encode cf < encode (comp cf cg)
lg : encode cg < encode (comp cf cg)
⊢ (Option.bind
(Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (comp cf cg)))))
(k' + 1, cg) n)
fun x =>
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (comp cf cg)))))
(k' + 1, cf) x) =
Option.bind (evaln (k' + 1) cg n) fun x => evaln (k' + 1) cf x
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
exact kn.elim (evaln_bound e)
simpa using kn
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
simp [evaln, nk, Bind.bind, Functor.map, Seq.seq, pure]
· cases' encode_lt_pair cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lf), hg (Nat.pair_lt_pair_right _ lg)]
cases evaln k cf n
· rfl
cases evaln k cg n <;> rfl
· cases' encode_lt_comp cf cg with lf lg
|
rw [hg (Nat.pair_lt_pair_right _ lg)]
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
simp [evaln, nk, Bind.bind, Functor.map, Seq.seq, pure]
· cases' encode_lt_pair cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lf), hg (Nat.pair_lt_pair_right _ lg)]
cases evaln k cf n
· rfl
cases evaln k cg n <;> rfl
· cases' encode_lt_comp cf cg with lf lg
|
Mathlib.Computability.PartrecCode.1088_0.A3c3Aev6SyIRjCJ
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2
|
Mathlib_Computability_PartrecCode
|
case succ.comp.intro
x✝ : Unit
p n : ℕ
this : List.range p = List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))
k' : ℕ
k : ℕ := k' + 1
nk : n ≤ k'
cf cg : Code
hg :
∀ {k' : ℕ} {c' : Code} {n : ℕ},
Nat.pair k' (encode c') < Nat.pair k (encode (comp cf cg)) →
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode (comp cf cg)))))
(k', c') n =
evaln k' c' n
lf : encode cf < encode (comp cf cg)
lg : encode cg < encode (comp cf cg)
⊢ (Option.bind (evaln k cg n) fun x =>
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (comp cf cg)))))
(k' + 1, cf) x) =
Option.bind (evaln (k' + 1) cg n) fun x => evaln (k' + 1) cf x
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
exact kn.elim (evaln_bound e)
simpa using kn
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
simp [evaln, nk, Bind.bind, Functor.map, Seq.seq, pure]
· cases' encode_lt_pair cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lf), hg (Nat.pair_lt_pair_right _ lg)]
cases evaln k cf n
· rfl
cases evaln k cg n <;> rfl
· cases' encode_lt_comp cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lg)]
|
cases evaln k cg n
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
simp [evaln, nk, Bind.bind, Functor.map, Seq.seq, pure]
· cases' encode_lt_pair cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lf), hg (Nat.pair_lt_pair_right _ lg)]
cases evaln k cf n
· rfl
cases evaln k cg n <;> rfl
· cases' encode_lt_comp cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lg)]
|
Mathlib.Computability.PartrecCode.1088_0.A3c3Aev6SyIRjCJ
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2
|
Mathlib_Computability_PartrecCode
|
case succ.comp.intro.none
x✝ : Unit
p n : ℕ
this : List.range p = List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))
k' : ℕ
k : ℕ := k' + 1
nk : n ≤ k'
cf cg : Code
hg :
∀ {k' : ℕ} {c' : Code} {n : ℕ},
Nat.pair k' (encode c') < Nat.pair k (encode (comp cf cg)) →
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode (comp cf cg)))))
(k', c') n =
evaln k' c' n
lf : encode cf < encode (comp cf cg)
lg : encode cg < encode (comp cf cg)
⊢ (Option.bind Option.none fun x =>
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (comp cf cg)))))
(k' + 1, cf) x) =
Option.bind Option.none fun x => evaln (k' + 1) cf x
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
exact kn.elim (evaln_bound e)
simpa using kn
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
simp [evaln, nk, Bind.bind, Functor.map, Seq.seq, pure]
· cases' encode_lt_pair cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lf), hg (Nat.pair_lt_pair_right _ lg)]
cases evaln k cf n
· rfl
cases evaln k cg n <;> rfl
· cases' encode_lt_comp cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lg)]
cases evaln k cg n
·
|
rfl
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
simp [evaln, nk, Bind.bind, Functor.map, Seq.seq, pure]
· cases' encode_lt_pair cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lf), hg (Nat.pair_lt_pair_right _ lg)]
cases evaln k cf n
· rfl
cases evaln k cg n <;> rfl
· cases' encode_lt_comp cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lg)]
cases evaln k cg n
·
|
Mathlib.Computability.PartrecCode.1088_0.A3c3Aev6SyIRjCJ
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2
|
Mathlib_Computability_PartrecCode
|
case succ.comp.intro.some
x✝ : Unit
p n : ℕ
this : List.range p = List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))
k' : ℕ
k : ℕ := k' + 1
nk : n ≤ k'
cf cg : Code
hg :
∀ {k' : ℕ} {c' : Code} {n : ℕ},
Nat.pair k' (encode c') < Nat.pair k (encode (comp cf cg)) →
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode (comp cf cg)))))
(k', c') n =
evaln k' c' n
lf : encode cf < encode (comp cf cg)
lg : encode cg < encode (comp cf cg)
val✝ : ℕ
⊢ (Option.bind (some val✝) fun x =>
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (comp cf cg)))))
(k' + 1, cf) x) =
Option.bind (some val✝) fun x => evaln (k' + 1) cf x
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
exact kn.elim (evaln_bound e)
simpa using kn
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
simp [evaln, nk, Bind.bind, Functor.map, Seq.seq, pure]
· cases' encode_lt_pair cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lf), hg (Nat.pair_lt_pair_right _ lg)]
cases evaln k cf n
· rfl
cases evaln k cg n <;> rfl
· cases' encode_lt_comp cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lg)]
cases evaln k cg n
· rfl
|
simp [hg (Nat.pair_lt_pair_right _ lf)]
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
simp [evaln, nk, Bind.bind, Functor.map, Seq.seq, pure]
· cases' encode_lt_pair cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lf), hg (Nat.pair_lt_pair_right _ lg)]
cases evaln k cf n
· rfl
cases evaln k cg n <;> rfl
· cases' encode_lt_comp cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lg)]
cases evaln k cg n
· rfl
|
Mathlib.Computability.PartrecCode.1088_0.A3c3Aev6SyIRjCJ
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2
|
Mathlib_Computability_PartrecCode
|
case succ.prec
x✝ : Unit
p n : ℕ
this : List.range p = List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))
k' : ℕ
k : ℕ := k' + 1
nk : n ≤ k'
cf cg : Code
hg :
∀ {k' : ℕ} {c' : Code} {n : ℕ},
Nat.pair k' (encode c') < Nat.pair k (encode (prec cf cg)) →
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode (prec cf cg)))))
(k', c') n =
evaln k' c' n
⊢ Nat.rec
(Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (prec cf cg)))))
(k' + 1, cf) (unpair n).1)
(fun n_1 n_ih =>
Option.bind
(Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (prec cf cg)))))
(k', prec cf cg) (Nat.pair (unpair n).1 n_1))
fun i =>
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (prec cf cg)))))
(k' + 1, cg) (Nat.pair (unpair n).1 (Nat.pair n_1 i)))
(unpair n).2 =
Nat.rec (evaln (k' + 1) cf (unpair n).1)
(fun n_1 n_ih =>
Option.bind (evaln k' (prec cf cg) (Nat.pair (unpair n).1 n_1)) fun i =>
evaln (k' + 1) cg (Nat.pair (unpair n).1 (Nat.pair n_1 i)))
(unpair n).2
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
exact kn.elim (evaln_bound e)
simpa using kn
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
simp [evaln, nk, Bind.bind, Functor.map, Seq.seq, pure]
· cases' encode_lt_pair cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lf), hg (Nat.pair_lt_pair_right _ lg)]
cases evaln k cf n
· rfl
cases evaln k cg n <;> rfl
· cases' encode_lt_comp cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lg)]
cases evaln k cg n
· rfl
simp [hg (Nat.pair_lt_pair_right _ lf)]
·
|
cases' encode_lt_prec cf cg with lf lg
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
simp [evaln, nk, Bind.bind, Functor.map, Seq.seq, pure]
· cases' encode_lt_pair cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lf), hg (Nat.pair_lt_pair_right _ lg)]
cases evaln k cf n
· rfl
cases evaln k cg n <;> rfl
· cases' encode_lt_comp cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lg)]
cases evaln k cg n
· rfl
simp [hg (Nat.pair_lt_pair_right _ lf)]
·
|
Mathlib.Computability.PartrecCode.1088_0.A3c3Aev6SyIRjCJ
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2
|
Mathlib_Computability_PartrecCode
|
case succ.prec.intro
x✝ : Unit
p n : ℕ
this : List.range p = List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))
k' : ℕ
k : ℕ := k' + 1
nk : n ≤ k'
cf cg : Code
hg :
∀ {k' : ℕ} {c' : Code} {n : ℕ},
Nat.pair k' (encode c') < Nat.pair k (encode (prec cf cg)) →
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode (prec cf cg)))))
(k', c') n =
evaln k' c' n
lf : encode cf < encode (prec cf cg)
lg : encode cg < encode (prec cf cg)
⊢ Nat.rec
(Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (prec cf cg)))))
(k' + 1, cf) (unpair n).1)
(fun n_1 n_ih =>
Option.bind
(Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (prec cf cg)))))
(k', prec cf cg) (Nat.pair (unpair n).1 n_1))
fun i =>
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (prec cf cg)))))
(k' + 1, cg) (Nat.pair (unpair n).1 (Nat.pair n_1 i)))
(unpair n).2 =
Nat.rec (evaln (k' + 1) cf (unpair n).1)
(fun n_1 n_ih =>
Option.bind (evaln k' (prec cf cg) (Nat.pair (unpair n).1 n_1)) fun i =>
evaln (k' + 1) cg (Nat.pair (unpair n).1 (Nat.pair n_1 i)))
(unpair n).2
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
exact kn.elim (evaln_bound e)
simpa using kn
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
simp [evaln, nk, Bind.bind, Functor.map, Seq.seq, pure]
· cases' encode_lt_pair cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lf), hg (Nat.pair_lt_pair_right _ lg)]
cases evaln k cf n
· rfl
cases evaln k cg n <;> rfl
· cases' encode_lt_comp cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lg)]
cases evaln k cg n
· rfl
simp [hg (Nat.pair_lt_pair_right _ lf)]
· cases' encode_lt_prec cf cg with lf lg
|
rw [hg (Nat.pair_lt_pair_right _ lf)]
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
simp [evaln, nk, Bind.bind, Functor.map, Seq.seq, pure]
· cases' encode_lt_pair cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lf), hg (Nat.pair_lt_pair_right _ lg)]
cases evaln k cf n
· rfl
cases evaln k cg n <;> rfl
· cases' encode_lt_comp cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lg)]
cases evaln k cg n
· rfl
simp [hg (Nat.pair_lt_pair_right _ lf)]
· cases' encode_lt_prec cf cg with lf lg
|
Mathlib.Computability.PartrecCode.1088_0.A3c3Aev6SyIRjCJ
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2
|
Mathlib_Computability_PartrecCode
|
case succ.prec.intro
x✝ : Unit
p n : ℕ
this : List.range p = List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))
k' : ℕ
k : ℕ := k' + 1
nk : n ≤ k'
cf cg : Code
hg :
∀ {k' : ℕ} {c' : Code} {n : ℕ},
Nat.pair k' (encode c') < Nat.pair k (encode (prec cf cg)) →
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode (prec cf cg)))))
(k', c') n =
evaln k' c' n
lf : encode cf < encode (prec cf cg)
lg : encode cg < encode (prec cf cg)
⊢ Nat.rec (evaln k cf (unpair n).1)
(fun n_1 n_ih =>
Option.bind
(Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (prec cf cg)))))
(k', prec cf cg) (Nat.pair (unpair n).1 n_1))
fun i =>
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (prec cf cg)))))
(k' + 1, cg) (Nat.pair (unpair n).1 (Nat.pair n_1 i)))
(unpair n).2 =
Nat.rec (evaln (k' + 1) cf (unpair n).1)
(fun n_1 n_ih =>
Option.bind (evaln k' (prec cf cg) (Nat.pair (unpair n).1 n_1)) fun i =>
evaln (k' + 1) cg (Nat.pair (unpair n).1 (Nat.pair n_1 i)))
(unpair n).2
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
exact kn.elim (evaln_bound e)
simpa using kn
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
simp [evaln, nk, Bind.bind, Functor.map, Seq.seq, pure]
· cases' encode_lt_pair cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lf), hg (Nat.pair_lt_pair_right _ lg)]
cases evaln k cf n
· rfl
cases evaln k cg n <;> rfl
· cases' encode_lt_comp cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lg)]
cases evaln k cg n
· rfl
simp [hg (Nat.pair_lt_pair_right _ lf)]
· cases' encode_lt_prec cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lf)]
|
cases n.unpair.2
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
simp [evaln, nk, Bind.bind, Functor.map, Seq.seq, pure]
· cases' encode_lt_pair cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lf), hg (Nat.pair_lt_pair_right _ lg)]
cases evaln k cf n
· rfl
cases evaln k cg n <;> rfl
· cases' encode_lt_comp cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lg)]
cases evaln k cg n
· rfl
simp [hg (Nat.pair_lt_pair_right _ lf)]
· cases' encode_lt_prec cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lf)]
|
Mathlib.Computability.PartrecCode.1088_0.A3c3Aev6SyIRjCJ
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2
|
Mathlib_Computability_PartrecCode
|
case succ.prec.intro.zero
x✝ : Unit
p n : ℕ
this : List.range p = List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))
k' : ℕ
k : ℕ := k' + 1
nk : n ≤ k'
cf cg : Code
hg :
∀ {k' : ℕ} {c' : Code} {n : ℕ},
Nat.pair k' (encode c') < Nat.pair k (encode (prec cf cg)) →
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode (prec cf cg)))))
(k', c') n =
evaln k' c' n
lf : encode cf < encode (prec cf cg)
lg : encode cg < encode (prec cf cg)
⊢ Nat.rec (evaln k cf (unpair n).1)
(fun n_1 n_ih =>
Option.bind
(Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (prec cf cg)))))
(k', prec cf cg) (Nat.pair (unpair n).1 n_1))
fun i =>
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (prec cf cg)))))
(k' + 1, cg) (Nat.pair (unpair n).1 (Nat.pair n_1 i)))
Nat.zero =
Nat.rec (evaln (k' + 1) cf (unpair n).1)
(fun n_1 n_ih =>
Option.bind (evaln k' (prec cf cg) (Nat.pair (unpair n).1 n_1)) fun i =>
evaln (k' + 1) cg (Nat.pair (unpair n).1 (Nat.pair n_1 i)))
Nat.zero
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
exact kn.elim (evaln_bound e)
simpa using kn
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
simp [evaln, nk, Bind.bind, Functor.map, Seq.seq, pure]
· cases' encode_lt_pair cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lf), hg (Nat.pair_lt_pair_right _ lg)]
cases evaln k cf n
· rfl
cases evaln k cg n <;> rfl
· cases' encode_lt_comp cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lg)]
cases evaln k cg n
· rfl
simp [hg (Nat.pair_lt_pair_right _ lf)]
· cases' encode_lt_prec cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lf)]
cases n.unpair.2
·
|
rfl
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
simp [evaln, nk, Bind.bind, Functor.map, Seq.seq, pure]
· cases' encode_lt_pair cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lf), hg (Nat.pair_lt_pair_right _ lg)]
cases evaln k cf n
· rfl
cases evaln k cg n <;> rfl
· cases' encode_lt_comp cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lg)]
cases evaln k cg n
· rfl
simp [hg (Nat.pair_lt_pair_right _ lf)]
· cases' encode_lt_prec cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lf)]
cases n.unpair.2
·
|
Mathlib.Computability.PartrecCode.1088_0.A3c3Aev6SyIRjCJ
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2
|
Mathlib_Computability_PartrecCode
|
case succ.prec.intro.succ
x✝ : Unit
p n : ℕ
this : List.range p = List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))
k' : ℕ
k : ℕ := k' + 1
nk : n ≤ k'
cf cg : Code
hg :
∀ {k' : ℕ} {c' : Code} {n : ℕ},
Nat.pair k' (encode c') < Nat.pair k (encode (prec cf cg)) →
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair k (encode (prec cf cg)))))
(k', c') n =
evaln k' c' n
lf : encode cf < encode (prec cf cg)
lg : encode cg < encode (prec cf cg)
n✝ : ℕ
⊢ Nat.rec (evaln k cf (unpair n).1)
(fun n_1 n_ih =>
Option.bind
(Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (prec cf cg)))))
(k', prec cf cg) (Nat.pair (unpair n).1 n_1))
fun i =>
Nat.Partrec.Code.lup
(List.map (fun n => List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
(List.range (Nat.pair (k' + 1) (encode (prec cf cg)))))
(k' + 1, cg) (Nat.pair (unpair n).1 (Nat.pair n_1 i)))
(Nat.succ n✝) =
Nat.rec (evaln (k' + 1) cf (unpair n).1)
(fun n_1 n_ih =>
Option.bind (evaln k' (prec cf cg) (Nat.pair (unpair n).1 n_1)) fun i =>
evaln (k' + 1) cg (Nat.pair (unpair n).1 (Nat.pair n_1 i)))
(Nat.succ n✝)
|
/-
Copyright (c) 2018 Mario Carneiro. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Mario Carneiro
-/
import Mathlib.Computability.Partrec
#align_import computability.partrec_code from "leanprover-community/mathlib"@"6155d4351090a6fad236e3d2e4e0e4e7342668e8"
/-!
# Gödel Numbering for Partial Recursive Functions.
This file defines `Nat.Partrec.Code`, an inductive datatype describing code for partial
recursive functions on ℕ. It defines an encoding for these codes, and proves that the constructors
are primitive recursive with respect to the encoding.
It also defines the evaluation of these codes as partial functions using `PFun`, and proves that a
function is partially recursive (as defined by `Nat.Partrec`) if and only if it is the evaluation
of some code.
## Main Definitions
* `Nat.Partrec.Code`: Inductive datatype for partial recursive codes.
* `Nat.Partrec.Code.encodeCode`: A (computable) encoding of codes as natural numbers.
* `Nat.Partrec.Code.ofNatCode`: The inverse of this encoding.
* `Nat.Partrec.Code.eval`: The interpretation of a `Nat.Partrec.Code` as a partial function.
## Main Results
* `Nat.Partrec.Code.rec_prim`: Recursion on `Nat.Partrec.Code` is primitive recursive.
* `Nat.Partrec.Code.rec_computable`: Recursion on `Nat.Partrec.Code` is computable.
* `Nat.Partrec.Code.smn`: The $S_n^m$ theorem.
* `Nat.Partrec.Code.exists_code`: Partial recursiveness is equivalent to being the eval of a code.
* `Nat.Partrec.Code.evaln_prim`: `evaln` is primitive recursive.
* `Nat.Partrec.Code.fixed_point`: Roger's fixed point theorem.
## References
* [Mario Carneiro, *Formalizing computability theory via partial recursive functions*][carneiro2019]
-/
open Encodable Denumerable Primrec
namespace Nat.Partrec
open Nat (pair)
theorem rfind' {f} (hf : Nat.Partrec f) :
Nat.Partrec
(Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + m))).map (· + m)) :=
Partrec₂.unpaired'.2 <| by
refine'
Partrec.map
((@Partrec₂.unpaired' fun a b : ℕ =>
Nat.rfind fun n => (fun m => m = 0) <$> f (Nat.pair a (n + b))).1
_)
(Primrec.nat_add.comp Primrec.snd <| Primrec.snd.comp Primrec.fst).to_comp.to₂
have : Nat.Partrec (fun a => Nat.rfind (fun n => (fun m => decide (m = 0)) <$>
Nat.unpaired (fun a b => f (Nat.pair (Nat.unpair a).1 (b + (Nat.unpair a).2)))
(Nat.pair a n))) :=
rfind
(Partrec₂.unpaired'.2
((Partrec.nat_iff.2 hf).comp
(Primrec₂.pair.comp (Primrec.fst.comp <| Primrec.unpair.comp Primrec.fst)
(Primrec.nat_add.comp Primrec.snd
(Primrec.snd.comp <| Primrec.unpair.comp Primrec.fst))).to_comp))
simp at this; exact this
#align nat.partrec.rfind' Nat.Partrec.rfind'
/-- Code for partial recursive functions from ℕ to ℕ.
See `Nat.Partrec.Code.eval` for the interpretation of these constructors.
-/
inductive Code : Type
| zero : Code
| succ : Code
| left : Code
| right : Code
| pair : Code → Code → Code
| comp : Code → Code → Code
| prec : Code → Code → Code
| rfind' : Code → Code
#align nat.partrec.code Nat.Partrec.Code
-- Porting note: `Nat.Partrec.Code.recOn` is noncomputable in Lean4, so we make it computable.
compile_inductive% Code
end Nat.Partrec
namespace Nat.Partrec.Code
open Nat (pair unpair)
open Nat.Partrec (Code)
instance instInhabited : Inhabited Code :=
⟨zero⟩
#align nat.partrec.code.inhabited Nat.Partrec.Code.instInhabited
/-- Returns a code for the constant function outputting a particular natural. -/
protected def const : ℕ → Code
| 0 => zero
| n + 1 => comp succ (Code.const n)
#align nat.partrec.code.const Nat.Partrec.Code.const
theorem const_inj : ∀ {n₁ n₂}, Nat.Partrec.Code.const n₁ = Nat.Partrec.Code.const n₂ → n₁ = n₂
| 0, 0, _ => by simp
| n₁ + 1, n₂ + 1, h => by
dsimp [Nat.add_one, Nat.Partrec.Code.const] at h
injection h with h₁ h₂
simp only [const_inj h₂]
#align nat.partrec.code.const_inj Nat.Partrec.Code.const_inj
/-- A code for the identity function. -/
protected def id : Code :=
pair left right
#align nat.partrec.code.id Nat.Partrec.Code.id
/-- Given a code `c` taking a pair as input, returns a code using `n` as the first argument to `c`.
-/
def curry (c : Code) (n : ℕ) : Code :=
comp c (pair (Code.const n) Code.id)
#align nat.partrec.code.curry Nat.Partrec.Code.curry
-- Porting note: `bit0` and `bit1` are deprecated.
/-- An encoding of a `Nat.Partrec.Code` as a ℕ. -/
def encodeCode : Code → ℕ
| zero => 0
| succ => 1
| left => 2
| right => 3
| pair cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 4
| comp cf cg => 2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg) + 1) + 4
| prec cf cg => (2 * (2 * Nat.pair (encodeCode cf) (encodeCode cg)) + 1) + 4
| rfind' cf => (2 * (2 * encodeCode cf + 1) + 1) + 4
#align nat.partrec.code.encode_code Nat.Partrec.Code.encodeCode
/--
A decoder for `Nat.Partrec.Code.encodeCode`, taking any ℕ to the `Nat.Partrec.Code` it represents.
-/
def ofNatCode : ℕ → Code
| 0 => zero
| 1 => succ
| 2 => left
| 3 => right
| n + 4 =>
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
match n.bodd, n.div2.bodd with
| false, false => pair (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| false, true => comp (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , false => prec (ofNatCode m.unpair.1) (ofNatCode m.unpair.2)
| true , true => rfind' (ofNatCode m)
#align nat.partrec.code.of_nat_code Nat.Partrec.Code.ofNatCode
/-- Proof that `Nat.Partrec.Code.ofNatCode` is the inverse of `Nat.Partrec.Code.encodeCode`-/
private theorem encode_ofNatCode : ∀ n, encodeCode (ofNatCode n) = n
| 0 => by simp [ofNatCode, encodeCode]
| 1 => by simp [ofNatCode, encodeCode]
| 2 => by simp [ofNatCode, encodeCode]
| 3 => by simp [ofNatCode, encodeCode]
| n + 4 => by
let m := n.div2.div2
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have _m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have _m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
have IH := encode_ofNatCode m
have IH1 := encode_ofNatCode m.unpair.1
have IH2 := encode_ofNatCode m.unpair.2
conv_rhs => rw [← Nat.bit_decomp n, ← Nat.bit_decomp n.div2]
simp only [ofNatCode._eq_5]
cases n.bodd <;> cases n.div2.bodd <;>
simp [encodeCode, ofNatCode, IH, IH1, IH2, Nat.bit_val]
instance instDenumerable : Denumerable Code :=
mk'
⟨encodeCode, ofNatCode, fun c => by
induction c <;> try {rfl} <;> simp [encodeCode, ofNatCode, Nat.div2_val, *],
encode_ofNatCode⟩
#align nat.partrec.code.denumerable Nat.Partrec.Code.instDenumerable
theorem encodeCode_eq : encode = encodeCode :=
rfl
#align nat.partrec.code.encode_code_eq Nat.Partrec.Code.encodeCode_eq
theorem ofNatCode_eq : ofNat Code = ofNatCode :=
rfl
#align nat.partrec.code.of_nat_code_eq Nat.Partrec.Code.ofNatCode_eq
theorem encode_lt_pair (cf cg) :
encode cf < encode (pair cf cg) ∧ encode cg < encode (pair cf cg) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right (Nat.pair cf.encodeCode cg.encodeCode) (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
have := lt_of_le_of_lt this (lt_add_of_pos_right _ (by decide : 0 < 4))
exact ⟨lt_of_le_of_lt (Nat.left_le_pair _ _) this, lt_of_le_of_lt (Nat.right_le_pair _ _) this⟩
#align nat.partrec.code.encode_lt_pair Nat.Partrec.Code.encode_lt_pair
theorem encode_lt_comp (cf cg) :
encode cf < encode (comp cf cg) ∧ encode cg < encode (comp cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_comp Nat.Partrec.Code.encode_lt_comp
theorem encode_lt_prec (cf cg) :
encode cf < encode (prec cf cg) ∧ encode cg < encode (prec cf cg) := by
suffices; exact (encode_lt_pair cf cg).imp (fun h => lt_trans h this) fun h => lt_trans h this
change _; simp [encodeCode_eq, encodeCode]
#align nat.partrec.code.encode_lt_prec Nat.Partrec.Code.encode_lt_prec
theorem encode_lt_rfind' (cf) : encode cf < encode (rfind' cf) := by
simp only [encodeCode_eq, encodeCode]
have := Nat.mul_le_mul_right cf.encodeCode (by decide : 1 ≤ 2 * 2)
rw [one_mul, mul_assoc] at this
refine' lt_of_le_of_lt (le_trans this _) (lt_add_of_pos_right _ (by decide : 0 < 4))
exact le_of_lt (Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_of_lt <|
Nat.lt_succ_of_le <| Nat.mul_le_mul_left _ <| le_rfl)
#align nat.partrec.code.encode_lt_rfind' Nat.Partrec.Code.encode_lt_rfind'
section
theorem pair_prim : Primrec₂ pair :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.pair_prim Nat.Partrec.Code.pair_prim
theorem comp_prim : Primrec₂ comp :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double.comp <|
nat_double_succ.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.comp_prim Nat.Partrec.Code.comp_prim
theorem prec_prim : Primrec₂ prec :=
Primrec₂.ofNat_iff.2 <|
Primrec₂.encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <|
nat_double.comp <|
Primrec₂.natPair.comp (encode_iff.2 <| (Primrec.ofNat Code).comp fst)
(encode_iff.2 <| (Primrec.ofNat Code).comp snd))
(Primrec₂.const 4)
#align nat.partrec.code.prec_prim Nat.Partrec.Code.prec_prim
theorem rfind_prim : Primrec rfind' :=
ofNat_iff.2 <|
encode_iff.1 <|
nat_add.comp
(nat_double_succ.comp <| nat_double_succ.comp <|
encode_iff.2 <| Primrec.ofNat Code)
(const 4)
#align nat.partrec.code.rfind_prim Nat.Partrec.Code.rfind_prim
theorem rec_prim' {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code × Code × σ × σ → σ} (hpr : Primrec₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Primrec₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Primrec₂ pc) {rf : α → Code × σ → σ} (hrf : Primrec₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Primrec (fun a => F a (c a) : α → σ) := by
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Primrec.unpair.comp (snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond (hrf.comp a (((Primrec.ofNat Code).comp m).pair s))
(hpc.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Primrec.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn
(list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim' Nat.Partrec.Code.rec_prim'
/-- Recursion on `Nat.Partrec.Code` is primitive recursive. -/
theorem rec_prim {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Primrec c) {z : α → σ}
(hz : Primrec z) {s : α → σ} (hs : Primrec s) {l : α → σ} (hl : Primrec l) {r : α → σ}
(hr : Primrec r) {pr : α → Code → Code → σ → σ → σ}
(hpr : Primrec fun a : α × Code × Code × σ × σ => pr a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{co : α → Code → Code → σ → σ → σ}
(hco : Primrec fun a : α × Code × Code × σ × σ => co a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{pc : α → Code → Code → σ → σ → σ}
(hpc : Primrec fun a : α × Code × Code × σ × σ => pc a.1 a.2.1 a.2.2.1 a.2.2.2.1 a.2.2.2.2)
{rf : α → Code → σ → σ} (hrf : Primrec fun a : α × Code × σ => rf a.1 a.2.1 a.2.2) :
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (pr a) (co a) (pc a) (rf a)
Primrec fun a => F a (c a) := by
intros F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m) s)
(pc a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂)
(pr a (ofNat Code m.unpair.1) (ofNat Code m.unpair.2) s₁ s₂))
have : Primrec G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Primrec₂
refine' option_bind ((list_get?.comp (snd.comp fst) (fst.comp <| Primrec.unpair.comp
(snd.comp snd))).comp fst) _
unfold Primrec₂
refine'
option_map
((list_get?.comp (snd.comp fst) (snd.comp <| Primrec.unpair.comp (snd.comp snd))).comp <|
fst.comp fst)
_
have a : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Primrec.unpair.comp m)
have m₂ := snd.comp (Primrec.unpair.comp m)
have s : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Primrec (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
have h₁ := hrf.comp <| a.pair (((Primrec.ofNat Code).comp m).pair s)
have h₂ := hpc.comp <| a.pair (((Primrec.ofNat Code).comp m₁).pair <|
((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₃ := hco.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
have h₄ := hpr.comp <| a.pair
(((Primrec.ofNat Code).comp m₁).pair <| ((Primrec.ofNat Code).comp m₂).pair <| s₁.pair s₂)
unfold Primrec₂
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond h₁ h₂)
(cond (nat_bodd.comp <| nat_div2.comp n) h₃ h₄)
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Primrec₂ G := by
unfold Primrec₂
refine nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
refine nat_casesOn snd (option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst))) ?_
unfold Primrec₂
exact this.comp <|
((fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp
_root_.Primrec.id <| encode_iff.2 hc).of_eq
fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_prim Nat.Partrec.Code.rec_prim
end
section
open Computable
/-- Recursion on `Nat.Partrec.Code` is computable. -/
theorem rec_computable {α σ} [Primcodable α] [Primcodable σ] {c : α → Code} (hc : Computable c)
{z : α → σ} (hz : Computable z) {s : α → σ} (hs : Computable s) {l : α → σ} (hl : Computable l)
{r : α → σ} (hr : Computable r) {pr : α → Code × Code × σ × σ → σ} (hpr : Computable₂ pr)
{co : α → Code × Code × σ × σ → σ} (hco : Computable₂ co) {pc : α → Code × Code × σ × σ → σ}
(hpc : Computable₂ pc) {rf : α → Code × σ → σ} (hrf : Computable₂ rf) :
let PR (a) cf cg hf hg := pr a (cf, cg, hf, hg)
let CO (a) cf cg hf hg := co a (cf, cg, hf, hg)
let PC (a) cf cg hf hg := pc a (cf, cg, hf, hg)
let RF (a) cf hf := rf a (cf, hf)
let F (a : α) (c : Code) : σ :=
Nat.Partrec.Code.recOn c (z a) (s a) (l a) (r a) (PR a) (CO a) (PC a) (RF a)
Computable fun a => F a (c a) := by
-- TODO(Mario): less copy-paste from previous proof
intros _ _ _ _ F
let G₁ : (α × List σ) × ℕ × ℕ → Option σ := fun p =>
let a := p.1.1
let IH := p.1.2
let n := p.2.1
let m := p.2.2
(IH.get? m).bind fun s =>
(IH.get? m.unpair.1).bind fun s₁ =>
(IH.get? m.unpair.2).map fun s₂ =>
cond n.bodd
(cond n.div2.bodd (rf a (ofNat Code m, s))
(pc a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
(cond n.div2.bodd (co a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂))
(pr a (ofNat Code m.unpair.1, ofNat Code m.unpair.2, s₁, s₂)))
have : Computable G₁ := by
refine' option_bind (list_get?.comp (snd.comp fst) (snd.comp snd)) _
unfold Computable₂
refine'
option_bind
((list_get?.comp (snd.comp fst)
(fst.comp <| Computable.unpair.comp (snd.comp snd))).comp fst) _
unfold Computable₂
refine'
option_map
((list_get?.comp (snd.comp fst)
(snd.comp <| Computable.unpair.comp (snd.comp snd))).comp <| fst.comp fst) _
have a : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.1.1) :=
fst.comp (fst.comp <| fst.comp <| fst.comp fst)
have n : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.1) :=
fst.comp (snd.comp <| fst.comp <| fst.comp fst)
have m : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.1.2.2) :=
snd.comp (snd.comp <| fst.comp <| fst.comp fst)
have m₁ := fst.comp (Computable.unpair.comp m)
have m₂ := snd.comp (Computable.unpair.comp m)
have s : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.1.2) :=
snd.comp (fst.comp fst)
have s₁ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.1.2) :=
snd.comp fst
have s₂ : Computable (fun p : ((((α × List σ) × ℕ × ℕ) × σ) × σ) × σ => p.2) :=
snd
exact
(nat_bodd.comp n).cond
((nat_bodd.comp <| nat_div2.comp n).cond
(hrf.comp a (((Computable.ofNat Code).comp m).pair s))
(hpc.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
(Computable.cond (nat_bodd.comp <| nat_div2.comp n)
(hco.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂))
(hpr.comp a
(((Computable.ofNat Code).comp m₁).pair <|
((Computable.ofNat Code).comp m₂).pair <| s₁.pair s₂)))
let G : α → List σ → Option σ := fun a IH =>
IH.length.casesOn (some (z a)) fun n =>
n.casesOn (some (s a)) fun n =>
n.casesOn (some (l a)) fun n =>
n.casesOn (some (r a)) fun n => G₁ ((a, IH), n, n.div2.div2)
have : Computable₂ G :=
Computable.nat_casesOn (list_length.comp snd) (option_some_iff.2 (hz.comp fst)) <|
Computable.nat_casesOn snd (option_some_iff.2 (hs.comp (fst.comp fst))) <|
Computable.nat_casesOn snd (option_some_iff.2 (hl.comp (fst.comp <| fst.comp fst))) <|
Computable.nat_casesOn snd
(option_some_iff.2 (hr.comp (fst.comp <| fst.comp <| fst.comp fst)))
(this.comp <|
((Computable.fst.pair snd).comp <| fst.comp <| fst.comp <| fst.comp <| fst).pair <|
snd.pair <| nat_div2.comp <| nat_div2.comp snd)
refine'
((nat_strong_rec (fun a n => F a (ofNat Code n)) this.to₂ fun a n => _).comp Computable.id <|
encode_iff.2 hc).of_eq fun a => by simp
simp (config := { zeta := false })
iterate 4 cases' n with n; · simp (config := { zeta := false }) [ofNatCode_eq, ofNatCode]; rfl
simp only []
rw [List.length_map, List.length_range]
let m := n.div2.div2
show
G₁ ((a, (List.range (n + 4)).map fun n => F a (ofNat Code n)), n, m) =
some (F a (ofNat Code (n + 4)))
have hm : m < n + 4 := by
simp only [div2_val]
exact
lt_of_le_of_lt (le_trans (Nat.div_le_self _ _) (Nat.div_le_self _ _))
(Nat.succ_le_succ (Nat.le_add_right _ _))
have m1 : m.unpair.1 < n + 4 := lt_of_le_of_lt m.unpair_left_le hm
have m2 : m.unpair.2 < n + 4 := lt_of_le_of_lt m.unpair_right_le hm
simp [List.get?_map, List.get?_range, hm, m1, m2]
rw [show ofNat Code (n + 4) = ofNatCode (n + 4) from rfl]
simp [ofNatCode]
cases n.bodd <;> cases n.div2.bodd <;> rfl
#align nat.partrec.code.rec_computable Nat.Partrec.Code.rec_computable
end
/-- The interpretation of a `Nat.Partrec.Code` as a partial function.
* `Nat.Partrec.Code.zero`: The constant zero function.
* `Nat.Partrec.Code.succ`: The successor function.
* `Nat.Partrec.Code.left`: Left unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.right`: Right unpairing of a pair of ℕ (encoded by `Nat.pair`)
* `Nat.Partrec.Code.pair`: Pairs the outputs of argument codes using `Nat.pair`.
* `Nat.Partrec.Code.comp`: Composition of two argument codes.
* `Nat.Partrec.Code.prec`: Primitive recursion. Given an argument of the form `Nat.pair a n`:
* If `n = 0`, returns `eval cf a`.
* If `n = succ k`, returns `eval cg (pair a (pair k (eval (prec cf cg) (pair a k))))`
* `Nat.Partrec.Code.rfind'`: Minimization. For `f` an argument of the form `Nat.pair a m`,
`rfind' f m` returns the least `a` such that `f a m = 0`, if one exists and `f b m` terminates
for `b < a`
-/
def eval : Code → ℕ →. ℕ
| zero => pure 0
| succ => Nat.succ
| left => ↑fun n : ℕ => n.unpair.1
| right => ↑fun n : ℕ => n.unpair.2
| pair cf cg => fun n => Nat.pair <$> eval cf n <*> eval cg n
| comp cf cg => fun n => eval cg n >>= eval cf
| prec cf cg =>
Nat.unpaired fun a n =>
n.rec (eval cf a) fun y IH => do
let i ← IH
eval cg (Nat.pair a (Nat.pair y i))
| rfind' cf =>
Nat.unpaired fun a m =>
(Nat.rfind fun n => (fun m => m = 0) <$> eval cf (Nat.pair a (n + m))).map (· + m)
#align nat.partrec.code.eval Nat.Partrec.Code.eval
/-- Helper lemma for the evaluation of `prec` in the base case. -/
@[simp]
theorem eval_prec_zero (cf cg : Code) (a : ℕ) : eval (prec cf cg) (Nat.pair a 0) = eval cf a := by
rw [eval, Nat.unpaired, Nat.unpair_pair]
simp (config := { Lean.Meta.Simp.neutralConfig with proj := true }) only []
rw [Nat.rec_zero]
#align nat.partrec.code.eval_prec_zero Nat.Partrec.Code.eval_prec_zero
/-- Helper lemma for the evaluation of `prec` in the recursive case. -/
theorem eval_prec_succ (cf cg : Code) (a k : ℕ) :
eval (prec cf cg) (Nat.pair a (Nat.succ k)) =
do {let ih ← eval (prec cf cg) (Nat.pair a k); eval cg (Nat.pair a (Nat.pair k ih))} := by
rw [eval, Nat.unpaired, Part.bind_eq_bind, Nat.unpair_pair]
simp
#align nat.partrec.code.eval_prec_succ Nat.Partrec.Code.eval_prec_succ
instance : Membership (ℕ →. ℕ) Code :=
⟨fun f c => eval c = f⟩
@[simp]
theorem eval_const : ∀ n m, eval (Code.const n) m = Part.some n
| 0, m => rfl
| n + 1, m => by simp! [eval_const n m]
#align nat.partrec.code.eval_const Nat.Partrec.Code.eval_const
@[simp]
theorem eval_id (n) : eval Code.id n = Part.some n := by simp! [Seq.seq]
#align nat.partrec.code.eval_id Nat.Partrec.Code.eval_id
@[simp]
theorem eval_curry (c n x) : eval (curry c n) x = eval c (Nat.pair n x) := by simp! [Seq.seq]
#align nat.partrec.code.eval_curry Nat.Partrec.Code.eval_curry
theorem const_prim : Primrec Code.const :=
(_root_.Primrec.id.nat_iterate (_root_.Primrec.const zero)
(comp_prim.comp (_root_.Primrec.const succ) Primrec.snd).to₂).of_eq
fun n => by simp; induction n <;>
simp [*, Code.const, Function.iterate_succ', -Function.iterate_succ]
#align nat.partrec.code.const_prim Nat.Partrec.Code.const_prim
theorem curry_prim : Primrec₂ curry :=
comp_prim.comp Primrec.fst <| pair_prim.comp (const_prim.comp Primrec.snd)
(_root_.Primrec.const Code.id)
#align nat.partrec.code.curry_prim Nat.Partrec.Code.curry_prim
theorem curry_inj {c₁ c₂ n₁ n₂} (h : curry c₁ n₁ = curry c₂ n₂) : c₁ = c₂ ∧ n₁ = n₂ :=
⟨by injection h, by
injection h with h₁ h₂
injection h₂ with h₃ h₄
exact const_inj h₃⟩
#align nat.partrec.code.curry_inj Nat.Partrec.Code.curry_inj
/--
The $S_n^m$ theorem: There is a computable function, namely `Nat.Partrec.Code.curry`, that takes a
program and a ℕ `n`, and returns a new program using `n` as the first argument.
-/
theorem smn :
∃ f : Code → ℕ → Code, Computable₂ f ∧ ∀ c n x, eval (f c n) x = eval c (Nat.pair n x) :=
⟨curry, Primrec₂.to_comp curry_prim, eval_curry⟩
#align nat.partrec.code.smn Nat.Partrec.Code.smn
/-- A function is partial recursive if and only if there is a code implementing it. -/
theorem exists_code {f : ℕ →. ℕ} : Nat.Partrec f ↔ ∃ c : Code, eval c = f :=
⟨fun h => by
induction h
case zero => exact ⟨zero, rfl⟩
case succ => exact ⟨succ, rfl⟩
case left => exact ⟨left, rfl⟩
case right => exact ⟨right, rfl⟩
case pair f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨pair cf cg, rfl⟩
case comp f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨comp cf cg, rfl⟩
case prec f g pf pg hf hg =>
rcases hf with ⟨cf, rfl⟩; rcases hg with ⟨cg, rfl⟩
exact ⟨prec cf cg, rfl⟩
case rfind f pf hf =>
rcases hf with ⟨cf, rfl⟩
refine' ⟨comp (rfind' cf) (pair Code.id zero), _⟩
simp [eval, Seq.seq, pure, PFun.pure, Part.map_id'],
fun h => by
rcases h with ⟨c, rfl⟩; induction c
case zero => exact Nat.Partrec.zero
case succ => exact Nat.Partrec.succ
case left => exact Nat.Partrec.left
case right => exact Nat.Partrec.right
case pair cf cg pf pg => exact pf.pair pg
case comp cf cg pf pg => exact pf.comp pg
case prec cf cg pf pg => exact pf.prec pg
case rfind' cf pf => exact pf.rfind'⟩
#align nat.partrec.code.exists_code Nat.Partrec.Code.exists_code
-- Porting note: `>>`s in `evaln` are now `>>=` because `>>`s are not elaborated well in Lean4.
/-- A modified evaluation for the code which returns an `Option ℕ` instead of a `Part ℕ`. To avoid
undecidability, `evaln` takes a parameter `k` and fails if it encounters a number ≥ k in the course
of its execution. Other than this, the semantics are the same as in `Nat.Partrec.Code.eval`.
-/
def evaln : ℕ → Code → ℕ → Option ℕ
| 0, _ => fun _ => Option.none
| k + 1, zero => fun n => do
guard (n ≤ k)
return 0
| k + 1, succ => fun n => do
guard (n ≤ k)
return (Nat.succ n)
| k + 1, left => fun n => do
guard (n ≤ k)
return n.unpair.1
| k + 1, right => fun n => do
guard (n ≤ k)
pure n.unpair.2
| k + 1, pair cf cg => fun n => do
guard (n ≤ k)
Nat.pair <$> evaln (k + 1) cf n <*> evaln (k + 1) cg n
| k + 1, comp cf cg => fun n => do
guard (n ≤ k)
let x ← evaln (k + 1) cg n
evaln (k + 1) cf x
| k + 1, prec cf cg => fun n => do
guard (n ≤ k)
n.unpaired fun a n =>
n.casesOn (evaln (k + 1) cf a) fun y => do
let i ← evaln k (prec cf cg) (Nat.pair a y)
evaln (k + 1) cg (Nat.pair a (Nat.pair y i))
| k + 1, rfind' cf => fun n => do
guard (n ≤ k)
n.unpaired fun a m => do
let x ← evaln (k + 1) cf (Nat.pair a m)
if x = 0 then
pure m
else
evaln k (rfind' cf) (Nat.pair a (m + 1))
termination_by evaln k c => (k, c)
decreasing_by { decreasing_with simp (config := { arith := true }) [Zero.zero]; done }
#align nat.partrec.code.evaln Nat.Partrec.Code.evaln
theorem evaln_bound : ∀ {k c n x}, x ∈ evaln k c n → n < k
| 0, c, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
suffices ∀ {o : Option ℕ}, x ∈ do { guard (n ≤ k); o } → n < k + 1 by
cases c <;> rw [evaln] at h <;> exact this h
simpa [Bind.bind] using Nat.lt_succ_of_le
#align nat.partrec.code.evaln_bound Nat.Partrec.Code.evaln_bound
theorem evaln_mono : ∀ {k₁ k₂ c n x}, k₁ ≤ k₂ → x ∈ evaln k₁ c n → x ∈ evaln k₂ c n
| 0, k₂, c, n, x, _, h => by simp [evaln] at h
| k + 1, k₂ + 1, c, n, x, hl, h => by
have hl' := Nat.le_of_succ_le_succ hl
have :
∀ {k k₂ n x : ℕ} {o₁ o₂ : Option ℕ},
k ≤ k₂ → (x ∈ o₁ → x ∈ o₂) →
x ∈ do { guard (n ≤ k); o₁ } → x ∈ do { guard (n ≤ k₂); o₂ } := by
simp only [Option.mem_def, bind, Option.bind_eq_some, Option.guard_eq_some', exists_and_left,
exists_const, and_imp]
introv h h₁ h₂ h₃
exact ⟨le_trans h₂ h, h₁ h₃⟩
simp? at h ⊢ says simp only [Option.mem_def] at h ⊢
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
rw [evaln] at h ⊢ <;> refine' this hl' (fun h => _) h
iterate 4 exact h
· -- pair cf cg
simp? [Seq.seq] at h ⊢ says
simp only [Seq.seq, Option.map_eq_map, Option.mem_def, Option.bind_eq_some,
Option.map_eq_some', exists_exists_and_eq_and] at h ⊢
exact h.imp fun a => And.imp (hf _ _) <| Exists.imp fun b => And.imp_left (hg _ _)
· -- comp cf cg
simp? [Bind.bind] at h ⊢ says simp only [bind, Option.mem_def, Option.bind_eq_some] at h ⊢
exact h.imp fun a => And.imp (hg _ _) (hf _ _)
· -- prec cf cg
revert h
simp only [unpaired, bind, Option.mem_def]
induction n.unpair.2 <;> simp
· apply hf
· exact fun y h₁ h₂ => ⟨y, evaln_mono hl' h₁, hg _ _ h₂⟩
· -- rfind' cf
simp? [Bind.bind] at h ⊢ says
simp only [unpaired, bind, pair_unpair, Option.mem_def, Option.bind_eq_some] at h ⊢
refine' h.imp fun x => And.imp (hf _ _) _
by_cases x0 : x = 0 <;> simp [x0]
exact evaln_mono hl'
#align nat.partrec.code.evaln_mono Nat.Partrec.Code.evaln_mono
theorem evaln_sound : ∀ {k c n x}, x ∈ evaln k c n → x ∈ eval c n
| 0, _, n, x, h => by simp [evaln] at h
| k + 1, c, n, x, h => by
induction' c with cf cg hf hg cf cg hf hg cf cg hf hg cf hf generalizing x n <;>
simp [eval, evaln, Bind.bind, Seq.seq] at h ⊢ <;>
cases' h with _ h
iterate 4 simpa [pure, PFun.pure, eq_comm] using h
· -- pair cf cg
rcases h with ⟨y, ef, z, eg, rfl⟩
exact ⟨_, hf _ _ ef, _, hg _ _ eg, rfl⟩
· --comp hf hg
rcases h with ⟨y, eg, ef⟩
exact ⟨_, hg _ _ eg, hf _ _ ef⟩
· -- prec cf cg
revert h
induction' n.unpair.2 with m IH generalizing x <;> simp
· apply hf
· refine' fun y h₁ h₂ => ⟨y, IH _ _, _⟩
· have := evaln_mono k.le_succ h₁
simp [evaln, Bind.bind] at this
exact this.2
· exact hg _ _ h₂
· -- rfind' cf
rcases h with ⟨m, h₁, h₂⟩
by_cases m0 : m = 0 <;> simp [m0] at h₂
· exact
⟨0, ⟨by simpa [m0] using hf _ _ h₁, fun {m} => (Nat.not_lt_zero _).elim⟩, by
injection h₂ with h₂; simp [h₂]⟩
· have := evaln_sound h₂
simp [eval] at this
rcases this with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
refine'
⟨y + 1, ⟨by simpa [add_comm, add_left_comm] using hy₁, fun {i} im => _⟩, by
simp [add_comm, add_left_comm]⟩
cases' i with i
· exact ⟨m, by simpa using hf _ _ h₁, m0⟩
· rcases hy₂ (Nat.lt_of_succ_lt_succ im) with ⟨z, hz, z0⟩
exact ⟨z, by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hz, z0⟩
#align nat.partrec.code.evaln_sound Nat.Partrec.Code.evaln_sound
theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
⟨fun h => by
rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
· exact ⟨k + 1, h⟩
induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
case pair cf cg hf hg =>
rcases h with ⟨x, hx, y, hy, rfl⟩
rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
refine'
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
case comp cf cg hf hg =>
rcases h with ⟨y, hy, hx⟩
rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
refine' ⟨max k₁ k₂, _⟩
exact
⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
case prec cf cg hf hg =>
revert h
generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
induction' n₂ with m IH generalizing x n <;> simp
· intro h
rcases hf h with ⟨k, hk⟩
exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
· intro y hy hx
rcases IH hy with ⟨k₁, nk₁, hk₁⟩
rcases hg hx with ⟨k₂, hk₂⟩
refine'
⟨(max k₁ k₂).succ,
Nat.le_succ_of_le <| le_max_of_le_left <|
le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
Option.guard_eq_some', exists_and_left, exists_const]
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
case rfind' cf hf =>
rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
simpa [evaln, Bind.bind]
revert hy₁ hy₂
generalize n.unpair.2 = m
intro hy₁ hy₂
induction' y with y IH generalizing m <;> simp [evaln, Bind.bind]
· simp at hy₁
rcases hf hy₁ with ⟨k, hk⟩
exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp; rfl⟩
· rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
rcases hf ha with ⟨k₁, hk₁⟩
rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
fun {i} hi => by
simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
hy₂ (Nat.succ_lt_succ hi) with
⟨k₂, hk₂⟩
use (max k₁ k₂).succ
rw [zero_add] at hk₁
use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
use a
use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
simpa [Nat.succ_eq_add_one, a0, -max_eq_left, -max_eq_right, add_comm, add_left_comm] using
evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂,
fun ⟨k, h⟩ => evaln_sound h⟩
#align nat.partrec.code.evaln_complete Nat.Partrec.Code.evaln_complete
section
open Primrec
private def lup (L : List (List (Option ℕ))) (p : ℕ × Code) (n : ℕ) := do
let l ← L.get? (encode p)
let o ← l.get? n
o
private theorem hlup : Primrec fun p : _ × (_ × _) × _ => lup p.1 p.2.1 p.2.2 :=
Primrec.option_bind
(Primrec.list_get?.comp Primrec.fst (Primrec.encode.comp <| Primrec.fst.comp Primrec.snd))
(Primrec.option_bind (Primrec.list_get?.comp Primrec.snd <| Primrec.snd.comp <|
Primrec.snd.comp Primrec.fst) Primrec.snd)
private def G (L : List (List (Option ℕ))) : Option (List (Option ℕ)) :=
Option.some <|
let a := ofNat (ℕ × Code) L.length
let k := a.1
let c := a.2
(List.range k).map fun n =>
k.casesOn Option.none fun k' =>
Nat.Partrec.Code.recOn c
(some 0) -- zero
(some (Nat.succ n))
(some n.unpair.1)
(some n.unpair.2)
(fun cf cg _ _ => do
let x ← lup L (k, cf) n
let y ← lup L (k, cg) n
some (Nat.pair x y))
(fun cf cg _ _ => do
let x ← lup L (k, cg) n
lup L (k, cf) x)
(fun cf cg _ _ =>
let z := n.unpair.1
n.unpair.2.casesOn (lup L (k, cf) z) fun y => do
let i ← lup L (k', c) (Nat.pair z y)
lup L (k, cg) (Nat.pair z (Nat.pair y i)))
(fun cf _ =>
let z := n.unpair.1
let m := n.unpair.2
do
let x ← lup L (k, cf) (Nat.pair z m)
x.casesOn (some m) fun _ => lup L (k', c) (Nat.pair z (m + 1)))
private theorem hG : Primrec G := by
have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
have k := Primrec.fst.comp a
refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
replace k := k.comp (Primrec.fst (β := ℕ))
have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
have k := k.comp (Primrec.fst (β := ℕ))
have n := n.comp (Primrec.fst (β := ℕ))
have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
apply
Nat.Partrec.Code.rec_prim c
(_root_.Primrec.const (some 0))
(Primrec.option_some.comp (_root_.Primrec.succ.comp n))
(Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
(Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
unfold Primrec₂
conv =>
congr
· ext p
dsimp only []
erw [Option.bind_eq_bind, ← Option.map_eq_bind]
refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
unfold Primrec₂
exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
unfold Primrec₂
have h :=
hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
exact h
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have cg := (Primrec.fst.comp Primrec.snd).comp
(Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Code × Option ℕ × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
refine'
Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
(hlup.comp <| L.pair <| (k.pair cf).pair z)
(_ : Primrec _)
have L := L.comp (Primrec.fst (β := ℕ))
have z := z.comp (Primrec.fst (β := ℕ))
have y := Primrec.snd
(α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
(Primrec₂.natPair.comp z y)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have z := z.comp (Primrec.fst (β := ℕ))
have y := y.comp (Primrec.fst (β := ℕ))
have i := Primrec.snd
(α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
(β := ℕ)
have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
exact h₂
· have L := (Primrec.fst.comp Primrec.fst).comp
(Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have k := k.comp (Primrec.fst (β := Code × Option ℕ))
have n := n.comp (Primrec.fst (β := Code × Option ℕ))
have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
(β := Code × Option ℕ))
have z := Primrec.fst.comp (Primrec.unpair.comp n)
have m := Primrec.snd.comp (Primrec.unpair.comp n)
have h₁ := hlup.comp <| L.pair <| (k.pair cf).pair (Primrec₂.natPair.comp z m)
refine' Primrec.option_bind h₁ (_ : Primrec _)
have m := m.comp (Primrec.fst (β := ℕ))
refine Primrec.nat_casesOn Primrec.snd (Primrec.option_some.comp m) ?_
unfold Primrec₂
exact (hlup.comp ((L.comp Primrec.fst).pair <|
((k'.pair c).comp <| Primrec.fst.comp Primrec.fst).pair
(Primrec₂.natPair.comp (z.comp Primrec.fst) (_root_.Primrec.succ.comp m)))).comp
Primrec.fst
private theorem evaln_map (k c n) :
((((List.range k).get? n).map (evaln k c)).bind fun b => b) = evaln k c n := by
by_cases kn : n < k
· simp [List.get?_range kn]
· rw [List.get?_len_le]
· cases e : evaln k c n
· rfl
exact kn.elim (evaln_bound e)
simpa using kn
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
simp [evaln, nk, Bind.bind, Functor.map, Seq.seq, pure]
· cases' encode_lt_pair cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lf), hg (Nat.pair_lt_pair_right _ lg)]
cases evaln k cf n
· rfl
cases evaln k cg n <;> rfl
· cases' encode_lt_comp cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lg)]
cases evaln k cg n
· rfl
simp [hg (Nat.pair_lt_pair_right _ lf)]
· cases' encode_lt_prec cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lf)]
cases n.unpair.2
· rfl
|
simp only [decode_eq_ofNat, Option.some.injEq]
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
have :
Primrec₂ fun (_ : Unit) (n : ℕ) =>
let a := ofNat (ℕ × Code) n
(List.range a.1).map (evaln a.1 a.2) :=
Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
Nat.pair_unpair, Option.some_inj]
refine List.map_congr fun n => ?_
have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
simp
rw [this]
generalize p.unpair.1 = k
generalize ofNat Code p.unpair.2 = c
intro nk
cases' k with k'
· simp [evaln]
let k := k' + 1
simp only [show k'.succ = k from rfl]
simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
have hg :
∀ {k' c' n},
Nat.pair k' (encode c') < Nat.pair k (encode c) →
lup ((List.range (Nat.pair k (encode c))).map fun n =>
(List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
evaln k' c' n := by
intro k₁ c₁ n₁ hl
simp [lup, List.get?_range hl, evaln_map, Bind.bind]
cases' c with cf cg cf cg cf cg cf <;>
simp [evaln, nk, Bind.bind, Functor.map, Seq.seq, pure]
· cases' encode_lt_pair cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lf), hg (Nat.pair_lt_pair_right _ lg)]
cases evaln k cf n
· rfl
cases evaln k cg n <;> rfl
· cases' encode_lt_comp cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lg)]
cases evaln k cg n
· rfl
simp [hg (Nat.pair_lt_pair_right _ lf)]
· cases' encode_lt_prec cf cg with lf lg
rw [hg (Nat.pair_lt_pair_right _ lf)]
cases n.unpair.2
· rfl
|
Mathlib.Computability.PartrecCode.1088_0.A3c3Aev6SyIRjCJ
|
/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2
|
Mathlib_Computability_PartrecCode
|
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