{-
(c) The University of Glasgow 2006
(c) The GRASP/AQUA Project, Glasgow University, 1998

\section[DataCon]{@DataCon@: Data Constructors}
-}

{-# LANGUAGE CPP, DeriveDataTypeable #-}

module DataCon (
        -- * Main data types
        DataCon, DataConRep(..),
        SrcStrictness(..), SrcUnpackedness(..),
        HsSrcBang(..), HsImplBang(..),
        StrictnessMark(..),
        ConTag,

        -- ** Equality specs
        EqSpec, mkEqSpec, eqSpecTyVar, eqSpecType,
        eqSpecPair, eqSpecPreds,
        substEqSpec, filterEqSpec,

        -- ** Field labels
        FieldLbl(..), FieldLabel, FieldLabelString,

        -- ** Type construction
        mkDataCon, buildAlgTyCon, fIRST_TAG,

        -- ** Type deconstruction
        dataConRepType, dataConSig, dataConInstSig, dataConFullSig,
        dataConName, dataConIdentity, dataConTag, dataConTyCon,
        dataConOrigTyCon, dataConUserType,
        dataConUnivTyVars, dataConUnivTyVarBinders,
        dataConExTyVars, dataConExTyVarBinders,
        dataConAllTyVars,
        dataConEqSpec, dataConTheta,
        dataConStupidTheta,
        dataConInstArgTys, dataConOrigArgTys, dataConOrigResTy,
        dataConInstOrigArgTys, dataConRepArgTys,
        dataConFieldLabels, dataConFieldType,
        dataConSrcBangs,
        dataConSourceArity, dataConRepArity,
        dataConIsInfix,
        dataConWorkId, dataConWrapId, dataConWrapId_maybe,
        dataConImplicitTyThings,
        dataConRepStrictness, dataConImplBangs, dataConBoxer,

        splitDataProductType_maybe,

        -- ** Predicates on DataCons
        isNullarySrcDataCon, isNullaryRepDataCon, isTupleDataCon, isUnboxedTupleCon,
        isUnboxedSumCon,
        isVanillaDataCon, classDataCon, dataConCannotMatch,
        isBanged, isMarkedStrict, eqHsBang, isSrcStrict, isSrcUnpacked,
        specialPromotedDc, isLegacyPromotableDataCon, isLegacyPromotableTyCon,

        -- ** Promotion related functions
        promoteDataCon
    ) where

#include "HsVersions.h"

import {-# SOURCE #-} MkId( DataConBoxer )
import Type
import ForeignCall ( CType )
import Coercion
import Unify
import TyCon
import FieldLabel
import Class
import Name
import PrelNames
import Var
import Outputable
import ListSetOps
import Util
import BasicTypes
import FastString
import Module
import Binary
import UniqSet
import UniqFM
import Unique( mkAlphaTyVarUnique )

import qualified Data.Data as Data
import Data.Char
import Data.Word
import Data.List( mapAccumL, find )

{-
Data constructor representation
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider the following Haskell data type declaration

        data T = T !Int ![Int]

Using the strictness annotations, GHC will represent this as

        data T = T Int# [Int]

That is, the Int has been unboxed.  Furthermore, the Haskell source construction

        T e1 e2

is translated to

        case e1 of { I# x ->
        case e2 of { r ->
        T x r }}

That is, the first argument is unboxed, and the second is evaluated.  Finally,
pattern matching is translated too:

        case e of { T a b -> ... }

becomes

        case e of { T a' b -> let a = I# a' in ... }

To keep ourselves sane, we name the different versions of the data constructor
differently, as follows.


Note [Data Constructor Naming]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Each data constructor C has two, and possibly up to four, Names associated with it:

                   OccName   Name space   Name of   Notes
 ---------------------------------------------------------------------------
 The "data con itself"   C     DataName   DataCon   In dom( GlobalRdrEnv )
 The "worker data con"   C     VarName    Id        The worker
 The "wrapper data con"  $WC   VarName    Id        The wrapper
 The "newtype coercion"  :CoT  TcClsName  TyCon

EVERY data constructor (incl for newtypes) has the former two (the
data con itself, and its worker.  But only some data constructors have a
wrapper (see Note [The need for a wrapper]).

Each of these three has a distinct Unique.  The "data con itself" name
appears in the output of the renamer, and names the Haskell-source
data constructor.  The type checker translates it into either the wrapper Id
(if it exists) or worker Id (otherwise).

The data con has one or two Ids associated with it:

The "worker Id", is the actual data constructor.
* Every data constructor (newtype or data type) has a worker

* The worker is very like a primop, in that it has no binding.

* For a *data* type, the worker *is* the data constructor;
  it has no unfolding

* For a *newtype*, the worker has a compulsory unfolding which
  does a cast, e.g.
        newtype T = MkT Int
        The worker for MkT has unfolding
                \\(x:Int). x `cast` sym CoT
  Here CoT is the type constructor, witnessing the FC axiom
        axiom CoT : T = Int

The "wrapper Id", \$WC, goes as follows

* Its type is exactly what it looks like in the source program.

* It is an ordinary function, and it gets a top-level binding
  like any other function.

* The wrapper Id isn't generated for a data type if there is
  nothing for the wrapper to do.  That is, if its defn would be
        \$wC = C

Note [The need for a wrapper]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Why might the wrapper have anything to do?  Two reasons:

* Unboxing strict fields (with -funbox-strict-fields)
        data T = MkT !(Int,Int)
        \$wMkT :: (Int,Int) -> T
        \$wMkT (x,y) = MkT x y
  Notice that the worker has two fields where the wapper has
  just one.  That is, the worker has type
                MkT :: Int -> Int -> T

* Equality constraints for GADTs
        data T a where { MkT :: a -> T [a] }

  The worker gets a type with explicit equality
  constraints, thus:
        MkT :: forall a b. (a=[b]) => b -> T a

  The wrapper has the programmer-specified type:
        \$wMkT :: a -> T [a]
        \$wMkT a x = MkT [a] a [a] x
  The third argument is a coercion
        [a] :: [a]~[a]

INVARIANT: the dictionary constructor for a class
           never has a wrapper.


A note about the stupid context
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Data types can have a context:

        data (Eq a, Ord b) => T a b = T1 a b | T2 a

and that makes the constructors have a context too
(notice that T2's context is "thinned"):

        T1 :: (Eq a, Ord b) => a -> b -> T a b
        T2 :: (Eq a) => a -> T a b

Furthermore, this context pops up when pattern matching
(though GHC hasn't implemented this, but it is in H98, and
I've fixed GHC so that it now does):

        f (T2 x) = x
gets inferred type
        f :: Eq a => T a b -> a

I say the context is "stupid" because the dictionaries passed
are immediately discarded -- they do nothing and have no benefit.
It's a flaw in the language.

        Up to now [March 2002] I have put this stupid context into the
        type of the "wrapper" constructors functions, T1 and T2, but
        that turned out to be jolly inconvenient for generics, and
        record update, and other functions that build values of type T
        (because they don't have suitable dictionaries available).

        So now I've taken the stupid context out.  I simply deal with
        it separately in the type checker on occurrences of a
        constructor, either in an expression or in a pattern.

        [May 2003: actually I think this decision could evasily be
        reversed now, and probably should be.  Generics could be
        disabled for types with a stupid context; record updates now
        (H98) needs the context too; etc.  It's an unforced change, so
        I'm leaving it for now --- but it does seem odd that the
        wrapper doesn't include the stupid context.]

[July 04] With the advent of generalised data types, it's less obvious
what the "stupid context" is.  Consider
        C :: forall a. Ord a => a -> a -> T (Foo a)
Does the C constructor in Core contain the Ord dictionary?  Yes, it must:

        f :: T b -> Ordering
        f = /\b. \x:T b.
            case x of
                C a (d:Ord a) (p:a) (q:a) -> compare d p q

Note that (Foo a) might not be an instance of Ord.

************************************************************************
*                                                                      *
\subsection{Data constructors}
*                                                                      *
************************************************************************
-}

-- | A data constructor
--
-- - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnOpen',
--             'ApiAnnotation.AnnClose','ApiAnnotation.AnnComma'

-- For details on above see note [Api annotations] in ApiAnnotation
data DataCon
  = MkData {
        dcName    :: Name,      -- This is the name of the *source data con*
                                -- (see "Note [Data Constructor Naming]" above)
        dcUnique :: Unique,     -- Cached from Name
        dcTag    :: ConTag,     -- ^ Tag, used for ordering 'DataCon's

        -- Running example:
        --
        --      *** As declared by the user
        --  data T a where
        --    MkT :: forall x y. (x~y,Ord x) => x -> y -> T (x,y)

        --      *** As represented internally
        --  data T a where
        --    MkT :: forall a. forall x y. (a~(x,y),x~y,Ord x) => x -> y -> T a
        --
        -- The next six fields express the type of the constructor, in pieces
        -- e.g.
        --
        --      dcUnivTyVars  = [a]
        --      dcExTyVars    = [x,y]
        --      dcEqSpec      = [a~(x,y)]
        --      dcOtherTheta  = [x~y, Ord x]
        --      dcOrigArgTys  = [x,y]
        --      dcRepTyCon       = T

        -- In general, the dcUnivTyVars are NOT NECESSARILY THE SAME AS THE TYVARS
        -- FOR THE PARENT TyCon. (This is a change (Oct05): previously, vanilla
        -- datacons guaranteed to have the same type variables as their parent TyCon,
        -- but that seems ugly.)

        dcVanilla :: Bool,      -- True <=> This is a vanilla Haskell 98 data constructor
                                --          Its type is of form
                                --              forall a1..an . t1 -> ... tm -> T a1..an
                                --          No existentials, no coercions, nothing.
                                -- That is: dcExTyVars = dcEqSpec = dcOtherTheta = []
                -- NB 1: newtypes always have a vanilla data con
                -- NB 2: a vanilla constructor can still be declared in GADT-style
                --       syntax, provided its type looks like the above.
                --       The declaration format is held in the TyCon (algTcGadtSyntax)

        -- Universally-quantified type vars [a,b,c]
        -- INVARIANT: length matches arity of the dcRepTyCon
        -- INVARIANT: result type of data con worker is exactly (T a b c)
        dcUnivTyVars    :: [TyVarBinder],

        -- Existentially-quantified type vars [x,y]
        dcExTyVars     :: [TyVarBinder],

        -- INVARIANT: the UnivTyVars and ExTyVars all have distinct OccNames
        -- Reason: less confusing, and easier to generate IfaceSyn

        dcEqSpec :: [EqSpec],   -- Equalities derived from the result type,
                                -- _as written by the programmer_

                -- This field allows us to move conveniently between the two ways
                -- of representing a GADT constructor's type:
                --      MkT :: forall a b. (a ~ [b]) => b -> T a
                --      MkT :: forall b. b -> T [b]
                -- Each equality is of the form (a ~ ty), where 'a' is one of
                -- the universally quantified type variables

                -- The next two fields give the type context of the data constructor
                --      (aside from the GADT constraints,
                --       which are given by the dcExpSpec)
                -- In GADT form, this is *exactly* what the programmer writes, even if
                -- the context constrains only universally quantified variables
                --      MkT :: forall a b. (a ~ b, Ord b) => a -> T a b
        dcOtherTheta :: ThetaType,  -- The other constraints in the data con's type
                                    -- other than those in the dcEqSpec

        dcStupidTheta :: ThetaType,     -- The context of the data type declaration
                                        --      data Eq a => T a = ...
                                        -- or, rather, a "thinned" version thereof
                -- "Thinned", because the Report says
                -- to eliminate any constraints that don't mention
                -- tyvars free in the arg types for this constructor
                --
                -- INVARIANT: the free tyvars of dcStupidTheta are a subset of dcUnivTyVars
                -- Reason: dcStupidTeta is gotten by thinning the stupid theta from the tycon
                --
                -- "Stupid", because the dictionaries aren't used for anything.
                -- Indeed, [as of March 02] they are no longer in the type of
                -- the wrapper Id, because that makes it harder to use the wrap-id
                -- to rebuild values after record selection or in generics.

        dcOrigArgTys :: [Type],         -- Original argument types
                                        -- (before unboxing and flattening of strict fields)
        dcOrigResTy :: Type,            -- Original result type, as seen by the user
                -- NB: for a data instance, the original user result type may
                -- differ from the DataCon's representation TyCon.  Example
                --      data instance T [a] where MkT :: a -> T [a]
                -- The OrigResTy is T [a], but the dcRepTyCon might be :T123

        -- Now the strictness annotations and field labels of the constructor
        dcSrcBangs :: [HsSrcBang],
                -- See Note [Bangs on data constructor arguments]
                --
                -- The [HsSrcBang] as written by the programmer.
                --
                -- Matches 1-1 with dcOrigArgTys
                -- Hence length = dataConSourceArity dataCon

        dcFields  :: [FieldLabel],
                -- Field labels for this constructor, in the
                -- same order as the dcOrigArgTys;
                -- length = 0 (if not a record) or dataConSourceArity.

        -- The curried worker function that corresponds to the constructor:
        -- It doesn't have an unfolding; the code generator saturates these Ids
        -- and allocates a real constructor when it finds one.
        dcWorkId :: Id,

        -- Constructor representation
        dcRep      :: DataConRep,

        -- Cached; see Note [DataCon arities]
        -- INVARIANT: dcRepArity    == length dataConRepArgTys
        -- INVARIANT: dcSourceArity == length dcOrigArgTys
        dcRepArity    :: Arity,
        dcSourceArity :: Arity,

        -- Result type of constructor is T t1..tn
        dcRepTyCon  :: TyCon,           -- Result tycon, T

        dcRepType   :: Type,    -- Type of the constructor
                                --      forall a x y. (a~(x,y), x~y, Ord x) =>
                                --        x -> y -> T a
                                -- (this is *not* of the constructor wrapper Id:
                                --  see Note [Data con representation] below)
        -- Notice that the existential type parameters come *second*.
        -- Reason: in a case expression we may find:
        --      case (e :: T t) of
        --        MkT x y co1 co2 (d:Ord x) (v:r) (w:F s) -> ...
        -- It's convenient to apply the rep-type of MkT to 't', to get
        --      forall x y. (t~(x,y), x~y, Ord x) => x -> y -> T t
        -- and use that to check the pattern.  Mind you, this is really only
        -- used in CoreLint.


        dcInfix :: Bool,        -- True <=> declared infix
                                -- Used for Template Haskell and 'deriving' only
                                -- The actual fixity is stored elsewhere

        dcPromoted :: TyCon    -- The promoted TyCon
                               -- See Note [Promoted data constructors] in TyCon
  }


{- Note [TyVarBinders in DataCons]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
For the TyVarBinders in a DataCon and PatSyn:

 * Each argument flag is Inferred or Specified.
   None are Required. (A DataCon is a term-level function; see
   Note [No Required TyBinder in terms] in TyCoRep.)

Why do we need the TyVarBinders, rather than just the TyVars?  So that
we can construct the right type for the DataCon with its foralls
attributed the correce visiblity.  That in turn governs whether you
can use visible type application at a call of the data constructor.

Note [DataCon arities]
~~~~~~~~~~~~~~~~~~~~~~
dcSourceArity does not take constraints into account,
but dcRepArity does.  For example:
   MkT :: Ord a => a -> T a
    dcSourceArity = 1
    dcRepArity    = 2
-}

data DataConRep
  = NoDataConRep              -- No wrapper

  | DCR { dcr_wrap_id :: Id   -- Takes src args, unboxes/flattens,
                              -- and constructs the representation

        , dcr_boxer   :: DataConBoxer

        , dcr_arg_tys :: [Type]  -- Final, representation argument types,
                                 -- after unboxing and flattening,
                                 -- and *including* all evidence args

        , dcr_stricts :: [StrictnessMark]  -- 1-1 with dcr_arg_tys
                -- See also Note [Data-con worker strictness] in MkId.hs

        , dcr_bangs :: [HsImplBang]  -- The actual decisions made (including failures)
                                     -- about the original arguments; 1-1 with orig_arg_tys
                                     -- See Note [Bangs on data constructor arguments]

    }
-- Algebraic data types always have a worker, and
-- may or may not have a wrapper, depending on whether
-- the wrapper does anything.
--
-- Data types have a worker with no unfolding
-- Newtypes just have a worker, which has a compulsory unfolding (just a cast)

-- _Neither_ the worker _nor_ the wrapper take the dcStupidTheta dicts as arguments

-- The wrapper (if it exists) takes dcOrigArgTys as its arguments
-- The worker takes dataConRepArgTys as its arguments
-- If the worker is absent, dataConRepArgTys is the same as dcOrigArgTys

-- The 'NoDataConRep' case is important
-- Not only is this efficient,
-- but it also ensures that the wrapper is replaced
-- by the worker (because it *is* the worker)
-- even when there are no args. E.g. in
--              f (:) x
-- the (:) *is* the worker.
-- This is really important in rule matching,
-- (We could match on the wrappers,
-- but that makes it less likely that rules will match
-- when we bring bits of unfoldings together.)

-------------------------

-- | Bangs on data constructor arguments as the user wrote them in the
-- source code.
--
-- (HsSrcBang _ SrcUnpack SrcLazy) and
-- (HsSrcBang _ SrcUnpack NoSrcStrict) (without StrictData) makes no sense, we
-- emit a warning (in checkValidDataCon) and treat it like
-- (HsSrcBang _ NoSrcUnpack SrcLazy)
data HsSrcBang =
  HsSrcBang (Maybe SourceText) -- Note [Pragma source text] in BasicTypes
            SrcUnpackedness
            SrcStrictness
  deriving Data.Data

-- | Bangs of data constructor arguments as generated by the compiler
-- after consulting HsSrcBang, flags, etc.
data HsImplBang
  = HsLazy  -- ^ Lazy field
  | HsStrict  -- ^ Strict but not unpacked field
  | HsUnpack (Maybe Coercion)
    -- ^ Strict and unpacked field
    -- co :: arg-ty ~ product-ty HsBang
  deriving Data.Data

-- | What strictness annotation the user wrote
data SrcStrictness = SrcLazy -- ^ Lazy, ie '~'
                   | SrcStrict -- ^ Strict, ie '!'
                   | NoSrcStrict -- ^ no strictness annotation
     deriving (Eq, Data.Data)

-- | What unpackedness the user requested
data SrcUnpackedness = SrcUnpack -- ^ {-# UNPACK #-} specified
                     | SrcNoUnpack -- ^ {-# NOUNPACK #-} specified
                     | NoSrcUnpack -- ^ no unpack pragma
     deriving (Eq, Data.Data)



-------------------------
-- StrictnessMark is internal only, used to indicate strictness
-- of the DataCon *worker* fields
data StrictnessMark = MarkedStrict | NotMarkedStrict

-- | An 'EqSpec' is a tyvar/type pair representing an equality made in
-- rejigging a GADT constructor
data EqSpec = EqSpec TyVar
                     Type

-- | Make an 'EqSpec'
mkEqSpec :: TyVar -> Type -> EqSpec
mkEqSpec tv ty = EqSpec tv ty

eqSpecTyVar :: EqSpec -> TyVar
eqSpecTyVar (EqSpec tv _) = tv

eqSpecType :: EqSpec -> Type
eqSpecType (EqSpec _ ty) = ty

eqSpecPair :: EqSpec -> (TyVar, Type)
eqSpecPair (EqSpec tv ty) = (tv, ty)

eqSpecPreds :: [EqSpec] -> ThetaType
eqSpecPreds spec = [ mkPrimEqPred (mkTyVarTy tv) ty
                   | EqSpec tv ty <- spec ]

-- | Substitute in an 'EqSpec'. Precondition: if the LHS of the EqSpec
-- is mapped in the substitution, it is mapped to a type variable, not
-- a full type.
substEqSpec :: TCvSubst -> EqSpec -> EqSpec
substEqSpec subst (EqSpec tv ty)
  = EqSpec tv' (substTy subst ty)
  where
    tv' = getTyVar "substEqSpec" (substTyVar subst tv)

-- | Filter out any TyBinders mentioned in an EqSpec
filterEqSpec :: [EqSpec] -> [TyVarBinder] -> [TyVarBinder]
filterEqSpec eq_spec
  = filter not_in_eq_spec
  where
    not_in_eq_spec bndr = let var = binderVar bndr in
                          all (not . (== var) . eqSpecTyVar) eq_spec

instance Outputable EqSpec where
  ppr (EqSpec tv ty) = ppr (tv, ty)

{- Note [Bangs on data constructor arguments]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider
  data T = MkT !Int {-# UNPACK #-} !Int Bool

When compiling the module, GHC will decide how to represent
MkT, depending on the optimisation level, and settings of
flags like -funbox-small-strict-fields.

Terminology:
  * HsSrcBang:  What the user wrote
                Constructors: HsSrcBang

  * HsImplBang: What GHC decided
                Constructors: HsLazy, HsStrict, HsUnpack

* If T was defined in this module, MkT's dcSrcBangs field
  records the [HsSrcBang] of what the user wrote; in the example
    [ HsSrcBang _ NoSrcUnpack SrcStrict
    , HsSrcBang _ SrcUnpack SrcStrict
    , HsSrcBang _ NoSrcUnpack NoSrcStrictness]

* However, if T was defined in an imported module, the importing module
  must follow the decisions made in the original module, regardless of
  the flag settings in the importing module.
  Also see Note [Bangs on imported data constructors] in MkId

* The dcr_bangs field of the dcRep field records the [HsImplBang]
  If T was defined in this module, Without -O the dcr_bangs might be
    [HsStrict, HsStrict, HsLazy]
  With -O it might be
    [HsStrict, HsUnpack _, HsLazy]
  With -funbox-small-strict-fields it might be
    [HsUnpack, HsUnpack _, HsLazy]
  With -XStrictData it might be
    [HsStrict, HsUnpack _, HsStrict]

Note [Data con representation]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The dcRepType field contains the type of the representation of a constructor
This may differ from the type of the constructor *Id* (built
by MkId.mkDataConId) for two reasons:
        a) the constructor Id may be overloaded, but the dictionary isn't stored
           e.g.    data Eq a => T a = MkT a a

        b) the constructor may store an unboxed version of a strict field.

Here's an example illustrating both:
        data Ord a => T a = MkT Int! a
Here
        T :: Ord a => Int -> a -> T a
but the rep type is
        Trep :: Int# -> a -> T a
Actually, the unboxed part isn't implemented yet!



************************************************************************
*                                                                      *
\subsection{Instances}
*                                                                      *
************************************************************************
-}

instance Eq DataCon where
    a == b = getUnique a == getUnique b
    a /= b = getUnique a /= getUnique b

instance Uniquable DataCon where
    getUnique = dcUnique

instance NamedThing DataCon where
    getName = dcName

instance Outputable DataCon where
    ppr con = ppr (dataConName con)

instance OutputableBndr DataCon where
    pprInfixOcc con = pprInfixName (dataConName con)
    pprPrefixOcc con = pprPrefixName (dataConName con)

instance Data.Data DataCon where
    -- don't traverse?
    toConstr _   = abstractConstr "DataCon"
    gunfold _ _  = error "gunfold"
    dataTypeOf _ = mkNoRepType "DataCon"

instance Outputable HsSrcBang where
    ppr (HsSrcBang _ prag mark) = ppr prag <+> ppr mark

instance Outputable HsImplBang where
    ppr HsLazy                  = text "Lazy"
    ppr (HsUnpack Nothing)      = text "Unpacked"
    ppr (HsUnpack (Just co))    = text "Unpacked" <> parens (ppr co)
    ppr HsStrict                = text "StrictNotUnpacked"

instance Outputable SrcStrictness where
    ppr SrcLazy     = char '~'
    ppr SrcStrict   = char '!'
    ppr NoSrcStrict = empty

instance Outputable SrcUnpackedness where
    ppr SrcUnpack   = text "{-# UNPACK #-}"
    ppr SrcNoUnpack = text "{-# NOUNPACK #-}"
    ppr NoSrcUnpack = empty

instance Outputable StrictnessMark where
    ppr MarkedStrict    = text "!"
    ppr NotMarkedStrict = empty

instance Binary SrcStrictness where
    put_ bh SrcLazy     = putByte bh 0
    put_ bh SrcStrict   = putByte bh 1
    put_ bh NoSrcStrict = putByte bh 2

    get bh =
      do h <- getByte bh
         case h of
           0 -> return SrcLazy
           1 -> return SrcLazy
           _ -> return NoSrcStrict

instance Binary SrcUnpackedness where
    put_ bh SrcNoUnpack = putByte bh 0
    put_ bh SrcUnpack   = putByte bh 1
    put_ bh NoSrcUnpack = putByte bh 2

    get bh =
      do h <- getByte bh
         case h of
           0 -> return SrcNoUnpack
           1 -> return SrcUnpack
           _ -> return NoSrcUnpack

-- | Compare strictness annotations
eqHsBang :: HsImplBang -> HsImplBang -> Bool
eqHsBang HsLazy               HsLazy              = True
eqHsBang HsStrict             HsStrict            = True
eqHsBang (HsUnpack Nothing)   (HsUnpack Nothing)  = True
eqHsBang (HsUnpack (Just c1)) (HsUnpack (Just c2))
  = eqType (coercionType c1) (coercionType c2)
eqHsBang _ _                                       = False

isBanged :: HsImplBang -> Bool
isBanged (HsUnpack {}) = True
isBanged (HsStrict {}) = True
isBanged HsLazy        = False

isSrcStrict :: SrcStrictness -> Bool
isSrcStrict SrcStrict = True
isSrcStrict _ = False

isSrcUnpacked :: SrcUnpackedness -> Bool
isSrcUnpacked SrcUnpack = True
isSrcUnpacked _ = False

isMarkedStrict :: StrictnessMark -> Bool
isMarkedStrict NotMarkedStrict = False
isMarkedStrict _               = True   -- All others are strict

{- *********************************************************************
*                                                                      *
\subsection{Construction}
*                                                                      *
********************************************************************* -}

-- | Build a new data constructor
mkDataCon :: Name
          -> Bool           -- ^ Is the constructor declared infix?
          -> TyConRepName   -- ^  TyConRepName for the promoted TyCon
          -> [HsSrcBang]    -- ^ Strictness/unpack annotations, from user
          -> [FieldLabel]   -- ^ Field labels for the constructor,
                            -- if it is a record, otherwise empty
          -> [TyVarBinder]  -- ^ Universals. See Note [TyVarBinders in DataCons]
          -> [TyVarBinder]  -- ^ Existentials.
                            -- (These last two must be Named and Inferred/Specified)
          -> [EqSpec]       -- ^ GADT equalities
          -> ThetaType      -- ^ Theta-type occuring before the arguments proper
          -> [Type]         -- ^ Original argument types
          -> Type           -- ^ Original result type
          -> RuntimeRepInfo -- ^ See comments on 'TyCon.RuntimeRepInfo'
          -> TyCon          -- ^ Representation type constructor
          -> ThetaType      -- ^ The "stupid theta", context of the data
                            -- declaration e.g. @data Eq a => T a ...@
          -> Id             -- ^ Worker Id
          -> DataConRep     -- ^ Representation
          -> DataCon
  -- Can get the tag from the TyCon

mkDataCon name declared_infix prom_info
          arg_stricts   -- Must match orig_arg_tys 1-1
          fields
          univ_tvs ex_tvs
          eq_spec theta
          orig_arg_tys orig_res_ty rep_info rep_tycon
          stupid_theta work_id rep
-- Warning: mkDataCon is not a good place to check invariants.
-- If the programmer writes the wrong result type in the decl, thus:
--      data T a where { MkT :: S }
-- then it's possible that the univ_tvs may hit an assertion failure
-- if you pull on univ_tvs.  This case is checked by checkValidDataCon,
-- so the error is detected properly... it's just that asaertions here
-- are a little dodgy.

  = con
  where
    is_vanilla = null ex_tvs && null eq_spec && null theta
    con = MkData {dcName = name, dcUnique = nameUnique name,
                  dcVanilla = is_vanilla, dcInfix = declared_infix,
                  dcUnivTyVars = univ_tvs,
                  dcExTyVars = ex_tvs,
                  dcEqSpec = eq_spec,
                  dcOtherTheta = theta,
                  dcStupidTheta = stupid_theta,
                  dcOrigArgTys = orig_arg_tys, dcOrigResTy = orig_res_ty,
                  dcRepTyCon = rep_tycon,
                  dcSrcBangs = arg_stricts,
                  dcFields = fields, dcTag = tag, dcRepType = rep_ty,
                  dcWorkId = work_id,
                  dcRep = rep,
                  dcSourceArity = length orig_arg_tys,
                  dcRepArity = length rep_arg_tys,
                  dcPromoted = promoted }

        -- The 'arg_stricts' passed to mkDataCon are simply those for the
        -- source-language arguments.  We add extra ones for the
        -- dictionary arguments right here.

    tag = assoc "mkDataCon" (tyConDataCons rep_tycon `zip` [fIRST_TAG..]) con
    rep_arg_tys = dataConRepArgTys con

    rep_ty = mkForAllTys univ_tvs $ mkForAllTys ex_tvs $
             mkFunTys rep_arg_tys $
             mkTyConApp rep_tycon (mkTyVarTys (binderVars univ_tvs))

      -- See Note [Promoted data constructors] in TyCon
    prom_tv_bndrs = [ mkNamedTyConBinder vis tv
                    | TvBndr tv vis <- filterEqSpec eq_spec univ_tvs ++ ex_tvs ]

    prom_arg_bndrs = mkCleanAnonTyConBinders prom_tv_bndrs (theta ++ orig_arg_tys)
    prom_res_kind  = orig_res_ty
    promoted       = mkPromotedDataCon con name prom_info
                                       (prom_tv_bndrs ++ prom_arg_bndrs)
                                       prom_res_kind roles rep_info

    roles = map (const Nominal) (univ_tvs ++ ex_tvs) ++
            map (const Representational) orig_arg_tys

mkCleanAnonTyConBinders :: [TyConBinder] -> [Type] -> [TyConBinder]
-- Make sure that the "anonymous" tyvars don't clash in
-- name or unique with the universal/existential ones.
-- Tiresome!  And unnecessary because these tyvars are never looked at
mkCleanAnonTyConBinders tc_bndrs tys
  = [ mkAnonTyConBinder (mkTyVar name ty)
    | (name, ty) <- fresh_names `zip` tys ]
  where
    fresh_names = freshNames (map getName (binderVars tc_bndrs))

freshNames :: [Name] -> [Name]
-- Make names whose Uniques and OccNames differ from
-- those in the 'avoid' list
freshNames avoids
  = [ mkSystemName uniq occ
    | n <- [0..]
    , let uniq = mkAlphaTyVarUnique n
          occ = mkTyVarOccFS (mkFastString ('x' : show n))

    , not (uniq `elementOfUniqSet` avoid_uniqs)
    , not (occ `elemOccSet` avoid_occs) ]

  where
    avoid_uniqs :: UniqSet Unique
    avoid_uniqs = mkUniqSet (map getUnique avoids)

    avoid_occs :: OccSet
    avoid_occs = mkOccSet (map getOccName avoids)

-- | The 'Name' of the 'DataCon', giving it a unique, rooted identification
dataConName :: DataCon -> Name
dataConName = dcName

-- | The tag used for ordering 'DataCon's
dataConTag :: DataCon -> ConTag
dataConTag  = dcTag

-- | The type constructor that we are building via this data constructor
dataConTyCon :: DataCon -> TyCon
dataConTyCon = dcRepTyCon

-- | The original type constructor used in the definition of this data
-- constructor.  In case of a data family instance, that will be the family
-- type constructor.
dataConOrigTyCon :: DataCon -> TyCon
dataConOrigTyCon dc
  | Just (tc, _) <- tyConFamInst_maybe (dcRepTyCon dc) = tc
  | otherwise                                          = dcRepTyCon dc

-- | The representation type of the data constructor, i.e. the sort
-- type that will represent values of this type at runtime
dataConRepType :: DataCon -> Type
dataConRepType = dcRepType

-- | Should the 'DataCon' be presented infix?
dataConIsInfix :: DataCon -> Bool
dataConIsInfix = dcInfix

-- | The universally-quantified type variables of the constructor
dataConUnivTyVars :: DataCon -> [TyVar]
dataConUnivTyVars (MkData { dcUnivTyVars = tvbs }) = binderVars tvbs

-- | 'TyBinder's for the universally-quantified type variables
dataConUnivTyVarBinders :: DataCon -> [TyVarBinder]
dataConUnivTyVarBinders = dcUnivTyVars

-- | The existentially-quantified type variables of the constructor
dataConExTyVars :: DataCon -> [TyVar]
dataConExTyVars (MkData { dcExTyVars = tvbs }) = binderVars tvbs

-- | 'TyBinder's for the existentially-quantified type variables
dataConExTyVarBinders :: DataCon -> [TyVarBinder]
dataConExTyVarBinders = dcExTyVars

-- | Both the universal and existentiatial type variables of the constructor
dataConAllTyVars :: DataCon -> [TyVar]
dataConAllTyVars (MkData { dcUnivTyVars = univ_tvs, dcExTyVars = ex_tvs })
  = binderVars (univ_tvs ++ ex_tvs)

-- | Equalities derived from the result type of the data constructor, as written
-- by the programmer in any GADT declaration. This includes *all* GADT-like
-- equalities, including those written in by hand by the programmer.
dataConEqSpec :: DataCon -> [EqSpec]
dataConEqSpec (MkData { dcEqSpec = eq_spec, dcOtherTheta = theta })
  = eq_spec ++
    [ spec   -- heterogeneous equality
    | Just (tc, [_k1, _k2, ty1, ty2]) <- map splitTyConApp_maybe theta
    , tc `hasKey` heqTyConKey
    , spec <- case (getTyVar_maybe ty1, getTyVar_maybe ty2) of
                    (Just tv1, _) -> [mkEqSpec tv1 ty2]
                    (_, Just tv2) -> [mkEqSpec tv2 ty1]
                    _             -> []
    ] ++
    [ spec   -- homogeneous equality
    | Just (tc, [_k, ty1, ty2]) <- map splitTyConApp_maybe theta
    , tc `hasKey` eqTyConKey
    , spec <- case (getTyVar_maybe ty1, getTyVar_maybe ty2) of
                    (Just tv1, _) -> [mkEqSpec tv1 ty2]
                    (_, Just tv2) -> [mkEqSpec tv2 ty1]
                    _             -> []
    ]


-- | The *full* constraints on the constructor type.
dataConTheta :: DataCon -> ThetaType
dataConTheta (MkData { dcEqSpec = eq_spec, dcOtherTheta = theta })
  = eqSpecPreds eq_spec ++ theta

-- | Get the Id of the 'DataCon' worker: a function that is the "actual"
-- constructor and has no top level binding in the program. The type may
-- be different from the obvious one written in the source program. Panics
-- if there is no such 'Id' for this 'DataCon'
dataConWorkId :: DataCon -> Id
dataConWorkId dc = dcWorkId dc

-- | Get the Id of the 'DataCon' wrapper: a function that wraps the "actual"
-- constructor so it has the type visible in the source program: c.f. 'dataConWorkId'.
-- Returns Nothing if there is no wrapper, which occurs for an algebraic data constructor
-- and also for a newtype (whose constructor is inlined compulsorily)
dataConWrapId_maybe :: DataCon -> Maybe Id
dataConWrapId_maybe dc = case dcRep dc of
                           NoDataConRep -> Nothing
                           DCR { dcr_wrap_id = wrap_id } -> Just wrap_id

-- | Returns an Id which looks like the Haskell-source constructor by using
-- the wrapper if it exists (see 'dataConWrapId_maybe') and failing over to
-- the worker (see 'dataConWorkId')
dataConWrapId :: DataCon -> Id
dataConWrapId dc = case dcRep dc of
                     NoDataConRep-> dcWorkId dc    -- worker=wrapper
                     DCR { dcr_wrap_id = wrap_id } -> wrap_id

-- | Find all the 'Id's implicitly brought into scope by the data constructor. Currently,
-- the union of the 'dataConWorkId' and the 'dataConWrapId'
dataConImplicitTyThings :: DataCon -> [TyThing]
dataConImplicitTyThings (MkData { dcWorkId = work, dcRep = rep })
  = [AnId work] ++ wrap_ids
  where
    wrap_ids = case rep of
                 NoDataConRep               -> []
                 DCR { dcr_wrap_id = wrap } -> [AnId wrap]

-- | The labels for the fields of this particular 'DataCon'
dataConFieldLabels :: DataCon -> [FieldLabel]
dataConFieldLabels = dcFields

-- | Extract the type for any given labelled field of the 'DataCon'
dataConFieldType :: DataCon -> FieldLabelString -> Type
dataConFieldType con label
  = case find ((== label) . flLabel . fst) (dcFields con `zip` dcOrigArgTys con) of
      Just (_, ty) -> ty
      Nothing -> pprPanic "dataConFieldType" (ppr con <+> ppr label)

-- | Strictness/unpack annotations, from user; or, for imported
-- DataCons, from the interface file
-- The list is in one-to-one correspondence with the arity of the 'DataCon'

dataConSrcBangs :: DataCon -> [HsSrcBang]
dataConSrcBangs = dcSrcBangs

-- | Source-level arity of the data constructor
dataConSourceArity :: DataCon -> Arity
dataConSourceArity (MkData { dcSourceArity = arity }) = arity

-- | Gives the number of actual fields in the /representation/ of the
-- data constructor. This may be more than appear in the source code;
-- the extra ones are the existentially quantified dictionaries
dataConRepArity :: DataCon -> Arity
dataConRepArity (MkData { dcRepArity = arity }) = arity

-- | Return whether there are any argument types for this 'DataCon's original source type
-- See Note [DataCon arities]
isNullarySrcDataCon :: DataCon -> Bool
isNullarySrcDataCon dc = dataConSourceArity dc == 0

-- | Return whether there are any argument types for this 'DataCon's runtime representation type
-- See Note [DataCon arities]
isNullaryRepDataCon :: DataCon -> Bool
isNullaryRepDataCon dc = dataConRepArity dc == 0

dataConRepStrictness :: DataCon -> [StrictnessMark]
-- ^ Give the demands on the arguments of a
-- Core constructor application (Con dc args)
dataConRepStrictness dc = case dcRep dc of
                            NoDataConRep -> [NotMarkedStrict | _ <- dataConRepArgTys dc]
                            DCR { dcr_stricts = strs } -> strs

dataConImplBangs :: DataCon -> [HsImplBang]
-- The implementation decisions about the strictness/unpack of each
-- source program argument to the data constructor
dataConImplBangs dc
  = case dcRep dc of
      NoDataConRep              -> replicate (dcSourceArity dc) HsLazy
      DCR { dcr_bangs = bangs } -> bangs

dataConBoxer :: DataCon -> Maybe DataConBoxer
dataConBoxer (MkData { dcRep = DCR { dcr_boxer = boxer } }) = Just boxer
dataConBoxer _ = Nothing

-- | The \"signature\" of the 'DataCon' returns, in order:
--
-- 1) The result of 'dataConAllTyVars',
--
-- 2) All the 'ThetaType's relating to the 'DataCon' (coercion, dictionary, implicit
--    parameter - whatever)
--
-- 3) The type arguments to the constructor
--
-- 4) The /original/ result type of the 'DataCon'
dataConSig :: DataCon -> ([TyVar], ThetaType, [Type], Type)
dataConSig con@(MkData {dcOrigArgTys = arg_tys, dcOrigResTy = res_ty})
  = (dataConAllTyVars con, dataConTheta con, arg_tys, res_ty)

dataConInstSig
  :: DataCon
  -> [Type]    -- Instantiate the *universal* tyvars with these types
  -> ([TyVar], ThetaType, [Type])  -- Return instantiated existentials
                                   -- theta and arg tys
-- ^ Instantantiate the universal tyvars of a data con,
--   returning the instantiated existentials, constraints, and args
dataConInstSig (MkData { dcUnivTyVars = univ_tvs, dcExTyVars = ex_tvs
                       , dcEqSpec = eq_spec, dcOtherTheta  = theta
                       , dcOrigArgTys = arg_tys })
               univ_tys
  = ( ex_tvs'
    , substTheta subst (eqSpecPreds eq_spec ++ theta)
    , substTys   subst arg_tys)
  where
    univ_subst = zipTvSubst (binderVars univ_tvs) univ_tys
    (subst, ex_tvs') = mapAccumL Type.substTyVarBndr univ_subst $
                       binderVars ex_tvs


-- | The \"full signature\" of the 'DataCon' returns, in order:
--
-- 1) The result of 'dataConUnivTyVars'
--
-- 2) The result of 'dataConExTyVars'
--
-- 3) The GADT equalities
--
-- 4) The result of 'dataConDictTheta'
--
-- 5) The original argument types to the 'DataCon' (i.e. before
--    any change of the representation of the type)
--
-- 6) The original result type of the 'DataCon'
dataConFullSig :: DataCon
               -> ([TyVar], [TyVar], [EqSpec], ThetaType, [Type], Type)
dataConFullSig (MkData {dcUnivTyVars = univ_tvs, dcExTyVars = ex_tvs,
                        dcEqSpec = eq_spec, dcOtherTheta = theta,
                        dcOrigArgTys = arg_tys, dcOrigResTy = res_ty})
  = (binderVars univ_tvs, binderVars ex_tvs, eq_spec, theta, arg_tys, res_ty)

dataConOrigResTy :: DataCon -> Type
dataConOrigResTy dc = dcOrigResTy dc

-- | The \"stupid theta\" of the 'DataCon', such as @data Eq a@ in:
--
-- > data Eq a => T a = ...
dataConStupidTheta :: DataCon -> ThetaType
dataConStupidTheta dc = dcStupidTheta dc

dataConUserType :: DataCon -> Type
-- ^ The user-declared type of the data constructor
-- in the nice-to-read form:
--
-- > T :: forall a b. a -> b -> T [a]
--
-- rather than:
--
-- > T :: forall a c. forall b. (c~[a]) => a -> b -> T c
--
-- NB: If the constructor is part of a data instance, the result type
-- mentions the family tycon, not the internal one.
dataConUserType (MkData { dcUnivTyVars = univ_tvs,
                          dcExTyVars = ex_tvs, dcEqSpec = eq_spec,
                          dcOtherTheta = theta, dcOrigArgTys = arg_tys,
                          dcOrigResTy = res_ty })
  = mkForAllTys (filterEqSpec eq_spec univ_tvs) $
    mkForAllTys ex_tvs $
    mkFunTys theta $
    mkFunTys arg_tys $
    res_ty
  where

-- | Finds the instantiated types of the arguments required to construct a 'DataCon' representation
-- NB: these INCLUDE any dictionary args
--     but EXCLUDE the data-declaration context, which is discarded
-- It's all post-flattening etc; this is a representation type
dataConInstArgTys :: DataCon    -- ^ A datacon with no existentials or equality constraints
                                -- However, it can have a dcTheta (notably it can be a
                                -- class dictionary, with superclasses)
                  -> [Type]     -- ^ Instantiated at these types
                  -> [Type]
dataConInstArgTys dc@(MkData {dcUnivTyVars = univ_tvs,
                              dcExTyVars = ex_tvs}) inst_tys
 = ASSERT2( length univ_tvs == length inst_tys
          , text "dataConInstArgTys" <+> ppr dc $$ ppr univ_tvs $$ ppr inst_tys)
   ASSERT2( null ex_tvs, ppr dc )
   map (substTyWith (binderVars univ_tvs) inst_tys) (dataConRepArgTys dc)

-- | Returns just the instantiated /value/ argument types of a 'DataCon',
-- (excluding dictionary args)
dataConInstOrigArgTys
        :: DataCon      -- Works for any DataCon
        -> [Type]       -- Includes existential tyvar args, but NOT
                        -- equality constraints or dicts
        -> [Type]
-- For vanilla datacons, it's all quite straightforward
-- But for the call in MatchCon, we really do want just the value args
dataConInstOrigArgTys dc@(MkData {dcOrigArgTys = arg_tys,
                                  dcUnivTyVars = univ_tvs,
                                  dcExTyVars = ex_tvs}) inst_tys
  = ASSERT2( length tyvars == length inst_tys
          , text "dataConInstOrigArgTys" <+> ppr dc $$ ppr tyvars $$ ppr inst_tys )
    map (substTyWith tyvars inst_tys) arg_tys
  where
    tyvars = binderVars (univ_tvs ++ ex_tvs)

-- | Returns the argument types of the wrapper, excluding all dictionary arguments
-- and without substituting for any type variables
dataConOrigArgTys :: DataCon -> [Type]
dataConOrigArgTys dc = dcOrigArgTys dc

-- | Returns the arg types of the worker, including *all*
-- evidence, after any flattening has been done and without substituting for
-- any type variables
dataConRepArgTys :: DataCon -> [Type]
dataConRepArgTys (MkData { dcRep = rep
                         , dcEqSpec = eq_spec
                         , dcOtherTheta = theta
                         , dcOrigArgTys = orig_arg_tys })
  = case rep of
      NoDataConRep -> ASSERT( null eq_spec ) theta ++ orig_arg_tys
      DCR { dcr_arg_tys = arg_tys } -> arg_tys

-- | The string @package:module.name@ identifying a constructor, which is attached
-- to its info table and used by the GHCi debugger and the heap profiler
dataConIdentity :: DataCon -> [Word8]
-- We want this string to be UTF-8, so we get the bytes directly from the FastStrings.
dataConIdentity dc = bytesFS (unitIdFS (moduleUnitId mod)) ++
                  fromIntegral (ord ':') : bytesFS (moduleNameFS (moduleName mod)) ++
                  fromIntegral (ord '.') : bytesFS (occNameFS (nameOccName name))
  where name = dataConName dc
        mod  = ASSERT( isExternalName name ) nameModule name

isTupleDataCon :: DataCon -> Bool
isTupleDataCon (MkData {dcRepTyCon = tc}) = isTupleTyCon tc

isUnboxedTupleCon :: DataCon -> Bool
isUnboxedTupleCon (MkData {dcRepTyCon = tc}) = isUnboxedTupleTyCon tc

isUnboxedSumCon :: DataCon -> Bool
isUnboxedSumCon (MkData {dcRepTyCon = tc}) = isUnboxedSumTyCon tc

-- | Vanilla 'DataCon's are those that are nice boring Haskell 98 constructors
isVanillaDataCon :: DataCon -> Bool
isVanillaDataCon dc = dcVanilla dc

-- | Should this DataCon be allowed in a type even without -XDataKinds?
-- Currently, only Lifted & Unlifted
specialPromotedDc :: DataCon -> Bool
specialPromotedDc = isKindTyCon . dataConTyCon

-- | Was this datacon promotable before GHC 8.0? That is, is it promotable
-- without -XTypeInType
isLegacyPromotableDataCon :: DataCon -> Bool
isLegacyPromotableDataCon dc
  =  null (dataConEqSpec dc)  -- no GADTs
  && null (dataConTheta dc)   -- no context
  && not (isFamInstTyCon (dataConTyCon dc))   -- no data instance constructors
  && allUFM isLegacyPromotableTyCon (tyConsOfType (dataConUserType dc))

-- | Was this tycon promotable before GHC 8.0? That is, is it promotable
-- without -XTypeInType
isLegacyPromotableTyCon :: TyCon -> Bool
isLegacyPromotableTyCon tc
  = isVanillaAlgTyCon tc ||
      -- This returns True more often than it should, but it's quite painful
      -- to make this fully accurate. And no harm is caused; we just don't
      -- require -XTypeInType every time we need to. (We'll always require
      -- -XDataKinds, though, so there's no standards-compliance issue.)
    isFunTyCon tc || isKindTyCon tc

classDataCon :: Class -> DataCon
classDataCon clas = case tyConDataCons (classTyCon clas) of
                      (dict_constr:no_more) -> ASSERT( null no_more ) dict_constr
                      [] -> panic "classDataCon"

dataConCannotMatch :: [Type] -> DataCon -> Bool
-- Returns True iff the data con *definitely cannot* match a
--                  scrutinee of type (T tys)
--                  where T is the dcRepTyCon for the data con
dataConCannotMatch tys con
  | null inst_theta   = False   -- Common
  | all isTyVarTy tys = False   -- Also common
  | otherwise         = typesCantMatch (concatMap predEqs inst_theta)
  where
    (_, inst_theta, _) = dataConInstSig con tys

    -- TODO: could gather equalities from superclasses too
    predEqs pred = case classifyPredType pred of
                     EqPred NomEq ty1 ty2       -> [(ty1, ty2)]
                     ClassPred eq [_, ty1, ty2]
                       | eq `hasKey` eqTyConKey -> [(ty1, ty2)]
                     _                          -> []

{-
%************************************************************************
%*                                                                      *
        Promoting of data types to the kind level
*                                                                      *
************************************************************************

-}

promoteDataCon :: DataCon -> TyCon
promoteDataCon (MkData { dcPromoted = tc }) = tc

{-
************************************************************************
*                                                                      *
\subsection{Splitting products}
*                                                                      *
************************************************************************
-}

-- | Extract the type constructor, type argument, data constructor and it's
-- /representation/ argument types from a type if it is a product type.
--
-- Precisely, we return @Just@ for any type that is all of:
--
--  * Concrete (i.e. constructors visible)
--
--  * Single-constructor
--
--  * Not existentially quantified
--
-- Whether the type is a @data@ type or a @newtype@
splitDataProductType_maybe
        :: Type                         -- ^ A product type, perhaps
        -> Maybe (TyCon,                -- The type constructor
                  [Type],               -- Type args of the tycon
                  DataCon,              -- The data constructor
                  [Type])               -- Its /representation/ arg types

        -- Rejecting existentials is conservative.  Maybe some things
        -- could be made to work with them, but I'm not going to sweat
        -- it through till someone finds it's important.

splitDataProductType_maybe ty
  | Just (tycon, ty_args) <- splitTyConApp_maybe ty
  , Just con <- isDataProductTyCon_maybe tycon
  = Just (tycon, ty_args, con, dataConInstArgTys con ty_args)
  | otherwise
  = Nothing

{-
************************************************************************
*                                                                      *
              Building an algebraic data type
*                                                                      *
************************************************************************

buildAlgTyCon is here because it is called from TysWiredIn, which can
depend on this module, but not on BuildTyCl.
-}

buildAlgTyCon :: Name
              -> [TyVar]               -- ^ Kind variables and type variables
              -> [Role]
              -> Maybe CType
              -> ThetaType             -- ^ Stupid theta
              -> AlgTyConRhs
              -> Bool                  -- ^ True <=> was declared in GADT syntax
              -> AlgTyConFlav
              -> TyCon

buildAlgTyCon tc_name ktvs roles cType stupid_theta rhs
              gadt_syn parent
  = mkAlgTyCon tc_name binders liftedTypeKind roles cType stupid_theta
               rhs parent gadt_syn
  where
    binders = mkTyConBindersPreferAnon ktvs liftedTypeKind
