 DEFER: lconcat
 
 : (lconcat) ( car cdr -- list )
-  over nil? [
-    nip lconcat
-  ] [
-    <lazy-concat>
-  ] if ;
+    over nil? [
+        nip lconcat
+    ] [
+        <lazy-concat>
+    ] if ;
 
 : lconcat ( list -- result )
-  dup nil? [
-    drop nil
-  ] [
-    uncons swap (lconcat)
-  ] if ;
+    dup nil? [
+        drop nil
+    ] [
+        uncons swap (lconcat)
+    ] if ;
 
 M: lazy-concat car ( lazy-concat -- car )
-  car>> car ;
+    car>> car ;
 
 M: lazy-concat cdr ( lazy-concat -- cdr )
-  [ car>> cdr ] keep cdr>> (lconcat) ;
+    [ car>> cdr ] keep cdr>> (lconcat) ;
 
 M: lazy-concat nil? ( lazy-concat -- bool )
-  dup car>> nil? [
-    cdr>> nil?
-  ] [
-    drop f
-  ] if ;
+    dup car>> nil? [
+        cdr>> nil?
+    ] [
+        drop f
+    ] if ;
 
 : lcartesian-product ( list1 list2 -- result )
-  swap [ swap [ 2array ] lmap-with ] lmap-with lconcat ;
+    swap [ swap [ 2array ] lmap-with ] lmap-with lconcat ;
 
 : lcartesian-product* ( lists -- result )
-  dup nil? [
-    drop nil
-  ] [
-    [ car ] keep cdr [ car lcartesian-product ] keep cdr list>array swap [
-      swap [ swap [ suffix ] lmap-with ] lmap-with lconcat
-    ] reduce
-  ] if ;
+    dup nil? [
+        drop nil
+    ] [
+        [ car ] keep cdr [ car lcartesian-product ] keep cdr list>array swap [
+            swap [ swap [ suffix ] lmap-with ] lmap-with lconcat
+        ] reduce
+    ] if ;
 
 : lcomp ( list quot -- result )
-  [ lcartesian-product* ] dip lmap ;
+    [ lcartesian-product* ] dip lmap ;
 
 : lcomp* ( list guards quot -- result )
-  [ [ lcartesian-product* ] dip [ lfilter ] each ] dip lmap ;
+    [ [ lcartesian-product* ] dip [ lfilter ] each ] dip lmap ;
 
 DEFER: lmerge
 
 : (lmerge) ( list1 list2 -- result )
-  over [ car ] curry -rot
-  [
-    dup [ car ] curry -rot
+    over [ car ] curry -rot
     [
-      [ cdr ] bi@ lmerge
-    ] 2curry lazy-cons
-  ] 2curry lazy-cons ;
+        dup [ car ] curry -rot
+        [
+            [ cdr ] bi@ lmerge
+        ] 2curry lazy-cons
+    ] 2curry lazy-cons ;
 
 : lmerge ( list1 list2 -- result )
-  {
-    { [ over nil? ] [ nip   ] }
-    { [ dup nil?  ]  [ drop ] }
-    { [ t         ]  [ (lmerge) ] }
-  } cond ;
+    {
+        { [ over nil? ] [ nip     ] }
+        { [ dup nil?    ]    [ drop ] }
+        { [ t                 ]    [ (lmerge) ] }
+    } cond ;
 
 TUPLE: lazy-io stream car cdr quot ;
 
 C: <lazy-io> lazy-io
 
 : lcontents ( stream -- result )
-  f f [ stream-read1 ] <lazy-io> ;
+    f f [ stream-read1 ] <lazy-io> ;
 
 : llines ( stream -- result )
-  f f [ stream-readln ] <lazy-io> ;
+    f f [ stream-readln ] <lazy-io> ;
 
 M: lazy-io car ( lazy-io -- car )
-  dup car>> dup [
-    nip
-  ] [
-    drop dup stream>> over quot>> call
-    swap dupd set-lazy-io-car
-  ] if ;
+    dup car>> dup [
+        nip
+    ] [
+        drop dup stream>> over quot>> call
+        swap dupd set-lazy-io-car
+    ] if ;
 
 M: lazy-io cdr ( lazy-io -- cdr )
-  dup cdr>> dup [
-    nip
-  ] [
-    drop dup
-    [ stream>> ] keep
-    [ quot>> ] keep
-    car [
-      [ f f ] dip <lazy-io> [ >>cdr drop ] keep
-    ] [
-      3drop nil
-    ] if
-  ] if ;
+    dup cdr>> dup [
+        nip
+    ] [
+        drop dup
+        [ stream>> ] keep
+        [ quot>> ] keep
+        car [
+            [ f f ] dip <lazy-io> [ >>cdr drop ] keep
+        ] [
+            3drop nil
+        ] if
+    ] if ;
 
 M: lazy-io nil? ( lazy-io -- bool )
-  car not ;
+    car not ;
 
 INSTANCE: sequence-cons list
 INSTANCE: memoized-cons list

    nil eq? ;

: 1lazy-list ( a -- lazy-cons )
  [ nil ] lazy-cons ;

: 2lazy-list ( a b -- lazy-cons )
  1lazy-list 1quotation lazy-cons ;

: 3lazy-list ( a b c -- lazy-cons )
  2lazy-list 1quotation lazy-cons ;

: lnth ( n list -- elt )
  swap [ cdr ] times car ;

: (llength) ( list acc -- n )
  over nil? [ nip ] [ [ cdr ] dip 1+ (llength) ] if ;

: llength ( list -- n )
  0 (llength) ;

: leach ( list quot -- )
  over nil? [ 2drop ] [ [ uncons ] dip tuck call leach ] if ; inline

: lreduce ( list identity quot -- result )
  swapd leach ; inline

TUPLE: memoized-cons original car cdr nil? ;

: not-memoized ( -- obj )
  { } ;

: not-memoized? ( obj -- bool )
  not-memoized eq? ;

: <memoized-cons> ( cons -- memoized-cons )
  not-memoized not-memoized not-memoized
  memoized-cons boa ;

M: memoized-cons car ( memoized-cons -- car )
  dup car>> not-memoized? [
    dup original>> car [ >>car drop ] keep
  ] [
    car>>
  ] if ;

M: memoized-cons cdr ( memoized-cons -- cdr )
  dup cdr>> not-memoized? [
    dup original>> cdr [ >>cdr drop ] keep
  ] [
    cdr>>
  ] if ;

M: memoized-cons nil? ( memoized-cons -- bool )
  dup nil?>> not-memoized? [
    dup original>> nil? [ >>nil? drop ] keep
  ] [
    nil?>>
  ] if ;

TUPLE: lazy-map cons quot ;

C: <lazy-map> lazy-map

: lmap ( list quot -- result )
    over nil? [ 2drop nil ] [ <lazy-map> <memoized-cons> ] if ;

M: lazy-map car ( lazy-map -- car )
  [ cons>> car ] keep
  quot>> call ;

M: lazy-map cdr ( lazy-map -- cdr )
  [ cons>> cdr ] keep
  quot>> lmap ;

M: lazy-map nil? ( lazy-map -- bool )
  cons>> nil? ;

: lmap-with ( value list quot -- result )
  with lmap ;

TUPLE: lazy-take n cons ;

C: <lazy-take> lazy-take

: ltake ( n list -- result )
    over zero? [ 2drop nil ] [ <lazy-take> ] if ;

M: lazy-take car ( lazy-take -- car )
  cons>> car ;

M: lazy-take cdr ( lazy-take -- cdr )
  [ n>> 1- ] keep
  cons>> cdr ltake ;

M: lazy-take nil? ( lazy-take -- bool )
  dup n>> zero? [
    drop t
  ] [
    cons>> nil?
  ] if ;

TUPLE: lazy-until cons quot ;

C: <lazy-until> lazy-until

: luntil ( list quot -- result )
  over nil? [ drop ] [ <lazy-until> ] if ;

M: lazy-until car ( lazy-until -- car )
   cons>> car ;

M: lazy-until cdr ( lazy-until -- cdr )
   [ cons>> uncons ] keep quot>> tuck call
   [ 2drop nil ] [ luntil ] if ;

M: lazy-until nil? ( lazy-until -- bool )
   drop f ;

TUPLE: lazy-while cons quot ;

C: <lazy-while> lazy-while

: lwhile ( list quot -- result )
  over nil? [ drop ] [ <lazy-while> ] if ;

M: lazy-while car ( lazy-while -- car )
   cons>> car ;

M: lazy-while cdr ( lazy-while -- cdr )
   [ cons>> cdr ] keep quot>> lwhile ;

M: lazy-while nil? ( lazy-while -- bool )
   [ car ] keep quot>> call not ;

TUPLE: lazy-filter cons quot ;

C: <lazy-filter> lazy-filter

: lfilter ( list quot -- result )
    over nil? [ 2drop nil ] [ <lazy-filter> <memoized-cons> ] if ;

: car-filter?  ( lazy-filter -- ? )
  [ cons>> car ] keep
  quot>> call ;

: skip ( lazy-filter -- )
  dup cons>> cdr >>cons ;

M: lazy-filter car ( lazy-filter -- car )
  dup car-filter? [ cons>> ] [ dup skip ] if car ;

M: lazy-filter cdr ( lazy-filter -- cdr )
  dup car-filter? [
    [ cons>> cdr ] keep
    quot>> lfilter
  ] [
    dup skip cdr
  ] if ;

M: lazy-filter nil? ( lazy-filter -- bool )
  dup cons>> nil? [
    drop t
  ] [
    dup car-filter? [
      drop f
    ] [
      dup skip nil?
    ] if
  ] if ;

: list>vector ( list -- vector )
  [ [ , ] leach ] V{ } make ;

: list>array ( list -- array )
  [ [ , ] leach ] { } make ;

TUPLE: lazy-append list1 list2 ;

C: <lazy-append> lazy-append

: lappend ( list1 list2 -- result )
  over nil? [ nip ] [ <lazy-append> ] if ;

M: lazy-append car ( lazy-append -- car )
  list1>> car ;

M: lazy-append cdr ( lazy-append -- cdr )
  [ list1>> cdr  ] keep
  list2>> lappend ;

M: lazy-append nil? ( lazy-append -- bool )
   drop f ;

TUPLE: lazy-from-by n quot ;

C: lfrom-by lazy-from-by ( n quot -- list )

: lfrom ( n -- list )
  [ 1+ ] lfrom-by ;

M: lazy-from-by car ( lazy-from-by -- car )
  n>> ;

M: lazy-from-by cdr ( lazy-from-by -- cdr )
  [ n>> ] keep
  quot>> dup slip lfrom-by ;

M: lazy-from-by nil? ( lazy-from-by -- bool )
  drop f ;

TUPLE: lazy-zip list1 list2 ;

C: <lazy-zip> lazy-zip

: lzip ( list1 list2 -- lazy-zip )
    over nil? over nil? or
    [ 2drop nil ] [ <lazy-zip> ] if ;

M: lazy-zip car ( lazy-zip -- car )
    [ list1>> car ] keep list2>> car 2array ;

M: lazy-zip cdr ( lazy-zip -- cdr )
    [ list1>> cdr ] keep list2>> cdr lzip ;

M: lazy-zip nil? ( lazy-zip -- bool )
    drop f ;

TUPLE: sequence-cons index seq ;

C: <sequence-cons> sequence-cons

: seq>list ( index seq -- list )
  2dup length >= [
    2drop nil
  ] [
    <sequence-cons>
  ] if ;

M: sequence-cons car ( sequence-cons -- car )
  [ index>> ] keep
  seq>> nth ;

M: sequence-cons cdr ( sequence-cons -- cdr )
  [ index>> 1+ ] keep
  seq>> seq>list ;

M: sequence-cons nil? ( sequence-cons -- bool )
    drop f ;

: >list ( object -- list )
  {
    { [ dup sequence? ] [ 0 swap seq>list ] }
    { [ dup list?     ] [ ] }
    [ "Could not convert object to a list" throw ]
  } cond ;

TUPLE: lazy-concat car cdr ;


IN: lazy-lists

M: promise car ( promise -- car )
    force car ;

M: promise cdr ( promise -- cdr )
    force cdr ;

M: promise nil? ( cons -- bool )
    force nil? ;
    
! Both 'car' and 'cdr' are promises
TUPLE: lazy-cons car cdr ;

