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|
(*
An attempt at encoding omega examples from the 2nd Central European
Functional Programming School:
Generic Programming in Omega, by Tim Sheard and Nathan Linger
http://web.cecs.pdx.edu/~sheard/
*)
(* Basic types *)
type ('a,'b) sum = Inl of 'a | Inr of 'b
type zero = Zero
type 'a succ = Succ of 'a
type _ nat =
| NZ : zero nat
| NS : 'a nat -> 'a succ nat
;;
(* 2: A simple example *)
type (_,_) seq =
| Snil : ('a,zero) seq
| Scons : 'a * ('a,'n) seq -> ('a, 'n succ) seq
;;
let l1 = Scons (3, Scons (5, Snil)) ;;
[%%expect{|
type ('a, 'b) sum = Inl of 'a | Inr of 'b
type zero = Zero
type 'a succ = Succ of 'a
type _ nat = NZ : zero nat | NS : 'a nat -> 'a succ nat
type (_, _) seq =
Snil : ('a, zero) seq
| Scons : 'a * ('a, 'n) seq -> ('a, 'n succ) seq
val l1 : (int, zero succ succ) seq = Scons (3, Scons (5, Snil))
|}];;
(* We do not have type level functions, so we need to use witnesses. *)
(* We copy here the definitions from section 3.9 *)
(* Note the addition of the ['a nat] argument to PlusZ, since we do not
have kinds *)
type (_,_,_) plus =
| PlusZ : 'a nat -> (zero, 'a, 'a) plus
| PlusS : ('a,'b,'c) plus -> ('a succ, 'b, 'c succ) plus
;;
let rec length : type a n. (a,n) seq -> n nat = function
| Snil -> NZ
| Scons (_, s) -> NS (length s)
;;
[%%expect{|
type (_, _, _) plus =
PlusZ : 'a nat -> (zero, 'a, 'a) plus
| PlusS : ('a, 'b, 'c) plus -> ('a succ, 'b, 'c succ) plus
val length : ('a, 'n) seq -> 'n nat = <fun>
|}];;
(* app returns the catenated lists with a witness proving that
the size is the sum of its two inputs *)
type (_,_,_) app = App : ('a,'p) seq * ('n,'m,'p) plus -> ('a,'n,'m) app
let rec app : type a n m. (a,n) seq -> (a,m) seq -> (a,n,m) app =
fun xs ys ->
match xs with
| Snil -> App (ys, PlusZ (length ys))
| Scons (x, xs') ->
let App (xs'', pl) = app xs' ys in
App (Scons (x, xs''), PlusS pl)
;;
[%%expect{|
type (_, _, _) app =
App : ('a, 'p) seq * ('n, 'm, 'p) plus -> ('a, 'n, 'm) app
val app : ('a, 'n) seq -> ('a, 'm) seq -> ('a, 'n, 'm) app = <fun>
|}];;
(* 3.1 Feature: kinds *)
(* We do not have kinds, but we can encode them as predicates *)
type tp = TP
type nd = ND
type ('a,'b) fk = FK
type _ shape =
| Tp : tp shape
| Nd : nd shape
| Fk : 'a shape * 'b shape -> ('a,'b) fk shape
;;
type tt = TT
type ff = FF
type _ boolean =
| BT : tt boolean
| BF : ff boolean
;;
(* 3.3 Feature : GADTs *)
type (_,_) path =
| Pnone : 'a -> (tp,'a) path
| Phere : (nd,'a) path
| Pleft : ('x,'a) path -> (('x,'y) fk, 'a) path
| Pright : ('y,'a) path -> (('x,'y) fk, 'a) path
;;
type (_,_) tree =
| Ttip : (tp,'a) tree
| Tnode : 'a -> (nd,'a) tree
| Tfork : ('x,'a) tree * ('y,'a) tree -> (('x,'y)fk, 'a) tree
;;
let tree1 = Tfork (Tfork (Ttip, Tnode 4), Tfork (Tnode 4, Tnode 3))
;;
[%%expect{|
type tp = TP
type nd = ND
type ('a, 'b) fk = FK
type _ shape =
Tp : tp shape
| Nd : nd shape
| Fk : 'a shape * 'b shape -> ('a, 'b) fk shape
type tt = TT
type ff = FF
type _ boolean = BT : tt boolean | BF : ff boolean
type (_, _) path =
Pnone : 'a -> (tp, 'a) path
| Phere : (nd, 'a) path
| Pleft : ('x, 'a) path -> (('x, 'y) fk, 'a) path
| Pright : ('y, 'a) path -> (('x, 'y) fk, 'a) path
type (_, _) tree =
Ttip : (tp, 'a) tree
| Tnode : 'a -> (nd, 'a) tree
| Tfork : ('x, 'a) tree * ('y, 'a) tree -> (('x, 'y) fk, 'a) tree
val tree1 : (((tp, nd) fk, (nd, nd) fk) fk, int) tree =
Tfork (Tfork (Ttip, Tnode 4), Tfork (Tnode 4, Tnode 3))
|}];;
let rec find : type sh.
('a -> 'a -> bool) -> 'a -> (sh,'a) tree -> (sh,'a) path list
= fun eq n t ->
match t with
| Ttip -> []
| Tnode m ->
if eq n m then [Phere] else []
| Tfork (x, y) ->
List.map (fun x -> Pleft x) (find eq n x) @
List.map (fun x -> Pright x) (find eq n y)
;;
[%%expect{|
val find : ('a -> 'a -> bool) -> 'a -> ('sh, 'a) tree -> ('sh, 'a) path list =
<fun>
|}];;
let rec extract : type sh. (sh,'a) path -> (sh,'a) tree -> 'a = fun p t ->
match (p, t) with
| Pnone x, Ttip -> x
| Phere, Tnode y -> y
| Pleft p, Tfork(l,_) -> extract p l
| Pright p, Tfork(_,r) -> extract p r
;;
[%%expect{|
val extract : ('sh, 'a) path -> ('sh, 'a) tree -> 'a = <fun>
|}];;
(* 3.4 Pattern : Witness *)
type (_,_) le =
| LeZ : 'a nat -> (zero, 'a) le
| LeS : ('n, 'm) le -> ('n succ, 'm succ) le
;;
type _ even =
| EvenZ : zero even
| EvenSS : 'n even -> 'n succ succ even
;;
type one = zero succ
type two = one succ
type three = two succ
type four = three succ
;;
let even0 : zero even = EvenZ
let even2 : two even = EvenSS EvenZ
let even4 : four even = EvenSS (EvenSS EvenZ)
;;
let p1 : (two, one, three) plus = PlusS (PlusS (PlusZ (NS NZ)))
;;
[%%expect{|
type (_, _) le =
LeZ : 'a nat -> (zero, 'a) le
| LeS : ('n, 'm) le -> ('n succ, 'm succ) le
type _ even = EvenZ : zero even | EvenSS : 'n even -> 'n succ succ even
type one = zero succ
type two = one succ
type three = two succ
type four = three succ
val even0 : zero even = EvenZ
val even2 : two even = EvenSS EvenZ
val even4 : four even = EvenSS (EvenSS EvenZ)
val p1 : (two, one, three) plus = PlusS (PlusS (PlusZ (NS NZ)))
|}];;
let rec summandLessThanSum : type a b c. (a,b,c) plus -> (a,c) le = fun p ->
match p with
| PlusZ n -> LeZ n
| PlusS p' -> LeS (summandLessThanSum p')
;;
[%%expect{|
val summandLessThanSum : ('a, 'b, 'c) plus -> ('a, 'c) le = <fun>
|}];;
(* 3.8 Pattern: Leibniz Equality *)
type (_,_) equal = Eq : ('a,'a) equal
let convert : type a b. (a,b) equal -> a -> b = fun Eq x -> x
[%%expect{|
type (_, _) equal = Eq : ('a, 'a) equal
val convert : ('a, 'b) equal -> 'a -> 'b = <fun>
|}];;
let rec sameNat : type a b. a nat -> b nat -> (a,b) equal option = fun a b ->
match a, b with
| NZ, NZ -> Some Eq
| NS a', NS b' ->
begin match sameNat a' b' with
| Some Eq -> Some Eq
| None -> None
end
| _ -> None
;;
[%%expect{|
val sameNat : 'a nat -> 'b nat -> ('a, 'b) equal option = <fun>
|}];;
(* Extra: associativity of addition *)
let rec plus_func : type a b m n.
(a,b,m) plus -> (a,b,n) plus -> (m,n) equal =
fun p1 p2 ->
match p1, p2 with
| PlusZ _, PlusZ _ -> Eq
| PlusS p1', PlusS p2' ->
let Eq = plus_func p1' p2' in Eq
;;
[%%expect{|
val plus_func : ('a, 'b, 'm) plus -> ('a, 'b, 'n) plus -> ('m, 'n) equal =
<fun>
|}];;
let rec plus_assoc : type a b c ab bc m n.
(a,b,ab) plus -> (ab,c,m) plus ->
(b,c,bc) plus -> (a,bc,n) plus -> (m,n) equal = fun p1 p2 p3 p4 ->
match p1, p4 with
| PlusZ b, PlusZ bc ->
let Eq = plus_func p2 p3 in Eq
| PlusS p1', PlusS p4' ->
let PlusS p2' = p2 in
let Eq = plus_assoc p1' p2' p3 p4' in Eq
;;
[%%expect{|
val plus_assoc :
('a, 'b, 'ab) plus ->
('ab, 'c, 'm) plus ->
('b, 'c, 'bc) plus -> ('a, 'bc, 'n) plus -> ('m, 'n) equal = <fun>
|}];;
(* 3.9 Computing Programs and Properties Simultaneously *)
(* Plus and app1 are moved to section 2 *)
let smaller : type a b. (a succ, b succ) le -> (a,b) le =
function LeS x -> x ;;
[%%expect{|
val smaller : ('a succ, 'b succ) le -> ('a, 'b) le = <fun>
|}];;
type (_,_) diff = Diff : 'c nat * ('a,'c,'b) plus -> ('a,'b) diff ;;
(*
let rec diff : type a b. (a,b) le -> a nat -> b nat -> (a,b) diff =
fun le a b ->
match a, b, le with
| NZ, m, _ -> Diff (m, PlusZ m)
| NS x, NZ, _ -> assert false
| NS x, NS y, q ->
match diff (smaller q) x y with Diff (m, p) -> Diff (m, PlusS p)
;;
*)
let rec diff : type a b. (a,b) le -> a nat -> b nat -> (a,b) diff =
fun le a b ->
match le, a, b with
| LeZ _, _, m -> Diff (m, PlusZ m)
| LeS q, NS x, NS y ->
match diff q x y with Diff (m, p) -> Diff (m, PlusS p)
;;
[%%expect{|
type (_, _) diff = Diff : 'c nat * ('a, 'c, 'b) plus -> ('a, 'b) diff
val diff : ('a, 'b) le -> 'a nat -> 'b nat -> ('a, 'b) diff = <fun>
|}];;
let rec diff : type a b. (a,b) le -> a nat -> b nat -> (a,b) diff =
fun le a b ->
match a, b,le with (* warning *)
| NZ, m, LeZ _ -> Diff (m, PlusZ m)
| NS x, NS y, LeS q ->
(match diff q x y with Diff (m, p) -> Diff (m, PlusS p))
| _ -> .
;;
[%%expect{|
val diff : ('a, 'b) le -> 'a nat -> 'b nat -> ('a, 'b) diff = <fun>
|}];;
let rec diff : type a b. (a,b) le -> b nat -> (a,b) diff =
fun le b ->
match b,le with
| m, LeZ _ -> Diff (m, PlusZ m)
| NS y, LeS q ->
match diff q y with Diff (m, p) -> Diff (m, PlusS p)
;;
[%%expect{|
val diff : ('a, 'b) le -> 'b nat -> ('a, 'b) diff = <fun>
|}];;
type (_,_) filter = Filter : ('m,'n) le * ('a,'m) seq -> ('a,'n) filter
let rec leS' : type m n. (m,n) le -> (m,n succ) le = function
| LeZ n -> LeZ (NS n)
| LeS le -> LeS (leS' le)
;;
[%%expect{|
type (_, _) filter = Filter : ('m, 'n) le * ('a, 'm) seq -> ('a, 'n) filter
val leS' : ('m, 'n) le -> ('m, 'n succ) le = <fun>
|}];;
let rec filter : type a n. (a -> bool) -> (a,n) seq -> (a,n) filter =
fun f s ->
match s with
| Snil -> Filter (LeZ NZ, Snil)
| Scons (a,l) ->
match filter f l with Filter (le, l') ->
if f a then Filter (LeS le, Scons (a, l'))
else Filter (leS' le, l')
;;
[%%expect{|
val filter : ('a -> bool) -> ('a, 'n) seq -> ('a, 'n) filter = <fun>
|}];;
(* 4.1 AVL trees *)
type (_,_,_) balance =
| Less : ('h, 'h succ, 'h succ) balance
| Same : ('h, 'h, 'h) balance
| More : ('h succ, 'h, 'h succ) balance
type _ avl =
| Leaf : zero avl
| Node :
('hL, 'hR, 'hMax) balance * 'hL avl * int * 'hR avl -> 'hMax succ avl
type avl' = Avl : 'h avl -> avl'
;;
let empty = Avl Leaf;;
[%%expect{|
type (_, _, _) balance =
Less : ('h, 'h succ, 'h succ) balance
| Same : ('h, 'h, 'h) balance
| More : ('h succ, 'h, 'h succ) balance
type _ avl =
Leaf : zero avl
| Node : ('hL, 'hR, 'hMax) balance * 'hL avl * int *
'hR avl -> 'hMax succ avl
type avl' = Avl : 'h avl -> avl'
val empty : avl' = Avl Leaf
|}];;
let rec elem : type h. int -> h avl -> bool = fun x t ->
match t with
| Leaf -> false
| Node (_, l, y, r) ->
x = y || if x < y then elem x l else elem x r
;;
[%%expect{|
val elem : int -> 'h avl -> bool = <fun>
|}];;
let rec rotr : type n. (n succ succ) avl -> int -> n avl ->
((n succ succ) avl, (n succ succ succ) avl) sum =
fun tL y tR ->
match tL with
| Node (Same, a, x, b) -> Inr (Node (Less, a, x, Node (More, b, y, tR)))
| Node (More, a, x, b) -> Inl (Node (Same, a, x, Node (Same, b, y, tR)))
| Node (Less, a, x, Node (Same, b, z, c)) ->
Inl (Node (Same, Node (Same, a, x, b), z, Node (Same, c, y, tR)))
| Node (Less, a, x, Node (Less, b, z, c)) ->
Inl (Node (Same, Node (More, a, x, b), z, Node (Same, c, y, tR)))
| Node (Less, a, x, Node (More, b, z, c)) ->
Inl (Node (Same, Node (Same, a, x, b), z, Node (Less, c, y, tR)))
;;
[%%expect{|
val rotr :
'n succ succ avl ->
int -> 'n avl -> ('n succ succ avl, 'n succ succ succ avl) sum = <fun>
|}];;
let rec rotl : type n. n avl -> int -> (n succ succ) avl ->
((n succ succ) avl, (n succ succ succ) avl) sum =
fun tL u tR ->
match tR with
| Node (Same, a, x, b) -> Inr (Node (More, Node (Less, tL, u, a), x, b))
| Node (Less, a, x, b) -> Inl (Node (Same, Node (Same, tL, u, a), x, b))
| Node (More, Node (Same, a, x, b), y, c) ->
Inl (Node (Same, Node (Same, tL, u, a), x, Node (Same, b, y, c)))
| Node (More, Node (Less, a, x, b), y, c) ->
Inl (Node (Same, Node (More, tL, u, a), x, Node (Same, b, y, c)))
| Node (More, Node (More, a, x, b), y, c) ->
Inl (Node (Same, Node (Same, tL, u, a), x, Node (Less, b, y, c)))
;;
[%%expect{|
val rotl :
'n avl ->
int -> 'n succ succ avl -> ('n succ succ avl, 'n succ succ succ avl) sum =
<fun>
|}];;
let rec ins : type n. int -> n avl -> (n avl, (n succ) avl) sum =
fun x t ->
match t with
| Leaf -> Inr (Node (Same, Leaf, x, Leaf))
| Node (bal, a, y, b) ->
if x = y then Inl t else
if x < y then begin
match ins x a with
| Inl a -> Inl (Node (bal, a, y, b))
| Inr a ->
match bal with
| Less -> Inl (Node (Same, a, y, b))
| Same -> Inr (Node (More, a, y, b))
| More -> rotr a y b
end else begin
match ins x b with
| Inl b -> Inl (Node (bal, a, y, b) : n avl)
| Inr b ->
match bal with
| More -> Inl (Node (Same, a, y, b) : n avl)
| Same -> Inr (Node (Less, a, y, b) : n succ avl)
| Less -> rotl a y b
end
;;
[%%expect{|
val ins : int -> 'n avl -> ('n avl, 'n succ avl) sum = <fun>
|}];;
let insert x (Avl t) =
match ins x t with
| Inl t -> Avl t
| Inr t -> Avl t
;;
[%%expect{|
val insert : int -> avl' -> avl' = <fun>
|}];;
let rec del_min : type n. (n succ) avl -> int * (n avl, (n succ) avl) sum =
function
| Node (Less, Leaf, x, r) -> (x, Inl r)
| Node (Same, Leaf, x, r) -> (x, Inl r)
| Node (bal, (Node _ as l) , x, r) ->
match del_min l with
| y, Inr l -> (y, Inr (Node (bal, l, x, r)))
| y, Inl l ->
(y, match bal with
| Same -> Inr (Node (Less, l, x, r))
| More -> Inl (Node (Same, l, x, r))
| Less -> rotl l x r)
;;
[%%expect{|
val del_min : 'n succ avl -> int * ('n avl, 'n succ avl) sum = <fun>
|}];;
type _ avl_del =
| Dsame : 'n avl -> 'n avl_del
| Ddecr : ('m succ, 'n) equal * 'm avl -> 'n avl_del
let rec del : type n. int -> n avl -> n avl_del = fun y t ->
match t with
| Leaf -> Dsame Leaf
| Node (bal, l, x, r) ->
if x = y then begin
match r with
| Leaf ->
begin match bal with
| Same -> Ddecr (Eq, l)
| More -> Ddecr (Eq, l)
end
| Node _ ->
begin match bal, del_min r with
| _, (z, Inr r) -> Dsame (Node (bal, l, z, r))
| Same, (z, Inl r) -> Dsame (Node (More, l, z, r))
| Less, (z, Inl r) -> Ddecr (Eq, Node (Same, l, z, r))
| More, (z, Inl r) ->
match rotr l z r with
| Inl t -> Ddecr (Eq, t)
| Inr t -> Dsame t
end
end else if y < x then begin
match del y l with
| Dsame l -> Dsame (Node (bal, l, x, r))
| Ddecr(Eq,l) ->
begin match bal with
| Same -> Dsame (Node (Less, l, x, r))
| More -> Ddecr (Eq, Node (Same, l, x, r))
| Less ->
match rotl l x r with
| Inl t -> Ddecr (Eq, t)
| Inr t -> Dsame t
end
end else begin
match del y r with
| Dsame r -> Dsame (Node (bal, l, x, r))
| Ddecr(Eq,r) ->
begin match bal with
| Same -> Dsame (Node (More, l, x, r))
| Less -> Ddecr (Eq, Node (Same, l, x, r))
| More ->
match rotr l x r with
| Inl t -> Ddecr (Eq, t)
| Inr t -> Dsame t
end
end
;;
[%%expect{|
type _ avl_del =
Dsame : 'n avl -> 'n avl_del
| Ddecr : ('m succ, 'n) equal * 'm avl -> 'n avl_del
val del : int -> 'n avl -> 'n avl_del = <fun>
|}];;
let delete x (Avl t) =
match del x t with
| Dsame t -> Avl t
| Ddecr (_, t) -> Avl t
;;
[%%expect{|
val delete : int -> avl' -> avl' = <fun>
|}];;
(* Exercise 22: Red-black trees *)
type red = RED
type black = BLACK
type (_,_) sub_tree =
| Bleaf : (black, zero) sub_tree
| Rnode :
(black, 'n) sub_tree * int * (black, 'n) sub_tree -> (red, 'n) sub_tree
| Bnode :
('cL, 'n) sub_tree * int * ('cR, 'n) sub_tree -> (black, 'n succ) sub_tree
type rb_tree = Root : (black, 'n) sub_tree -> rb_tree
;;
type dir = LeftD | RightD
type (_,_) ctxt =
| CNil : (black,'n) ctxt
| CRed : int * dir * (black,'n) sub_tree * (red,'n) ctxt -> (black,'n) ctxt
| CBlk : int * dir * ('c1,'n) sub_tree * (black, 'n succ) ctxt -> ('c,'n) ctxt
;;
let blacken = function
Rnode (l, e, r) -> Bnode (l, e, r)
;;
[%%expect{|
type red = RED
type black = BLACK
type (_, _) sub_tree =
Bleaf : (black, zero) sub_tree
| Rnode : (black, 'n) sub_tree * int *
(black, 'n) sub_tree -> (red, 'n) sub_tree
| Bnode : ('cL, 'n) sub_tree * int *
('cR, 'n) sub_tree -> (black, 'n succ) sub_tree
type rb_tree = Root : (black, 'n) sub_tree -> rb_tree
type dir = LeftD | RightD
type (_, _) ctxt =
CNil : (black, 'n) ctxt
| CRed : int * dir * (black, 'n) sub_tree *
(red, 'n) ctxt -> (black, 'n) ctxt
| CBlk : int * dir * ('c1, 'n) sub_tree *
(black, 'n succ) ctxt -> ('c, 'n) ctxt
val blacken : (red, 'a) sub_tree -> (black, 'a succ) sub_tree = <fun>
|}];;
type _ crep =
| Red : red crep
| Black : black crep
let color : type c n. (c,n) sub_tree -> c crep = function
| Bleaf -> Black
| Rnode _ -> Red
| Bnode _ -> Black
;;
[%%expect{|
type _ crep = Red : red crep | Black : black crep
val color : ('c, 'n) sub_tree -> 'c crep = <fun>
|}];;
let rec fill : type c n. (c,n) ctxt -> (c,n) sub_tree -> rb_tree =
fun ct t ->
match ct with
| CNil -> Root t
| CRed (e, LeftD, uncle, c) -> fill c (Rnode (uncle, e, t))
| CRed (e, RightD, uncle, c) -> fill c (Rnode (t, e, uncle))
| CBlk (e, LeftD, uncle, c) -> fill c (Bnode (uncle, e, t))
| CBlk (e, RightD, uncle, c) -> fill c (Bnode (t, e, uncle))
;;
[%%expect{|
val fill : ('c, 'n) ctxt -> ('c, 'n) sub_tree -> rb_tree = <fun>
|}];;
let recolor d1 pE sib d2 gE uncle t =
match d1, d2 with
| LeftD, RightD -> Rnode (Bnode (sib, pE, t), gE, uncle)
| RightD, RightD -> Rnode (Bnode (t, pE, sib), gE, uncle)
| LeftD, LeftD -> Rnode (uncle, gE, Bnode (sib, pE, t))
| RightD, LeftD -> Rnode (uncle, gE, Bnode (t, pE, sib))
;;
[%%expect{|
val recolor :
dir ->
int ->
('a, 'b) sub_tree ->
dir ->
int ->
(black, 'b succ) sub_tree -> ('c, 'b) sub_tree -> (red, 'b succ) sub_tree =
<fun>
|}];;
let rotate d1 pE sib d2 gE uncle (Rnode (x, e, y)) =
match d1, d2 with
| RightD, RightD -> Bnode (Rnode (x,e,y), pE, Rnode (sib, gE, uncle))
| LeftD, RightD -> Bnode (Rnode (sib, pE, x), e, Rnode (y, gE, uncle))
| LeftD, LeftD -> Bnode (Rnode (uncle, gE, sib), pE, Rnode (x,e,y))
| RightD, LeftD -> Bnode (Rnode (uncle, gE, x), e, Rnode (y, pE, sib))
;;
[%%expect{|
val rotate :
dir ->
int ->
(black, 'a) sub_tree ->
dir ->
int ->
(black, 'a) sub_tree -> (red, 'a) sub_tree -> (black, 'a succ) sub_tree =
<fun>
|}];;
let rec repair : type c n. (red,n) sub_tree -> (c,n) ctxt -> rb_tree =
fun t ct ->
match ct with
| CNil -> Root (blacken t)
| CBlk (e, LeftD, sib, c) -> fill c (Bnode (sib, e, t))
| CBlk (e, RightD, sib, c) -> fill c (Bnode (t, e, sib))
| CRed (e, dir, sib, CBlk (e', dir', uncle, ct)) ->
match color uncle with
| Red -> repair (recolor dir e sib dir' e' (blacken uncle) t) ct
| Black -> fill ct (rotate dir e sib dir' e' uncle t)
;;
[%%expect{|
val repair : (red, 'n) sub_tree -> ('c, 'n) ctxt -> rb_tree = <fun>
|}];;
let rec ins : type c n. int -> (c,n) sub_tree -> (c,n) ctxt -> rb_tree =
fun e t ct ->
match t with
| Rnode (l, e', r) ->
if e < e' then ins e l (CRed (e', RightD, r, ct))
else ins e r (CRed (e', LeftD, l, ct))
| Bnode (l, e', r) ->
if e < e' then ins e l (CBlk (e', RightD, r, ct))
else ins e r (CBlk (e', LeftD, l, ct))
| Bleaf -> repair (Rnode (Bleaf, e, Bleaf)) ct
;;
[%%expect{|
val ins : int -> ('c, 'n) sub_tree -> ('c, 'n) ctxt -> rb_tree = <fun>
|}];;
let insert e (Root t) = ins e t CNil
;;
[%%expect{|
val insert : int -> rb_tree -> rb_tree = <fun>
|}];;
(* 5.7 typed object languages using GADTs *)
type _ term =
| Const : int -> int term
| Add : (int * int -> int) term
| LT : (int * int -> bool) term
| Ap : ('a -> 'b) term * 'a term -> 'b term
| Pair : 'a term * 'b term -> ('a * 'b) term
let ex1 = Ap (Add, Pair (Const 3, Const 5))
let ex2 = Pair (ex1, Const 1)
;;
[%%expect{|
type _ term =
Const : int -> int term
| Add : (int * int -> int) term
| LT : (int * int -> bool) term
| Ap : ('a -> 'b) term * 'a term -> 'b term
| Pair : 'a term * 'b term -> ('a * 'b) term
val ex1 : int term = Ap (Add, Pair (Const 3, Const 5))
val ex2 : (int * int) term =
Pair (Ap (Add, Pair (Const 3, Const 5)), Const 1)
|}];;
let rec eval_term : type a. a term -> a = function
| Const x -> x
| Add -> fun (x,y) -> x+y
| LT -> fun (x,y) -> x<y
| Ap(f,x) -> eval_term f (eval_term x)
| Pair(x,y) -> (eval_term x, eval_term y)
;;
[%%expect{|
val eval_term : 'a term -> 'a = <fun>
|}];;
type _ rep =
| Rint : int rep
| Rbool : bool rep
| Rpair : 'a rep * 'b rep -> ('a * 'b) rep
| Rfun : 'a rep * 'b rep -> ('a -> 'b) rep
type (_,_) equal = Eq : ('a,'a) equal
let rec rep_equal : type a b. a rep -> b rep -> (a, b) equal option =
fun ra rb ->
match ra, rb with
| Rint, Rint -> Some Eq
| Rbool, Rbool -> Some Eq
| Rpair (a1, a2), Rpair (b1, b2) ->
begin match rep_equal a1 b1 with
| None -> None
| Some Eq -> match rep_equal a2 b2 with
| None -> None
| Some Eq -> Some Eq
end
| Rfun (a1, a2), Rfun (b1, b2) ->
begin match rep_equal a1 b1 with
| None -> None
| Some Eq -> match rep_equal a2 b2 with
| None -> None
| Some Eq -> Some Eq
end
| _ -> None
;;
[%%expect{|
type _ rep =
Rint : int rep
| Rbool : bool rep
| Rpair : 'a rep * 'b rep -> ('a * 'b) rep
| Rfun : 'a rep * 'b rep -> ('a -> 'b) rep
type (_, _) equal = Eq : ('a, 'a) equal
val rep_equal : 'a rep -> 'b rep -> ('a, 'b) equal option = <fun>
|}];;
type assoc = Assoc : string * 'a rep * 'a -> assoc
let rec assoc : type a. string -> a rep -> assoc list -> a =
fun x r -> function
| [] -> raise Not_found
| Assoc (x', r', v) :: env ->
if x = x' then
match rep_equal r r' with
| None -> failwith ("Wrong type for " ^ x)
| Some Eq -> v
else assoc x r env
;;
[%%expect{|
type assoc = Assoc : string * 'a rep * 'a -> assoc
val assoc : string -> 'a rep -> assoc list -> 'a = <fun>
|}];;
type _ term =
| Var : string * 'a rep -> 'a term
| Abs : string * 'a rep * 'b term -> ('a -> 'b) term
| Const : int -> int term
| Add : (int * int -> int) term
| LT : (int * int -> bool) term
| Ap : ('a -> 'b) term * 'a term -> 'b term
| Pair : 'a term * 'b term -> ('a * 'b) term
let rec eval_term : type a. assoc list -> a term -> a =
fun env -> function
| Var (x, r) -> assoc x r env
| Abs (x, r, e) -> fun v -> eval_term (Assoc (x, r, v) :: env) e
| Const x -> x
| Add -> fun (x,y) -> x+y
| LT -> fun (x,y) -> x<y
| Ap(f,x) -> eval_term env f (eval_term env x)
| Pair(x,y) -> (eval_term env x, eval_term env y)
;;
[%%expect{|
type _ term =
Var : string * 'a rep -> 'a term
| Abs : string * 'a rep * 'b term -> ('a -> 'b) term
| Const : int -> int term
| Add : (int * int -> int) term
| LT : (int * int -> bool) term
| Ap : ('a -> 'b) term * 'a term -> 'b term
| Pair : 'a term * 'b term -> ('a * 'b) term
val eval_term : assoc list -> 'a term -> 'a = <fun>
|}];;
let ex3 = Abs ("x", Rint, Ap (Add, Pair (Var("x",Rint), Var("x",Rint))))
let ex4 = Ap (ex3, Const 3)
let v4 = eval_term [] ex4
;;
[%%expect{|
val ex3 : (int -> int) term =
Abs ("x", Rint, Ap (Add, Pair (Var ("x", Rint), Var ("x", Rint))))
val ex4 : int term =
Ap (Abs ("x", Rint, Ap (Add, Pair (Var ("x", Rint), Var ("x", Rint)))),
Const 3)
val v4 : int = 6
|}];;
(* 5.9/5.10 Language with binding *)
type rnil = RNIL
type ('a,'b,'c) rcons = RCons of 'a * 'b * 'c
type _ is_row =
| Rnil : rnil is_row
| Rcons : 'c is_row -> ('a,'b,'c) rcons is_row
type (_,_) lam =
| Const : int -> ('e, int) lam
| Var : 'a -> (('a,'t,'e) rcons, 't) lam
| Shift : ('e,'t) lam -> (('a,'q,'e) rcons, 't) lam
| Abs : 'a * (('a,'s,'e) rcons, 't) lam -> ('e, 's -> 't) lam
| App : ('e, 's -> 't) lam * ('e, 's) lam -> ('e, 't) lam
type x = X
type y = Y
let ex1 = App (Var X, Shift (Var Y))
let ex2 = Abs (X, Abs (Y, App (Shift (Var X), Var Y)))
;;
[%%expect{|
type rnil = RNIL
type ('a, 'b, 'c) rcons = RCons of 'a * 'b * 'c
type _ is_row =
Rnil : rnil is_row
| Rcons : 'c is_row -> ('a, 'b, 'c) rcons is_row
type (_, _) lam =
Const : int -> ('e, int) lam
| Var : 'a -> (('a, 't, 'e) rcons, 't) lam
| Shift : ('e, 't) lam -> (('a, 'q, 'e) rcons, 't) lam
| Abs : 'a * (('a, 's, 'e) rcons, 't) lam -> ('e, 's -> 't) lam
| App : ('e, 's -> 't) lam * ('e, 's) lam -> ('e, 't) lam
type x = X
type y = Y
val ex1 : ((x, 'a -> 'b, (y, 'a, 'c) rcons) rcons, 'b) lam =
App (Var X, Shift (Var Y))
val ex2 : ('a, ('b -> 'c) -> 'b -> 'c) lam =
Abs (<poly>, Abs (<poly>, App (Shift (Var <poly>), Var <poly>)))
|}];;
type _ env =
| Enil : rnil env
| Econs : 'a * 't * 'e env -> ('a, 't, 'e) rcons env
let rec eval_lam : type e t. e env -> (e, t) lam -> t =
fun env m ->
match env, m with
| _, Const n -> n
| Econs (_, v, r), Var _ -> v
| Econs (_, _, r), Shift e -> eval_lam r e
| _, Abs (n, body) -> fun x -> eval_lam (Econs (n, x, env)) body
| _, App (f, x) -> eval_lam env f (eval_lam env x)
;;
[%%expect{|
type _ env =
Enil : rnil env
| Econs : 'a * 't * 'e env -> ('a, 't, 'e) rcons env
val eval_lam : 'e env -> ('e, 't) lam -> 't = <fun>
|}];;
type add = Add
type suc = Suc
let env0 = Econs (Zero, 0, Econs (Suc, succ, Econs (Add, (+), Enil)))
let _0 : (_, int) lam = Var Zero
let suc x = App (Shift (Var Suc : (_, int -> int) lam), x)
let _1 = suc _0
let _2 = suc _1
let _3 = suc _2
let add = Shift (Shift (Var Add : (_, int -> int -> int) lam))
let double = Abs (X, App (App (Shift add, Var X), Var X))
let ex3 = App (double, _3)
;;
[%%expect{|
type add = Add
type suc = Suc
val env0 :
(zero, int, (suc, int -> int, (add, int -> int -> int, rnil) rcons) rcons)
rcons env = Econs (Zero, 0, Econs (Suc, <fun>, Econs (Add, <fun>, Enil)))
val _0 : ((zero, int, 'a) rcons, int) lam = Var Zero
val suc :
(('a, 'b, (suc, int -> int, 'c) rcons) rcons, int) lam ->
(('a, 'b, (suc, int -> int, 'c) rcons) rcons, int) lam = <fun>
val _1 : ((zero, int, (suc, int -> int, '_weak1) rcons) rcons, int) lam =
App (Shift (Var Suc), Var Zero)
val _2 : ((zero, int, (suc, int -> int, '_weak2) rcons) rcons, int) lam =
App (Shift (Var Suc), App (Shift (Var Suc), Var Zero))
val _3 : ((zero, int, (suc, int -> int, '_weak3) rcons) rcons, int) lam =
App (Shift (Var Suc),
App (Shift (Var Suc), App (Shift (Var Suc), Var Zero)))
val add :
(('a, 'b, ('c, 'd, (add, int -> int -> int, 'e) rcons) rcons) rcons,
int -> int -> int)
lam = Shift (Shift (Var Add))
val double :
(('a, 'b, ('c, 'd, (add, int -> int -> int, 'e) rcons) rcons) rcons,
int -> int)
lam =
Abs (<poly>,
App (App (Shift (Shift (Shift (Var Add))), Var <poly>), Var <poly>))
val ex3 :
((zero, int,
(suc, int -> int, (add, int -> int -> int, '_weak4) rcons) rcons)
rcons, int)
lam =
App
(Abs (<poly>,
App (App (Shift (Shift (Shift (Var Add))), Var <poly>), Var <poly>)),
App (Shift (Var Suc),
App (Shift (Var Suc), App (Shift (Var Suc), Var Zero))))
|}];;
let v3 = eval_lam env0 ex3
;;
[%%expect{|
val v3 : int = 6
|}];;
(* 5.13: Constructing typing derivations at runtime *)
(* Modified slightly to use the language of 5.10, since this is more fun.
Of course this works also with the language of 5.12. *)
type _ rep =
| I : int rep
| Ar : 'a rep * 'b rep -> ('a -> 'b) rep
let rec compare : type a b. a rep -> b rep -> (string, (a,b) equal) sum =
fun a b ->
match a, b with
| I, I -> Inr Eq
| Ar(x,y), Ar(s,t) ->
begin match compare x s with
| Inl _ as e -> e
| Inr Eq -> match compare y t with
| Inl _ as e -> e
| Inr Eq as e -> e
end
| I, Ar _ -> Inl "I <> Ar _"
| Ar _, I -> Inl "Ar _ <> I"
;;
[%%expect{|
type _ rep = I : int rep | Ar : 'a rep * 'b rep -> ('a -> 'b) rep
val compare : 'a rep -> 'b rep -> (string, ('a, 'b) equal) sum = <fun>
|}];;
type term =
| C of int
| Ab : string * 'a rep * term -> term
| Ap of term * term
| V of string
type _ ctx =
| Cnil : rnil ctx
| Ccons : 't * string * 'x rep * 'e ctx -> ('t,'x,'e) rcons ctx
;;
type _ checked =
| Cerror of string
| Cok : ('e,'t) lam * 't rep -> 'e checked
let rec lookup : type e. string -> e ctx -> e checked =
fun name ctx ->
match ctx with
| Cnil -> Cerror ("Name not found: " ^ name)
| Ccons (l,s,t,rs) ->
if s = name then Cok (Var l,t) else
match lookup name rs with
| Cerror m -> Cerror m
| Cok (v, t) -> Cok (Shift v, t)
;;
[%%expect{|
type term =
C of int
| Ab : string * 'a rep * term -> term
| Ap of term * term
| V of string
type _ ctx =
Cnil : rnil ctx
| Ccons : 't * string * 'x rep * 'e ctx -> ('t, 'x, 'e) rcons ctx
type _ checked = Cerror of string | Cok : ('e, 't) lam * 't rep -> 'e checked
val lookup : string -> 'e ctx -> 'e checked = <fun>
|}];;
let rec tc : type n e. n nat -> e ctx -> term -> e checked =
fun n ctx t ->
match t with
| V s -> lookup s ctx
| Ap(f,x) ->
begin match tc n ctx f with
| Cerror _ as e -> e
| Cok (f', ft) -> match tc n ctx x with
| Cerror _ as e -> e
| Cok (x', xt) ->
match ft with
| Ar (a, b) ->
begin match compare a xt with
| Inl s -> Cerror s
| Inr Eq -> Cok (App (f',x'), b)
end
| _ -> Cerror "Non fun in Ap"
end
| Ab(s,t,body) ->
begin match tc (NS n) (Ccons (n, s, t, ctx)) body with
| Cerror _ as e -> e
| Cok (body', et) -> Cok (Abs (n, body'), Ar (t, et))
end
| C m -> Cok (Const m, I)
;;
[%%expect{|
val tc : 'n nat -> 'e ctx -> term -> 'e checked = <fun>
|}];;
let ctx0 =
Ccons (Zero, "0", I,
Ccons (Suc, "S", Ar(I,I),
Ccons (Add, "+", Ar(I,Ar(I,I)), Cnil)))
let ex1 = Ab ("x", I, Ap(Ap(V"+",V"x"),V"x"));;
let c1 = tc NZ ctx0 ex1;;
let ex2 = Ap (ex1, C 3);;
let c2 = tc NZ ctx0 ex2;;
[%%expect{|
val ctx0 :
(zero, int, (suc, int -> int, (add, int -> int -> int, rnil) rcons) rcons)
rcons ctx =
Ccons (Zero, "0", I,
Ccons (Suc, "S", Ar (I, I), Ccons (Add, "+", Ar (I, Ar (I, I)), Cnil)))
val ex1 : term = Ab ("x", I, Ap (Ap (V "+", V "x"), V "x"))
val c1 :
(zero, int, (suc, int -> int, (add, int -> int -> int, rnil) rcons) rcons)
rcons checked =
Cok
(Abs (<poly>,
App (App (Shift (Shift (Shift (Var Add))), Var <poly>), Var <poly>)),
Ar (I, I))
val ex2 : term = Ap (Ab ("x", I, Ap (Ap (V "+", V "x"), V "x")), C 3)
val c2 :
(zero, int, (suc, int -> int, (add, int -> int -> int, rnil) rcons) rcons)
rcons checked =
Cok
(App
(Abs (<poly>,
App (App (Shift (Shift (Shift (Var Add))), Var <poly>), Var <poly>)),
Const 3),
I)
|}];;
let eval_checked env = function
| Cerror s -> failwith s
| Cok (e, I) -> (eval_lam env e : int)
| Cok _ -> failwith "Can only evaluate expressions of type I"
;;
[%%expect{|
val eval_checked : 'a env -> 'a checked -> int = <fun>
|}];;
let v2 = eval_checked env0 c2 ;;
[%%expect{|
val v2 : int = 6
|}];;
(* 5.12 Soundness *)
type pexp = PEXP
type pval = PVAL
type _ mode =
| Pexp : pexp mode
| Pval : pval mode
type ('a,'b) tarr = TARR
type tint = TINT
type (_,_) rel =
| IntR : (tint, int) rel
| IntTo : ('b, 's) rel -> ((tint, 'b) tarr, int -> 's) rel
type (_,_,_) lam =
| Const : ('a,'b) rel * 'b -> (pval, 'env, 'a) lam
| Var : 'a -> (pval, ('a,'t,'e) rcons, 't) lam
| Shift : ('m,'e,'t) lam -> ('m, ('a,'q,'e) rcons, 't) lam
| Lam : 'a * ('m, ('a,'s,'e) rcons, 't) lam -> (pval, 'e, ('s,'t) tarr) lam
| App : ('m1, 'e, ('s,'t) tarr) lam * ('m2, 'e, 's) lam -> (pexp, 'e, 't) lam
;;
let ex1 = App (Lam (X, Var X), Const (IntR, 3))
[%%expect{|
type pexp = PEXP
type pval = PVAL
type _ mode = Pexp : pexp mode | Pval : pval mode
type ('a, 'b) tarr = TARR
type tint = TINT
type (_, _) rel =
IntR : (tint, int) rel
| IntTo : ('b, 's) rel -> ((tint, 'b) tarr, int -> 's) rel
type (_, _, _) lam =
Const : ('a, 'b) rel * 'b -> (pval, 'env, 'a) lam
| Var : 'a -> (pval, ('a, 't, 'e) rcons, 't) lam
| Shift : ('m, 'e, 't) lam -> ('m, ('a, 'q, 'e) rcons, 't) lam
| Lam : 'a *
('m, ('a, 's, 'e) rcons, 't) lam -> (pval, 'e, ('s, 't) tarr) lam
| App : ('m1, 'e, ('s, 't) tarr) lam *
('m2, 'e, 's) lam -> (pexp, 'e, 't) lam
val ex1 : (pexp, 'a, tint) lam =
App (Lam (<poly>, Var <poly>), Const (IntR, <poly>))
|}];;
let rec mode : type m e t. (m,e,t) lam -> m mode = function
| Lam (v, body) -> Pval
| Var v -> Pval
| Const (r, v) -> Pval
| Shift e -> mode e
| App _ -> Pexp
;;
[%%expect{|
val mode : ('m, 'e, 't) lam -> 'm mode = <fun>
|}];;
type (_,_) sub =
| Id : ('r,'r) sub
| Bind : 't * ('m,'r2,'x) lam * ('r,'r2) sub -> (('t,'x,'r) rcons, 'r2) sub
| Push : ('r1,'r2) sub -> (('a,'b,'r1) rcons, ('a,'b,'r2) rcons) sub
type (_,_) lam' = Ex : ('m, 's, 't) lam -> ('s,'t) lam'
;;
let rec subst : type m1 r t s. (m1,r,t) lam -> (r,s) sub -> (s,t) lam' =
fun t s ->
match t, s with
| _, Id -> Ex t
| Const(r,c), sub -> Ex (Const (r,c))
| Var v, Bind (x, e, r) -> Ex e
| Var v, Push sub -> Ex (Var v)
| Shift e, Bind (_, _, r) -> subst e r
| Shift e, Push sub ->
(match subst e sub with Ex a -> Ex (Shift a))
| App(f,x), sub ->
(match subst f sub, subst x sub with Ex g, Ex y -> Ex (App (g,y)))
| Lam(v,x), sub ->
(match subst x (Push sub) with Ex body -> Ex (Lam (v, body)))
;;
[%%expect{|
type (_, _) sub =
Id : ('r, 'r) sub
| Bind : 't * ('m, 'r2, 'x) lam *
('r, 'r2) sub -> (('t, 'x, 'r) rcons, 'r2) sub
| Push : ('r1, 'r2) sub -> (('a, 'b, 'r1) rcons, ('a, 'b, 'r2) rcons) sub
type (_, _) lam' = Ex : ('m, 's, 't) lam -> ('s, 't) lam'
val subst : ('m1, 'r, 't) lam -> ('r, 's) sub -> ('s, 't) lam' = <fun>
|}];;
type closed = rnil
type 'a rlam = ((pexp,closed,'a) lam, (pval,closed,'a) lam) sum ;;
let rec rule : type a b.
(pval, closed, (a,b) tarr) lam -> (pval, closed, a) lam -> b rlam =
fun v1 v2 ->
match v1, v2 with
| Lam(x,body), v ->
begin
match subst body (Bind (x, v, Id)) with Ex term ->
match mode term with
| Pexp -> Inl term
| Pval -> Inr term
end
| Const (IntTo b, f), Const (IntR, x) ->
Inr (Const (b, f x))
;;
[%%expect{|
type closed = rnil
type 'a rlam = ((pexp, closed, 'a) lam, (pval, closed, 'a) lam) sum
val rule :
(pval, closed, ('a, 'b) tarr) lam -> (pval, closed, 'a) lam -> 'b rlam =
<fun>
|}];;
let rec onestep : type m t. (m,closed,t) lam -> t rlam = function
| Lam (v, body) -> Inr (Lam (v, body))
| Const (r, v) -> Inr (Const (r, v))
| App (e1, e2) ->
match mode e1, mode e2 with
| Pexp, _->
begin match onestep e1 with
| Inl e -> Inl(App(e,e2))
| Inr v -> Inl(App(v,e2))
end
| Pval, Pexp ->
begin match onestep e2 with
| Inl e -> Inl(App(e1,e))
| Inr v -> Inl(App(e1,v))
end
| Pval, Pval -> rule e1 e2
;;
[%%expect{|
val onestep : ('m, closed, 't) lam -> 't rlam = <fun>
|}];;
|