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Typing Haskell in ML

This post presents a quick translation of the haskell code of the article Typing Haskell in Haskell by Mark P. Jones to OCaml.


I did this translation to experiment with type-class capable type inference for my Photon compiler project. I decided upon OCaml because:

  • It has good LLVM bindings.
  • I find it easier when I have to mix mutable state, error management and I/O (I am not quite at ease with monad transformers).

I share it because:

  • It could be of some use for someone else.
  • It can serve as a comparison point between the two languages (although I don't pretend my OCaml version is pretty).


While I intend to rewrite this code completely from scratch in my project (indeed, my type system is not even the same as Haskell), I kept this translation nearly as close to the original as possible to:

  • allow for easy comparison;
  • allow to follow the article with the OCaml code (and vice-versa).

One thing I changed is switching from monads to exceptions, as OCaml has no syntactic support for monads. I even used exceptions when I could have used the option type for the same reason. Maybe values in the haskell code are used monadically.

There are also some re-ordering, as OCaml process files linearly (i.e. values must be defined before being used). I even had to fuse sections 11.3 to 11.6 due to mutual recursion.

The type inference monad has been replaced by mutable state which is passed explicitely such that a function which had the haskell type

f :: a -> TI b

has the OCaml type

val f : ti -> a -> b

The Code
 * This file is a translation of the code pertaining to the article
 * `Typing Haskell in Haskell' by Mark P. Jones [1], from Haskell to OCaml.
 * 1:
 * Copyright (C) 1999 - 2000  Mark P. Jones
 * Copyright (C) 2010 Cyril Soldani
 * Here follows the license (license of the original Haskell code from the
 * article).
 * `Typing Haskell in Haskell' is Copyright (c) Mark P Jones
 * and the Oregon Graduate Institute of Science and Technology,
 * 1999-2000, All rights reserved, and is distributed as
 * free software under the following license.
 * Redistribution and use in source and binary forms, with or
 * without modification, are permitted provided that the following
 * conditions are met:
 * - Redistributions of source code must retain the above copyright
 * notice, this list of conditions and the following disclaimer.
 * - Redistributions in binary form must reproduce the above
 * copyright notice, this list of conditions and the following
 * disclaimer in the documentation and/or other materials provided
 * with the distribution.
 * - Neither name of the copyright holders nor the names of its
 * contributors may be used to endorse or promote products derived
 * from this software without specific prior written permission.
open Big_int
open List
open Num
(* 2 Preliminaries *)
let union xs ys = filter (fun x -> not (mem x ys)) xs @ ys
let intersect xs ys = filter (fun x -> mem x ys) xs
let nub xs =
   let addToSet ys y = if mem y ys then ys else y :: ys in
   fold_left addToSet [] xs
let isEmpty = function
     [] -> true
   | _ -> false
let fold_left1 f = function
     [] -> invalid_arg "empty list"
   | [x] -> x
   | x :: xs -> fold_left f x xs
let rec deleteFirst x = function
     [] -> []
   | y :: ys ->
        if x = y then ys
        else y :: deleteFirst x ys
let rec diff xs = function
     [] -> xs
   | y :: ys -> diff (deleteFirst y xs) ys
let split3 xs =
   let rec loop (ws, xs, ys) = function
        [] -> (rev ws, rev xs, rev ys)
      | (w, x, y) :: zs -> loop (w :: ws, x :: xs, y :: ys) zs in
   loop ([], [], []) xs
type id = string
let enumId n : id = "v" ^ string_of_int n
(* 3 Kinds *)
type kind = Star | Kfun of kind * kind
(* 4 Types *)
type tyvar = Tyvar of id * kind
type tycon = Tycon of id * kind
type type_ = TVar of tyvar | TCon of tycon | TAp of type_ * type_ | TGen of int
let tUnit = TCon(Tycon("()", Star))
let tChar = TCon(Tycon("Char", Star))
let tInt = TCon(Tycon("Int", Star))
let tInteger = TCon(Tycon("Integer", Star))
let tFloat = TCon(Tycon("Float", Star))
let tDouble = TCon(Tycon("Double", Star))
let tList = TCon(Tycon("[]", Kfun(Star, Star)))
let tArrow = TCon(Tycon("(->)", Kfun(Star, Kfun(Star, Star))))
let tTuple2 = TCon(Tycon("(,)", Kfun(Star, Kfun(Star, Star))))
let fn a b = TAp(TAp(tArrow, a), b)
let list t = TAp(tList, t)
let tString = list tChar
let pair a b = TAp(TAp(tTuple2, a), b)
let tyvarKind (Tyvar(_, k)) = k
let tyconKind (Tycon(_, k)) = k
let rec typeKind = function
     TCon tc -> tyconKind tc
   | TVar u -> tyvarKind u
   | TAp(t, _) ->
        begin match typeKind t with
             Kfun(_, k) -> k
           | _ -> failwith "inconsistent type"
   | TGen _ -> failwith "generic type variables have no kind"
(* 5 Substitutions *)
type subst = (tyvar * type_) list
let nullSubst : subst = []
let (+->) u t : subst = [(u, t)]
let rec typeApply (s : subst) = function
     TVar u as t ->
        begin try assoc u s
        with Not_found -> t
   | TAp(l, r) -> TAp(typeApply s l, typeApply s r)
   | t -> t
let rec typeTv = function
     TVar u -> [u]
   | TAp(l, r) -> union (typeTv l) (typeTv r)
   | _ -> []
let listApply (apply : subst -> 'a -> 'a) (s : subst) xs = map (apply s) xs
let listTv tv xs : tyvar list = nub (concat (map tv xs))
let (@@) s1 (s2 : subst) : subst =
   map (fun (u, t) -> (u, typeApply s1 t)) s2 @ s1
let merge s1 s2 : subst =
   let agree =
      let agreeOnVar v = typeApply s1 (TVar v) = typeApply s2 (TVar v) in
      for_all agreeOnVar (intersect (map fst s1) (map fst s2)) in
   if agree then s1 @ s2 else failwith "substitutions do not agree"
(* 6 Unification and Matching *)
let rec mgu t1 t2 =
   match t1, t2 with
        TAp(l, r), TAp(l', r') ->
           let s1 = mgu l l' in
           let s2 = mgu (typeApply s1 r) (typeApply s1 r') in
           s2 @@ s1
      | TVar u, t | t, TVar u -> varBind u t
      | TCon tc1, TCon tc2 when tc1 = tc2 -> nullSubst
      | _ -> failwith "types do not unify"
and varBind u t =
   if t = TVar u then nullSubst
   else if mem u (typeTv t) then failwith "occurs check fails"
   else if tyvarKind u <> typeKind t then failwith "kinds do not match"
   else u +-> t
let rec match_ t1 t2 =
   match t1, t2 with
        TAp(l, r), TAp(l', r') ->
           let sl = match_ l l' in
           let sr = match_ r r' in
           merge sl sr
      | TVar u, t when tyvarKind u = typeKind t -> u +-> t
      | TCon tc1, TCon tc2 when tc1 = tc2 -> nullSubst
      | _ -> failwith "types do not match"
(* 7 Type Classes, Predicates and Qualified Types *)
(* 7.1 Basic definitions *)
type pred = IsIn of id * type_
type 't qual = Qual of pred list * 't
let predApply s (IsIn(i, t)) = IsIn(i, typeApply s t)
let predsApply = listApply predApply
let qualTypeApply s (Qual(ps, t)) = Qual(predsApply s ps, typeApply s t)
let predTv (IsIn(_, t)) = typeTv t
let predsTv = listTv predTv
let qualTypeTv (Qual(ps, t)) = union (predsTv ps) (typeTv t)
let lift m (IsIn(i, t)) (IsIn(i', t')) =
   if i = i' then m t t'
   else failwith "classes differ"
let mguPred = lift mgu
let matchPred = lift match_
type inst = pred qual
type class_ = id list * inst list
(* 7.2 Class Environments *)
type classEnv = {
   classes : (id -> class_);
   defaults : type_ list;
let super ce i = fst (ce.classes i)
let insts ce i = snd (ce.classes i)
let defined ce i =
      ignore (ce.classes i);
   with Not_found -> false
let modify ce i c =
   { ce with classes = fun j -> if i = j then c else ce.classes j; }
let initalEnv = {
   classes = (fun i -> raise Not_found);
   defaults = [tInteger; tDouble]
type envTransformer = classEnv -> classEnv
let (<:>) (f : envTransformer) (g : envTransformer) : envTransformer =
   fun ce -> g (f ce)
let addClass i is : envTransformer =
   fun ce ->
      if defined ce i then failwith "class already defined"
      else if exists (fun i -> not (defined ce i)) is then
         failwith "superclass not defined"
      else modify ce i (is, [])
let addCoreClasses =
       addClass "Eq" []
   <:> addClass "Ord" ["Eq"]
   <:> addClass "Show" []
   <:> addClass "Read" []
   <:> addClass "Bounded" []
   <:> addClass "Enum" []
   <:> addClass "Functor" []
   <:> addClass "Monad" []
let addNumClasses =
       addClass "Num" ["Eq"; "Show"]
   <:> addClass "Real" ["Num"; "Ord"]
   <:> addClass "Fractional" ["Num"]
   <:> addClass "Integral" ["Real"; "Enum"]
   <:> addClass "RealFrac" ["Real"; "Fractional"]
   <:> addClass "Floating" ["Fractional"]
   <:> addClass "RealFloat" ["RealFrac"; "Floating"]
let addPreludeClasses = addCoreClasses <:> addNumClasses
let overlap p q =
      ignore (mguPred p q);
   with _ -> false
let addInst ps (IsIn(i, _) as p) : envTransformer =
   fun ce ->
      if not (defined ce i) then failwith "no class for instance"
         let its = insts ce i in
         let qs = map (fun (Qual(_, q)) -> q) its in
         if exists (overlap p) qs then failwith "overlapping instance"
            let c = super ce i, Qual(ps, p) :: its in
            modify ce i c
let exampleInsts =
   <:> addInst [] (IsIn("Ord", tUnit))
   <:> addInst [] (IsIn("Ord", tChar))
   <:> addInst [] (IsIn("Ord", tInt))
   <:> addInst [IsIn("Ord", TVar(Tyvar("a", Star)));
                IsIn("Ord", TVar(Tyvar("b", Star)))]
               (IsIn("Ord", pair (TVar(Tyvar("a", Star)))
                                 (TVar(Tyvar("b", Star)))))
(* 7.3 Entailment *)
let rec bySuper ce (IsIn(i, t) as p) =
   p :: concat (map (fun i' -> bySuper ce (IsIn(i', t))) (super ce i))
let byInst ce (IsIn(i, t) as p) =
   let tryInst (Qual(ps, h)) =
      let u = matchPred h p in
      map (predApply u) ps in
   concat (map tryInst (insts ce i))
let rec entail ce ps p =
   exists (mem p) (map (bySuper ce) ps) ||
   try for_all (entail ce ps) (byInst ce p)
   with _ -> false
(* 7.4 Context Reduction *)
let inHnf (IsIn(_, t)) =
   let rec hnf = function
        TVar _ -> true
      | TCon _ -> false
      | TAp(t, _) -> hnf t
      | TGen _ -> failwith "context reduction on generic variable" in
   hnf t
let rec toHnfs ce ps = concat (map (toHnf ce) ps)
and toHnf ce p =
   if inHnf p then [p]
      try toHnfs ce (byInst ce p)
      with _ -> failwith "context reduction"
let simplify ce ps =
   let rec loop rs = function
        [] -> rs
      | p :: ps ->
           if entail ce (rs @ ps) p then loop rs ps
           else loop (p :: rs) ps in
   loop [] ps
let reduce ce ps = simplify ce (toHnfs ce ps)
let scEntail ce ps p = exists (mem p) (map (bySuper ce) ps)
(* 8 Type Schemes *)
type scheme = Forall of kind list * type_ qual
let schemeApply s (Forall(ks, qt)) = Forall(ks, qualTypeApply s qt)
let schemeTv (Forall(_, qt)) = qualTypeTv qt
let quantify vs qt =
   let vs' = filter (fun v -> mem v vs) (qualTypeTv qt) in
   let ks = map tyvarKind vs' in
   let newGen v =
      let count = ref 0 in
      let t = TGen !count in
      incr count;
      (v, t) in
   let s = map newGen vs' in
   Forall(ks, qualTypeApply s qt)
let toScheme t = Forall([], (Qual([], t)))
(* 9 Assumptions *)
type assump = Assump of id * scheme
let assumpApply s (Assump(i, sc)) = Assump(i, schemeApply s sc)
let assumpTv (Assump(_, sc)) = schemeTv sc
let assumpsApply = listApply assumpApply
let assumpsTv = listTv assumpTv
let find i as_ =
   let Assump(_, sc) = List.find (fun (Assump(i', _)) -> i = i') as_ in
(* 10 A Type Inference Monad *)
type ti = subst ref * int ref
let runTI (f : ti -> 'a) = f (ref nullSubst, ref 0)
let getSubst ((s, _) : ti) = !s
let extSubst ((s, _) : ti) u = s := u @@ !s
let unify ti t1 t2 =
   let s = getSubst ti in
   let u = mgu (typeApply s t1) (typeApply s t2) in
   extSubst ti u
let newTVar ((_, n) : ti) k =
   let v = Tyvar(enumId !n, k) in
   incr n;
   TVar v
let rec typeInst ts = function
     TAp(l, r) -> TAp(typeInst ts l, typeInst ts r)
   | TGen n -> nth ts n
   | t -> t
let listInst inst (ts : type_ list) (xs : 'a list) : 'a list = map (inst ts) xs
let predInst ts (IsIn(c, t)) = IsIn(c, typeInst ts t)
let qualTypeInst ts (Qual(ps, t)) = Qual(listInst predInst ts ps, typeInst ts t)
let freshInst ti (Forall(ks, qt)) =
   let ts = map (newTVar ti) ks in
   qualTypeInst ts qt
(* 11 Type Inference *)
type ('e, 't) infer = ti -> classEnv -> assump list -> 'e -> pred list * 't
(* 11.1 Literals *)
type literal =
     LitInt of big_int
   | LitChar of char
   | LitRat of num
   | LitStr of string
let tiLit ti = function
     LitChar _ -> ([], tChar)
   | LitInt _ ->
        let v = newTVar ti Star in
        ([IsIn("Num", v)], v)
   | LitStr _ -> ([], tString)
   | LitRat _ ->
        let v = newTVar ti Star in
        ([IsIn("Fractional", v)], v)
(* 11.2 Patterns *)
type pat =
     PVar of id
   | PWildcard
   | PAs of id * pat
   | PLit of literal
   | PNpk of id * big_int
   | PCon of assump * pat list
let rec tiPat ti = function
     PVar i ->
        let t = newTVar ti Star in
        ([], [Assump(i, toScheme t)], t)
   | PWildcard -> ([], [], newTVar ti Star)
   | PAs(i, pat) ->
        let (ps, as_, t) = tiPat ti pat in
        (ps, Assump(i, toScheme t) :: as_, t)
   | PLit l ->
        let (ps, t) = tiLit ti l in
        (ps, [], t)
   | PNpk(i, k) ->
        let t = newTVar ti Star in
        ([IsIn("Integral", t)], [Assump(i, toScheme t)], t)
   | PCon(Assump(i, sc), pats) ->
        let (ps, as_, ts) = tiPats ti pats in
        let t' = newTVar ti Star in
        let Qual(qs, t) = freshInst ti sc in
        unify ti t (fold_right fn ts t');
        (ps @ qs, as_, t')
and tiPats ti pats =
   let (pss, ass, ts) = split3 (map (tiPat ti) pats) in
   (concat pss, concat ass, ts)
(* 11.3 Expressions
 * 11.4 Alternatives
 * 11.5 From Types to Type Schemes
 * 11.6 Binding Groups *)
type ambiguity = tyvar * pred list
let ambiguities vs ps : ambiguity list =
   let vs' = diff (predsTv ps) vs in
   map (fun v -> (v, filter (fun p -> mem v (predTv p)) ps)) vs'
let numClasses : id list = [
   "Num"; "Integral"; "Floating"; "Fractional"; "Real"; "RealFloat";
let stdClasses : id list = [
   "Eq"; "Ord"; "Show"; "Read"; "Bounded"; "Enum"; "Ix"; "Functor"; "Monad";
   "MonadPlus"] @ numClasses
let candidates ce ((v, qs) : ambiguity) =
   let is = map (fun (IsIn(i, _)) -> i) qs in
   let ts = map (fun (IsIn(_, t)) -> t) qs in
   if for_all (fun t -> t = TVar v) ts &&
      exists (fun i -> mem i numClasses) is &&
      for_all (fun i -> mem i stdClasses) is then
      let isCandidate t' =
         for_all (entail ce []) (map (fun i -> IsIn(i, t')) is) in
      filter isCandidate ce.defaults
   else []
let withDefaults f ce vs ps =
   let vps = ambiguities vs ps in
   let tss = map (candidates ce) vps in
   if exists isEmpty tss then failwith "cannot resolve ambiguity"
   else f vps (map hd tss)
let defaultedPreds = withDefaults (fun vps ts -> concat (map snd vps))
let defaultSubst ce vs ps : subst =
   withDefaults (fun vps ts -> combine (map fst vps) ts) ce vs ps
let split ce fs gs ps =
   let ps' = reduce ce ps in
   let (ds, rs) =
      partition (fun p -> for_all (fun t -> mem t fs) (predTv p)) ps' in
   let rs' = defaultedPreds ce (fs @ gs) rs in
   (ds, diff rs rs')
type alt = pat list * expr
and expl = id * scheme * alt list
and impl = id * alt list
and bindGroup = expl list * impl list list
and expr =
     Var of id
   | Lit of literal
   | Const of assump
   | Ap of expr * expr
   | Let of bindGroup * expr
let restricted (bs : impl list) =
   let simple (i, alts) = exists (fun alt -> isEmpty (fst alt)) alts in
   exists simple bs
let rec tiSeq (f : ('bg, assump list) infer) : ('bg list, assump list) infer =
   fun ti ce as_ -> function
        [] -> ([], [])
      | bs :: bss ->
           let (ps, as') = f ti ce as_ bs in
           let (qs, as'') = tiSeq f ti ce (as' @ as_) bss in
           (ps @ qs, as'' @ as')
let rec tiExpr ti ce as_ = function
     Var i ->
        let sc = find i as_ in
        let Qual(ps, t) = freshInst ti sc in
        (ps, t)
   | Const(Assump(_, sc)) ->
        let Qual(ps, t) = freshInst ti sc in
        (ps, t)
   | Lit l -> tiLit ti l
   | Ap(e, f) ->
        let (ps, te) = tiExpr ti ce as_ e in
        let (qs, tf) = tiExpr ti ce as_ f in
        let t = newTVar ti Star in
        unify ti (fn tf t) te;
        (ps @ qs, t)
   | Let(bg, e) ->
        let (ps, as') = tiBindGroup ti ce as_ bg in
        let (qs, t) = tiExpr ti ce (as' @ as_) e in
        (ps @ qs, t)
and tiAlt : (alt, type_) infer =
   fun ti ce as_ (pats, e) ->
      let (ps, as', ts) = tiPats ti pats in
      let (qs, t) = tiExpr ti ce (as' @ as_) e in
      (ps @ qs, fold_right fn ts t)
and tiAlts ti ce as_ alts t =
   let (ps, ts) = List.split (map (tiAlt ti ce as_) alts) in
   iter (unify ti t) ts;
   concat ps
and tiExpl ti ce as_ ((i, sc, alts) : expl) =
   let Qual(qs, t) = freshInst ti sc in
   let ps = tiAlts ti ce as_ alts t in
   let s = getSubst ti in
   let qs' = predsApply s qs in
   let t' = typeApply s t in
   let fs = assumpsTv (assumpsApply s as_) in
   let gs = diff (typeTv t') fs in
   let sc' = quantify gs (Qual(qs', t')) in
   let ps' = filter (fun p -> not (entail ce qs' p)) (predsApply s ps) in
   let (ds, rs) = split ce fs gs ps' in
   if sc <> sc' then failwith "signature too general"
   else if not (isEmpty rs) then failwith "context too weak"
   else ds
and tiImpls : (impl list, assump list) infer =
   fun ti ce as_ bs ->
      let ts = map (fun _ -> newTVar ti Star) bs in
      let (is, altss) = List.split bs in
      let scs = map toScheme ts in
      let as' = map2 (fun i sc -> Assump(i, sc)) is scs @ as_ in
      let pss = map2 (tiAlts ti ce as') altss ts in
      let s = getSubst ti in
      let ps' = map (predApply s) (concat pss) in
      let ts' = map (typeApply s) ts in
      let fs = assumpsTv (assumpsApply s as_) in
      let vss = map typeTv ts' in
      let gs = diff (fold_left1 union vss) fs in
      let (ds, rs) = split ce fs (fold_left1 intersect vss) ps' in
      if restricted bs then
         let gs' = diff gs (predsTv rs) in
         let scs' = map (fun t -> quantify gs' (Qual([], t))) ts' in
         (ds @ rs, map2 (fun i sc -> Assump(i, sc)) is scs')
            let scs' = map (fun t -> quantify gs (Qual(rs, t))) ts' in
            (ds, map2 (fun i sc -> Assump(i, sc)) is scs')
and tiBindGroup : (bindGroup, assump list) infer =
   fun ti ce as_ (es, iss) ->
      let as' = map (fun (v, sc, _) -> Assump(v, sc)) es in
      let (ps, as'') = tiSeq tiImpls ti ce (as' @ as_) iss in
      let qss = map (tiExpl ti ce (as'' @ as' @ as_)) es in
      (ps @ concat qss, as'' @ as')
type program = bindGroup list
let tiProgram ce as_ (bgs : program) =
   runTI (fun ti ->
             let (ps, as') = tiSeq tiBindGroup ti ce as_ bgs in
             let s = getSubst ti in
             let rs = reduce ce (predsApply s ps) in
             let s' = defaultSubst ce [] rs in
             assumpsApply (s' @@ s) as')


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blog/2010/08/04/typing_haskell_in_ml.1280924575.txt.gz · Last modified: 2011/02/20 20:28 (external edit)