src/Pure/type_infer.ML

new distributive laws

(*  Title:      Pure/type_infer.ML
    ID:         $Id$
    Author:     Stefan Berghofer and Markus Wenzel, TU Muenchen

Type inference.
*)

signature TYPE_INFER =
sig
  val anyT: sort -> typ
  val logicT: typ
  val infer_types: (term -> Pretty.T) -> (typ -> Pretty.T)
    -> (string -> typ option) -> Sorts.classrel -> Sorts.arities
    -> string list -> bool -> (indexname -> bool) -> term list -> typ list
    -> term list * typ list * (indexname * typ) list
  val appl_error: (term -> Pretty.T) -> (typ -> Pretty.T)
    -> string -> term -> typ -> term -> typ -> string list
end;

structure TypeInfer: TYPE_INFER =
struct


(** term encodings **)

(*
  Flavours of term encodings:

    parse trees (type term):
      A very complicated structure produced by the syntax module's
      read functions.  Encodes types and sorts as terms; may contain
      explicit constraints and partial typing information (where
      dummies serve as wildcards).

      Parse trees are INTERNAL! Users should never encounter them,
      except in parse / print translation functions.

    raw terms (type term):
      Provide the user interface to type inferences.  They may contain
      partial type information (dummies are wildcards) or explicit
      type constraints (introduced via constrain: term -> typ ->
      term).

      The type inference function also lets users specify a certain
      subset of TVars to be treated as non-rigid inference parameters.

    preterms (type preterm):
      The internal representation for type inference.

    well-typed term (type term):
      Fully typed lambda terms to be accepted by appropriate
      certification functions.
*)



(** pretyps and preterms **)

(*links to parameters may get instantiated, anything else is rigid*)
datatype pretyp =
  PType of string * pretyp list |
  PTFree of string * sort |
  PTVar of indexname * sort |
  Param of sort |
  Link of pretyp ref;

datatype preterm =
  PConst of string * pretyp |
  PFree of string * pretyp |
  PVar of indexname * pretyp |
  PBound of int |
  PAbs of string * pretyp * preterm |
  PAppl of preterm * preterm |
  Constraint of preterm * pretyp;


(* utils *)

val mk_param = Link o ref o Param;

fun deref (T as Link (ref (Param _))) = T
  | deref (Link (ref T)) = deref T
  | deref T = T;

fun foldl_pretyps f (x, PConst (_, T)) = f (x, T)
  | foldl_pretyps f (x, PFree (_, T)) = f (x, T)
  | foldl_pretyps f (x, PVar (_, T)) = f (x, T)
  | foldl_pretyps _ (x, PBound _) = x
  | foldl_pretyps f (x, PAbs (_, T, t)) = foldl_pretyps f (f (x, T), t)
  | foldl_pretyps f (x, PAppl (t, u)) = foldl_pretyps f (foldl_pretyps f (x, t), u)
  | foldl_pretyps f (x, Constraint (t, T)) = f (foldl_pretyps f (x, t), T);



(** raw typs/terms to pretyps/preterms **)

(* pretyp(s)_of *)

fun anyT S = TFree ("'_dummy_", S);
val logicT = anyT logicS;

fun pretyp_of is_param (params, typ) =
  let
    fun add_parms (ps, TVar (xi as (x, _), S)) =
          if is_param xi andalso is_none (assoc (ps, xi))
          then (xi, mk_param S) :: ps else ps
      | add_parms (ps, TFree _) = ps
      | add_parms (ps, Type (_, Ts)) = foldl add_parms (ps, Ts);

    val params' = add_parms (params, typ);

    fun pre_of (TVar (v as (xi, _))) =
          (case assoc (params', xi) of
            None => PTVar v
          | Some p => p)
      | pre_of (TFree ("'_dummy_", S)) = mk_param S
      | pre_of (TFree v) = PTFree v
      | pre_of (T as Type (a, Ts)) =
          if T = dummyT then mk_param []
          else PType (a, map pre_of Ts);
  in (params', pre_of typ) end;

fun pretyps_of is_param = foldl_map (pretyp_of is_param);


(* preterm(s)_of *)

fun preterm_of const_type is_param ((vparams, params), tm) =
  let
    fun add_vparm (ps, xi) =
      if is_none (assoc (ps, xi)) then
        (xi, mk_param []) :: ps
      else ps;

    fun add_vparms (ps, Var (xi, _)) = add_vparm (ps, xi)
      | add_vparms (ps, Free (x, _)) = add_vparm (ps, (x, ~1))
      | add_vparms (ps, Abs (_, _, t)) = add_vparms (ps, t)
      | add_vparms (ps, t $ u) = add_vparms (add_vparms (ps, t), u)
      | add_vparms (ps, _) = ps;

    val vparams' = add_vparms (vparams, tm);
    fun var_param xi = the (assoc (vparams', xi));


    val preT_of = pretyp_of is_param;

    fun constrain (ps, t) T =
      if T = dummyT then (ps, t)
      else
        let val (ps', T') = preT_of (ps, T)
        in (ps', Constraint (t, T')) end;

    fun pre_of (ps, Const (c, T)) =
          (case const_type c of
            Some U => constrain (ps, PConst (c, snd (pretyp_of (K true) ([], U)))) T
          | None => raise TYPE ("No such constant: " ^ quote c, [], []))
      | pre_of (ps, Free (x, T)) = constrain (ps, PFree (x, var_param (x, ~1))) T
      | pre_of (ps, Var (xi, T)) = constrain (ps, PVar (xi, var_param xi)) T
      | pre_of (ps, Const ("_type_constraint_", T) $ t) = constrain (pre_of (ps, t)) T
      | pre_of (ps, Bound i) = (ps, PBound i)
      | pre_of (ps, Abs (x, T, t)) =
          let
            val (ps', T') = preT_of (ps, T);
            val (ps'', t') = pre_of (ps', t);
          in (ps'', PAbs (x, T', t')) end
      | pre_of (ps, t $ u) =
          let
            val (ps', t') = pre_of (ps, t);
            val (ps'', u') = pre_of (ps', u);
          in (ps'', PAppl (t', u')) end;


    val (params', tm') = pre_of (params, tm);
  in
    ((vparams', params'), tm')
  end;

fun preterms_of const_type is_param = foldl_map (preterm_of const_type is_param);



(** pretyps/terms to typs/terms **)

(* add_parms *)

fun add_parmsT (rs, PType (_, Ts)) = foldl add_parmsT (rs, Ts)
  | add_parmsT (rs, Link (r as ref (Param _))) = r ins rs
  | add_parmsT (rs, Link (ref T)) = add_parmsT (rs, T)
  | add_parmsT (rs, _) = rs;

val add_parms = foldl_pretyps add_parmsT;


(* add_names *)

fun add_namesT (xs, PType (_, Ts)) = foldl add_namesT (xs, Ts)
  | add_namesT (xs, PTFree (x, _)) = x ins xs
  | add_namesT (xs, PTVar ((x, _), _)) = x ins xs
  | add_namesT (xs, Link (ref T)) = add_namesT (xs, T)
  | add_namesT (xs, Param _) = xs;

val add_names = foldl_pretyps add_namesT;


(* simple_typ/term_of *)

(*deref links, fail on params*)
fun simple_typ_of (PType (a, Ts)) = Type (a, map simple_typ_of Ts)
  | simple_typ_of (PTFree v) = TFree v
  | simple_typ_of (PTVar v) = TVar v
  | simple_typ_of (Link (ref T)) = simple_typ_of T
  | simple_typ_of (Param _) = sys_error "simple_typ_of: illegal Param";

(*convert types, drop constraints*)
fun simple_term_of (PConst (c, T)) = Const (c, simple_typ_of T)
  | simple_term_of (PFree (x, T)) = Free (x, simple_typ_of T)
  | simple_term_of (PVar (xi, T)) = Var (xi, simple_typ_of T)
  | simple_term_of (PBound i) = Bound i
  | simple_term_of (PAbs (x, T, t)) = Abs (x, simple_typ_of T, simple_term_of t)
  | simple_term_of (PAppl (t, u)) = simple_term_of t $ simple_term_of u
  | simple_term_of (Constraint (t, _)) = simple_term_of t;


(* typs_terms_of *)                             (*DESTRUCTIVE*)

fun typs_terms_of used mk_var prfx (Ts, ts) =
  let
    fun elim (r as ref (Param S), x) = r := mk_var (x, S)
      | elim _ = ();

    val used' = foldl add_names (foldl add_namesT (used, Ts), ts);
    val parms = rev (foldl add_parms (foldl add_parmsT ([], Ts), ts));
    val pre_names = replicate (length parms) (prfx ^ "'");
    val names = variantlist (pre_names, prfx ^ "'" :: used');
  in
    seq2 elim (parms, names);
    (map simple_typ_of Ts, map simple_term_of ts)
  end;



(** order-sorted unification of types **)       (*DESTRUCTIVE*)

exception NO_UNIFIER of string;


fun unify classrel arities =
  let

    (* adjust sorts of parameters *)

    fun not_in_sort x S' S =
      "Variable " ^ x ^ "::" ^ Sorts.str_of_sort S' ^ " not of sort " ^
        Sorts.str_of_sort S ^ ".";

    fun no_domain (a, c) = "No way to get " ^ a ^ "::" ^ c ^ ".";

    fun meet (_, []) = ()
      | meet (Link (r as (ref (Param S'))), S) =
          if Sorts.sort_le classrel (S', S) then ()
          else r := mk_param (Sorts.inter_sort classrel (S', S))
      | meet (Link (ref T), S) = meet (T, S)
      | meet (PType (a, Ts), S) =
          seq2 meet (Ts, Sorts.mg_domain (classrel, arities) a S
            handle Sorts.DOMAIN ac => raise NO_UNIFIER (no_domain ac))
      | meet (PTFree (x, S'), S) =
          if Sorts.sort_le classrel (S', S) then ()
          else raise NO_UNIFIER (not_in_sort x S' S)
      | meet (PTVar (xi, S'), S) =
          if Sorts.sort_le classrel (S', S) then ()
          else raise NO_UNIFIER (not_in_sort (Syntax.string_of_vname xi) S' S)
      | meet (Param _, _) = sys_error "meet";


    (* occurs check and assigment *)

    fun occurs_check r (Link (r' as ref T)) =
          if r = r' then raise NO_UNIFIER "Occurs check!"
          else occurs_check r T
      | occurs_check r (PType (_, Ts)) = seq (occurs_check r) Ts
      | occurs_check _ _ = ();

    fun assign r T S =
      (case deref T of
        T' as Link (r' as ref (Param _)) =>
          if r = r' then () else (meet (T', S); r := T')
      | T' => (occurs_check r T'; meet (T', S); r := T'));


    (* unification *)

    fun unif (Link (r as ref (Param S)), T) = assign r T S
      | unif (T, Link (r as ref (Param S))) = assign r T S
      | unif (Link (ref T), U) = unif (T, U)
      | unif (T, Link (ref U)) = unif (T, U)
      | unif (PType (a, Ts), PType (b, Us)) =
          if a <> b then
            raise NO_UNIFIER ("Clash of types " ^ quote a ^ " and " ^ quote b ^ ".")
          else seq2 unif (Ts, Us)
      | unif (T, U) = if T = U then () else raise NO_UNIFIER "";

  in unif end;



(** type inference **)

fun appl_error prt prT why t T u U =
 ["Type error in application: " ^ why,
  "",
  Pretty.string_of (Pretty.block
    [Pretty.str "Operator:", Pretty.brk 2, prt t, Pretty.str " ::", Pretty.brk 1, prT T]),
  Pretty.string_of (Pretty.block
    [Pretty.str "Operand:", Pretty.brk 3, prt u, Pretty.str " ::", Pretty.brk 1, prT U]),
  ""];


(* infer *)                                     (*DESTRUCTIVE*)

fun infer prt prT classrel arities =
  let
    (* errors *)

    fun unif_failed msg =
      "Type unification failed" ^ (if msg = "" then "." else ": " ^ msg) ^ "\n";

    fun prep_output bs ts Ts =
      let
        val (Ts_bTs', ts') = typs_terms_of [] PTFree "??" (Ts @ map snd bs, ts);
        val len = length Ts;
        val Ts' = take (len, Ts_bTs');
        val xs = map Free (map fst bs ~~ drop (len, Ts_bTs'));
        val ts'' = map (fn t => subst_bounds (xs, t)) ts';
      in (ts'', Ts') end;

    fun err_loose i =
      raise TYPE ("Loose bound variable: B." ^ string_of_int i, [], []);

    fun err_appl msg bs t T u U =
      let
        val ([t', u'], [T', U']) = prep_output bs [t, u] [T, U];
        val why =
          (case T' of
            Type ("fun", _) => "Incompatible operand type."
          | _ => "Operator not of function type.");
        val text = unif_failed msg ^
                   cat_lines (appl_error prt prT why t' T' u' U');
      in raise TYPE (text, [T', U'], [t', u']) end;

    fun err_constraint msg bs t T U =
      let
        val ([t'], [T', U']) = prep_output bs [t] [T, U];
        val text = cat_lines
         [unif_failed msg,
          "Cannot meet type constraint:", "",
          Pretty.string_of
           (Pretty.block [Pretty.str "Term:", Pretty.brk 2, prt t',
                          Pretty.str " ::", Pretty.brk 1, prT T']),
          Pretty.string_of
           (Pretty.block [Pretty.str "Type:", Pretty.brk 2, prT U']), ""];
      in raise TYPE (text, [T', U'], [t']) end;


    (* main *)

    val unif = unify classrel arities;

    fun inf _ (PConst (_, T)) = T
      | inf _ (PFree (_, T)) = T
      | inf _ (PVar (_, T)) = T
      | inf bs (PBound i) = snd (nth_elem (i, bs) handle LIST _ => err_loose i)
      | inf bs (PAbs (x, T, t)) = PType ("fun", [T, inf ((x, T) :: bs) t])
      | inf bs (PAppl (t, u)) =
          let
            val T = inf bs t;
            val U = inf bs u;
            val V = mk_param [];
            val U_to_V = PType ("fun", [U, V]);
            val _ = unif (U_to_V, T) handle NO_UNIFIER msg => err_appl msg bs t T u U;
          in V end
      | inf bs (Constraint (t, U)) =
          let val T = inf bs t in
            unif (T, U) handle NO_UNIFIER msg => err_constraint msg bs t T U;
            T
          end;

  in inf [] end;


(* infer_types *)

fun infer_types prt prT const_type classrel arities used freeze is_param ts Ts =
  let
    (*convert to preterms/typs*)
    val (Tps, Ts') = pretyps_of (K true) ([], Ts);
    val ((vps, ps), ts') = preterms_of const_type is_param (([], Tps), ts);

    (*run type inference*)
    val tTs' = ListPair.map Constraint (ts', Ts');
    val _ = seq (fn t => (infer prt prT classrel arities t; ())) tTs';

    (*collect result unifier*)
    fun ch_var (xi, Link (r as ref (Param S))) = (r := PTVar (xi, S); None)
      | ch_var xi_T = Some xi_T;
    val env = mapfilter ch_var Tps;

    (*convert back to terms/typs*)
    val mk_var =
      if freeze then PTFree
      else (fn (x, S) => PTVar ((x, 0), S));
    val (final_Ts, final_ts) = typs_terms_of used mk_var "" (Ts', ts');
    val final_env = map (apsnd simple_typ_of) env;
  in
    (final_ts, final_Ts, final_env)
  end;


end;