path: root/typing/typetexp.ml

(**************************************************************************)
(*                                                                        *)
(*                                 OCaml                                  *)
(*                                                                        *)
(*             Xavier Leroy, projet Cristal, INRIA Rocquencourt           *)
(*                                                                        *)
(*   Copyright 1996 Institut National de Recherche en Informatique et     *)
(*     en Automatique.                                                    *)
(*                                                                        *)
(*   All rights reserved.  This file is distributed under the terms of    *)
(*   the GNU Lesser General Public License version 2.1, with the          *)
(*   special exception on linking described in the file LICENSE.          *)
(*                                                                        *)
(**************************************************************************)

(* typetexp.ml,v 1.34.4.9 2002/01/07 08:39:16 garrigue Exp *)

(* Typechecking of type expressions for the core language *)

open Asttypes
open Misc
open Parsetree
open Typedtree
open Types
open Ctype

exception Already_bound

type error =
    Unbound_type_variable of string
  | Undefined_type_constructor of Path.t
  | Type_arity_mismatch of Longident.t * int * int
  | Bound_type_variable of string
  | Recursive_type
  | Unbound_row_variable of Longident.t
  | Type_mismatch of Errortrace.unification_error
  | Alias_type_mismatch of Errortrace.unification_error
  | Present_has_conjunction of string
  | Present_has_no_type of string
  | Constructor_mismatch of type_expr * type_expr
  | Not_a_variant of type_expr
  | Variant_tags of string * string
  | Invalid_variable_name of string
  | Cannot_quantify of string * type_expr
  | Multiple_constraints_on_type of Longident.t
  | Method_mismatch of string * type_expr * type_expr
  | Opened_object of Path.t option
  | Not_an_object of type_expr

exception Error of Location.t * Env.t * error
exception Error_forward of Location.error

(** Map indexed by type variable names. *)
module TyVarMap = Misc.Stdlib.String.Map

(* Support for first-class modules. *)

let transl_modtype_longident = ref (fun _ -> assert false)
let transl_modtype = ref (fun _ -> assert false)

let sort_constraints_no_duplicates loc env l =
  List.sort
    (fun (s1, _t1) (s2, _t2) ->
       if s1.txt = s2.txt then
         raise (Error (loc, env, Multiple_constraints_on_type s1.txt));
       compare s1.txt s2.txt)
    l

let create_package_mty loc p l =
  List.fold_left
    (fun mty (s, _) ->
      let d = {ptype_name = mkloc (Longident.last s.txt) s.loc;
               ptype_params = [];
               ptype_cstrs = [];
               ptype_kind = Ptype_abstract;
               ptype_private = Asttypes.Public;
               ptype_manifest = None;
               ptype_attributes = [];
               ptype_loc = loc} in
      Ast_helper.Mty.mk ~loc
        (Pmty_with (mty, [ Pwith_type ({ txt = s.txt; loc }, d) ]))
    )
    (Ast_helper.Mty.mk ~loc (Pmty_ident p))
    l

(* Translation of type expressions *)

let type_variables = ref (TyVarMap.empty : type_expr TyVarMap.t)
let univars        = ref ([] : (string * type_expr) list)
let pre_univars    = ref ([] : type_expr list)
let used_variables = ref (TyVarMap.empty : (type_expr * Location.t) TyVarMap.t)

let reset_type_variables () =
  reset_global_level ();
  Ctype.reset_reified_var_counter ();
  type_variables := TyVarMap.empty

let narrow () =
  (increase_global_level (), !type_variables)

let widen (gl, tv) =
  restore_global_level gl;
  type_variables := tv

let with_local_type_variable_scope f =
  let context = narrow () in
  let r = f () in
  widen context;
  r

let generalize_ctyp typ = generalize typ.ctyp_type

let strict_ident c = (c = '_' || c >= 'a' && c <= 'z' || c >= 'A' && c <= 'Z')

let validate_name = function
    None -> None
  | Some name as s ->
      if name <> "" && strict_ident name.[0] then s else None

let new_global_var ?name () =
  new_global_var ?name:(validate_name name) ()
let newvar ?name () =
  newvar ?name:(validate_name name) ()

let type_variable loc name =
  try
    TyVarMap.find name !type_variables
  with Not_found ->
    raise(Error(loc, Env.empty, Unbound_type_variable ("'" ^ name)))

let valid_tyvar_name name =
  name <> "" && name.[0] <> '_'

let transl_type_param env styp =
  let loc = styp.ptyp_loc in
  match styp.ptyp_desc with
    Ptyp_any ->
      let ty = new_global_var ~name:"_" () in
        { ctyp_desc = Ttyp_any; ctyp_type = ty; ctyp_env = env;
          ctyp_loc = loc; ctyp_attributes = styp.ptyp_attributes; }
  | Ptyp_var name ->
      let ty =
        try
          if not (valid_tyvar_name name) then
            raise (Error (loc, Env.empty, Invalid_variable_name ("'" ^ name)));
          ignore (TyVarMap.find name !type_variables);
          raise Already_bound
        with Not_found ->
          let v = new_global_var ~name () in
            type_variables := TyVarMap.add name v !type_variables;
            v
      in
        { ctyp_desc = Ttyp_var name; ctyp_type = ty; ctyp_env = env;
          ctyp_loc = loc; ctyp_attributes = styp.ptyp_attributes; }
  | _ -> assert false

let transl_type_param env styp =
  (* Currently useless, since type parameters cannot hold attributes
     (but this could easily be lifted in the future). *)
  Builtin_attributes.warning_scope styp.ptyp_attributes
    (fun () -> transl_type_param env styp)


let new_pre_univar ?name () =
  let v = newvar ?name () in pre_univars := v :: !pre_univars; v

type poly_univars = (string * type_expr) list
let make_poly_univars vars =
  List.map (fun name -> name, newvar ~name ()) vars

let check_poly_univars env loc vars =
  vars |> List.iter (fun (_, v) -> generalize v);
  vars |> List.map (fun (name, ty1) ->
    let v = Btype.proxy ty1 in
    begin match get_desc v with
    | Tvar name when get_level v = Btype.generic_level ->
       set_type_desc v (Tunivar name)
    | _ ->
       raise (Error (loc, env, Cannot_quantify(name, v)))
    end;
    v)

let instance_poly_univars env loc vars =
  let vs = check_poly_univars env loc vars in
  vs |> List.iter (fun v ->
    match get_desc v with
    | Tunivar name ->
       set_type_desc v (Tvar name)
    | _ -> assert false);
  vs


type policy = Fixed | Extensible | Univars

let rec transl_type env policy styp =
  Builtin_attributes.warning_scope styp.ptyp_attributes
    (fun () -> transl_type_aux env policy styp)

and transl_type_aux env policy styp =
  let loc = styp.ptyp_loc in
  let ctyp ctyp_desc ctyp_type =
    { ctyp_desc; ctyp_type; ctyp_env = env;
      ctyp_loc = loc; ctyp_attributes = styp.ptyp_attributes }
  in
  match styp.ptyp_desc with
    Ptyp_any ->
      let ty =
        if policy = Univars then new_pre_univar () else
          if policy = Fixed then
            raise (Error (styp.ptyp_loc, env, Unbound_type_variable "_"))
          else newvar ()
      in
      ctyp Ttyp_any ty
  | Ptyp_var name ->
    let ty =
      if not (valid_tyvar_name name) then
        raise (Error (styp.ptyp_loc, env, Invalid_variable_name ("'" ^ name)));
      begin try
        instance (List.assoc name !univars)
      with Not_found -> try
        instance (fst (TyVarMap.find name !used_variables))
      with Not_found ->
        let v =
          if policy = Univars then new_pre_univar ~name () else newvar ~name ()
        in
        used_variables := TyVarMap.add name (v, styp.ptyp_loc) !used_variables;
        v
      end
    in
    ctyp (Ttyp_var name) ty
  | Ptyp_arrow(l, st1, st2) ->
    let cty1 = transl_type env policy st1 in
    let cty2 = transl_type env policy st2 in
    let ty1 = cty1.ctyp_type in
    let ty1 =
      if Btype.is_optional l
      then newty (Tconstr(Predef.path_option,[ty1], ref Mnil))
      else ty1 in
    let ty = newty (Tarrow(l, ty1, cty2.ctyp_type, commu_ok)) in
    ctyp (Ttyp_arrow (l, cty1, cty2)) ty
  | Ptyp_tuple stl ->
    assert (List.length stl >= 2);
    let ctys = List.map (transl_type env policy) stl in
    let ty = newty (Ttuple (List.map (fun ctyp -> ctyp.ctyp_type) ctys)) in
    ctyp (Ttyp_tuple ctys) ty
  | Ptyp_constr(lid, stl) ->
      let (path, decl) = Env.lookup_type ~loc:lid.loc lid.txt env in
      let stl =
        match stl with
        | [ {ptyp_desc=Ptyp_any} as t ] when decl.type_arity > 1 ->
            List.map (fun _ -> t) decl.type_params
        | _ -> stl
      in
      if List.length stl <> decl.type_arity then
        raise(Error(styp.ptyp_loc, env,
                    Type_arity_mismatch(lid.txt, decl.type_arity,
                                        List.length stl)));
      let args = List.map (transl_type env policy) stl in
      let params = instance_list decl.type_params in
      let unify_param =
        match decl.type_manifest with
          None -> unify_var
        | Some ty ->
            if get_level ty = Btype.generic_level then unify_var else unify
      in
      List.iter2
        (fun (sty, cty) ty' ->
           try unify_param env ty' cty.ctyp_type with Unify err ->
             let err = Errortrace.swap_unification_error err in
             raise (Error(sty.ptyp_loc, env, Type_mismatch err))
        )
        (List.combine stl args) params;
      let constr =
        newconstr path (List.map (fun ctyp -> ctyp.ctyp_type) args) in
      ctyp (Ttyp_constr (path, lid, args)) constr
  | Ptyp_object (fields, o) ->
      let ty, fields = transl_fields env policy o fields in
      ctyp (Ttyp_object (fields, o)) (newobj ty)
  | Ptyp_class(lid, stl) ->
      let (path, decl) =
        let path, decl = Env.lookup_cltype ~loc:lid.loc lid.txt env in
        (path, decl.clty_hash_type)
      in
      if List.length stl <> decl.type_arity then
        raise(Error(styp.ptyp_loc, env,
                    Type_arity_mismatch(lid.txt, decl.type_arity,
                                        List.length stl)));
      let args = List.map (transl_type env policy) stl in
      let body = Option.get decl.type_manifest in
      let (params, body) = instance_parameterized_type decl.type_params body in
      List.iter2
        (fun (sty, cty) ty' ->
           try unify_var env ty' cty.ctyp_type with Unify err ->
             let err = Errortrace.swap_unification_error err in
             raise (Error(sty.ptyp_loc, env, Type_mismatch err))
        )
        (List.combine stl args) params;
      let ty_args = List.map (fun ctyp -> ctyp.ctyp_type) args in
      let ty = Ctype.apply ~use_current_level:true env params body ty_args in
      let ty = match get_desc ty with
        | Tobject (fi, _) ->
            let _, tv = flatten_fields fi in
            if policy = Univars then pre_univars := tv :: !pre_univars;
            ty
        | _ ->
            assert false
      in
      ctyp (Ttyp_class (path, lid, args)) ty
  | Ptyp_alias(st, alias) ->
      let cty =
        try
          let t =
            try List.assoc alias !univars
            with Not_found ->
              instance (fst(TyVarMap.find alias !used_variables))
          in
          let ty = transl_type env policy st in
          begin try unify_var env t ty.ctyp_type with Unify err ->
            let err = Errortrace.swap_unification_error err in
            raise(Error(styp.ptyp_loc, env, Alias_type_mismatch err))
          end;
          ty
        with Not_found ->
          let t, ty =
            with_local_level_if_principal begin fun () ->
              let t = newvar () in
              used_variables :=
                TyVarMap.add alias (t, styp.ptyp_loc) !used_variables;
              let ty = transl_type env policy st in
              begin try unify_var env t ty.ctyp_type with Unify err ->
                let err = Errortrace.swap_unification_error err in
                raise(Error(styp.ptyp_loc, env, Alias_type_mismatch err))
              end;
              (t, ty)
            end
            ~post: (fun (t, _) -> generalize_structure t)
          in
          let t = instance t in
          let px = Btype.proxy t in
          begin match get_desc px with
          | Tvar None -> set_type_desc px (Tvar (Some alias))
          | Tunivar None -> set_type_desc px (Tunivar (Some alias))
          | _ -> ()
          end;
          { ty with ctyp_type = t }
      in
      ctyp (Ttyp_alias (cty, alias)) cty.ctyp_type
  | Ptyp_variant(fields, closed, present) ->
      let name = ref None in
      let mkfield l f =
        newty (Tvariant (create_row ~fields:[l,f] ~more:(newvar())
                           ~closed:true ~fixed:None ~name:None)) in
      let hfields = Hashtbl.create 17 in
      let add_typed_field loc l f =
        let h = Btype.hash_variant l in
        try
          let (l',f') = Hashtbl.find hfields h in
          (* Check for tag conflicts *)
          if l <> l' then raise(Error(styp.ptyp_loc, env, Variant_tags(l, l')));
          let ty = mkfield l f and ty' = mkfield l f' in
          if is_equal env false [ty] [ty'] then () else
          try unify env ty ty'
          with Unify _trace ->
            raise(Error(loc, env, Constructor_mismatch (ty,ty')))
        with Not_found ->
          Hashtbl.add hfields h (l,f)
      in
      let add_field field =
        let rf_loc = field.prf_loc in
        let rf_attributes = field.prf_attributes in
        let rf_desc = match field.prf_desc with
        | Rtag (l, c, stl) ->
            name := None;
            let tl =
              Builtin_attributes.warning_scope rf_attributes
                (fun () -> List.map (transl_type env policy) stl)
            in
            let f = match present with
              Some present when not (List.mem l.txt present) ->
                let ty_tl = List.map (fun cty -> cty.ctyp_type) tl in
                rf_either ty_tl ~no_arg:c ~matched:false
            | _ ->
                if List.length stl > 1 || c && stl <> [] then
                  raise(Error(styp.ptyp_loc, env,
                              Present_has_conjunction l.txt));
                match tl with [] -> rf_present None
                | st :: _ -> rf_present (Some st.ctyp_type)
            in
            add_typed_field styp.ptyp_loc l.txt f;
              Ttag (l,c,tl)
        | Rinherit sty ->
            let cty = transl_type env policy sty in
            let ty = cty.ctyp_type in
            let nm =
              match get_desc cty.ctyp_type with
                Tconstr(p, tl, _) -> Some(p, tl)
              | _                 -> None
            in
            name := if Hashtbl.length hfields <> 0 then None else nm;
            let fl = match get_desc (expand_head env cty.ctyp_type), nm with
              Tvariant row, _ when Btype.static_row row ->
                row_fields row
            | Tvar _, Some(p, _) ->
                raise(Error(sty.ptyp_loc, env, Undefined_type_constructor p))
            | _ ->
                raise(Error(sty.ptyp_loc, env, Not_a_variant ty))
            in
            List.iter
              (fun (l, f) ->
                let f = match present with
                  Some present when not (List.mem l present) ->
                    begin match row_field_repr f with
                      Rpresent oty -> rf_either_of oty
                    | _ -> assert false
                    end
                | _ -> f
                in
                add_typed_field sty.ptyp_loc l f)
              fl;
              Tinherit cty
        in
        { rf_desc; rf_loc; rf_attributes; }
      in
      let tfields = List.map add_field fields in
      let fields = List.rev (Hashtbl.fold (fun _ p l -> p :: l) hfields []) in
      begin match present with None -> ()
      | Some present ->
          List.iter
            (fun l -> if not (List.mem_assoc l fields) then
              raise(Error(styp.ptyp_loc, env, Present_has_no_type l)))
            present
      end;
      let name = !name in
      let make_row more =
        create_row ~fields ~more ~closed:(closed = Closed) ~fixed:None ~name
      in
      let more =
        if Btype.static_row (make_row (newvar ())) then newty Tnil else
        if policy = Univars then new_pre_univar () else newvar ()
      in
      let ty = newty (Tvariant (make_row more)) in
      ctyp (Ttyp_variant (tfields, closed, present)) ty
  | Ptyp_poly(vars, st) ->
      let vars = List.map (fun v -> v.txt) vars in
      let new_univars, cty =
        with_local_level begin fun () ->
          let new_univars = make_poly_univars vars in
          let old_univars = !univars in
          univars := new_univars @ !univars;
          let cty = transl_type env policy st in
          univars := old_univars;
          (new_univars, cty)
        end
        ~post:(fun (_,cty) -> generalize_ctyp cty)
      in
      let ty = cty.ctyp_type in
      let ty_list = check_poly_univars env styp.ptyp_loc new_univars in
      let ty_list = List.filter (fun v -> deep_occur v ty) ty_list in
      let ty' = Btype.newgenty (Tpoly(ty, ty_list)) in
      unify_var env (newvar()) ty';
      ctyp (Ttyp_poly (vars, cty)) ty'
  | Ptyp_package (p, l) ->
      let loc = styp.ptyp_loc in
      let l = sort_constraints_no_duplicates loc env l in
      let mty = create_package_mty loc p l in
      let mty =
        with_local_type_variable_scope (fun () -> !transl_modtype env mty) in
      let ptys = List.map (fun (s, pty) ->
                             s, transl_type env policy pty
                          ) l in
      let path = !transl_modtype_longident loc env p.txt in
      let ty = newty (Tpackage (path,
                       List.map (fun (s, cty) -> (s.txt, cty.ctyp_type)) ptys))
      in
      ctyp (Ttyp_package {
            pack_path = path;
            pack_type = mty.mty_type;
            pack_fields = ptys;
            pack_txt = p;
           }) ty
  | Ptyp_extension ext ->
      raise (Error_forward (Builtin_attributes.error_of_extension ext))

and transl_fields env policy o fields =
  let hfields = Hashtbl.create 17 in
  let add_typed_field loc l ty =
    try
      let ty' = Hashtbl.find hfields l in
      if is_equal env false [ty] [ty'] then () else
        try unify env ty ty'
        with Unify _trace ->
          raise(Error(loc, env, Method_mismatch (l, ty, ty')))
    with Not_found ->
      Hashtbl.add hfields l ty in
  let add_field {pof_desc; pof_loc; pof_attributes;} =
    let of_loc = pof_loc in
    let of_attributes = pof_attributes in
    let of_desc = match pof_desc with
    | Otag (s, ty1) -> begin
        let ty1 =
          Builtin_attributes.warning_scope of_attributes
            (fun () -> transl_type env policy (Ast_helper.Typ.force_poly ty1))
        in
        let field = OTtag (s, ty1) in
        add_typed_field ty1.ctyp_loc s.txt ty1.ctyp_type;
        field
      end
    | Oinherit sty -> begin
        let cty = transl_type env policy sty in
        let nm =
          match get_desc cty.ctyp_type with
            Tconstr(p, _, _) -> Some p
          | _                -> None in
        let t = expand_head env cty.ctyp_type in
        match get_desc t, nm with
          Tobject (tf, _), _
          when (match get_desc tf with Tfield _ | Tnil -> true | _ -> false) ->
            begin
              if opened_object t then
                raise (Error (sty.ptyp_loc, env, Opened_object nm));
              let rec iter_add ty =
                match get_desc ty with
                | Tfield (s, _k, ty1, ty2) ->
                    add_typed_field sty.ptyp_loc s ty1;
                    iter_add ty2
                | Tnil -> ()
                | _ -> assert false
              in
              iter_add tf;
              OTinherit cty
            end
        | Tvar _, Some p ->
            raise (Error (sty.ptyp_loc, env, Undefined_type_constructor p))
        | _ -> raise (Error (sty.ptyp_loc, env, Not_an_object t))
      end in
    { of_desc; of_loc; of_attributes; }
  in
  let object_fields = List.map add_field fields in
  let fields = Hashtbl.fold (fun s ty l -> (s, ty) :: l) hfields [] in
  let ty_init =
     match o, policy with
     | Closed, _ -> newty Tnil
     | Open, Univars -> new_pre_univar ()
     | Open, _ -> newvar () in
  let ty = List.fold_left (fun ty (s, ty') ->
      newty (Tfield (s, field_public, ty', ty))) ty_init fields in
  ty, object_fields


(* Make the rows "fixed" in this type, to make universal check easier *)
let rec make_fixed_univars ty =
  if Btype.try_mark_node ty then
    begin match get_desc ty with
    | Tvariant row ->
        let Row {fields; more; name; closed} = row_repr row in
        if Btype.is_Tunivar more then
          let fields =
            List.map
              (fun (s,f as p) -> match row_field_repr f with
                Reither (no_arg, tl, _m) ->
                  s, rf_either tl ~use_ext_of:f ~no_arg ~matched:true
              | _ -> p)
              fields
          in
          set_type_desc ty
            (Tvariant
               (create_row ~fields ~more ~name ~closed
                  ~fixed:(Some (Univar more))));
        Btype.iter_row make_fixed_univars row
    | _ ->
        Btype.iter_type_expr make_fixed_univars ty
    end

let make_fixed_univars ty =
  make_fixed_univars ty;
  Btype.unmark_type ty

let globalize_used_variables env fixed =
  let r = ref [] in
  TyVarMap.iter
    (fun name (ty, loc) ->
      let v = new_global_var () in
      let snap = Btype.snapshot () in
      if try unify env v ty; true with _ -> Btype.backtrack snap; false
      then try
        r := (loc, v,  TyVarMap.find name !type_variables) :: !r
      with Not_found ->
        if fixed && Btype.is_Tvar ty then
          raise(Error(loc, env, Unbound_type_variable ("'"^name)));
        let v2 = new_global_var () in
        r := (loc, v, v2) :: !r;
        type_variables := TyVarMap.add name v2 !type_variables)
    !used_variables;
  used_variables := TyVarMap.empty;
  fun () ->
    List.iter
      (function (loc, t1, t2) ->
        try unify env t1 t2 with Unify err ->
          raise (Error(loc, env, Type_mismatch err)))
      !r

let transl_simple_type env ?univars:(uvs=[]) fixed styp =
  univars := uvs; used_variables := TyVarMap.empty;
  let typ = transl_type env (if fixed then Fixed else Extensible) styp in
  globalize_used_variables env fixed ();
  make_fixed_univars typ.ctyp_type;
  typ

let transl_simple_type_univars env styp =
  univars := []; used_variables := TyVarMap.empty; pre_univars := [];
  let typ =
    with_local_level ~post:generalize_ctyp begin fun () ->
      let typ = transl_type env Univars styp in
      (* Only keep already global variables in used_variables *)
      let new_variables = !used_variables in
      used_variables := TyVarMap.empty;
      TyVarMap.iter
        (fun name p ->
          if TyVarMap.mem name !type_variables then
            used_variables := TyVarMap.add name p !used_variables)
        new_variables;
      globalize_used_variables env false ();
      typ
    end
  in
  let univs =
    List.fold_left
      (fun acc v ->
        match get_desc v with
          Tvar name when get_level v = Btype.generic_level ->
            set_type_desc v (Tunivar name); v :: acc
        | _ -> acc)
      [] !pre_univars
  in
  make_fixed_univars typ.ctyp_type;
    { typ with ctyp_type =
        instance (Btype.newgenty (Tpoly (typ.ctyp_type, univs))) }

let transl_simple_type_delayed env styp =
  univars := []; used_variables := TyVarMap.empty;
  let typ, force =
    with_local_level begin fun () ->
      let typ = transl_type env Extensible styp in
      make_fixed_univars typ.ctyp_type;
      (* This brings the used variables to the global level, but doesn't link
         them to their other occurrences just yet. This will be done when
         [force] is  called. *)
      let force = globalize_used_variables env false in
      (typ, force)
    end
    (* Generalize everything except the variables that were just globalized. *)
    ~post:(fun (typ,_) -> generalize_ctyp typ)
  in
  (typ, instance typ.ctyp_type, force)

let transl_type_scheme env styp =
  reset_type_variables();
  match styp.ptyp_desc with
  | Ptyp_poly (vars, st) ->
     let vars = List.map (fun v -> v.txt) vars in
     let univars, typ =
       with_local_level begin fun () ->
         let univars = make_poly_univars vars in
         let typ = transl_simple_type env ~univars true st in
         (univars, typ)
       end
       ~post:(fun (_,typ) -> generalize_ctyp typ)
     in
     let _ = instance_poly_univars env styp.ptyp_loc univars in
     { ctyp_desc = Ttyp_poly (vars, typ);
       ctyp_type = typ.ctyp_type;
       ctyp_env = env;
       ctyp_loc = styp.ptyp_loc;
       ctyp_attributes = styp.ptyp_attributes }
  | _ ->
      with_local_level (fun () -> transl_simple_type env false styp)
        ~post:generalize_ctyp


(* Error report *)

open Format
open Printtyp

let report_error env ppf = function
  | Unbound_type_variable name ->
      let add_name name _ l = if name = "_" then l else ("'" ^ name) :: l in
      let names = TyVarMap.fold add_name !type_variables [] in
    fprintf ppf "The type variable %s is unbound in this type declaration.@ %a"
      name
      did_you_mean (fun () -> Misc.spellcheck names name )
  | Undefined_type_constructor p ->
    fprintf ppf "The type constructor@ %a@ is not yet completely defined"
      path p
  | Type_arity_mismatch(lid, expected, provided) ->
    fprintf ppf
      "@[The type constructor %a@ expects %i argument(s),@ \
        but is here applied to %i argument(s)@]"
      longident lid expected provided
  | Bound_type_variable name ->
    fprintf ppf "Already bound type parameter %a" Pprintast.tyvar name
  | Recursive_type ->
    fprintf ppf "This type is recursive"
  | Unbound_row_variable lid ->
      (* we don't use "spellcheck" here: this error is not raised
         anywhere so it's unclear how it should be handled *)
      fprintf ppf "Unbound row variable in #%a" longident lid
  | Type_mismatch trace ->
      Printtyp.report_unification_error ppf Env.empty trace
        (function ppf ->
           fprintf ppf "This type")
        (function ppf ->
           fprintf ppf "should be an instance of type")
  | Alias_type_mismatch trace ->
      Printtyp.report_unification_error ppf Env.empty trace
        (function ppf ->
           fprintf ppf "This alias is bound to type")
        (function ppf ->
           fprintf ppf "but is used as an instance of type")
  | Present_has_conjunction l ->
      fprintf ppf "The present constructor %s has a conjunctive type" l
  | Present_has_no_type l ->
      fprintf ppf
        "@[<v>@[The constructor %s is missing from the upper bound@ \
         (between '<'@ and '>')@ of this polymorphic variant@ \
         but is present in@ its lower bound (after '>').@]@,\
         @[@{<hint>Hint@}: Either add `%s in the upper bound,@ \
         or remove it@ from the lower bound.@]@]"
         l l
  | Constructor_mismatch (ty, ty') ->
      wrap_printing_env ~error:true env (fun ()  ->
        Printtyp.prepare_for_printing [ty; ty'];
        fprintf ppf "@[<hov>%s %a@ %s@ %a@]"
          "This variant type contains a constructor"
          !Oprint.out_type (tree_of_typexp Type ty)
          "which should be"
           !Oprint.out_type (tree_of_typexp Type ty'))
  | Not_a_variant ty ->
      fprintf ppf
        "@[The type %a@ does not expand to a polymorphic variant type@]"
        Printtyp.type_expr ty;
      begin match get_desc ty with
        | Tvar (Some s) ->
           (* PR#7012: help the user that wrote 'Foo instead of `Foo *)
           Misc.did_you_mean ppf (fun () -> ["`" ^ s])
        | _ -> ()
      end
  | Variant_tags (lab1, lab2) ->
      fprintf ppf
        "@[Variant tags `%s@ and `%s have the same hash value.@ %s@]"
        lab1 lab2 "Change one of them."
  | Invalid_variable_name name ->
      fprintf ppf "The type variable name %s is not allowed in programs" name
  | Cannot_quantify (name, v) ->
      fprintf ppf
        "@[<hov>The universal type variable %a cannot be generalized:@ "
        Pprintast.tyvar name;
      if Btype.is_Tvar v then
        fprintf ppf "it escapes its scope"
      else if Btype.is_Tunivar v then
        fprintf ppf "it is already bound to another variable"
      else
        fprintf ppf "it is bound to@ %a" Printtyp.type_expr v;
      fprintf ppf ".@]";
  | Multiple_constraints_on_type s ->
      fprintf ppf "Multiple constraints for type %a" longident s
  | Method_mismatch (l, ty, ty') ->
      wrap_printing_env ~error:true env (fun ()  ->
        fprintf ppf "@[<hov>Method '%s' has type %a,@ which should be %a@]"
          l Printtyp.type_expr ty Printtyp.type_expr ty')
  | Opened_object nm ->
      fprintf ppf
        "Illegal open object type%a"
        (fun ppf -> function
             Some p -> fprintf ppf "@ %a" path p
           | None -> fprintf ppf "") nm
  | Not_an_object ty ->
      fprintf ppf "@[The type %a@ is not an object type@]"
        Printtyp.type_expr ty

let () =
  Location.register_error_of_exn
    (function
      | Error (loc, env, err) ->
        Some (Location.error_of_printer ~loc (report_error env) err)
      | Error_forward err ->
        Some err
      | _ ->
        None
    )