rust/src/comp/middle/ty.rs

import std._str;
import std._uint;
import std._vec;
import std.Box;
import std.UFind;
import std.map;
import std.map.hashmap;
import std.option;
import std.option.none;
import std.option.some;

import driver.session;
import front.ast;
import front.ast.mutability;
import front.creader;
import middle.metadata;
import util.common;
import util.common.new_def_hash;
import util.common.span;
import util.typestate_ann.ts_ann;

// Data types

type arg = rec(ast.mode mode, t ty);
type field = rec(ast.ident ident, mt mt);
type method = rec(ast.proto proto,
                  ast.ident ident,
                  vec[arg] inputs,
                  t output);

type mt = rec(t ty, ast.mutability mut);

// Convert from method type to function type.  Pretty easy; we just drop
// 'ident'.
fn method_ty_to_fn_ty(@type_store tystore, method m) -> t {
    ret mk_fn(tystore, m.proto, m.inputs, m.output);
}

// Never construct these manually. These are interned. Also don't assume that
// you can access the fields of this type directly; soon these will just be
// uints, and that won't work anymore.
//
// TODO: It'd be really nice to be able to hide this definition from the
// outside world, to enforce the above invariants.
type raw_t = rec(sty struct, option.t[str] cname, uint hash);
type t = @raw_t;

// NB: If you change this, you'll probably want to change the corresponding
// AST structure in front/ast.rs as well.
tag sty {
    ty_nil;
    ty_bool;
    ty_int;
    ty_float;
    ty_uint;
    ty_machine(util.common.ty_mach);
    ty_char;
    ty_str;
    ty_tag(ast.def_id, vec[t]);
    ty_box(mt);
    ty_vec(mt);
    ty_port(t);
    ty_chan(t);
    ty_task;
    ty_tup(vec[mt]);
    ty_rec(vec[field]);
    ty_fn(ast.proto, vec[arg], t);
    ty_native_fn(ast.native_abi, vec[arg], t);
    ty_obj(vec[method]);
    ty_var(int);                                    // ephemeral type var
    ty_local(ast.def_id);                           // type of a local var
    ty_param(uint);                                 // fn/tag type param
    ty_bound_param(uint);                           // bound param, only paths
    ty_type;
    ty_native;
    // TODO: ty_fn_arg(t), for a possibly-aliased function argument
}

// Data structures used in type unification

type unify_handler = obj {
    fn resolve_local(ast.def_id id) -> option.t[t];
    fn record_local(ast.def_id id, t ty);  // TODO: -> Unify.result
    fn record_param(uint index, t binding) -> Unify.result;
};

tag type_err {
    terr_mismatch;
    terr_box_mutability;
    terr_vec_mutability;
    terr_tuple_size(uint, uint);
    terr_tuple_mutability;
    terr_record_size(uint, uint);
    terr_record_mutability;
    terr_record_fields(ast.ident,ast.ident);
    terr_meth_count;
    terr_obj_meths(ast.ident,ast.ident);
    terr_arg_count;
}


type ty_param_count_and_ty = tup(uint, t);
type type_cache = hashmap[ast.def_id,ty_param_count_and_ty];


type type_store = hashmap[t,t];

fn mk_type_store() -> @type_store {
    auto hasher = hash_ty;
    auto eqer = eq_ty_full;
    ret @map.mk_hashmap[t,t](hasher, eqer);
}

// Type constructors

// These are private constructors to this module. External users should always
// use the mk_foo() functions below.
fn gen_ty(@type_store tystore, &sty st) -> t {
    ret gen_ty_full(tystore, st, none[str]);
}

fn gen_ty_full(@type_store tystore, &sty st, option.t[str] cname) -> t {
    auto h = hash_type_structure(st);
    auto new_type = @rec(struct=st, cname=cname, hash=h);

    // Is it interned?
    alt (tystore.find(new_type)) {
        case (some[t](?typ)) {
            ret typ;
        }
        case (none[t]) {
            // Nope. Insert it and return.
            tystore.insert(new_type, new_type);
            ret new_type;
        }
    }
}

fn mk_nil(@type_store ts) -> t          { ret gen_ty(ts, ty_nil); }
fn mk_bool(@type_store ts) -> t         { ret gen_ty(ts, ty_bool); }
fn mk_int(@type_store ts) -> t          { ret gen_ty(ts, ty_int); }
fn mk_float(@type_store ts) -> t        { ret gen_ty(ts, ty_float); }
fn mk_uint(@type_store ts) -> t         { ret gen_ty(ts, ty_uint); }

fn mk_mach(@type_store ts, util.common.ty_mach tm) -> t {
    ret gen_ty(ts, ty_machine(tm));
}

fn mk_char(@type_store ts) -> t         { ret gen_ty(ts, ty_char); }
fn mk_str(@type_store ts) -> t          { ret gen_ty(ts, ty_str); }

fn mk_tag(@type_store ts, ast.def_id did, vec[t] tys) -> t {
    ret gen_ty(ts, ty_tag(did, tys));
}

fn mk_box(@type_store ts, mt tm) -> t {
    ret gen_ty(ts, ty_box(tm));
}

fn mk_imm_box(@type_store ts, t ty) -> t {
    ret mk_box(ts, rec(ty=ty, mut=ast.imm));
}

fn mk_vec(@type_store ts, mt tm) -> t   { ret gen_ty(ts, ty_vec(tm)); }
fn mk_port(@type_store ts, t ty) -> t   { ret gen_ty(ts, ty_port(ty)); }
fn mk_chan(@type_store ts, t ty) -> t   { ret gen_ty(ts, ty_chan(ty)); }
fn mk_task(@type_store ts) -> t         { ret gen_ty(ts, ty_task); }

fn mk_tup(@type_store ts, vec[mt] tms) -> t {
    ret gen_ty(ts, ty_tup(tms));
}

fn mk_imm_tup(@type_store ts, vec[t] tys) -> t {
    // TODO: map
    let vec[ty.mt] mts = vec();
    for (t typ in tys) {
        mts += vec(rec(ty=typ, mut=ast.imm));
    }
    ret mk_tup(ts, mts);
}

fn mk_rec(@type_store ts, vec[field] fs) -> t {
    ret gen_ty(ts, ty_rec(fs));
}

fn mk_fn(@type_store ts, ast.proto proto, vec[arg] args, t ty) -> t {
    ret gen_ty(ts, ty_fn(proto, args, ty));
}

fn mk_native_fn(@type_store ts, ast.native_abi abi, vec[arg] args, t ty)
        -> t {
    ret gen_ty(ts, ty_native_fn(abi, args, ty));
}

fn mk_obj(@type_store ts, vec[method] meths) -> t {
    ret gen_ty(ts, ty_obj(meths));
}

fn mk_var(@type_store ts, int v) -> t    { ret gen_ty(ts, ty_var(v)); }

fn mk_local(@type_store ts, ast.def_id did) -> t {
    ret gen_ty(ts, ty_local(did));
}

fn mk_param(@type_store ts, uint n) -> t {
    ret gen_ty(ts, ty_param(n));
}

fn mk_bound_param(@type_store ts, uint n) -> t {
    ret gen_ty(ts, ty_bound_param(n));
}

fn mk_type(@type_store ts) -> t          { ret gen_ty(ts, ty_type); }
fn mk_native(@type_store ts) -> t        { ret gen_ty(ts, ty_native); }


// Returns the one-level-deep type structure of the given type.
fn struct(@type_store tystore, t typ) -> sty { ret typ.struct; }

// Returns the canonical name of the given type.
fn cname(@type_store tystore, t typ) -> option.t[str] { ret typ.cname; }


// Stringification

fn path_to_str(&ast.path pth) -> str {
    auto result = _str.connect(pth.node.idents,  ".");
    if (_vec.len[@ast.ty](pth.node.types) > 0u) {
        auto f = pretty.pprust.ty_to_str;
        result += "[";
        result += _str.connect(_vec.map[@ast.ty,str](f, pth.node.types), ",");
        result += "]";
    }
    ret result;
}

fn ty_to_str(@type_store ts, &t typ) -> str {

    fn fn_input_to_str(@type_store tystore,
                       &rec(ast.mode mode, t ty) input) -> str {
        auto s;
        if (mode_is_alias(input.mode)) {
            s = "&";
        } else {
            s = "";
        }

        ret s + ty_to_str(tystore, input.ty);
    }

    fn fn_to_str(@type_store tystore,
                 ast.proto proto,
                 option.t[ast.ident] ident,
                 vec[arg] inputs, t output) -> str {
            auto f = bind fn_input_to_str(tystore, _);

            auto s;
            alt (proto) {
                case (ast.proto_iter) {
                    s = "iter";
                }
                case (ast.proto_fn) {
                    s = "fn";
                }
            }

            alt (ident) {
                case (some[ast.ident](?i)) {
                    s += " ";
                    s += i;
                }
                case (_) { }
            }

            s += "(";
            s += _str.connect(_vec.map[arg,str](f, inputs), ", ");
            s += ")";

            if (struct(tystore, output) != ty_nil) {
                s += " -> " + ty_to_str(tystore, output);
            }
            ret s;
    }

    fn method_to_str(@type_store tystore, &method m) -> str {
        ret fn_to_str(tystore, m.proto, some[ast.ident](m.ident),
                      m.inputs, m.output) + ";";
    }

    fn field_to_str(@type_store tystore, &field f) -> str {
        ret mt_to_str(tystore, f.mt) + " " + f.ident;
    }

    fn mt_to_str(@type_store tystore, &mt m) -> str {
        auto mstr;
        alt (m.mut) {
            case (ast.mut)       { mstr = "mutable "; }
            case (ast.imm)       { mstr = "";         }
            case (ast.maybe_mut) { mstr = "mutable? "; }
        }

        ret mstr + ty_to_str(tystore, m.ty);
    }

    auto s = "";
    alt (struct(ts, typ)) {
        case (ty_native)       { s += "native";                         }
        case (ty_nil)          { s += "()";                             }
        case (ty_bool)         { s += "bool";                           }
        case (ty_int)          { s += "int";                            }
        case (ty_float)        { s += "float";                          }
        case (ty_uint)         { s += "uint";                           }
        case (ty_machine(?tm)) { s += common.ty_mach_to_str(tm);        }
        case (ty_char)         { s += "char";                           }
        case (ty_str)          { s += "str";                            }
        case (ty_box(?tm))     { s += "@" + mt_to_str(ts, tm);          }
        case (ty_vec(?tm))     { s += "vec[" + mt_to_str(ts, tm) + "]"; }
        case (ty_port(?t))     { s += "port[" + ty_to_str(ts, t) + "]"; }
        case (ty_chan(?t))     { s += "chan[" + ty_to_str(ts, t) + "]"; }
        case (ty_type)         { s += "type";                           }

        case (ty_tup(?elems)) {
            auto f = bind mt_to_str(ts, _);
            auto strs = _vec.map[mt,str](f, elems);
            s += "tup(" + _str.connect(strs, ",") + ")";
        }

        case (ty_rec(?elems)) {
            auto f = bind field_to_str(ts, _);
            auto strs = _vec.map[field,str](f, elems);
            s += "rec(" + _str.connect(strs, ",") + ")";
        }

        case (ty_tag(?id, ?tps)) {
            // The user should never see this if the cname is set properly!
            s += "<tag#" + util.common.istr(id._0) + ":" +
                util.common.istr(id._1) + ">";
            if (_vec.len[t](tps) > 0u) {
                auto f = bind ty_to_str(ts, _);
                auto strs = _vec.map[t,str](f, tps);
                s += "[" + _str.connect(strs, ",") + "]";
            }
        }

        case (ty_fn(?proto, ?inputs, ?output)) {
            s += fn_to_str(ts, proto, none[ast.ident], inputs, output);
        }

        case (ty_native_fn(_, ?inputs, ?output)) {
            s += fn_to_str(ts, ast.proto_fn, none[ast.ident], inputs, output);
        }

        case (ty_obj(?meths)) {
            alt (cname(ts, typ)) {
                case (some[str](?cs)) {
                    s += cs;
                }
                case (_) {
                    auto f = bind method_to_str(ts, _);
                    auto m = _vec.map[method,str](f, meths);
                    s += "obj {\n\t" + _str.connect(m, "\n\t") + "\n}";
                }
            }
        }

        case (ty_var(?v)) {
            s += "<T" + util.common.istr(v) + ">";
        }

        case (ty_local(?id)) {
            s += "<L" + util.common.istr(id._0) + ":" +
                util.common.istr(id._1) + ">";
        }

        case (ty_param(?id)) {
            s += "'" + _str.unsafe_from_bytes(vec(('a' as u8) + (id as u8)));
        }

        case (ty_bound_param(?id)) {
            s += "''" + _str.unsafe_from_bytes(vec(('a' as u8) + (id as u8)));
        }
    }

    ret s;
}

// Type folds

type ty_walk = fn(t);

fn walk_ty(@type_store tystore, ty_walk walker, t ty) {
    alt (struct(tystore, ty)) {
        case (ty_nil)           { /* no-op */ }
        case (ty_bool)          { /* no-op */ }
        case (ty_int)           { /* no-op */ }
        case (ty_uint)          { /* no-op */ }
        case (ty_float)         { /* no-op */ }
        case (ty_machine(_))    { /* no-op */ }
        case (ty_char)          { /* no-op */ }
        case (ty_str)           { /* no-op */ }
        case (ty_type)          { /* no-op */ }
        case (ty_native)        { /* no-op */ }
        case (ty_box(?tm))      { walk_ty(tystore, walker, tm.ty); }
        case (ty_vec(?tm))      { walk_ty(tystore, walker, tm.ty); }
        case (ty_port(?subty))  { walk_ty(tystore, walker, subty); }
        case (ty_chan(?subty))  { walk_ty(tystore, walker, subty); }
        case (ty_tag(?tid, ?subtys)) {
            for (t subty in subtys) {
                walk_ty(tystore, walker, subty);
            }
        }
        case (ty_tup(?mts)) {
            for (mt tm in mts) {
                walk_ty(tystore, walker, tm.ty);
            }
        }
        case (ty_rec(?fields)) {
            for (field fl in fields) {
                walk_ty(tystore, walker, fl.mt.ty);
            }
        }
        case (ty_fn(?proto, ?args, ?ret_ty)) {
            for (arg a in args) {
                walk_ty(tystore, walker, a.ty);
            }
            walk_ty(tystore, walker, ret_ty);
        }
        case (ty_native_fn(?abi, ?args, ?ret_ty)) {
            for (arg a in args) {
                walk_ty(tystore, walker, a.ty);
            }
            walk_ty(tystore, walker, ret_ty);
        }
        case (ty_obj(?methods)) {
            let vec[method] new_methods = vec();
            for (method m in methods) {
                for (arg a in m.inputs) {
                    walk_ty(tystore, walker, a.ty);
                }
                walk_ty(tystore, walker, m.output);
            }
        }
        case (ty_var(_))         { /* no-op */ }
        case (ty_local(_))       { /* no-op */ }
        case (ty_param(_))       { /* no-op */ }
        case (ty_bound_param(_)) { /* no-op */ }
    }

    walker(ty);
}

type ty_fold = fn(t) -> t;

fn fold_ty(@type_store tystore, ty_fold fld, t ty_0) -> t {
    auto ty = ty_0;
    alt (struct(tystore, ty)) {
        case (ty_nil)           { /* no-op */ }
        case (ty_bool)          { /* no-op */ }
        case (ty_int)           { /* no-op */ }
        case (ty_uint)          { /* no-op */ }
        case (ty_float)         { /* no-op */ }
        case (ty_machine(_))    { /* no-op */ }
        case (ty_char)          { /* no-op */ }
        case (ty_str)           { /* no-op */ }
        case (ty_type)          { /* no-op */ }
        case (ty_native)        { /* no-op */ }
        case (ty_box(?tm)) {
            ty = copy_cname(tystore,
                            mk_box(tystore,
                                   rec(ty=fold_ty(tystore, fld, tm.ty),
                                       mut=tm.mut)), ty);
        }
        case (ty_vec(?tm)) {
            ty = copy_cname(tystore,
                mk_vec(tystore, rec(ty=fold_ty(tystore, fld, tm.ty),
                                    mut=tm.mut)), ty);
        }
        case (ty_port(?subty)) {
            ty = copy_cname(tystore,
                mk_port(tystore, fold_ty(tystore, fld, subty)), ty);
        }
        case (ty_chan(?subty)) {
            ty = copy_cname(tystore,
                mk_chan(tystore, fold_ty(tystore, fld, subty)), ty);
        }
        case (ty_tag(?tid, ?subtys)) {
            let vec[t] new_subtys = vec();
            for (t subty in subtys) {
                new_subtys += vec(fold_ty(tystore, fld, subty));
            }
            ty = copy_cname(tystore, mk_tag(tystore, tid, new_subtys), ty);
        }
        case (ty_tup(?mts)) {
            let vec[mt] new_mts = vec();
            for (mt tm in mts) {
                auto new_subty = fold_ty(tystore, fld, tm.ty);
                new_mts += vec(rec(ty=new_subty, mut=tm.mut));
            }
            ty = copy_cname(tystore, mk_tup(tystore, new_mts), ty);
        }
        case (ty_rec(?fields)) {
            let vec[field] new_fields = vec();
            for (field fl in fields) {
                auto new_ty = fold_ty(tystore, fld, fl.mt.ty);
                auto new_mt = rec(ty=new_ty, mut=fl.mt.mut);
                new_fields += vec(rec(ident=fl.ident, mt=new_mt));
            }
            ty = copy_cname(tystore, mk_rec(tystore, new_fields), ty);
        }
        case (ty_fn(?proto, ?args, ?ret_ty)) {
            let vec[arg] new_args = vec();
            for (arg a in args) {
                auto new_ty = fold_ty(tystore, fld, a.ty);
                new_args += vec(rec(mode=a.mode, ty=new_ty));
            }
            ty = copy_cname(tystore, mk_fn(tystore, proto, new_args,
                                  fold_ty(tystore, fld, ret_ty)),
                            ty);
        }
        case (ty_native_fn(?abi, ?args, ?ret_ty)) {
            let vec[arg] new_args = vec();
            for (arg a in args) {
                auto new_ty = fold_ty(tystore, fld, a.ty);
                new_args += vec(rec(mode=a.mode, ty=new_ty));
            }
            ty = copy_cname(tystore, mk_native_fn(tystore, abi, new_args,
                                         fold_ty(tystore, fld, ret_ty)),
                            ty);
        }
        case (ty_obj(?methods)) {
            let vec[method] new_methods = vec();
            for (method m in methods) {
                let vec[arg] new_args = vec();
                for (arg a in m.inputs) {
                    new_args += vec(rec(mode=a.mode,
                                        ty=fold_ty(tystore, fld, a.ty)));
                }
                new_methods += vec(rec(proto=m.proto, ident=m.ident,
                                       inputs=new_args,
                                       output=fold_ty(tystore, fld,
                                                      m.output)));
            }
            ty = copy_cname(tystore, mk_obj(tystore, new_methods), ty);
        }
        case (ty_var(_))         { /* no-op */ }
        case (ty_local(_))       { /* no-op */ }
        case (ty_param(_))       { /* no-op */ }
        case (ty_bound_param(_)) { /* no-op */ }
    }

    ret fld(ty);
}

// Type utilities

fn rename(@type_store tystore, t typ, str new_cname) -> t {
    ret gen_ty_full(tystore, struct(tystore, typ), some[str](new_cname));
}

// Returns a type with the structural part taken from `struct_ty` and the
// canonical name from `cname_ty`.
fn copy_cname(@type_store tystore, t struct_ty, t cname_ty) -> t {
    ret gen_ty_full(tystore, struct(tystore, struct_ty), cname_ty.cname);
}

// FIXME: remove me when == works on these tags.
fn mode_is_alias(ast.mode m) -> bool {
    alt (m) {
        case (ast.val) { ret false; }
        case (ast.alias) { ret true; }
    }
    fail;
}

fn type_is_nil(@type_store tystore, t ty) -> bool {
    alt (struct(tystore, ty)) {
        case (ty_nil) { ret true; }
        case (_) { ret false; }
    }
    fail;
}

fn type_is_bool(@type_store tystore, t ty) -> bool {
    alt (struct(tystore, ty)) {
        case (ty_bool) { ret true; }
        case (_) { ret false; }
    }
}


fn type_is_structural(@type_store tystore, t ty) -> bool {
    alt (struct(tystore, ty)) {
        case (ty_tup(_))    { ret true; }
        case (ty_rec(_))    { ret true; }
        case (ty_tag(_,_))  { ret true; }
        case (ty_fn(_,_,_)) { ret true; }
        case (ty_obj(_))    { ret true; }
        case (_)            { ret false; }
    }
    fail;
}

fn type_is_sequence(@type_store tystore, t ty) -> bool {
    alt (struct(tystore, ty)) {
        case (ty_str)    { ret true; }
        case (ty_vec(_))    { ret true; }
        case (_)            { ret false; }
    }
    fail;
}

fn sequence_element_type(@type_store tystore, t ty) -> t {
    alt (struct(tystore, ty)) {
        case (ty_str)      { ret mk_mach(tystore, common.ty_u8); }
        case (ty_vec(?mt)) { ret mt.ty; }
    }
    fail;
}


fn type_is_tup_like(@type_store tystore, t ty) -> bool {
    alt (struct(tystore, ty)) {
        case (ty_box(_))    { ret true; }
        case (ty_tup(_))    { ret true; }
        case (ty_rec(_))    { ret true; }
        case (ty_tag(_,_))  { ret true; }
        case (_)            { ret false; }
    }
    fail;
}

fn get_element_type(@type_store tystore, t ty, uint i) -> t {
    check (type_is_tup_like(tystore, ty));
    alt (struct(tystore, ty)) {
        case (ty_tup(?mts)) {
            ret mts.(i).ty;
        }
        case (ty_rec(?flds)) {
            ret flds.(i).mt.ty;
        }
    }
    fail;
}

fn type_is_box(@type_store tystore, t ty) -> bool {
    alt (struct(tystore, ty)) {
        case (ty_box(_)) { ret true; }
        case (_) { ret false; }
    }
    fail;
}

fn type_is_boxed(@type_store tystore, t ty) -> bool {
    alt (struct(tystore, ty)) {
        case (ty_str) { ret true; }
        case (ty_vec(_)) { ret true; }
        case (ty_box(_)) { ret true; }
        case (ty_port(_)) { ret true; }
        case (ty_chan(_)) { ret true; }
        case (_) { ret false; }
    }
    fail;
}

fn type_is_scalar(@type_store tystore, t ty) -> bool {
    alt (struct(tystore, ty)) {
        case (ty_nil) { ret true; }
        case (ty_bool) { ret true; }
        case (ty_int) { ret true; }
        case (ty_float) { ret true; }
        case (ty_uint) { ret true; }
        case (ty_machine(_)) { ret true; }
        case (ty_char) { ret true; }
        case (ty_type) { ret true; }
        case (ty_native) { ret true; }
        case (_) { ret false; }
    }
    fail;
}

// FIXME: should we just return true for native types in
// type_is_scalar?
fn type_is_native(@type_store tystore, t ty) -> bool {
    alt (struct(tystore, ty)) {
        case (ty_native) { ret true; }
        case (_) { ret false; }
    }
    fail;
}

fn type_has_dynamic_size(@type_store tystore, t ty) -> bool {
    alt (struct(tystore, ty)) {
        case (ty_tup(?mts)) {
            auto i = 0u;
            while (i < _vec.len[mt](mts)) {
                if (type_has_dynamic_size(tystore, mts.(i).ty)) { ret true; }
                i += 1u;
            }
        }
        case (ty_rec(?fields)) {
            auto i = 0u;
            while (i < _vec.len[field](fields)) {
                if (type_has_dynamic_size(tystore, fields.(i).mt.ty)) {
                    ret true;
                }
                i += 1u;
            }
        }
        case (ty_tag(_, ?subtys)) {
            auto i = 0u;
            while (i < _vec.len[t](subtys)) {
                if (type_has_dynamic_size(tystore, subtys.(i))) { ret true; }
                i += 1u;
            }
        }
        case (ty_param(_)) { ret true; }
        case (_) { /* fall through */ }
    }
    ret false;
}

fn type_is_integral(@type_store tystore, t ty) -> bool {
    alt (struct(tystore, ty)) {
        case (ty_int) { ret true; }
        case (ty_uint) { ret true; }
        case (ty_machine(?m)) {
            alt (m) {
                case (common.ty_i8) { ret true; }
                case (common.ty_i16) { ret true; }
                case (common.ty_i32) { ret true; }
                case (common.ty_i64) { ret true; }

                case (common.ty_u8) { ret true; }
                case (common.ty_u16) { ret true; }
                case (common.ty_u32) { ret true; }
                case (common.ty_u64) { ret true; }
                case (_) { ret false; }
            }
        }
        case (ty_char) { ret true; }
        case (_) { ret false; }
    }
    fail;
}

fn type_is_fp(@type_store tystore, t ty) -> bool {
    alt (struct(tystore, ty)) {
        case (ty_machine(?tm)) {
            alt (tm) {
                case (common.ty_f32) { ret true; }
                case (common.ty_f64) { ret true; }
                case (_) { ret false; }
            }
        }
        case (ty_float) {
            ret true;
        }
        case (_) { ret false; }
    }
    fail;
}

fn type_is_signed(@type_store tystore, t ty) -> bool {
    alt (struct(tystore, ty)) {
        case (ty_int) { ret true; }
        case (ty_machine(?tm)) {
            alt (tm) {
                case (common.ty_i8) { ret true; }
                case (common.ty_i16) { ret true; }
                case (common.ty_i32) { ret true; }
                case (common.ty_i64) { ret true; }
                case (_) { ret false; }
            }
        }
        case (_) { ret false; }
    }
    fail;
}

fn type_param(@type_store tystore, t ty) -> option.t[uint] {
    alt (struct(tystore, ty)) {
        case (ty_param(?id)) { ret some[uint](id); }
        case (_)             { /* fall through */  }
    }
    ret none[uint];
}

fn def_to_str(ast.def_id did) -> str {
    ret #fmt("%d:%d", did._0, did._1);
}

// Type hashing. This function is private to this module (and slow); external
// users should use `hash_ty()` instead.
fn hash_type_structure(&sty st) -> uint {
    fn hash_uint(uint id, uint n) -> uint {
        auto h = id;
        h += h << 5u + n;
        ret h;
    }

    fn hash_def(uint id, ast.def_id did) -> uint {
        auto h = id;
        h += h << 5u + (did._0 as uint);
        h += h << 5u + (did._1 as uint);
        ret h;
    }

    fn hash_subty(uint id, t subty) -> uint {
        auto h = id;
        h += h << 5u + hash_ty(subty);
        ret h;
    }

    fn hash_fn(uint id, vec[arg] args, t rty) -> uint {
        auto h = id;
        for (arg a in args) {
            h += h << 5u + hash_ty(a.ty);
        }
        h += h << 5u + hash_ty(rty);
        ret h;
    }

    alt (st) {
        case (ty_nil) { ret 0u; }
        case (ty_bool) { ret 1u; }
        case (ty_int) { ret 2u; }
        case (ty_float) { ret 3u; }
        case (ty_uint) { ret 4u; }
        case (ty_machine(?tm)) {
            alt (tm) {
                case (common.ty_i8) { ret 5u; }
                case (common.ty_i16) { ret 6u; }
                case (common.ty_i32) { ret 7u; }
                case (common.ty_i64) { ret 8u; }

                case (common.ty_u8) { ret 9u; }
                case (common.ty_u16) { ret 10u; }
                case (common.ty_u32) { ret 11u; }
                case (common.ty_u64) { ret 12u; }

                case (common.ty_f32) { ret 13u; }
                case (common.ty_f64) { ret 14u; }
            }
        }
        case (ty_char) { ret 15u; }
        case (ty_str) { ret 16u; }
        case (ty_tag(?did, ?tys)) {
            auto h = hash_def(17u, did);
            for (t typ in tys) {
                h += h << 5u + hash_ty(typ);
            }
            ret h;
        }
        case (ty_box(?mt)) { ret hash_subty(18u, mt.ty); }
        case (ty_vec(?mt)) { ret hash_subty(19u, mt.ty); }
        case (ty_port(?typ)) { ret hash_subty(20u, typ); }
        case (ty_chan(?typ)) { ret hash_subty(21u, typ); }
        case (ty_task) { ret 22u; }
        case (ty_tup(?mts)) {
            auto h = 23u;
            for (mt tm in mts) {
                h += h << 5u + hash_ty(tm.ty);
            }
            ret h;
        }
        case (ty_rec(?fields)) {
            auto h = 24u;
            for (field f in fields) {
                h += h << 5u + hash_ty(f.mt.ty);
            }
            ret h;
        }
        case (ty_fn(_, ?args, ?rty)) { ret hash_fn(25u, args, rty); }
        case (ty_native_fn(_, ?args, ?rty)) { ret hash_fn(26u, args, rty); }
        case (ty_obj(?methods)) {
            auto h = 27u;
            for (method m in methods) {
                h += h << 5u + _str.hash(m.ident);
            }
            ret h;
        }
        case (ty_var(?v)) { ret hash_uint(28u, v as uint); }
        case (ty_local(?did)) { ret hash_def(29u, did); }
        case (ty_param(?pid)) { ret hash_uint(30u, pid); }
        case (ty_bound_param(?pid)) { ret hash_uint(31u, pid); }
        case (ty_type) { ret 32u; }
        case (ty_native) { ret 33u; }
    }
}

fn hash_ty(&t typ) -> uint { ret typ.hash; }


// Type equality. This function is private to this module (and slow); external
// users should use `eq_ty()` instead.
fn equal_type_structures(&sty a, &sty b) -> bool {
    fn equal_ty(t a, t b) -> bool { ret Box.ptr_eq[raw_t](a, b); }

    fn equal_proto(ast.proto a, ast.proto b) -> bool {
        alt (a) {
            case (ast.proto_iter) {
                alt (b) {
                    case (ast.proto_iter) { ret true; }
                    case (_) { ret false; }
                }
            }
            case (ast.proto_fn) {
                alt (b) {
                    case (ast.proto_fn) { ret true; }
                    case (_) { ret false; }
                }
            }
        }
    }

    fn equal_abi(ast.native_abi a, ast.native_abi b) -> bool {
        alt (a) {
            case (ast.native_abi_rust) {
                alt (b) {
                    case (ast.native_abi_rust) { ret true; }
                    case (_) { ret false; }
                }
            }
            case (ast.native_abi_cdecl) {
                alt (b) {
                    case (ast.native_abi_cdecl) { ret true; }
                    case (_) { ret false; }
                }
            }
            case (ast.native_abi_llvm) {
                alt (b) {
                    case (ast.native_abi_llvm) { ret true; }
                    case (_) { ret false; }
                }
            }
        }
    }

    fn equal_mut(ast.mutability a, ast.mutability b) -> bool {
        alt (a) {
            case (ast.mut) {
                alt (b) {
                    case (ast.mut) { ret true; }
                    case (_) { ret false; }
                }
            }
            case (ast.imm) {
                alt (b) {
                    case (ast.imm) { ret true; }
                    case (_) { ret false; }
                }
            }
            case (ast.maybe_mut) {
                alt (b) {
                    case (ast.maybe_mut) { ret true; }
                    case (_) { ret false; }
                }
            }
        }
    }

    fn equal_mode(ast.mode a, ast.mode b) -> bool {
        alt (a) {
            case (ast.val) {
                alt (b) {
                    case (ast.val) { ret true; }
                    case (_) { ret false; }
                }
            }
            case (ast.alias) {
                alt (b) {
                    case (ast.alias) { ret true; }
                    case (_) { ret false; }
                }
            }
        }
    }

    fn equal_mt(&mt a, &mt b) -> bool {
        ret equal_mut(a.mut, b.mut) && equal_ty(a.ty, b.ty);
    }

    fn equal_fn(vec[arg] args_a, t rty_a,
                vec[arg] args_b, t rty_b) -> bool {
        if (!equal_ty(rty_a, rty_b)) { ret false; }

        auto len = _vec.len[arg](args_a);
        if (len != _vec.len[arg](args_b)) { ret false; }
        auto i = 0u;
        while (i < len) {
            auto arg_a = args_a.(i); auto arg_b = args_b.(i);
            if (!equal_mode(arg_a.mode, arg_b.mode) ||
                    !equal_ty(arg_a.ty, arg_b.ty)) {
                ret false;
            }
            i += 1u;
        }
        ret true;
    }

    fn equal_def(ast.def_id did_a, ast.def_id did_b) -> bool {
        ret did_a._0 == did_b._0 && did_a._1 == did_b._1;
    }

    alt (a) {
        case (ty_nil) {
            alt (b) {
                case (ty_nil) { ret true; }
                case (_) { ret false; }
            }
        }
        case (ty_bool) {
            alt (b) {
                case (ty_bool) { ret true; }
                case (_) { ret false; }
            }
        }
        case (ty_int) {
            alt (b) {
                case (ty_int) { ret true; }
                case (_) { ret false; }
            }
        }
        case (ty_float) {
            alt (b) {
                case (ty_float) { ret true; }
                case (_) { ret false; }
            }
        }
        case (ty_uint) {
            alt (b) {
                case (ty_uint) { ret true; }
                case (_) { ret false; }
            }
        }
        case (ty_machine(?tm_a)) {
            alt (b) {
                case (ty_machine(?tm_b)) {
                    ret hash_type_structure(a) == hash_type_structure(b);
                }
                case (_) { ret false; }
            }
        }
        case (ty_char) {
            alt (b) {
                case (ty_char) { ret true; }
                case (_) { ret false; }
            }
        }
        case (ty_str) {
            alt (b) {
                case (ty_str) { ret true; }
                case (_) { ret false; }
            }
        }
        case (ty_tag(?id_a, ?tys_a)) {
            alt (b) {
                case (ty_tag(?id_b, ?tys_b)) {
                    if (!equal_def(id_a, id_b)) { ret false; }

                    auto len = _vec.len[t](tys_a);
                    if (len != _vec.len[t](tys_b)) { ret false; }
                    auto i = 0u;
                    while (i < len) {
                        if (!equal_ty(tys_a.(i), tys_b.(i))) { ret false; }
                        i += 1u;
                    }
                    ret true;
                }
                case (_) { ret false; }
            }
        }
        case (ty_box(?mt_a)) {
            alt (b) {
                case (ty_box(?mt_b)) { ret equal_mt(mt_a, mt_b); }
                case (_) { ret false; }
            }
        }
        case (ty_vec(?mt_a)) {
            alt (b) {
                case (ty_vec(?mt_b)) { ret equal_mt(mt_a, mt_b); }
                case (_) { ret false; }
            }
        }
        case (ty_port(?t_a)) {
            alt (b) {
                case (ty_port(?t_b)) { ret equal_ty(t_a, t_b); }
                case (_) { ret false; }
            }
        }
        case (ty_chan(?t_a)) {
            alt (b) {
                case (ty_chan(?t_b)) { ret equal_ty(t_a, t_b); }
                case (_) { ret false; }
            }
        }
        case (ty_task) {
            alt (b) {
                case (ty_task) { ret true; }
                case (_) { ret false; }
            }
        }
        case (ty_tup(?mts_a)) {
            alt (b) {
                case (ty_tup(?mts_b)) {
                    auto len = _vec.len[mt](mts_a);
                    if (len != _vec.len[mt](mts_b)) { ret false; }
                    auto i = 0u;
                    while (i < len) {
                        if (!equal_mt(mts_a.(i), mts_b.(i))) { ret false; }
                        i += 1u;
                    }
                    ret true;
                }
                case (_) { ret false; }
            }
        }
        case (ty_rec(?flds_a)) {
            alt (b) {
                case (ty_rec(?flds_b)) {
                    auto len = _vec.len[field](flds_a);
                    if (len != _vec.len[field](flds_b)) { ret false; }
                    auto i = 0u;
                    while (i < len) {
                        auto fld_a = flds_a.(i); auto fld_b = flds_b.(i);
                        if (!_str.eq(fld_a.ident, fld_b.ident) ||
                                !equal_mt(fld_a.mt, fld_b.mt)) {
                            ret false;
                        }
                        i += 1u;
                    }
                    ret true;
                }
                case (_) { ret false; }
            }
        }
        case (ty_fn(?p_a, ?args_a, ?rty_a)) {
            alt (b) {
                case (ty_fn(?p_b, ?args_b, ?rty_b)) {
                    ret equal_proto(p_a, p_b) &&
                        equal_fn(args_a, rty_a, args_b, rty_b);
                }
                case (_) { ret false; }
            }
        }
        case (ty_native_fn(?abi_a, ?args_a, ?rty_a)) {
            alt (b) {
                case (ty_native_fn(?abi_b, ?args_b, ?rty_b)) {
                    ret equal_abi(abi_a, abi_b) &&
                        equal_fn(args_a, rty_a, args_b, rty_b);
                }
                case (_) { ret false; }
            }
        }
        case (ty_obj(?methods_a)) {
            alt (b) {
                case (ty_obj(?methods_b)) {
                    auto len = _vec.len[method](methods_a);
                    if (len != _vec.len[method](methods_b)) { ret false; }
                    auto i = 0u;
                    while (i < len) {
                        auto m_a = methods_a.(i); auto m_b = methods_b.(i);
                        if (!equal_proto(m_a.proto, m_b.proto) ||
                                !_str.eq(m_a.ident, m_b.ident) ||
                                !equal_fn(m_a.inputs, m_a.output,
                                          m_b.inputs, m_b.output)) {
                            ret false;
                        }
                        i += 1u;
                    }
                    ret true;
                }
                case (_) { ret false; }
            }
        }
        case (ty_var(?v_a)) {
            alt (b) {
                case (ty_var(?v_b)) { ret v_a == v_b; }
                case (_) { ret false; }
            }
        }
        case (ty_local(?did_a)) {
            alt (b) {
                case (ty_local(?did_b)) { ret equal_def(did_a, did_b); }
                case (_) { ret false; }
            }
        }
        case (ty_param(?pid_a)) {
            alt (b) {
                case (ty_param(?pid_b)) { ret pid_a == pid_b; }
                case (_) { ret false; }
            }
        }
        case (ty_bound_param(?pid_a)) {
            alt (b) {
                case (ty_bound_param(?pid_b)) { ret pid_a == pid_b; }
                case (_) { ret false; }
            }
        }
        case (ty_type) {
            alt (b) {
                case (ty_type) { ret true; }
                case (_) { ret false; }
            }
        }
        case (ty_native) {
            alt (b) {
                case (ty_native) { ret true; }
                case (_) { ret false; }
            }
        }
    }
}

// An expensive type equality function. This function is private to this
// module.
fn eq_ty_full(&t a, &t b) -> bool {
    // Check hashes (fast path).
    if (a.hash != b.hash) {
        ret false;
    }

    // Check canonical names.
    alt (a.cname) {
        case (none[str]) {
            alt (b.cname) {
                case (none[str]) { /* ok */ }
                case (_) { ret false; }
            }
        }
        case (some[str](?s_a)) {
            alt (b.cname) {
                case (some[str](?s_b)) {
                    if (!_str.eq(s_a, s_b)) { ret false; }
                }
                case (_) { ret false; }
            }
        }
    }

    // Check structures.
    ret equal_type_structures(a.struct, b.struct);
}

// This is the equality function the public should use. It works as long as
// the types are interned.
fn eq_ty(&t a, &t b) -> bool { ret Box.ptr_eq[raw_t](a, b); }


fn ann_to_type(&ast.ann ann) -> t {
    alt (ann) {
        case (ast.ann_none) {
            log_err "ann_to_type() called on node with no type";
            fail;
        }
        case (ast.ann_type(?ty, _, _)) {
            ret ty;
        }
    }
}

fn ann_to_type_params(&ast.ann ann) -> vec[t] {
    alt (ann) {
        case (ast.ann_none) {
            log_err "ann_to_type_params() called on node with no type params";
            fail;
        }
        case (ast.ann_type(_, ?tps, _)) {
            alt (tps) {
                case (none[vec[t]]) {
                    let vec[t] result = vec();
                    ret result;
                }
                case (some[vec[t]](?tps)) { ret tps; }
            }
        }
    }
}

// Returns the type of an annotation, with type parameter substitutions
// performed if applicable.
fn ann_to_monotype(@type_store tystore, ast.ann a) -> t {
    // TODO: Refactor to use recursive pattern matching when we're more
    // confident that it works.
    alt (a) {
        case (ast.ann_none) {
            log_err "ann_to_monotype() called on expression with no type!";
            fail;
        }
        case (ast.ann_type(?typ, ?tps_opt, _)) {
            alt (tps_opt) {
                case (none[vec[t]]) { ret typ; }
                case (some[vec[t]](?tps)) {
                    ret substitute_type_params(tystore, tps, typ);
                }
            }
        }
    }
}

// Turns a type into an ann_type, using defaults for other fields.
fn triv_ann(t typ) -> ast.ann {
    ret ast.ann_type(typ, none[vec[t]], none[@ts_ann]);
}

// Returns the number of distinct type parameters in the given type.
fn count_ty_params(@type_store tystore, t ty) -> uint {
    fn counter(@type_store tystore, @mutable vec[uint] param_indices, t ty) {
        alt (struct(tystore, ty)) {
            case (ty_param(?param_idx)) {
                auto seen = false;
                for (uint other_param_idx in *param_indices) {
                    if (param_idx == other_param_idx) {
                        seen = true;
                    }
                }
                if (!seen) {
                    *param_indices += vec(param_idx);
                }
            }
            case (_) { /* fall through */ }
        }
    }

    let vec[uint] v = vec();    // FIXME: typechecker botch
    let @mutable vec[uint] param_indices = @mutable v;
    auto f = bind counter(tystore, param_indices, _);
    walk_ty(tystore, f, ty);
    ret _vec.len[uint](*param_indices);
}

fn type_contains_vars(@type_store tystore, t typ) -> bool {
    fn checker(@type_store tystore, @mutable bool flag, t typ) {
        alt (struct(tystore, typ)) {
            case (ty_var(_)) { *flag = true; }
            case (_) { /* fall through */ }
        }
    }

    let @mutable bool flag = @mutable false;
    auto f = bind checker(tystore, flag, _);
    walk_ty(tystore, f, typ);
    ret *flag;
}

// Type accessors for substructures of types

fn ty_fn_args(@type_store tystore, t fty) -> vec[arg] {
    alt (struct(tystore, fty)) {
        case (ty.ty_fn(_, ?a, _)) { ret a; }
        case (ty.ty_native_fn(_, ?a, _)) { ret a; }
    }
    fail;
}

fn ty_fn_proto(@type_store tystore, t fty) -> ast.proto {
    alt (struct(tystore, fty)) {
        case (ty.ty_fn(?p, _, _)) { ret p; }
    }
    fail;
}

fn ty_fn_abi(@type_store tystore, t fty) -> ast.native_abi {
    alt (struct(tystore, fty)) {
        case (ty.ty_native_fn(?a, _, _)) { ret a; }
    }
    fail;
}

fn ty_fn_ret(@type_store tystore, t fty) -> t {
    alt (struct(tystore, fty)) {
        case (ty.ty_fn(_, _, ?r)) { ret r; }
        case (ty.ty_native_fn(_, _, ?r)) { ret r; }
    }
    fail;
}

fn is_fn_ty(@type_store tystore, t fty) -> bool {
    alt (struct(tystore, fty)) {
        case (ty.ty_fn(_, _, _)) { ret true; }
        case (ty.ty_native_fn(_, _, _)) { ret true; }
        case (_) { ret false; }
    }
    ret false;
}


// Type accessors for AST nodes

// Given an item, returns the associated type as well as the number of type
// parameters it has.
fn native_item_ty(@ast.native_item it) -> ty_param_count_and_ty {
    auto ty_param_count;
    auto result_ty;
    alt (it.node) {
        case (ast.native_item_fn(_, _, _, ?tps, _, ?ann)) {
            ty_param_count = _vec.len[ast.ty_param](tps);
            result_ty = ann_to_type(ann);
        }
    }
    ret tup(ty_param_count, result_ty);
}

fn item_ty(@ast.item it) -> ty_param_count_and_ty {
    auto ty_param_count;
    auto result_ty;
    alt (it.node) {
        case (ast.item_const(_, _, _, _, ?ann)) {
            ty_param_count = 0u;
            result_ty = ann_to_type(ann);
        }
        case (ast.item_fn(_, _, ?tps, _, ?ann)) {
            ty_param_count = _vec.len[ast.ty_param](tps);
            result_ty = ann_to_type(ann);
        }
        case (ast.item_mod(_, _, _)) {
            fail;   // modules are typeless
        }
        case (ast.item_ty(_, _, ?tps, _, ?ann)) {
            ty_param_count = _vec.len[ast.ty_param](tps);
            result_ty = ann_to_type(ann);
        }
        case (ast.item_tag(_, _, ?tps, ?did, ?ann)) {
            ty_param_count = _vec.len[ast.ty_param](tps);
            result_ty = ann_to_type(ann);
        }
        case (ast.item_obj(_, _, ?tps, _, ?ann)) {
            ty_param_count = _vec.len[ast.ty_param](tps);
            result_ty = ann_to_type(ann);
        }
    }

    ret tup(ty_param_count, result_ty);
}

fn stmt_ty(@type_store tystore, @ast.stmt s) -> t {
    alt (s.node) {
        case (ast.stmt_expr(?e,_)) {
            ret expr_ty(tystore, e);
        }
        case (_) {
            ret mk_nil(tystore);
        }
    }
}

fn block_ty(@type_store tystore, &ast.block b) -> t {
    alt (b.node.expr) {
        case (some[@ast.expr](?e)) { ret expr_ty(tystore, e); }
        case (none[@ast.expr])     { ret mk_nil(tystore); }
    }
}

// Returns the type of a pattern as a monotype. Like @expr_ty, this function
// doesn't provide type parameter substitutions.
fn pat_ty(@type_store ts, @ast.pat pat) -> t {
    alt (pat.node) {
        case (ast.pat_wild(?ann))           { ret ann_to_monotype(ts, ann); }
        case (ast.pat_lit(_, ?ann))         { ret ann_to_monotype(ts, ann); }
        case (ast.pat_bind(_, _, ?ann))     { ret ann_to_monotype(ts, ann); }
        case (ast.pat_tag(_, _, _, ?ann))   { ret ann_to_monotype(ts, ann); }
    }
    fail;   // not reached
}

fn expr_ann(@ast.expr expr) -> option.t[ast.ann] {
    alt (expr.node) {
        case (ast.expr_vec(_, _, ?ann))       { ret some[ast.ann](ann); }
        case (ast.expr_tup(_, ?ann))          { ret some[ast.ann](ann); }
        case (ast.expr_rec(_, _, ?ann))       { ret some[ast.ann](ann); }
        case (ast.expr_bind(_, _, ?ann))      { ret some[ast.ann](ann); }
        case (ast.expr_call(_, _, ?ann))      { ret some[ast.ann](ann); }
        case (ast.expr_self_method(_, ?ann))  { ret some[ast.ann](ann); }
        case (ast.expr_spawn(_, _, _, _, ?ann))
                                              { ret some[ast.ann](ann); }
        case (ast.expr_binary(_, _, _, ?ann)) { ret some[ast.ann](ann); }
        case (ast.expr_unary(_, _, ?ann))     { ret some[ast.ann](ann); }
        case (ast.expr_lit(_, ?ann))          { ret some[ast.ann](ann); }
        case (ast.expr_cast(_, _, ?ann))      { ret some[ast.ann](ann); }
        case (ast.expr_if(_, _, _, ?ann))     { ret some[ast.ann](ann); }
        case (ast.expr_for(_, _, _, ?ann))    { ret some[ast.ann](ann); }
        case (ast.expr_for_each(_, _, _, ?ann))
                                              { ret some[ast.ann](ann); }
        case (ast.expr_while(_, _, ?ann))     { ret some[ast.ann](ann); }
        case (ast.expr_do_while(_, _, ?ann))  { ret some[ast.ann](ann); }
        case (ast.expr_alt(_, _, ?ann))       { ret some[ast.ann](ann); }
        case (ast.expr_block(_, ?ann))        { ret some[ast.ann](ann); }
        case (ast.expr_assign(_, _, ?ann))    { ret some[ast.ann](ann); }
        case (ast.expr_assign_op(_, _, _, ?ann))
                                              { ret some[ast.ann](ann); }
        case (ast.expr_field(_, _, ?ann))     { ret some[ast.ann](ann); }
        case (ast.expr_index(_, _, ?ann))     { ret some[ast.ann](ann); }
        case (ast.expr_path(_, _, ?ann))      { ret some[ast.ann](ann); }
        case (ast.expr_ext(_, _, _, _, ?ann)) { ret some[ast.ann](ann); }
        case (ast.expr_port(?ann))            { ret some[ast.ann](ann); }
        case (ast.expr_chan(_, ?ann))         { ret some[ast.ann](ann); }
        case (ast.expr_send(_, _, ?ann))      { ret some[ast.ann](ann); }
        case (ast.expr_recv(_, _, ?ann))      { ret some[ast.ann](ann); }

        case (ast.expr_fail(_))               { ret none[ast.ann]; }
        case (ast.expr_break(_))              { ret none[ast.ann]; }
        case (ast.expr_cont(_))               { ret none[ast.ann]; }
        case (ast.expr_log(_,_,_))            { ret none[ast.ann]; }
        case (ast.expr_check_expr(_,_))       { ret none[ast.ann]; }
        case (ast.expr_ret(_,_))              { ret none[ast.ann]; }
        case (ast.expr_put(_,_))              { ret none[ast.ann]; }
        case (ast.expr_be(_,_))               { ret none[ast.ann]; }
    }
    fail;
}

// Returns the type of an expression as a monotype.
//
// NB: This type doesn't provide type parameter substitutions; e.g. if you
// ask for the type of "id" in "id(3)", it will return "fn(&int) -> int"
// instead of "fn(&T) -> T with T = int". If this isn't what you want, see
// expr_ty_params_and_ty() below.
fn expr_ty(@type_store tystore, @ast.expr expr) -> t {
    alt (expr_ann(expr)) {
        case (none[ast.ann])     { ret mk_nil(tystore); }
        case (some[ast.ann](?a)) { ret ann_to_monotype(tystore, a); }
    }
}

fn expr_ty_params_and_ty(@type_store tystore, @ast.expr expr)
        -> tup(vec[t], t) {
    alt (expr_ann(expr)) {
        case (none[ast.ann]) {
            let vec[t] tps = vec();
            ret tup(tps, mk_nil(tystore));
        }
        case (some[ast.ann](?a)) {
            ret tup(ann_to_type_params(a), ann_to_type(a));
        }
    }
}

fn expr_has_ty_params(@ast.expr expr) -> bool {
    // FIXME: Rewrite using complex patterns when they're trustworthy.
    alt (expr_ann(expr)) {
        case (none[ast.ann]) { fail; }
        case (some[ast.ann](?a)) {
            alt (a) {
                case (ast.ann_none) { fail; }
                case (ast.ann_type(_, ?tps_opt, _)) {
                    ret !option.is_none[vec[t]](tps_opt);
                }
            }
        }
    }
}

// FIXME: At the moment this works only for call, bind, and path expressions.
fn replace_expr_type(@ast.expr expr, tup(vec[t], t) new_tyt) -> @ast.expr {
    auto new_tps;
    if (expr_has_ty_params(expr)) {
        new_tps = some[vec[t]](new_tyt._0);
    } else {
        new_tps = none[vec[t]];
    }

    auto ann = ast.ann_type(new_tyt._1, new_tps, none[@ts_ann]);

    alt (expr.node) {
        case (ast.expr_call(?callee, ?args, _)) {
            ret @fold.respan[ast.expr_](expr.span,
                                        ast.expr_call(callee, args, ann));
        }
        case (ast.expr_self_method(?ident, _)) {
            ret @fold.respan[ast.expr_](expr.span,
                                        ast.expr_self_method(ident, ann));
        }
        case (ast.expr_bind(?callee, ?args, _)) {
            ret @fold.respan[ast.expr_](expr.span,
                                        ast.expr_bind(callee, args, ann));
        }
        case (ast.expr_field(?e, ?i, _)) {
            ret @fold.respan[ast.expr_](expr.span,
                                        ast.expr_field(e, i, ann));
        }
        case (ast.expr_path(?p, ?dopt, _)) {
            ret @fold.respan[ast.expr_](expr.span,
                                        ast.expr_path(p, dopt, ann));
        }
        case (_) {
            log_err "unhandled expr type in replace_expr_type(): " +
                pretty.pprust.expr_to_str(expr);
            fail;
        }
    }
}

// Expression utilities

fn field_num(session.session sess, &span sp, &ast.ident id) -> uint {
    let uint accum = 0u;
    let uint i = 0u;
    for (u8 c in id) {
        if (i == 0u) {
            if (c != ('_' as u8)) {
                sess.span_err(sp,
                              "bad numeric field on tuple: "
                              + "missing leading underscore");
            }
        } else {
            if (('0' as u8) <= c && c <= ('9' as u8)) {
                accum *= 10u;
                accum += (c as uint) - ('0' as uint);
            } else {
                auto s = "";
                s += _str.unsafe_from_byte(c);
                sess.span_err(sp,
                              "bad numeric field on tuple: "
                              + " non-digit character: "
                              + s);
            }
        }
        i += 1u;
    }
    ret accum;
}

fn field_idx(session.session sess, &span sp,
             &ast.ident id, vec[field] fields) -> uint {
    let uint i = 0u;
    for (field f in fields) {
        if (_str.eq(f.ident, id)) {
            ret i;
        }
        i += 1u;
    }
    sess.span_err(sp, "unknown field '" + id + "' of record");
    fail;
}

fn method_idx(session.session sess, &span sp,
              &ast.ident id, vec[method] meths) -> uint {
    let uint i = 0u;
    for (method m in meths) {
        if (_str.eq(m.ident, id)) {
            ret i;
        }
        i += 1u;
    }
    sess.span_err(sp, "unknown method '" + id + "' of obj");
    fail;
}

fn sort_methods(vec[method] meths) -> vec[method] {
    fn method_lteq(&method a, &method b) -> bool {
        ret _str.lteq(a.ident, b.ident);
    }

    ret std.sort.merge_sort[method](bind method_lteq(_,_), meths);
}

fn is_lval(@ast.expr expr) -> bool {
    alt (expr.node) {
        case (ast.expr_field(_,_,_))    { ret true;  }
        case (ast.expr_index(_,_,_))    { ret true;  }
        case (ast.expr_path(_,_,_))     { ret true;  }
        case (_)                        { ret false; }
    }
}

// Type unification via Robinson's algorithm (Robinson 1965). Implemented as
// described in Hoder and Voronkov:
//
//     http://www.cs.man.ac.uk/~hoderk/ubench/unification_full.pdf

mod Unify {
    tag result {
        ures_ok(t);
        ures_err(type_err, t, t);
    }

    type ctxt = rec(UFind.ufind sets,
                    hashmap[int,uint] var_ids,
                    mutable vec[mutable vec[t]] types,
                    unify_handler handler,
                    @type_store tystore);

    // Wraps the given type in an appropriate cname.
    //
    // TODO: This doesn't do anything yet. We should carry the cname up from
    // the expected and/or actual types when unification results in a type
    // identical to one or both of the two. The precise algorithm for this is
    // something we'll probably need to develop over time.

    // Simple structural type comparison.
    fn struct_cmp(@ctxt cx, t expected, t actual) -> result {
        if (struct(cx.tystore, expected) == struct(cx.tystore, actual)) {
            ret ures_ok(expected);
        }

        ret ures_err(terr_mismatch, expected, actual);
    }

    // Unifies two mutability flags.
    fn unify_mut(ast.mutability expected, ast.mutability actual)
            -> option.t[ast.mutability] {
        if (expected == actual) {
            ret some[ast.mutability](expected);
        }
        if (expected == ast.maybe_mut) {
            ret some[ast.mutability](actual);
        }
        if (actual == ast.maybe_mut) {
            ret some[ast.mutability](expected);
        }
        ret none[ast.mutability];
    }

    tag fn_common_res {
        fn_common_res_err(result);
        fn_common_res_ok(vec[arg], t);
    }

    fn unify_fn_common(@ctxt cx,
                       t expected,
                       t actual,
                       vec[arg] expected_inputs, t expected_output,
                       vec[arg] actual_inputs, t actual_output)
        -> fn_common_res {
        auto expected_len = _vec.len[arg](expected_inputs);
        auto actual_len = _vec.len[arg](actual_inputs);
        if (expected_len != actual_len) {
            ret fn_common_res_err(ures_err(terr_arg_count,
                                           expected, actual));
        }

        // TODO: as above, we should have an iter2 iterator.
        let vec[arg] result_ins = vec();
        auto i = 0u;
        while (i < expected_len) {
            auto expected_input = expected_inputs.(i);
            auto actual_input = actual_inputs.(i);

            // This should be safe, I think?
            auto result_mode;
            if (mode_is_alias(expected_input.mode) ||
                mode_is_alias(actual_input.mode)) {
                result_mode = ast.alias;
            } else {
                result_mode = ast.val;
            }

            auto result = unify_step(cx, actual_input.ty, expected_input.ty);

            alt (result) {
                case (ures_ok(?rty)) {
                    result_ins += vec(rec(mode=result_mode, ty=rty));
                }

                case (_) {
                    ret fn_common_res_err(result);
                }
            }

            i += 1u;
        }

        // Check the output.
        auto result = unify_step(cx, expected_output, actual_output);
        alt (result) {
            case (ures_ok(?rty)) {
                ret fn_common_res_ok(result_ins, rty);
            }

            case (_) {
                ret fn_common_res_err(result);
            }
        }
    }

    fn unify_fn(@ctxt cx,
                ast.proto e_proto,
                ast.proto a_proto,
                t expected,
                t actual,
                vec[arg] expected_inputs, t expected_output,
                vec[arg] actual_inputs, t actual_output)
        -> result {

        if (e_proto != a_proto) {
            ret ures_err(terr_mismatch, expected, actual);
        }
        auto t = unify_fn_common(cx, expected, actual,
                                 expected_inputs, expected_output,
                                 actual_inputs, actual_output);
        alt (t) {
            case (fn_common_res_err(?r)) {
                ret r;
            }
            case (fn_common_res_ok(?result_ins, ?result_out)) {
                auto t2 = mk_fn(cx.tystore, e_proto, result_ins, result_out);
                ret ures_ok(t2);
            }
        }
    }

    fn unify_native_fn(@ctxt cx,
                       ast.native_abi e_abi,
                       ast.native_abi a_abi,
                       t expected,
                       t actual,
                       vec[arg] expected_inputs, t expected_output,
                       vec[arg] actual_inputs, t actual_output)
        -> result {
        if (e_abi != a_abi) {
            ret ures_err(terr_mismatch, expected, actual);
        }

        auto t = unify_fn_common(cx, expected, actual,
                                 expected_inputs, expected_output,
                                 actual_inputs, actual_output);
        alt (t) {
            case (fn_common_res_err(?r)) {
                ret r;
            }
            case (fn_common_res_ok(?result_ins, ?result_out)) {
                auto t2 = mk_native_fn(cx.tystore, e_abi, result_ins,
                                       result_out);
                ret ures_ok(t2);
            }
        }
    }

    fn unify_obj(@ctxt cx,
                 t expected,
                 t actual,
                 vec[method] expected_meths,
                 vec[method] actual_meths) -> result {
      let vec[method] result_meths = vec();
      let uint i = 0u;
      let uint expected_len = _vec.len[method](expected_meths);
      let uint actual_len = _vec.len[method](actual_meths);

      if (expected_len != actual_len) {
        ret ures_err(terr_meth_count, expected, actual);
      }

      while (i < expected_len) {
        auto e_meth = expected_meths.(i);
        auto a_meth = actual_meths.(i);
        if (! _str.eq(e_meth.ident, a_meth.ident)) {
          ret ures_err(terr_obj_meths(e_meth.ident, a_meth.ident),
                       expected, actual);
        }
        auto r = unify_fn(cx,
                          e_meth.proto, a_meth.proto,
                          expected, actual,
                          e_meth.inputs, e_meth.output,
                          a_meth.inputs, a_meth.output);
        alt (r) {
            case (ures_ok(?tfn)) {
                alt (struct(cx.tystore, tfn)) {
                    case (ty_fn(?proto, ?ins, ?out)) {
                        result_meths += vec(rec(inputs = ins,
                                                output = out
                                                with e_meth));
                    }
                }
            }
            case (_) {
                ret r;
            }
        }
        i += 1u;
      }
      auto t = mk_obj(cx.tystore, result_meths);
      ret ures_ok(t);
    }

    fn get_or_create_set(@ctxt cx, int id) -> uint {
        auto set_num;
        alt (cx.var_ids.find(id)) {
        case (none[uint]) {
            set_num = UFind.make_set(cx.sets);
            cx.var_ids.insert(id, set_num);
        }
        case (some[uint](?n)) { set_num = n; }
        }
        ret set_num;
    }

    fn unify_step(@ctxt cx, t expected, t actual) -> result {
        // TODO: rewrite this using tuple pattern matching when available, to
        // avoid all this rightward drift and spikiness.

        // TODO: occurs check, to make sure we don't loop forever when
        // unifying e.g. 'a and option['a]

        // Fast path.
        if (eq_ty(expected, actual)) { ret ures_ok(expected); }

        alt (struct(cx.tystore, actual)) {
            // If the RHS is a variable type, then just do the appropriate
            // binding.
            case (ty.ty_var(?actual_id)) {
                auto actual_n = get_or_create_set(cx, actual_id);
                alt (struct(cx.tystore, expected)) {
                    case (ty.ty_var(?expected_id)) {
                        auto expected_n = get_or_create_set(cx, expected_id);
                        UFind.union(cx.sets, expected_n, actual_n);
                    }

                    case (_) {
                        // Just bind the type variable to the expected type.
                        auto vlen = _vec.len[mutable vec[t]](cx.types);
                        if (actual_n < vlen) {
                            cx.types.(actual_n) += vec(expected);
                        } else {
                            check (actual_n == vlen);
                            cx.types += vec(mutable vec(expected));
                        }
                    }
                }
                ret ures_ok(actual);
            }
            case (ty.ty_local(?actual_id)) {
                auto result_ty;
                alt (cx.handler.resolve_local(actual_id)) {
                    case (none[t]) { result_ty = expected; }
                    case (some[t](?actual_ty)) {
                        auto result = unify_step(cx, expected, actual_ty);
                        alt (result) {
                            case (ures_ok(?rty)) { result_ty = rty; }
                            case (_) { ret result; }
                        }
                    }
                }

                cx.handler.record_local(actual_id, result_ty);
                ret ures_ok(result_ty);
            }
            case (ty.ty_bound_param(?actual_id)) {
                alt (struct(cx.tystore, expected)) {
                    case (ty.ty_local(_)) {
                        log_err "TODO: bound param unifying with local";
                        fail;
                    }

                    case (_) {
                        ret cx.handler.record_param(actual_id, expected);
                    }
                }
            }
            case (_) { /* empty */ }
        }

        alt (struct(cx.tystore, expected)) {
            case (ty.ty_nil)        { ret struct_cmp(cx, expected, actual); }
            case (ty.ty_bool)       { ret struct_cmp(cx, expected, actual); }
            case (ty.ty_int)        { ret struct_cmp(cx, expected, actual); }
            case (ty.ty_uint)       { ret struct_cmp(cx, expected, actual); }
            case (ty.ty_machine(_)) { ret struct_cmp(cx, expected, actual); }
            case (ty.ty_float)      { ret struct_cmp(cx, expected, actual); }
            case (ty.ty_char)       { ret struct_cmp(cx, expected, actual); }
            case (ty.ty_str)        { ret struct_cmp(cx, expected, actual); }
            case (ty.ty_type)       { ret struct_cmp(cx, expected, actual); }
            case (ty.ty_native)     { ret struct_cmp(cx, expected, actual); }
            case (ty.ty_param(_))   { ret struct_cmp(cx, expected, actual); }

            case (ty.ty_tag(?expected_id, ?expected_tps)) {
                alt (struct(cx.tystore, actual)) {
                    case (ty.ty_tag(?actual_id, ?actual_tps)) {
                        if (expected_id._0 != actual_id._0 ||
                                expected_id._1 != actual_id._1) {
                            ret ures_err(terr_mismatch, expected, actual);
                        }

                        // TODO: factor this cruft out, see the TODO in the
                        // ty.ty_tup case
                        let vec[t] result_tps = vec();
                        auto i = 0u;
                        auto expected_len = _vec.len[t](expected_tps);
                        while (i < expected_len) {
                            auto expected_tp = expected_tps.(i);
                            auto actual_tp = actual_tps.(i);

                            auto result = unify_step(cx,
                                                     expected_tp,
                                                     actual_tp);

                            alt (result) {
                                case (ures_ok(?rty)) {
                                    _vec.push[t](result_tps, rty);
                                }
                                case (_) {
                                    ret result;
                                }
                            }

                            i += 1u;
                        }

                        ret ures_ok(mk_tag(cx.tystore, expected_id,
                                           result_tps));
                    }
                    case (_) { /* fall through */ }
                }

                ret ures_err(terr_mismatch, expected, actual);
            }

            case (ty.ty_box(?expected_mt)) {
                alt (struct(cx.tystore, actual)) {
                    case (ty.ty_box(?actual_mt)) {
                        auto mut;
                        alt (unify_mut(expected_mt.mut, actual_mt.mut)) {
                            case (none[ast.mutability]) {
                                ret ures_err(terr_box_mutability, expected,
                                             actual);
                            }
                            case (some[ast.mutability](?m)) { mut = m; }
                        }

                        auto result = unify_step(cx,
                                                 expected_mt.ty,
                                                 actual_mt.ty);
                        alt (result) {
                            case (ures_ok(?result_sub)) {
                                auto mt = rec(ty=result_sub, mut=mut);
                                ret ures_ok(mk_box(cx.tystore, mt));
                            }
                            case (_) {
                                ret result;
                            }
                        }
                    }

                    case (_) {
                        ret ures_err(terr_mismatch, expected, actual);
                    }
                }
            }

            case (ty.ty_vec(?expected_mt)) {
                alt (struct(cx.tystore, actual)) {
                    case (ty.ty_vec(?actual_mt)) {
                        auto mut;
                        alt (unify_mut(expected_mt.mut, actual_mt.mut)) {
                            case (none[ast.mutability]) {
                                ret ures_err(terr_vec_mutability, expected,
                                             actual);
                            }
                            case (some[ast.mutability](?m)) { mut = m; }
                        }

                        auto result = unify_step(cx,
                                                 expected_mt.ty,
                                                 actual_mt.ty);
                        alt (result) {
                            case (ures_ok(?result_sub)) {
                                auto mt = rec(ty=result_sub, mut=mut);
                                ret ures_ok(mk_vec(cx.tystore, mt));
                            }
                            case (_) {
                                ret result;
                            }
                        }
                    }

                    case (_) {
                        ret ures_err(terr_mismatch, expected, actual);
                   }
                }
            }

            case (ty.ty_port(?expected_sub)) {
                alt (struct(cx.tystore, actual)) {
                    case (ty.ty_port(?actual_sub)) {
                        auto result = unify_step(cx,
                                                 expected_sub,
                                                 actual_sub);
                        alt (result) {
                            case (ures_ok(?result_sub)) {
                                ret ures_ok(mk_port(cx.tystore, result_sub));
                            }
                            case (_) {
                                ret result;
                            }
                        }
                    }

                    case (_) {
                        ret ures_err(terr_mismatch, expected, actual);
                    }
                }
            }

            case (ty.ty_chan(?expected_sub)) {
                alt (struct(cx.tystore, actual)) {
                    case (ty.ty_chan(?actual_sub)) {
                        auto result = unify_step(cx,
                                                 expected_sub,
                                                 actual_sub);
                        alt (result) {
                            case (ures_ok(?result_sub)) {
                                ret ures_ok(mk_chan(cx.tystore, result_sub));
                            }
                            case (_) {
                                ret result;
                            }
                        }
                    }

                    case (_) {
                        ret ures_err(terr_mismatch, expected, actual);
                    }
                }
            }

            case (ty.ty_tup(?expected_elems)) {
                alt (struct(cx.tystore, actual)) {
                    case (ty.ty_tup(?actual_elems)) {
                        auto expected_len = _vec.len[ty.mt](expected_elems);
                        auto actual_len = _vec.len[ty.mt](actual_elems);
                        if (expected_len != actual_len) {
                            auto err = terr_tuple_size(expected_len,
                                                       actual_len);
                            ret ures_err(err, expected, actual);
                        }

                        // TODO: implement an iterator that can iterate over
                        // two arrays simultaneously.
                        let vec[ty.mt] result_elems = vec();
                        auto i = 0u;
                        while (i < expected_len) {
                            auto expected_elem = expected_elems.(i);
                            auto actual_elem = actual_elems.(i);

                            auto mut;
                            alt (unify_mut(expected_elem.mut,
                                           actual_elem.mut)) {
                                case (none[ast.mutability]) {
                                    auto err = terr_tuple_mutability;
                                    ret ures_err(err, expected, actual);
                                }
                                case (some[ast.mutability](?m)) { mut = m; }
                            }

                            auto result = unify_step(cx,
                                                     expected_elem.ty,
                                                     actual_elem.ty);
                            alt (result) {
                                case (ures_ok(?rty)) {
                                    auto mt = rec(ty=rty, mut=mut);
                                    result_elems += vec(mt);
                                }
                                case (_) {
                                    ret result;
                                }
                            }

                            i += 1u;
                        }

                        ret ures_ok(mk_tup(cx.tystore, result_elems));
                    }

                    case (_) {
                        ret ures_err(terr_mismatch, expected, actual);
                    }
                }
            }

            case (ty.ty_rec(?expected_fields)) {
                alt (struct(cx.tystore, actual)) {
                    case (ty.ty_rec(?actual_fields)) {
                        auto expected_len = _vec.len[field](expected_fields);
                        auto actual_len = _vec.len[field](actual_fields);
                        if (expected_len != actual_len) {
                            auto err = terr_record_size(expected_len,
                                                        actual_len);
                            ret ures_err(err, expected, actual);
                        }

                        // TODO: implement an iterator that can iterate over
                        // two arrays simultaneously.
                        let vec[field] result_fields = vec();
                        auto i = 0u;
                        while (i < expected_len) {
                            auto expected_field = expected_fields.(i);
                            auto actual_field = actual_fields.(i);

                            auto mut;
                            alt (unify_mut(expected_field.mt.mut,
                                           actual_field.mt.mut)) {
                                case (none[ast.mutability]) {
                                    ret ures_err(terr_record_mutability,
                                                 expected, actual);
                                }
                                case (some[ast.mutability](?m)) { mut = m; }
                            }

                            if (!_str.eq(expected_field.ident,
                                         actual_field.ident)) {
                                auto err =
                                    terr_record_fields(expected_field.ident,
                                                       actual_field.ident);
                                ret ures_err(err, expected, actual);
                            }

                            auto result = unify_step(cx,
                                                     expected_field.mt.ty,
                                                     actual_field.mt.ty);
                            alt (result) {
                                case (ures_ok(?rty)) {
                                    auto mt = rec(ty=rty, mut=mut);
                                    _vec.push[field]
                                        (result_fields,
                                         rec(mt=mt with expected_field));
                                }
                                case (_) {
                                    ret result;
                                }
                            }

                            i += 1u;
                        }

                        ret ures_ok(mk_rec(cx.tystore, result_fields));
                    }

                    case (_) {
                        ret ures_err(terr_mismatch, expected, actual);
                    }
                }
            }

            case (ty.ty_fn(?ep, ?expected_inputs, ?expected_output)) {
                alt (struct(cx.tystore, actual)) {
                    case (ty.ty_fn(?ap, ?actual_inputs, ?actual_output)) {
                        ret unify_fn(cx, ep, ap,
                                     expected, actual,
                                     expected_inputs, expected_output,
                                     actual_inputs, actual_output);
                    }

                    case (_) {
                        ret ures_err(terr_mismatch, expected, actual);
                    }
                }
            }

            case (ty.ty_native_fn(?e_abi, ?expected_inputs,
                                  ?expected_output)) {
                alt (struct(cx.tystore, actual)) {
                    case (ty.ty_native_fn(?a_abi, ?actual_inputs,
                                          ?actual_output)) {
                        ret unify_native_fn(cx, e_abi, a_abi,
                                            expected, actual,
                                            expected_inputs, expected_output,
                                            actual_inputs, actual_output);
                    }
                    case (_) {
                        ret ures_err(terr_mismatch, expected, actual);
                    }
                }
            }

            case (ty.ty_obj(?expected_meths)) {
                alt (struct(cx.tystore, actual)) {
                    case (ty.ty_obj(?actual_meths)) {
                        ret unify_obj(cx, expected, actual,
                                      expected_meths, actual_meths);
                    }
                    case (_) {
                        ret ures_err(terr_mismatch, expected, actual);
                    }
                }
            }

            case (ty.ty_var(?expected_id)) {
                // Add a binding.
                auto expected_n = get_or_create_set(cx, expected_id);
                auto vlen = _vec.len[mutable vec[t]](cx.types);
                if (expected_n < vlen) {
                    cx.types.(expected_n) += vec(actual);
                } else {
                    check (expected_n == vlen);
                    cx.types += vec(mutable vec(actual));
                }
                ret ures_ok(expected);
            }

            case (ty.ty_local(?expected_id)) {
                auto result_ty;
                alt (cx.handler.resolve_local(expected_id)) {
                    case (none[t]) { result_ty = actual; }
                    case (some[t](?expected_ty)) {
                        auto result = unify_step(cx, expected_ty, actual);
                        alt (result) {
                            case (ures_ok(?rty)) { result_ty = rty; }
                            case (_) { ret result; }
                        }
                    }
                }

                cx.handler.record_local(expected_id, result_ty);
                ret ures_ok(result_ty);
            }

            case (ty.ty_bound_param(?expected_id)) {
                ret cx.handler.record_param(expected_id, actual);
            }
        }

        // TODO: remove me once match-exhaustiveness checking works
        fail;
    }

    // Performs type binding substitution.
    fn substitute(@ctxt cx, vec[t] set_types, t typ) -> t {
        fn substituter(@ctxt cx, vec[t] types, t typ) -> t {
            alt (struct(cx.tystore, typ)) {
                case (ty_var(?id)) {
                    alt (cx.var_ids.find(id)) {
                        case (some[uint](?n)) {
                            auto root = UFind.find(cx.sets, n);
                            ret types.(root);
                        }
                        case (none[uint]) { ret typ; }
                    }
                }
                case (_) { ret typ; }
            }
        }

        auto f = bind substituter(cx, set_types, _);
        ret fold_ty(cx.tystore, f, typ);
    }

    fn unify_sets(@ctxt cx) -> vec[t] {
        let vec[t] throwaway = vec();
        let vec[mutable vec[t]] set_types = vec(mutable throwaway);
        _vec.pop[mutable vec[t]](set_types);   // FIXME: botch

        for (UFind.node node in cx.sets.nodes) {
            let vec[t] v = vec();
            set_types += vec(mutable v);
        }

        auto i = 0u;
        while (i < _vec.len[mutable vec[t]](set_types)) {
            auto root = UFind.find(cx.sets, i);
            set_types.(root) += cx.types.(i);
            i += 1u;
        }

        let vec[t] result = vec();
        for (vec[t] types in set_types) {
            if (_vec.len[t](types) > 1u) {
                log_err "unification of > 1 types in a type set is " +
                    "unimplemented";
                fail;
            }
            result += vec(types.(0));
        }

        ret result;
    }

    fn unify(t expected,
             t actual,
             &unify_handler handler,
             @type_store tystore) -> result {
        let vec[t] throwaway = vec();
        let vec[mutable vec[t]] types = vec(mutable throwaway);
        _vec.pop[mutable vec[t]](types);   // FIXME: botch

        auto cx = @rec(sets=UFind.make(),
                       var_ids=common.new_int_hash[uint](),
                       mutable types=types,
                       handler=handler,
                       tystore=tystore);

        auto ures = unify_step(cx, expected, actual);
        alt (ures) {
        case (ures_ok(?typ)) {
            // Fast path: if there are no local variables, don't perform
            // substitutions.
            if (_vec.len[mutable UFind.node](cx.sets.nodes) == 0u) {
                ret ures_ok(typ);
            }

            auto set_types = unify_sets(cx);
            auto t2 = substitute(cx, set_types, typ);
            ret ures_ok(t2);
        }
        case (_) { ret ures; }
        }
        fail;   // not reached
    }
}

fn type_err_to_str(&ty.type_err err) -> str {
    alt (err) {
        case (terr_mismatch) {
            ret "types differ";
        }
        case (terr_box_mutability) {
            ret "boxed values differ in mutability";
        }
        case (terr_vec_mutability) {
            ret "vectors differ in mutability";
        }
        case (terr_tuple_size(?e_sz, ?a_sz)) {
            ret "expected a tuple with " + _uint.to_str(e_sz, 10u) +
                " elements but found one with " + _uint.to_str(a_sz, 10u) +
                " elements";
        }
        case (terr_tuple_mutability) {
            ret "tuple elements differ in mutability";
        }
        case (terr_record_size(?e_sz, ?a_sz)) {
            ret "expected a record with " + _uint.to_str(e_sz, 10u) +
                " fields but found one with " + _uint.to_str(a_sz, 10u) +
                " fields";
        }
        case (terr_record_mutability) {
            ret "record elements differ in mutability";
        }
        case (terr_record_fields(?e_fld, ?a_fld)) {
            ret "expected a record with field '" + e_fld +
                "' but found one with field '" + a_fld +
                "'";
        }
        case (terr_arg_count) {
            ret "incorrect number of function parameters";
        }
        case (terr_meth_count) {
            ret "incorrect number of object methods";
        }
        case (terr_obj_meths(?e_meth, ?a_meth)) {
            ret "expected an obj with method '" + e_meth +
                "' but found one with method '" + a_meth +
                "'";
        }
    }
}

// Performs bound type parameter replacement using the supplied mapping from
// parameter IDs to types.
fn substitute_type_params(@type_store tystore,
                          vec[t] bindings,
                          t typ) -> t {
    fn replacer(@type_store tystore, vec[t] bindings, t typ) -> t {
        alt (struct(tystore, typ)) {
            case (ty_bound_param(?param_index)) {
                ret bindings.(param_index);
            }
            case (_) { ret typ; }
        }
    }

    auto f = bind replacer(tystore, bindings, _);
    ret fold_ty(tystore, f, typ);
}

// Converts type parameters in a type to bound type parameters.
fn bind_params_in_type(@type_store tystore, t typ) -> t {
    fn binder(@type_store tystore, t typ) -> t {
        alt (struct(tystore, typ)) {
            case (ty_bound_param(?index)) {
                log_err "bind_params_in_type() called on type that already " +
                    "has bound params in it";
                fail;
            }
            case (ty_param(?index)) { ret mk_bound_param(tystore, index); }
            case (_) { ret typ; }
        }
    }

    auto f = bind binder(tystore, _);
    ret fold_ty(tystore, f, typ);
}


fn def_has_ty_params(&ast.def def) -> bool {
    alt (def) {
        case (ast.def_fn(_))            { ret true;  }
        case (ast.def_obj(_))           { ret true;  }
        case (ast.def_obj_field(_))     { ret false; }
        case (ast.def_mod(_))           { ret false; }
        case (ast.def_const(_))         { ret false; }
        case (ast.def_arg(_))           { ret false; }
        case (ast.def_local(_))         { ret false; }
        case (ast.def_variant(_, _))    { ret true;  }
        case (ast.def_ty(_))            { ret false; }
        case (ast.def_ty_arg(_))        { ret false; }
        case (ast.def_binding(_))       { ret false; }
        case (ast.def_use(_))           { ret false; }
        case (ast.def_native_ty(_))     { ret false; }
        case (ast.def_native_fn(_))     { ret true;  }
    }
}

// If the given item is in an external crate, looks up its type and adds it to
// the type cache. Returns the type parameters and type.
fn lookup_item_type(session.session sess,
                    @type_store tystore,
                    &type_cache cache,
                    ast.def_id did) -> ty_param_count_and_ty {
    if (did._0 == sess.get_targ_crate_num()) {
        // The item is in this crate. The caller should have added it to the
        // type cache already; we simply return it.
        ret cache.get(did);
    }

    alt (cache.find(did)) {
        case (some[ty_param_count_and_ty](?tpt)) { ret tpt; }
        case (none[ty_param_count_and_ty]) {
            auto tyt = creader.get_type(sess, tystore, did);
            cache.insert(did, tyt);
            ret tyt;
        }
    }
}


// Local Variables:
// mode: rust
// fill-column: 78;
// indent-tabs-mode: nil
// c-basic-offset: 4
// buffer-file-coding-system: utf-8-unix
// compile-command: "make -k -C $RBUILD 2>&1 | sed -e 's/\\/x\\//x:\\//g'";
// End: