diff --git a/src/comp/front/creader.rs b/src/comp/front/creader.rs
index b922b3780ed..89a19fc237b 100644
--- a/src/comp/front/creader.rs
+++ b/src/comp/front/creader.rs
@@ -6,11 +6,14 @@ import lib.llvm.llvmext;
 import lib.llvm.mk_object_file;
 import lib.llvm.mk_section_iter;
 import middle.fold;
+import middle.ty;
 import util.common;
 import util.common.span;
 
 import std._str;
+import std._vec;
 import std.fs;
+import std.option;
 import std.os;
 import std.map.hashmap;
 
@@ -20,6 +23,174 @@ type env = @rec(
     vec[str] library_search_paths
 );
 
+// Type decoding
+
+// Compact string representation for ty.t values. API ty_str & parse_from_str.
+// (The second has to be authed pure.) Extra parameters are for converting
+// to/from def_ids in the string rep. Whatever format you choose should not
+// contain pipe characters.
+
+// Callback to translate defs to strs or back.
+type str_def = fn(str) -> ast.def_id;
+
+type pstate = rec(str rep, mutable uint pos, uint len);
+
+fn peek(@pstate st) -> char {
+    if (st.pos < st.len) {ret st.rep.(st.pos) as char;}
+    else {ret ' ';}
+}
+impure fn next(@pstate st) -> char { // ?? somehow not recognized as impure
+    if (st.pos >= st.len) {fail;}
+    auto ch = st.rep.(st.pos);
+    st.pos = st.pos + 1u;
+    ret ch as char;
+}
+
+fn parse_ty_str(str rep, str_def sd) -> @ty.t {
+    auto len = _str.byte_len(rep);
+    auto st = @rec(rep=rep, mutable pos=0u, len=len);
+    auto result = parse_ty(st, sd);
+    check(st.pos == len);
+    ret result;
+}
+
+impure fn parse_ty(@pstate st, str_def sd) -> @ty.t {
+    ret @rec(struct=parse_sty(st, sd),
+             cname=option.none[str]);
+}
+
+impure fn parse_sty(@pstate st, str_def sd) -> ty.sty {
+    alt (next(st)) {
+        case ('n') {ret ty.ty_nil;}
+        case ('b') {ret ty.ty_bool;}
+        case ('i') {ret ty.ty_int;}
+        case ('u') {ret ty.ty_uint;}
+        case ('M') {
+            alt (next(st)) {
+                case ('b') {ret ty.ty_machine(common.ty_u8);}
+                case ('w') {ret ty.ty_machine(common.ty_u16);}
+                case ('l') {ret ty.ty_machine(common.ty_u32);}
+                case ('d') {ret ty.ty_machine(common.ty_u64);}
+                case ('B') {ret ty.ty_machine(common.ty_i8);}
+                case ('W') {ret ty.ty_machine(common.ty_i16);}
+                case ('L') {ret ty.ty_machine(common.ty_i32);}
+                case ('D') {ret ty.ty_machine(common.ty_i64);}
+                case ('f') {ret ty.ty_machine(common.ty_f32);}
+                case ('F') {ret ty.ty_machine(common.ty_f64);}
+            }
+        }
+        case ('c') {ret ty.ty_char;}
+        case ('s') {ret ty.ty_str;}
+        case ('t') {
+            check(next(st) == '[');
+            auto def = "";
+            while (peek(st) != '|') {def += _str.from_char(next(st));}
+            st.pos = st.pos + 1u;
+            let vec[@ty.t] params = vec();
+            while (peek(st) != ']') {
+                params = _vec.push[@ty.t](params, parse_ty(st, sd));
+            }
+            st.pos = st.pos + 1u;
+            ret ty.ty_tag(sd(def), params);
+        }      
+        case ('@') {ret ty.ty_box(parse_ty(st, sd));}
+        case ('V') {ret ty.ty_vec(parse_ty(st, sd));}
+        case ('P') {ret ty.ty_port(parse_ty(st, sd));}
+        case ('C') {ret ty.ty_chan(parse_ty(st, sd));}
+        case ('T') {
+            check(next(st) == '[');
+            let vec[@ty.t] params = vec();
+            while (peek(st) != ']') {
+                params = _vec.push[@ty.t](params, parse_ty(st, sd));
+            }
+            st.pos = st.pos + 1u;
+            ret ty.ty_tup(params);
+        }
+        case ('R') {
+            check(next(st) == '[');
+            let vec[ty.field] fields = vec();
+            while (peek(st) != ']') {
+                auto name = "";
+                while (peek(st) != '=') {name += _str.from_char(next(st));}
+                st.pos = st.pos + 1u;
+                fields = _vec.push[ty.field]
+                    (fields, rec(ident=name, ty=parse_ty(st, sd)));
+            }
+            st.pos = st.pos + 1u;
+            ret ty.ty_rec(fields);
+        }
+        case ('F') {
+            auto func = parse_ty_fn(st, sd);
+            ret ty.ty_fn(ast.proto_fn, func._0, func._1);
+        }
+        case ('W') {
+            auto func = parse_ty_fn(st, sd);
+            ret ty.ty_fn(ast.proto_iter, func._0, func._1);
+        }
+        case ('N') {
+            auto abi;
+            alt (next(st)) {
+                case ('r') {abi = ast.native_abi_rust;}
+                case ('c') {abi = ast.native_abi_cdecl;}
+            }
+            auto func = parse_ty_fn(st, sd);
+            ret ty.ty_native_fn(abi,func._0,func._1);
+        }
+        case ('O') {
+            check(next(st) == '[');
+            let vec[ty.method] methods = vec();
+            while (peek(st) != ']') {
+                auto proto;
+                alt (next(st)) {
+                    case ('W') {proto = ast.proto_iter;}
+                    case ('F') {proto = ast.proto_fn;}
+                }
+                auto name = "";
+                while (peek(st) != '[') {name += _str.from_char(next(st));}
+                auto func = parse_ty_fn(st, sd);
+                methods = _vec.push[ty.method]
+                    (methods, rec(proto=proto,
+                                  ident=name,
+                                  inputs=func._0,
+                                  output=func._1));
+            }
+            ret ty.ty_obj(methods);
+        }
+        case ('X') {ret ty.ty_var(parse_int(st));}
+        case ('E') {ret ty.ty_native;}
+    }
+}
+
+impure fn parse_int(@pstate st) -> int {
+    auto n = 0;
+    while (true) {
+        auto cur = peek(st);
+        if (cur < '0' || cur > '9') {break;}
+        st.pos = st.pos + 1u;
+        n *= 10;
+        n += (cur as int) - ('0' as int);
+    }
+    ret n;
+}
+
+impure fn parse_ty_fn(@pstate st, str_def sd) -> tup(vec[ty.arg], @ty.t) {
+    check(next(st) == '[');
+    let vec[ty.arg] inputs = vec();
+    while (peek(st) != ']') {
+        auto mode = ast.val;
+        if (peek(st) == '&') {
+            mode = ast.alias;
+            st.pos = st.pos + 1u;
+        }
+        inputs = _vec.push[ty.arg]
+            (inputs, rec(mode=mode, ty=parse_ty(st, sd)));
+    }
+    st.pos = st.pos + 1u;
+    ret tup(inputs, parse_ty(st, sd));
+}
+
+
+
 // TODO: return something
 fn load_crate(ast.ident ident, vec[str] library_search_paths) -> @() {
     for (str library_search_path in library_search_paths) {
diff --git a/src/comp/middle/metadata.rs b/src/comp/middle/metadata.rs
index 433d7147024..bff0ac94e19 100644
--- a/src/comp/middle/metadata.rs
+++ b/src/comp/middle/metadata.rs
@@ -1,12 +1,118 @@
 import std._str;
+import std._vec;
+import std.option;
+
 import front.ast;
 import middle.trans;
+import middle.ty;
 import back.x86;
+import util.common;
 
 import lib.llvm.llvm;
 import lib.llvm.llvm.ValueRef;
 import lib.llvm.False;
 
+// Type encoding
+
+// Compact string representation for ty.t values. API ty_str & parse_from_str.
+// (The second has to be authed pure.) Extra parameters are for converting
+// to/from def_ids in the string rep. Whatever format you choose should not
+// contain pipe characters.
+
+// Callback to translate defs to strs or back.
+type def_str = fn(ast.def_id) -> str;
+
+fn ty_str(@ty.t t, def_str ds) -> str {
+    ret sty_str(t.struct, ds);
+}
+
+fn sty_str(ty.sty st, def_str ds) -> str {
+    alt (st) {
+        case (ty.ty_nil) {ret "n";}
+        case (ty.ty_bool) {ret "b";}
+        case (ty.ty_int) {ret "i";}
+        case (ty.ty_uint) {ret "u";}
+        case (ty.ty_machine(?mach)) {
+            alt (mach) {
+                case (common.ty_u8) {ret "Mb";}
+                case (common.ty_u16) {ret "Mw";}
+                case (common.ty_u32) {ret "Ml";}
+                case (common.ty_u64) {ret "Md";}
+                case (common.ty_i8) {ret "MB";}
+                case (common.ty_i16) {ret "MW";}
+                case (common.ty_i32) {ret "ML";}
+                case (common.ty_i64) {ret "MD";}
+                case (common.ty_f32) {ret "Mf";}
+                case (common.ty_f64) {ret "MF";}
+            }
+        }
+        case (ty.ty_char) {ret "c";}
+        case (ty.ty_str) {ret "s";}
+        case (ty.ty_tag(?def,?tys)) { // TODO restore def_id
+            auto acc = "t[" + ds(def) + "|";
+            for (@ty.t t in tys) {acc += ty_str(t, ds);}
+            ret acc + "]";
+        }
+        case (ty.ty_box(?t)) {ret "@" + ty_str(t, ds);}
+        case (ty.ty_vec(?t)) {ret "V" + ty_str(t, ds);}
+        case (ty.ty_port(?t)) {ret "P" + ty_str(t, ds);}
+        case (ty.ty_chan(?t)) {ret "C" + ty_str(t, ds);}
+        case (ty.ty_tup(?tys)) {
+            auto acc = "T[";
+            for (@ty.t t in tys) {acc += ty_str(t, ds);}
+            ret acc + "]";
+        }
+        case (ty.ty_rec(?fields)) {
+            auto acc = "R[";
+            for (ty.field field in fields) {
+                acc += field.ident + "=";
+                acc += ty_str(field.ty, ds);
+            }
+            ret acc + "]";
+        }
+        case (ty.ty_fn(?proto,?args,?out)) {
+            ret proto_str(proto) + ty_fn_str(args, out, ds);
+        }
+        case (ty.ty_native_fn(?abi,?args,?out)) {
+            auto abistr;
+            alt (abi) {
+                case (ast.native_abi_rust) {abistr = "r";}
+                case (ast.native_abi_cdecl) {abistr = "c";}
+            }
+            ret "N" + abistr + ty_fn_str(args, out, ds);
+        }
+        case (ty.ty_obj(?methods)) {
+            auto acc = "O[";
+            for (ty.method m in methods) {
+                acc += proto_str(m.proto);
+                acc += m.ident;
+                acc += ty_fn_str(m.inputs, m.output, ds);
+            }
+            ret acc + "]";
+        }
+        case (ty.ty_var(?id)) {ret "X" + common.istr(id);}
+        case (ty.ty_native) {ret "E";}
+        // TODO (maybe?)   ty_param(ast.def_id), ty_type;
+    }
+}
+
+fn proto_str(ast.proto proto) -> str {
+    alt (proto) {
+        case (ast.proto_iter) {ret "W";}
+        case (ast.proto_fn) {ret "F";}
+    }
+}
+
+fn ty_fn_str(vec[ty.arg] args, @ty.t out, def_str ds) -> str {
+    auto acc = "[";
+    for (ty.arg arg in args) {
+        if (arg.mode == ast.alias) {acc += "&";}
+        acc += ty_str(arg.ty, ds);
+    }
+    ret acc + "]" + ty_str(out, ds);
+}
+
+
 // Returns a Plain Old LLVM String.
 fn C_postr(str s) -> ValueRef {
     ret llvm.LLVMConstString(_str.buf(s), _str.byte_len(s), False);