diff --git a/src/comp/middle/walk.rs b/src/comp/middle/walk.rs
new file mode 100644
index 00000000000..c0248864a73
--- /dev/null
+++ b/src/comp/middle/walk.rs
@@ -0,0 +1,471 @@
+import front.ast;
+
+import std.option;
+import std.option.some;
+import std.option.none;
+
+type ast_visitor =
+    rec(fn () -> bool                 keep_going,
+        fn () -> bool                 want_crate_directives,
+        fn (&ast.crate c)             visit_crate_pre,
+        fn (&ast.crate c)             visit_crate_post,
+        fn (@ast.crate_directive cd)  visit_crate_directive_pre,
+        fn (@ast.crate_directive cd)  visit_crate_directive_post,
+        fn (@ast.view_item i)         visit_view_item_pre,
+        fn (@ast.view_item i)         visit_view_item_post,
+        fn (@ast.native_item i)       visit_native_item_pre,
+        fn (@ast.native_item i)       visit_native_item_post,
+        fn (@ast.item i)              visit_item_pre,
+        fn (@ast.item i)              visit_item_post,
+        fn (&ast.block b)             visit_block_pre,
+        fn (&ast.block b)             visit_block_post,
+        fn (@ast.stmt s)              visit_stmt_pre,
+        fn (@ast.stmt s)              visit_stmt_post,
+        fn (@ast.decl d)              visit_decl_pre,
+        fn (@ast.decl d)              visit_decl_post,
+        fn (@ast.expr e)              visit_expr_pre,
+        fn (@ast.expr e)              visit_expr_post,
+        fn (@ast.ty t)                visit_ty_pre,
+        fn (@ast.ty t)                visit_ty_post);
+
+fn walk_crate(&ast_visitor v, &ast.crate c) {
+    if (!v.keep_going()) { ret; }
+    v.visit_crate_pre(c);
+    walk_mod(v, c.node.module);
+    v.visit_crate_post(c);
+}
+
+fn walk_crate_directive(&ast_visitor v, @ast.crate_directive cd) {
+    if (!v.keep_going()) { ret; }
+    if (!v.want_crate_directives()) { ret; }
+    v.visit_crate_directive_pre(cd);
+    alt (cd.node) {
+        case (ast.cdir_let(_, ?e, ?cdirs)) {
+            walk_expr(v, e);
+            for (@ast.crate_directive cdir in cdirs) {
+                walk_crate_directive(v, cdir);
+            }
+        }
+        case (ast.cdir_src_mod(_, _)) {}
+        case (ast.cdir_dir_mod(_, _, ?cdirs)) {
+            for (@ast.crate_directive cdir in cdirs) {
+                walk_crate_directive(v, cdir);
+            }
+        }
+        case (ast.cdir_view_item(?vi)) {
+            walk_view_item(v, vi);
+        }
+        case (ast.cdir_meta(_)) {}
+        case (ast.cdir_syntax(_)) {}
+        case (ast.cdir_auth(_, _)) {}
+    }
+    v.visit_crate_directive_post(cd);
+}
+
+fn walk_mod(&ast_visitor v, &ast._mod m) {
+    if (!v.keep_going()) { ret; }
+    for (@ast.view_item vi in m.view_items) {
+        walk_view_item(v, vi);
+    }
+    for (@ast.item i in m.items) {
+        walk_item(v, i);
+    }
+}
+
+fn walk_view_item(&ast_visitor v, @ast.view_item vi) {
+    if (!v.keep_going()) { ret; }
+    v.visit_view_item_pre(vi);
+    v.visit_view_item_post(vi);
+}
+
+fn walk_item(&ast_visitor v, @ast.item i) {
+    if (!v.keep_going()) { ret; }
+    v.visit_item_pre(i);
+    alt (i.node) {
+        case (ast.item_const(_, ?t, ?e, _, _)) {
+            walk_ty(v, t);
+            walk_expr(v, e);
+        }
+        case (ast.item_fn(_, ?f, _, _, _)) {
+            walk_fn(v, f);
+        }
+        case (ast.item_mod(_, ?m, _)) {
+            walk_mod(v, m);
+        }
+        case (ast.item_native_mod(_, ?nm, _)) {
+            walk_native_mod(v, nm);
+        }
+        case (ast.item_ty(_, ?t, _, _, _)) {
+            walk_ty(v, t);
+        }
+        case (ast.item_tag(_, ?variants, _, _, _)) {
+            for (ast.variant vr in variants) {
+                for (ast.variant_arg va in vr.node.args) {
+                    walk_ty(v, va.ty);
+                }
+            }
+        }
+        case (ast.item_obj(_, ?ob, _, _, _)) {
+            for (ast.obj_field f in ob.fields) {
+                walk_ty(v, f.ty);
+            }
+            for (@ast.method m in ob.methods) {
+                walk_fn(v, m.node.meth);
+            }
+            alt (ob.dtor) {
+                case (none[@ast.method]) {}
+                case (some[@ast.method](?m)) {
+                    walk_fn(v, m.node.meth);
+                }
+            }
+        }
+
+    }
+    v.visit_item_post(i);
+}
+
+fn walk_ty(&ast_visitor v, @ast.ty t) {
+    if (!v.keep_going()) { ret; }
+    v.visit_ty_pre(t);
+    alt (t.node) {
+        case (ast.ty_nil) {}
+        case (ast.ty_bool) {}
+        case (ast.ty_int) {}
+        case (ast.ty_uint) {}
+        case (ast.ty_float) {}
+        case (ast.ty_machine(_)) {}
+        case (ast.ty_char) {}
+        case (ast.ty_str) {}
+        case (ast.ty_box(?mt)) { walk_ty(v, mt.ty); }
+        case (ast.ty_vec(?mt)) { walk_ty(v, mt.ty); }
+        case (ast.ty_port(?t)) { walk_ty(v, t); }
+        case (ast.ty_chan(?t)) { walk_ty(v, t); }
+        case (ast.ty_tup(?mts)) {
+            for (ast.mt mt in mts) {
+                walk_ty(v, mt.ty);
+            }
+        }
+        case (ast.ty_rec(?flds)) {
+            for (ast.ty_field f in flds) {
+                walk_ty(v, f.mt.ty);
+            }
+        }
+        case (ast.ty_fn(_, _, ?args, ?out)) {
+            for (ast.ty_arg a in args) {
+                walk_ty(v, a.ty);
+            }
+            walk_ty(v, out);
+        }
+        case (ast.ty_obj(?tmeths)) {
+            for (ast.ty_method m in tmeths) {
+                for (ast.ty_arg a in m.inputs) {
+                    walk_ty(v, a.ty);
+                }
+                walk_ty(v, m.output);
+            }
+        }
+        case (ast.ty_path(_, _)) {}
+        case (ast.ty_type) {}
+        case (ast.ty_constr(?t, _)) { walk_ty(v, t); }
+    }
+    v.visit_ty_post(t);
+}
+
+fn walk_native_mod(&ast_visitor v, &ast.native_mod nm) {
+    if (!v.keep_going()) { ret; }
+    for (@ast.view_item vi in nm.view_items) {
+        walk_view_item(v, vi);
+    }
+    for (@ast.native_item ni in nm.items) {
+        walk_native_item(v, ni);
+    }
+}
+
+fn walk_native_item(&ast_visitor v, @ast.native_item ni) {
+    if (!v.keep_going()) { ret; }
+    v.visit_native_item_pre(ni);
+    alt (ni.node) {
+        case (ast.native_item_fn(_, _, ?fd, _, _, _)) {
+            walk_fn_decl(v, fd);
+        }
+        case (ast.native_item_ty(_, _)) {
+        }
+    }
+    v.visit_native_item_post(ni);
+}
+
+fn walk_fn_decl(&ast_visitor v, &ast.fn_decl fd) {
+    for (ast.arg a in fd.inputs) {
+        walk_ty(v, a.ty);
+    }
+    walk_ty(v, fd.output);
+}
+
+fn walk_fn(&ast_visitor v, &ast._fn f) {
+    if (!v.keep_going()) { ret; }
+    walk_fn_decl(v, f.decl);
+    walk_block(v, f.body);
+}
+
+fn walk_block(&ast_visitor v, &ast.block b) {
+    if (!v.keep_going()) { ret; }
+    v.visit_block_pre(b);
+    for (@ast.stmt s in b.node.stmts) {
+        walk_stmt(v, s);
+    }
+    walk_expr_opt(v, b.node.expr);
+    v.visit_block_post(b);
+}
+
+fn walk_stmt(&ast_visitor v, @ast.stmt s) {
+    if (!v.keep_going()) { ret; }
+    v.visit_stmt_pre(s);
+    alt (s.node) {
+        case (ast.stmt_decl(?d, _)) {
+            walk_decl(v, d);
+        }
+        case (ast.stmt_expr(?e, _)) {
+            walk_expr(v, e);
+        }
+        case (ast.stmt_crate_directive(?cdir)) {
+            walk_crate_directive(v, cdir);
+        }
+    }
+    v.visit_stmt_post(s);
+}
+
+fn walk_decl(&ast_visitor v, @ast.decl d) {
+    if (!v.keep_going()) { ret; }
+    v.visit_decl_pre(d);
+    alt (d.node) {
+        case (ast.decl_local(?loc)) {
+            alt (loc.ty) {
+                case (none[@ast.ty]) {}
+                case (some[@ast.ty](?t)) { walk_ty(v, t); }
+            }
+            alt (loc.init) {
+                case (none[ast.initializer]) {}
+                case (some[ast.initializer](?i)) {
+                    walk_expr(v, i.expr);
+                }
+            }
+        }
+        case (ast.decl_item(?it)) {
+            walk_item(v, it);
+        }
+    }
+    v.visit_decl_post(d);
+}
+
+fn walk_expr_opt(&ast_visitor v, option.t[@ast.expr] eo) {
+    alt (eo) {
+        case (none[@ast.expr]) {}
+        case (some[@ast.expr](?e)) {
+            walk_expr(v, e);
+        }
+    }
+}
+
+fn walk_exprs(&ast_visitor v, vec[@ast.expr] exprs) {
+    for (@ast.expr e in exprs) {
+        walk_expr(v, e);
+    }
+}
+
+fn walk_expr(&ast_visitor v, @ast.expr e) {
+    if (!v.keep_going()) { ret; }
+    v.visit_expr_pre(e);
+    alt (e.node) {
+        case (ast.expr_vec(?es, _, _)) {
+            walk_exprs(v, es);
+        }
+        case (ast.expr_tup(?elts, _)) {
+            for (ast.elt e in elts) {
+                walk_expr(v, e.expr);
+            }
+        }
+        case (ast.expr_rec(?flds, ?base, _)) {
+            for (ast.field f in flds) {
+                walk_expr(v, f.expr);
+            }
+            walk_expr_opt(v, base);
+        }
+        case (ast.expr_call(?callee, ?args, _)) {
+            walk_expr(v, callee);
+            walk_exprs(v, args);
+        }
+        case (ast.expr_self_method(_, _)) { }
+        case (ast.expr_bind(?callee, ?args, _)) {
+            walk_expr(v, callee);
+            for (option.t[@ast.expr] eo in args) {
+                walk_expr_opt(v, eo);
+            }
+        }
+        case (ast.expr_spawn(_, _, ?callee, ?args, _)) {
+            walk_expr(v, callee);
+            walk_exprs(v, args);
+        }
+        case (ast.expr_binary(_, ?a, ?b, _)) {
+            walk_expr(v, a);
+            walk_expr(v, b);
+        }
+        case (ast.expr_unary(_, ?a, _)) {
+            walk_expr(v, a);
+        }
+        case (ast.expr_lit(_, _)) { }
+        case (ast.expr_cast(?x, ?t, _)) {
+            walk_expr(v, x);
+            walk_ty(v, t);
+        }
+        case (ast.expr_if(?x, ?b, ?eo, _)) {
+            walk_expr(v, x);
+            walk_block(v, b);
+            walk_expr_opt(v, eo);
+        }
+        case (ast.expr_while(?x, ?b, _)) {
+            walk_expr(v, x);
+            walk_block(v, b);
+        }
+        case (ast.expr_for(?dcl, ?x, ?b, _)) {
+            walk_decl(v, dcl);
+            walk_expr(v, x);
+            walk_block(v, b);
+        }
+        case (ast.expr_for_each(?dcl, ?x, ?b, _)) {
+            walk_decl(v, dcl);
+            walk_expr(v, x);
+            walk_block(v, b);
+        }
+        case (ast.expr_do_while(?b, ?x, _)) {
+            walk_block(v, b);
+            walk_expr(v, x);
+        }
+        case (ast.expr_alt(?x, ?arms, _)) {
+            walk_expr(v, x);
+            for (ast.arm a in arms) {
+                walk_block(v, a.block);
+            }
+        }
+        case (ast.expr_block(?b, _)) {
+            walk_block(v, b);
+        }
+        case (ast.expr_assign(?a, ?b, _)) {
+            walk_expr(v, a);
+            walk_expr(v, b);
+        }
+        case (ast.expr_assign_op(_, ?a, ?b, _)) {
+            walk_expr(v, a);
+            walk_expr(v, b);
+        }
+        case (ast.expr_send(?a, ?b, _)) {
+            walk_expr(v, a);
+            walk_expr(v, b);
+        }
+        case (ast.expr_recv(?a, ?b, _)) {
+            walk_expr(v, a);
+            walk_expr(v, b);
+        }
+        case (ast.expr_field(?x, _, _)) {
+            walk_expr(v, x);
+        }
+        case (ast.expr_index(?a, ?b, _)) {
+            walk_expr(v, a);
+            walk_expr(v, b);
+        }
+        case (ast.expr_path(_, _, _)) { }
+        case (ast.expr_ext(_, ?es, ?eo, ?x, _)) {
+            walk_exprs(v, es);
+            walk_expr_opt(v, eo);
+            walk_expr(v, x);
+        }
+        case (ast.expr_fail(_)) { }
+        case (ast.expr_break(_)) { }
+        case (ast.expr_cont(_)) { }
+        case (ast.expr_ret(?eo, _)) {
+            walk_expr_opt(v, eo);
+        }
+        case (ast.expr_put(?eo, _)) {
+            walk_expr_opt(v, eo);
+        }
+        case (ast.expr_be(?x, _)) {
+            walk_expr(v, x);
+        }
+        case (ast.expr_log(?x, _)) {
+            walk_expr(v, x);
+        }
+        case (ast.expr_check_expr(?x, _)) {
+            walk_expr(v, x);
+        }
+        case (ast.expr_port(_)) { }
+        case (ast.expr_chan(?x, _)) {
+            walk_expr(v, x);
+        }
+    }
+    v.visit_expr_post(e);
+}
+
+fn def_keep_going() -> bool { ret true; }
+fn def_want_crate_directives() -> bool { ret false; }
+fn def_visit_crate(&ast.crate c) { }
+fn def_visit_crate_directive(@ast.crate_directive c) { }
+fn def_visit_view_item(@ast.view_item vi) { }
+fn def_visit_native_item(@ast.native_item ni) { }
+fn def_visit_item(@ast.item i) { }
+fn def_visit_block(&ast.block b) { }
+fn def_visit_stmt(@ast.stmt s) { }
+fn def_visit_decl(@ast.decl d) { }
+fn def_visit_expr(@ast.expr e) { }
+fn def_visit_ty(@ast.ty t) { }
+
+fn default_visitor() -> ast_visitor {
+
+    auto d_keep_going = def_keep_going;
+    auto d_want_crate_directives = def_want_crate_directives;
+    auto d_visit_crate = def_visit_crate;
+    auto d_visit_crate_directive = def_visit_crate_directive;
+    auto d_visit_view_item = def_visit_view_item;
+    auto d_visit_native_item = def_visit_native_item;
+    auto d_visit_item = def_visit_item;
+    auto d_visit_block = def_visit_block;
+    auto d_visit_stmt = def_visit_stmt;
+    auto d_visit_decl = def_visit_decl;
+    auto d_visit_expr = def_visit_expr;
+    auto d_visit_ty = def_visit_ty;
+
+    ret rec(keep_going = d_keep_going,
+            want_crate_directives = d_want_crate_directives,
+            visit_crate_pre = d_visit_crate,
+            visit_crate_post = d_visit_crate,
+            visit_crate_directive_pre = d_visit_crate_directive,
+            visit_crate_directive_post = d_visit_crate_directive,
+            visit_view_item_pre = d_visit_view_item,
+            visit_view_item_post = d_visit_view_item,
+            visit_native_item_pre = d_visit_native_item,
+            visit_native_item_post = d_visit_native_item,
+            visit_item_pre = d_visit_item,
+            visit_item_post = d_visit_item,
+            visit_block_pre = d_visit_block,
+            visit_block_post = d_visit_block,
+            visit_stmt_pre = d_visit_stmt,
+            visit_stmt_post = d_visit_stmt,
+            visit_decl_pre = d_visit_decl,
+            visit_decl_post = d_visit_decl,
+            visit_expr_pre = d_visit_expr,
+            visit_expr_post = d_visit_expr,
+            visit_ty_pre = d_visit_ty,
+            visit_ty_post = d_visit_ty);
+}
+
+//
+// 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:
+//
+
+
+