diff --git a/compiler/dcalc/ast.ml b/compiler/dcalc/ast.ml
index 3c19da3f..11350503 100644
--- a/compiler/dcalc/ast.ml
+++ b/compiler/dcalc/ast.ml
@@ -408,6 +408,16 @@ let map_exprs_in_scopes ~f ~varf scopes =
         new_scope_body_expr new_next)
     ~init:(Bindlib.box Nil) scopes
 
+let untype_program prg =
+  {
+    prg with
+    scopes =
+      Bindlib.unbox
+        (map_exprs_in_scopes
+           ~f:(fun e -> Bindlib.box (untype_expr e))
+           ~varf:translate_var prg.scopes);
+  }
+
 type 'm var = 'm expr Bindlib.var
 type 'm vars = 'm expr Bindlib.mvar
 
diff --git a/compiler/dcalc/ast.mli b/compiler/dcalc/ast.mli
index 072460ee..7de3bffd 100644
--- a/compiler/dcalc/ast.mli
+++ b/compiler/dcalc/ast.mli
@@ -251,6 +251,7 @@ val fold_marks :
 
 val get_scope_body_mark : ('expr, 'm) scope_body -> 'm mark
 val untype_expr : 'm marked_expr -> untyped marked_expr
+val untype_program : 'm program -> untyped program
 
 (** {2 Boxed constructors} *)
 
diff --git a/compiler/dcalc/optimizations.ml b/compiler/dcalc/optimizations.ml
index 4439968b..34d4dc7c 100644
--- a/compiler/dcalc/optimizations.ml
+++ b/compiler/dcalc/optimizations.ml
@@ -247,8 +247,9 @@ let program_map
     (fun new_scopes -> { p with scopes = new_scopes })
     (scopes_map t ctx p.scopes)
 
-let optimize_program (p : 'm program) : 'm program =
+let optimize_program (p : 'm program) : untyped program =
   Bindlib.unbox
     (program_map partial_evaluation
        { var_values = VarMap.empty; decl_ctx = p.decl_ctx }
        p)
+  |> untype_program
diff --git a/compiler/dcalc/optimizations.mli b/compiler/dcalc/optimizations.mli
index 27090edc..53c7a600 100644
--- a/compiler/dcalc/optimizations.mli
+++ b/compiler/dcalc/optimizations.mli
@@ -20,4 +20,4 @@
 open Ast
 
 val optimize_expr : decl_ctx -> 'm marked_expr -> 'm marked_expr Bindlib.box
-val optimize_program : 'm program -> 'm program
+val optimize_program : 'm program -> untyped program
diff --git a/compiler/driver.ml b/compiler/driver.ml
index 6b752eb1..8b4fbe2d 100644
--- a/compiler/driver.ml
+++ b/compiler/driver.ml
@@ -284,7 +284,7 @@ let driver source_file (options : Cli.options) : int =
                 Cli.debug_print "Optimizing lambda calculus...";
                 Lcalc.Optimizations.optimize_program prgm
               end
-              else prgm
+              else Lcalc.Ast.untype_program prgm
             in
             let prgm =
               if options.closure_conversion then (
diff --git a/compiler/lcalc/ast.ml b/compiler/lcalc/ast.ml
index 63cbcd31..3fa07c24 100644
--- a/compiler/lcalc/ast.ml
+++ b/compiler/lcalc/ast.ml
@@ -118,13 +118,17 @@ let eraise e1 pos = Bindlib.box (ERaise e1, pos)
 let ecatch e1 exn e2 pos =
   Bindlib.box_apply2 (fun e1 e2 -> ECatch (e1, exn, e2), pos) e1 e2
 
+let translate_var v = Bindlib.copy_var v (fun x -> EVar x) (Bindlib.name_of v)
+
 let map_expr ctx ~f e =
+  let m = Marked.get_mark e in
   match Marked.unmark e with
-  | EVar v -> evar v (Marked.get_mark e)
+  | EVar v -> evar (translate_var v) (Marked.get_mark e)
   | EApp (e1, args) ->
     eapp (f ctx e1) (List.map (f ctx) args) (Marked.get_mark e)
   | EAbs (binder, typs) ->
-    eabs (Bindlib.box_mbinder (f ctx) binder) typs (Marked.get_mark e)
+    let vars, body = Bindlib.unmbind binder in
+    eabs (Bindlib.bind_mvar (Array.map translate_var vars) (f ctx body)) typs m
   | ETuple (args, s) -> etuple (List.map (f ctx) args) s (Marked.get_mark e)
   | ETupleAccess (e1, n, s_name, typs) ->
     etupleaccess ((f ctx) e1) n s_name typs (Marked.get_mark e)
@@ -141,6 +145,26 @@ let map_expr ctx ~f e =
     eifthenelse ((f ctx) e1) ((f ctx) e2) ((f ctx) e3) (Marked.get_mark e)
   | ECatch (e1, exn, e2) -> ecatch (f ctx e1) exn (f ctx e2) (Marked.get_mark e)
 
+let rec map_expr_top_down ~f e =
+  map_expr () ~f:(fun () -> map_expr_top_down ~f) (f e)
+
+let map_expr_marks ~f e =
+  Bindlib.unbox
+  @@ map_expr_top_down ~f:(fun e -> Marked.(mark (f (get_mark e)) (unmark e))) e
+
+let untype_expr e =
+  map_expr_marks ~f:(fun m -> Untyped { pos = D.mark_pos m }) e
+
+let untype_program prg =
+  {
+    prg with
+    scopes =
+      Bindlib.unbox
+        (D.map_exprs_in_scopes
+           ~f:(fun e -> Bindlib.box (untype_expr e))
+           ~varf:translate_var prg.scopes);
+  }
+
 (** See [Bindlib.box_term] documentation for why we are doing that. *)
 let box_expr (e : 'm marked_expr) : 'm marked_expr Bindlib.box =
   let rec id_t () e = map_expr () ~f:id_t e in
diff --git a/compiler/lcalc/ast.mli b/compiler/lcalc/ast.mli
index bae1a9f0..10a595e5 100644
--- a/compiler/lcalc/ast.mli
+++ b/compiler/lcalc/ast.mli
@@ -91,6 +91,28 @@ val new_var : string -> 'm var
 
 type 'm binder = ('m expr, 'm marked_expr) Bindlib.binder
 
+(** {2 Program traversal} *)
+
+val map_expr :
+  'a ->
+  f:('a -> 'm1 marked_expr -> 'm2 marked_expr Bindlib.box) ->
+  ('m1 expr, 'm2 mark) Marked.t ->
+  'm2 marked_expr Bindlib.box
+(** See [Dcalc.Ast.map_expr] *)
+
+val map_expr_top_down :
+  f:('m1 marked_expr -> ('m1 expr, 'm2 mark) Marked.t) ->
+  'm1 marked_expr ->
+  'm2 marked_expr Bindlib.box
+(** See [Dcalc.Ast.map_expr_top_down] *)
+
+val map_expr_marks :
+  f:('m1 mark -> 'm2 mark) -> 'm1 marked_expr -> 'm2 marked_expr
+(** See [Dcalc.Ast.map_expr_marks] *)
+
+val untype_expr : 'm marked_expr -> Dcalc.Ast.untyped marked_expr
+val untype_program : 'm program -> Dcalc.Ast.untyped program
+
 (** {1 Boxed constructors} *)
 
 val evar : 'm expr Bindlib.var -> 'm mark -> 'm marked_expr Bindlib.box
diff --git a/compiler/lcalc/optimizations.ml b/compiler/lcalc/optimizations.ml
index ef521799..fc477470 100644
--- a/compiler/lcalc/optimizations.ml
+++ b/compiler/lcalc/optimizations.ml
@@ -150,5 +150,5 @@ let peephole_optimizations (p : 'm program) : 'm program =
   in
   { p with scopes = Bindlib.unbox new_scopes }
 
-let optimize_program (p : 'm program) : 'm program =
-  p |> iota_optimizations |> peephole_optimizations
+let optimize_program (p : 'm program) : Dcalc.Ast.untyped program =
+  p |> iota_optimizations |> peephole_optimizations |> untype_program
diff --git a/compiler/lcalc/optimizations.mli b/compiler/lcalc/optimizations.mli
index 59776c96..da3af2c5 100644
--- a/compiler/lcalc/optimizations.mli
+++ b/compiler/lcalc/optimizations.mli
@@ -16,6 +16,6 @@
 
 open Ast
 
-val optimize_program : 'm program -> 'm program
+val optimize_program : 'm program -> Dcalc.Ast.untyped program
 (** Warning/todo: no effort was yet made to ensure correct propagation of type
     annotations in the typed case *)
diff --git a/compiler/plugin.ml b/compiler/plugin.ml
index 09713b0a..08fce637 100644
--- a/compiler/plugin.ml
+++ b/compiler/plugin.ml
@@ -21,7 +21,7 @@ type 'ast gen = {
 }
 
 type t =
-  | Lcalc of Dcalc.Ast.typed Lcalc.Ast.program gen
+  | Lcalc of Dcalc.Ast.untyped Lcalc.Ast.program gen
   | Scalc of Scalc.Ast.program gen
 
 let name = function Lcalc { name; _ } | Scalc { name; _ } -> name
diff --git a/compiler/plugin.mli b/compiler/plugin.mli
index 3f7d0c14..27acbef7 100644
--- a/compiler/plugin.mli
+++ b/compiler/plugin.mli
@@ -23,7 +23,7 @@ type 'ast gen = {
 }
 
 type t =
-  | Lcalc of Dcalc.Ast.typed Lcalc.Ast.program gen
+  | Lcalc of Dcalc.Ast.untyped Lcalc.Ast.program gen
   | Scalc of Scalc.Ast.program gen
 
 val find : string -> t
@@ -42,7 +42,7 @@ module PluginAPI : sig
     name:string ->
     extension:string ->
     (string option ->
-    Dcalc.Ast.typed Lcalc.Ast.program ->
+    Dcalc.Ast.untyped Lcalc.Ast.program ->
     Scopelang.Dependency.TVertex.t list ->
     unit) ->
     unit