diff --git a/build_system/clerk_driver.ml b/build_system/clerk_driver.ml
index f377c377..2237d103 100644
--- a/build_system/clerk_driver.ml
+++ b/build_system/clerk_driver.ml
@@ -548,8 +548,11 @@ let[@ocamlformat "disable"] static_base_rules =
     Nj.rule "out-test"
       ~command: [
         !catala_exe; !test_command; "--plugin-dir="; "-o -"; !catala_flags;
-        !input; ">"; !output; "2>&1";
-        "||"; "true";
+        !input; "2>&1";
+        "|"; "sed";
+        "'s/\"CM0|[a-zA-Z0-9|]*\"/\"CMX|XXXXXXXX|XXXXXXXX|XXXXXXXX\"/g'";
+        ">"; !output;
+        "||"; "true"
       ]
       ~description:
         ["<catala>"; "test"; !test_id; "⇐"; !input; "(" ^ !test_command ^ ")"];
diff --git a/build_system/clerk_runtest.ml b/build_system/clerk_runtest.ml
index 4f262be8..b9daad31 100644
--- a/build_system/clerk_runtest.ml
+++ b/build_system/clerk_runtest.ml
@@ -16,10 +16,32 @@
 
 open Catala_utils
 
+let sanitize =
+  let re_endtest = Re.(compile @@ seq [bol; str "```"]) in
+  let re_modhash =
+    Re.(
+      compile
+      @@ seq
+           [
+             str "\"CM0|";
+             repn xdigit 8 (Some 8);
+             char '|';
+             repn xdigit 8 (Some 8);
+             char '|';
+             repn xdigit 8 (Some 8);
+             char '"';
+           ])
+  in
+  fun str ->
+    str
+    |> Re.replace_string re_endtest ~by:"\\```"
+    |> Re.replace_string re_modhash ~by:"\"CMX|XXXXXXXX|XXXXXXXX|XXXXXXXX\""
+
 let run_catala_test test_flags catala_exe catala_opts file program args oc =
-  let cmd_in_rd, cmd_in_wr = Unix.pipe () in
-  Unix.set_close_on_exec cmd_in_wr;
+  let cmd_in_rd, cmd_in_wr = Unix.pipe ~cloexec:true () in
+  let cmd_out_rd, cmd_out_wr = Unix.pipe ~cloexec:true () in
   let command_oc = Unix.out_channel_of_descr cmd_in_wr in
+  let command_ic = Unix.in_channel_of_descr cmd_out_rd in
   let catala_exe =
     (* If the exe name contains directories, make it absolute. Otherwise don't
        modify it so that it can be looked up in PATH. *)
@@ -59,12 +81,21 @@ let run_catala_test test_flags catala_exe catala_opts file program args oc =
     |> Seq.cons "CATALA_PLUGINS="
     |> Array.of_seq
   in
-  flush oc;
-  let ocfd = Unix.descr_of_out_channel oc in
-  let pid = Unix.create_process_env catala_exe cmd env cmd_in_rd ocfd ocfd in
+  let pid =
+    Unix.create_process_env catala_exe cmd env cmd_in_rd cmd_out_wr cmd_out_wr
+  in
   Unix.close cmd_in_rd;
+  Unix.close cmd_out_wr;
   Seq.iter (output_string command_oc) program;
   close_out command_oc;
+  let out_lines =
+    Seq.of_dispenser (fun () -> In_channel.input_line command_ic)
+  in
+  Seq.iter
+    (fun line ->
+      output_string oc (sanitize line);
+      output_char oc '\n')
+    out_lines;
   let return_code =
     match Unix.waitpid [] pid with
     | _, Unix.WEXITED n -> n
diff --git a/compiler/catala_utils/hash.ml b/compiler/catala_utils/hash.ml
new file mode 100644
index 00000000..4854ffab
--- /dev/null
+++ b/compiler/catala_utils/hash.ml
@@ -0,0 +1,110 @@
+(* This file is part of the Catala compiler, a specification language for tax
+   and social benefits computation rules. Copyright (C) 2024 Inria, contributor:
+   Louis Gesbert <louis.gesbert@inria.fr>
+
+   Licensed under the Apache License, Version 2.0 (the "License"); you may not
+   use this file except in compliance with the License. You may obtain a copy of
+   the License at
+
+   http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+   WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+   License for the specific language governing permissions and limitations under
+   the License. *)
+
+type t = int
+
+let mix (h1 : t) (h2 : t) : t = Hashtbl.hash (h1, h2)
+let raw = Hashtbl.hash
+
+module Op = struct
+  let ( % ) = mix
+  let ( ! ) = raw
+end
+
+open Op
+
+let option f = function None -> !`None | Some x -> !`Some % f x
+let list hf l = List.fold_left (fun acc x -> acc % hf x) !`ListEmpty l
+
+let map fold_fun kh vh map =
+  fold_fun (fun k v acc -> acc lxor (kh k % vh v)) map !`HashMapDelim
+
+module Flags : sig
+  type nonrec t = private t
+
+  val pass :
+    (t -> 'a) ->
+    avoid_exceptions:bool ->
+    closure_conversion:bool ->
+    monomorphize_types:bool ->
+    'a
+
+  val of_t : int -> t
+end = struct
+  type nonrec t = t
+
+  let pass k ~avoid_exceptions ~closure_conversion ~monomorphize_types =
+    (* Should not affect the call convention or actual interfaces: include,
+       optimize, check_invariants, typed *)
+    !(avoid_exceptions : bool)
+    % !(closure_conversion : bool)
+    % !(monomorphize_types : bool)
+    % (* The following may not affect the call convention, but we want it set in
+         an homogeneous way *)
+    !(Global.options.trace : bool)
+    % !(Global.options.max_prec_digits : int)
+    |> k
+
+  let of_t t = t
+end
+
+type full = { catala_version : t; flags_hash : Flags.t; contents : t }
+
+let finalise t =
+  Flags.pass (fun flags_hash ->
+      { catala_version = !(Version.v : string); flags_hash; contents = t })
+
+let to_string full =
+  Printf.sprintf "CM0|%08x|%08x|%08x" full.catala_version
+    (full.flags_hash :> int)
+    full.contents
+
+(* Putting color inside the hash makes them much easier to differentiate at a
+   glance *)
+let format ppf full =
+  let open Ocolor_types in
+  let pcolor col f x =
+    Format.pp_open_stag ppf Ocolor_format.(Ocolor_style_tag (Fg (C24 col)));
+    f x;
+    Format.pp_close_stag ppf ()
+  in
+  let tag = pcolor { r24 = 172; g24 = 172; b24 = 172 } in
+  let auto i =
+    {
+      r24 = 128 + (i mod 128);
+      g24 = 128 + ((i lsr 10) mod 128);
+      b24 = 128 + ((i lsr 20) mod 128);
+    }
+  in
+  let phash h =
+    let col = auto h in
+    pcolor col (Format.fprintf ppf "%08x") h
+  in
+  tag (Format.pp_print_string ppf) "CM0|";
+  phash full.catala_version;
+  tag (Format.pp_print_string ppf) "|";
+  phash (full.flags_hash :> int);
+  tag (Format.pp_print_string ppf) "|";
+  phash full.contents
+
+let of_string s =
+  try
+    Scanf.sscanf s "CM0|%08x|%08x|%08x"
+      (fun catala_version flags_hash contents ->
+        { catala_version; flags_hash = Flags.of_t flags_hash; contents })
+  with Scanf.Scan_failure _ -> failwith "Hash.of_string"
+
+let external_placeholder = "*external*"
diff --git a/compiler/catala_utils/hash.mli b/compiler/catala_utils/hash.mli
new file mode 100644
index 00000000..b70ba2f1
--- /dev/null
+++ b/compiler/catala_utils/hash.mli
@@ -0,0 +1,81 @@
+(* This file is part of the Catala compiler, a specification language for tax
+   and social benefits computation rules. Copyright (C) 2024 Inria, contributor:
+   Louis Gesbert <louis.gesbert@inria.fr>
+
+   Licensed under the Apache License, Version 2.0 (the "License"); you may not
+   use this file except in compliance with the License. You may obtain a copy of
+   the License at
+
+   http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+   WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+   License for the specific language governing permissions and limitations under
+   the License. *)
+
+(** Hashes for the identification of modules.
+
+    In contrast with OCaml's basic `Hashtbl.hash`, they process the full depth
+    of terms. Any meaningful interface change in a module should only be in hash
+    collision with a 1/2^30 probability. *)
+
+type t = private int
+(** Native Hasthbl.hash hashes, value is truncated to 30 bits whatever the
+    architecture (positive 31-bit integers) *)
+
+type full
+(** A "full" hash includes the Catala version and compilation flags, alongside
+    the module interface *)
+
+val raw : 'a -> t
+(** [Hashtbl.hash]. Do not use on deep types (it has a bounded depth), use
+    specific hashing functions. *)
+
+module Op : sig
+  val ( ! ) : 'a -> t
+  (** Shortcut to [raw]. Same warning: use with an explicit type annotation
+      [!(foo: string)] to ensure it's not called on types that are recursive or
+      include annotations.
+
+      Hint: we use [!`Foo] as a fancy way to generate constants for
+      discriminating constructions *)
+
+  val ( % ) : t -> t -> t
+  (** Safe combination of two hashes (non commutative or associative, etc.) *)
+end
+
+val option : ('a -> t) -> 'a option -> t
+val list : ('a -> t) -> 'a list -> t
+
+val map :
+  (('k -> 'v -> t -> t) -> 'map -> t -> t) ->
+  ('k -> t) ->
+  ('v -> t) ->
+  'map ->
+  t
+(** [map fold_f key_hash_f value_hash_f map] computes the hash of a map. The
+    first argument is expected to be a [Foo.Map.fold] function. The result is
+    independent of the ordering of the map. *)
+
+val finalise :
+  t ->
+  avoid_exceptions:bool ->
+  closure_conversion:bool ->
+  monomorphize_types:bool ->
+  full
+(** Turns a raw interface hash into a full hash, ready for printing *)
+
+val to_string : full -> string
+val format : Format.formatter -> full -> unit
+
+val of_string : string -> full
+(** @raise Failure *)
+
+val external_placeholder : string
+(** It's inconvenient to need hash updates on external modules. This string is
+    uses as a hash instead for those cases.
+
+    NOTE: This is a temporary solution A future approach could be to have Catala
+    generate a module loader (with the proper hash), relieving the user
+    implementation from having to do the registration. *)
diff --git a/compiler/catala_utils/mark.ml b/compiler/catala_utils/mark.ml
index 8d50f263..ef1be20c 100644
--- a/compiler/catala_utils/mark.ml
+++ b/compiler/catala_utils/mark.ml
@@ -29,6 +29,7 @@ let fold f (x, _) = f x
 let fold2 f (x, _) (y, _) = f x y
 let compare cmp a b = fold2 cmp a b
 let equal eq a b = fold2 eq a b
+let hash f (x, _) = f x
 
 class ['self] marked_map =
   object (_self : 'self)
diff --git a/compiler/catala_utils/mark.mli b/compiler/catala_utils/mark.mli
index 95a5ace5..ebbf44a7 100644
--- a/compiler/catala_utils/mark.mli
+++ b/compiler/catala_utils/mark.mli
@@ -41,6 +41,10 @@ val compare : ('a -> 'a -> int) -> ('a, 'm) ed -> ('a, 'm) ed -> int
 val equal : ('a -> 'a -> bool) -> ('a, 'm) ed -> ('a, 'm) ed -> bool
 (** Tests equality of two marked values {b ignoring marks} *)
 
+val hash : ('a -> Hash.t) -> ('a, 'm) ed -> Hash.t
+(** Computes the hash of the marked values using the given function
+    {b ignoring mark} *)
+
 (** Visitors *)
 
 class ['self] marked_map : object ('self)
diff --git a/compiler/catala_utils/string.ml b/compiler/catala_utils/string.ml
index 44dc5e6f..0af8ec76 100644
--- a/compiler/catala_utils/string.ml
+++ b/compiler/catala_utils/string.ml
@@ -101,6 +101,7 @@ module Arg = struct
 end
 
 let compare = Arg.compare
+let hash t = Hash.raw t
 
 module Set = Set.Make (Arg)
 module Map = Map.Make (Arg)
diff --git a/compiler/catala_utils/string.mli b/compiler/catala_utils/string.mli
index ee7248be..b16b6723 100644
--- a/compiler/catala_utils/string.mli
+++ b/compiler/catala_utils/string.mli
@@ -23,6 +23,8 @@ module Map : Map.S with type key = string
 val compare : string -> string -> int
 (** String comparison with natural ordering of numbers within strings *)
 
+val hash : string -> Hash.t
+
 val to_ascii : string -> string
 (** Removes all non-ASCII diacritics from a string by converting them to their
     base letter in the Latin alphabet. *)
diff --git a/compiler/catala_utils/uid.ml b/compiler/catala_utils/uid.ml
index 23beedae..83c89e52 100644
--- a/compiler/catala_utils/uid.ml
+++ b/compiler/catala_utils/uid.ml
@@ -21,6 +21,7 @@ module type Info = sig
   val format : Format.formatter -> info -> unit
   val equal : info -> info -> bool
   val compare : info -> info -> int
+  val hash : info -> Hash.t
 end
 
 module type Id = sig
@@ -33,7 +34,8 @@ module type Id = sig
   val equal : t -> t -> bool
   val format : Format.formatter -> t -> unit
   val to_string : t -> string
-  val hash : t -> int
+  val id : t -> int
+  val hash : t -> Hash.t
 
   module Set : Set.S with type elt = t
   module Map : Map.S with type key = t
@@ -68,8 +70,9 @@ module Make (X : Info) (S : Style) () : Id with type info = X.info = struct
     { id = !counter; info }
 
   let get_info (uid : t) : X.info = uid.info
-  let hash (x : t) : int = x.id
+  let id (x : t) : int = x.id
   let to_string t = X.to_string t.info
+  let hash t = X.hash t.info
 
   module Set = Set.Make (Ordering)
   module Map = Map.Make (Ordering)
@@ -84,6 +87,7 @@ module MarkedString = struct
   let format fmt i = String.format fmt (to_string i)
   let equal = Mark.equal String.equal
   let compare = Mark.compare String.compare
+  let hash = Mark.hash String.hash
 end
 
 module Gen (S : Style) () = Make (MarkedString) (S) ()
@@ -109,6 +113,15 @@ module Path = struct
   let to_string p = String.concat "." (List.map Module.to_string p)
   let equal = List.equal Module.equal
   let compare = List.compare Module.compare
+
+  let strip prefix p0 =
+    let rec aux prefix p =
+      match prefix, p with
+      | pfx1 :: pfx, p1 :: p -> if Module.equal pfx1 p1 then aux pfx p else p0
+      | [], p -> p
+      | _ -> p0
+    in
+    aux prefix p0
 end
 
 module QualifiedMarkedString = struct
@@ -125,12 +138,21 @@ module QualifiedMarkedString = struct
 
   let compare (p1, i1) (p2, i2) =
     match Path.compare p1 p2 with 0 -> MarkedString.compare i1 i2 | n -> n
+
+  let hash (p, i) =
+    let open Hash.Op in
+    Hash.list Module.hash p % MarkedString.hash i
 end
 
 module Gen_qualified (S : Style) () = struct
   include Make (QualifiedMarkedString) (S) ()
 
   let fresh path t = fresh (path, t)
+
+  let hash ~strip t =
+    let p, i = get_info t in
+    QualifiedMarkedString.hash (Path.strip strip p, i)
+
   let path t = fst (get_info t)
   let get_info t = snd (get_info t)
 end
diff --git a/compiler/catala_utils/uid.mli b/compiler/catala_utils/uid.mli
index 4ed2e8d7..a51e6cf4 100644
--- a/compiler/catala_utils/uid.mli
+++ b/compiler/catala_utils/uid.mli
@@ -28,6 +28,9 @@ module type Info = sig
 
   val compare : info -> info -> int
   (** Comparison disregards position *)
+
+  val hash : info -> Hash.t
+  (** Hashing disregards position *)
 end
 
 module MarkedString : Info with type info = string Mark.pos
@@ -48,7 +51,15 @@ module type Id = sig
   val equal : t -> t -> bool
   val format : Format.formatter -> t -> unit
   val to_string : t -> string
-  val hash : t -> int
+
+  val id : t -> int
+  (** Returns the unique ID of the identifier *)
+
+  val hash : t -> Hash.t
+  (** While [id] returns a unique ID valable for a given Uid instance within a
+      given run of catala, this is a raw hash of the identifier string.
+      Therefore, it may collide within a given program, but remains meaninful
+      across separate compilations. *)
 
   module Set : Set.S with type elt = t
   module Map : Map.S with type key = t
@@ -79,6 +90,10 @@ module Path : sig
   val format : Format.formatter -> t -> unit
   val equal : t -> t -> bool
   val compare : t -> t -> int
+
+  val strip : t -> t -> t
+  (** [strip pfx p] removed [pfx] from the start of [p]. if [p] doesn't start
+      with [pfx], it is returned unchanged *)
 end
 
 (** Same as [Gen] but also registers path information *)
@@ -88,4 +103,6 @@ module Gen_qualified (_ : Style) () : sig
   val fresh : Path.t -> MarkedString.info -> t
   val path : t -> Path.t
   val get_info : t -> MarkedString.info
+  val hash : strip:Path.t -> t -> Hash.t
+  (* [strip] strips that prefix from the start of the path before hashing *)
 end
diff --git a/compiler/desugared/ast.ml b/compiler/desugared/ast.ml
index 9ac16a16..19ded35b 100644
--- a/compiler/desugared/ast.ml
+++ b/compiler/desugared/ast.ml
@@ -71,12 +71,17 @@ module ScopeDef = struct
       ScopeVar.format ppf (Mark.remove v);
       format_kind ppf k
 
-    let hash_kind = function
-      | Var None -> 0
-      | Var (Some st) -> StateName.hash st
-      | SubScopeInput { var_within_origin_scope = v; _ } -> ScopeVar.hash v
+    open Hash.Op
 
-    let hash (v, k) = Int.logxor (ScopeVar.hash (Mark.remove v)) (hash_kind k)
+    let hash_kind ~strip = function
+      | Var v -> !`Var % Hash.option StateName.hash v
+      | SubScopeInput { name; var_within_origin_scope } ->
+        !`SubScopeInput
+        % ScopeName.hash ~strip name
+        % ScopeVar.hash var_within_origin_scope
+
+    let hash ~strip (v, k) =
+      Hash.Op.(ScopeVar.hash (Mark.remove v) % hash_kind ~strip k)
   end
 
   include Base
@@ -231,6 +236,8 @@ type scope_def = {
 
 type var_or_states = WholeVar | States of StateName.t list
 
+(* If fields are added, make sure to consider including them in the hash
+   computations below *)
 type scope = {
   scope_vars : var_or_states ScopeVar.Map.t;
   scope_sub_scopes : ScopeName.t ScopeVar.Map.t;
@@ -239,21 +246,76 @@ type scope = {
   scope_assertions : assertion AssertionName.Map.t;
   scope_options : catala_option Mark.pos list;
   scope_meta_assertions : meta_assertion list;
+  scope_visibility : visibility;
+}
+
+type topdef = {
+  topdef_expr : expr option;
+  topdef_type : typ;
+  topdef_visibility : visibility;
 }
 
 type modul = {
   module_scopes : scope ScopeName.Map.t;
-  module_topdefs : (expr option * typ) TopdefName.Map.t;
+  module_topdefs : topdef TopdefName.Map.t;
 }
 
 type program = {
-  program_module_name : Ident.t Mark.pos option;
+  program_module_name : (ModuleName.t * module_intf_id) option;
   program_ctx : decl_ctx;
   program_modules : modul ModuleName.Map.t;
   program_root : modul;
   program_lang : Global.backend_lang;
 }
 
+module Hash = struct
+  open Hash.Op
+
+  let var_or_state = function
+    | WholeVar -> !`WholeVar
+    | States s -> !`States % Hash.list StateName.hash s
+
+  let io x =
+    !(Mark.remove x.io_input : Runtime.io_input)
+    % !(Mark.remove x.io_output : bool)
+
+  let scope_decl ~strip d =
+    (* scope_def_rules is ignored (not part of the interface) *)
+    Type.hash ~strip d.scope_def_typ
+    % Hash.option
+        (fun (lst, _) ->
+          List.fold_left
+            (fun acc (name, ty) ->
+              acc % Uid.MarkedString.hash name % Type.hash ~strip ty)
+            !`SDparams lst)
+        d.scope_def_parameters
+    % !(d.scope_def_is_condition : bool)
+    % io d.scope_def_io
+
+  let scope ~strip s =
+    Hash.map ScopeVar.Map.fold ScopeVar.hash var_or_state s.scope_vars
+    % Hash.map ScopeVar.Map.fold ScopeVar.hash (ScopeName.hash ~strip)
+        s.scope_sub_scopes
+    % ScopeName.hash ~strip s.scope_uid
+    % Hash.map ScopeDef.Map.fold (ScopeDef.hash ~strip) (scope_decl ~strip)
+        s.scope_defs
+  (* assertions, options, etc. are not expected to be part of interfaces *)
+
+  let modul ?(strip = []) m =
+    Hash.map ScopeName.Map.fold (ScopeName.hash ~strip) (scope ~strip)
+      (ScopeName.Map.filter
+         (fun _ s -> s.scope_visibility = Public)
+         m.module_scopes)
+    % Hash.map TopdefName.Map.fold (TopdefName.hash ~strip)
+        (fun td -> Type.hash ~strip td.topdef_type)
+        (TopdefName.Map.filter
+           (fun _ td -> td.topdef_visibility = Public)
+           m.module_topdefs)
+
+  let module_binding modname m =
+    ModuleName.hash modname % modul ~strip:[modname] m
+end
+
 let rec locations_used e : LocationSet.t =
   match e with
   | ELocation l, m -> LocationSet.singleton (l, Expr.mark_pos m)
@@ -311,5 +373,5 @@ let fold_exprs ~(f : 'a -> expr -> 'a) ~(init : 'a) (p : program) : 'a =
       p.program_root.module_scopes init
   in
   TopdefName.Map.fold
-    (fun _ (e, _) acc -> Option.fold ~none:acc ~some:(f acc) e)
+    (fun _ tdef acc -> Option.fold ~none:acc ~some:(f acc) tdef.topdef_expr)
     p.program_root.module_topdefs acc
diff --git a/compiler/desugared/ast.mli b/compiler/desugared/ast.mli
index b8f787bf..88f263b2 100644
--- a/compiler/desugared/ast.mli
+++ b/compiler/desugared/ast.mli
@@ -32,7 +32,7 @@ module ScopeDef : sig
   val equal_kind : kind -> kind -> bool
   val compare_kind : kind -> kind -> int
   val format_kind : Format.formatter -> kind -> unit
-  val hash_kind : kind -> int
+  val hash_kind : strip:Uid.Path.t -> kind -> Hash.t
 
   type t = ScopeVar.t Mark.pos * kind
 
@@ -40,7 +40,7 @@ module ScopeDef : sig
   val compare : t -> t -> int
   val get_position : t -> Pos.t
   val format : Format.formatter -> t -> unit
-  val hash : t -> int
+  val hash : strip:Uid.Path.t -> t -> Hash.t
 
   module Map : Map.S with type key = t
   module Set : Set.S with type elt = t
@@ -123,16 +123,23 @@ type scope = {
       (** empty outside of the root module *)
   scope_options : catala_option Mark.pos list;
   scope_meta_assertions : meta_assertion list;
+  scope_visibility : visibility;
+}
+
+type topdef = {
+  topdef_expr : expr option;  (** Always [None] outside of the root module *)
+  topdef_type : typ;
+  topdef_visibility : visibility;
+      (** Necessarily [Public] outside of the root module *)
 }
 
 type modul = {
   module_scopes : scope ScopeName.Map.t;
-  module_topdefs : (expr option * typ) TopdefName.Map.t;
-      (** the expr is [None] outside of the root module *)
+  module_topdefs : topdef TopdefName.Map.t;
 }
 
 type program = {
-  program_module_name : Ident.t Mark.pos option;
+  program_module_name : (ModuleName.t * module_intf_id) option;
   program_ctx : decl_ctx;
   program_modules : modul ModuleName.Map.t;
       (** Contains all submodules of the program, in a flattened structure *)
@@ -140,6 +147,18 @@ type program = {
   program_lang : Global.backend_lang;
 }
 
+(** {1 Interface hash computations} *)
+
+(** These hashes are computed on interfaces: only signatures are considered. *)
+module Hash : sig
+  (** The [strip] argument below strips as many leading path components before
+      hashing *)
+
+  val scope : strip:Uid.Path.t -> scope -> Hash.t
+  val modul : ?strip:Uid.Path.t -> modul -> Hash.t
+  val module_binding : ModuleName.t -> modul -> Hash.t
+end
+
 (** {1 Helpers} *)
 
 val locations_used : expr -> LocationSet.t
diff --git a/compiler/desugared/dependency.ml b/compiler/desugared/dependency.ml
index 51490cdf..3006560c 100644
--- a/compiler/desugared/dependency.ml
+++ b/compiler/desugared/dependency.ml
@@ -39,9 +39,9 @@ module Vertex = struct
 
   let hash x =
     match x with
-    | Var (x, None) -> ScopeVar.hash x
-    | Var (x, Some sx) -> Int.logxor (ScopeVar.hash x) (StateName.hash sx)
-    | Assertion a -> Ast.AssertionName.hash a
+    | Var (x, None) -> ScopeVar.id x
+    | Var (x, Some sx) -> Hashtbl.hash (ScopeVar.id x, StateName.id sx)
+    | Assertion a -> Hashtbl.hash (`Assert (Ast.AssertionName.id a))
 
   let compare x y =
     match x, y with
@@ -252,7 +252,7 @@ module ExceptionVertex = struct
 
   let hash (x : t) : int =
     RuleName.Map.fold
-      (fun r _ acc -> Int.logxor (RuleName.hash r) acc)
+      (fun r _ acc -> Hashtbl.hash (RuleName.id r, acc))
       x.rules 0
 
   let equal x y = compare x y = 0
diff --git a/compiler/desugared/disambiguate.ml b/compiler/desugared/disambiguate.ml
index 024645f6..2de18f72 100644
--- a/compiler/desugared/disambiguate.ml
+++ b/compiler/desugared/disambiguate.ml
@@ -98,10 +98,14 @@ let program prg =
   in
   let module_topdefs =
     TopdefName.Map.map
-      (function
-        | Some e, ty ->
-          Some (Expr.unbox (expr prg.program_ctx env (Expr.box e))), ty
-        | None, ty -> None, ty)
+      (fun def ->
+        {
+          def with
+          topdef_expr =
+            Option.map
+              (fun e -> Expr.unbox (expr prg.program_ctx env (Expr.box e)))
+              def.topdef_expr;
+        })
       prg.program_root.module_topdefs
   in
   let module_scopes =
diff --git a/compiler/desugared/from_surface.ml b/compiler/desugared/from_surface.ml
index d9af0fc5..01d4ff47 100644
--- a/compiler/desugared/from_surface.ml
+++ b/compiler/desugared/from_surface.ml
@@ -313,7 +313,7 @@ let rec translate_expr
             in
             let e2 = rec_helper ~local_vars e2 in
             Expr.make_abs [| binding_var |] e2 [tau] pos_op)
-        (EnumName.Map.find enum_uid ctxt.enums)
+        (fst (EnumName.Map.find enum_uid ctxt.enums))
     in
     Expr.ematch ~e:(rec_helper e1_sub) ~name:enum_uid ~cases emark
   | Binop ((((S.And | S.Or | S.Xor), _) as op), e1, e2) ->
@@ -613,7 +613,7 @@ let rec translate_expr
           StructField.Map.add f_uid f_e s_fields)
         StructField.Map.empty fields
     in
-    let expected_s_fields = StructName.Map.find s_uid ctxt.structs in
+    let expected_s_fields, _ = StructName.Map.find s_uid ctxt.structs in
     if
       StructField.Map.exists
         (fun expected_f _ -> not (StructField.Map.mem expected_f s_fields))
@@ -719,7 +719,7 @@ let rec translate_expr
           Expr.make_abs [| nop_var |]
             (Expr.elit (LBool (EnumConstructor.compare c_uid c_uid' = 0)) emark)
             [tau] pos)
-        (EnumName.Map.find enum_uid ctxt.enums)
+        (fst (EnumName.Map.find enum_uid ctxt.enums))
     in
     Expr.ematch ~e:(rec_helper e1) ~name:enum_uid ~cases emark
   | ArrayLit es -> Expr.earray (List.map rec_helper es) emark
@@ -970,7 +970,7 @@ and disambiguate_match_and_build_expression
     Expr.eabs e_binder
       [
         EnumConstructor.Map.find c_uid
-          (EnumName.Map.find e_uid ctxt.Name_resolution.enums);
+          (fst (EnumName.Map.find e_uid ctxt.Name_resolution.enums));
       ]
       (Mark.get case_body)
   in
@@ -1037,6 +1037,7 @@ and disambiguate_match_and_build_expression
         if curr_index < nb_cases - 1 then raise_wildcard_not_last_case_err ();
         let missing_constructors =
           EnumName.Map.find e_uid ctxt.Name_resolution.enums
+          |> fst
           |> EnumConstructor.Map.filter_map (fun c_uid _ ->
                  match EnumConstructor.Map.find_opt c_uid cases_d with
                  | Some _ -> None
@@ -1451,6 +1452,7 @@ let process_scope_use
 let process_topdef
     (ctxt : Name_resolution.context)
     (prgm : Ast.program)
+    (is_public : bool)
     (def : S.top_def) : Ast.program =
   let id =
     Ident.Map.find
@@ -1493,12 +1495,14 @@ let process_topdef
       in
       Some (Expr.unbox_closed e)
   in
+  let topdef_visibility = if is_public then Public else Private in
   let module_topdefs =
     TopdefName.Map.update id
       (fun def0 ->
         match def0, expr_opt with
-        | None, eopt -> Some (eopt, typ)
-        | Some (eopt0, ty0), eopt -> (
+        | None, eopt ->
+          Some { Ast.topdef_expr = eopt; topdef_visibility; topdef_type = typ }
+        | Some def0, eopt -> (
           let err msg =
             Message.error
               ~extra_pos:
@@ -1508,13 +1512,16 @@ let process_topdef
                 ]
               (msg ^^ " for %a") TopdefName.format id
           in
-          if not (Type.equal ty0 typ) then err "Conflicting type definitions"
+          if not (Type.equal def0.Ast.topdef_type typ) then
+            err "Conflicting type definitions"
           else
-            match eopt0, eopt with
+            match def0.Ast.topdef_expr, eopt with
             | None, None -> err "Multiple declarations"
             | Some _, Some _ -> err "Multiple definitions"
-            | Some e, None -> Some (Some e, typ)
-            | None, Some e -> Some (Some e, ty0)))
+            | (Some _ as topdef_expr), None ->
+              Some { Ast.topdef_expr; topdef_visibility; topdef_type = typ }
+            | None, (Some _ as topdef_expr) ->
+              Some { def0 with Ast.topdef_expr }))
       prgm.Ast.program_root.module_topdefs
   in
   { prgm with program_root = { prgm.program_root with module_topdefs } }
@@ -1680,6 +1687,7 @@ let translate_program (ctxt : Name_resolution.context) (surface : S.program) :
       scope_meta_assertions = [];
       scope_options = [];
       scope_uid = s_uid;
+      scope_visibility = s_context.Name_resolution.scope_visibility;
     }
   in
   let get_scopes mctx =
@@ -1692,22 +1700,32 @@ let translate_program (ctxt : Name_resolution.context) (surface : S.program) :
       mctx.Name_resolution.typedefs ScopeName.Map.empty
   in
   let program_modules =
-    ModuleName.Map.map
-      (fun mctx ->
-        {
-          Ast.module_scopes = get_scopes mctx;
-          Ast.module_topdefs =
-            Ident.Map.fold
-              (fun _ name acc ->
-                TopdefName.Map.add name
-                  ( None,
-                    TopdefName.Map.find name ctxt.Name_resolution.topdef_types
-                  )
-                  acc)
-              mctx.topdefs TopdefName.Map.empty;
-        })
+    ModuleName.Map.mapi
+      (fun mname mctx ->
+        let m =
+          {
+            Ast.module_scopes = get_scopes mctx;
+            Ast.module_topdefs =
+              Ident.Map.fold
+                (fun _ name acc ->
+                  let topdef_type, topdef_visibility =
+                    TopdefName.Map.find name ctxt.Name_resolution.topdefs
+                  in
+                  TopdefName.Map.add name
+                    { Ast.topdef_expr = None; topdef_visibility; topdef_type }
+                    acc)
+                mctx.topdefs TopdefName.Map.empty;
+          }
+        in
+        m, Ast.Hash.module_binding mname m)
       ctxt.modules
   in
+  let program_root =
+    {
+      Ast.module_scopes = get_scopes ctxt.Name_resolution.local;
+      Ast.module_topdefs = TopdefName.Map.empty;
+    }
+  in
   let program_ctx =
     let open Name_resolution in
     let ctx_scopes mctx acc =
@@ -1720,23 +1738,30 @@ let translate_program (ctxt : Name_resolution.context) (surface : S.program) :
     in
     let ctx_modules =
       let rec aux mctx =
-        Ident.Map.fold
-          (fun _ m (M acc) ->
-            let sub = aux (ModuleName.Map.find m ctxt.modules) in
-            M (ModuleName.Map.add m sub acc))
-          mctx.used_modules (M ModuleName.Map.empty)
+        let subs =
+          Ident.Map.fold
+            (fun _ m acc ->
+              let mctx = ModuleName.Map.find m ctxt.Name_resolution.modules in
+              let deps = aux mctx in
+              let hash = snd (ModuleName.Map.find m program_modules) in
+              ModuleName.Map.add m
+                { deps; intf_id = { hash; is_external = mctx.is_external } }
+                acc)
+            mctx.used_modules ModuleName.Map.empty
+        in
+        subs
       in
       aux ctxt.local
     in
     {
-      ctx_structs = ctxt.structs;
-      ctx_enums = ctxt.enums;
+      ctx_structs = StructName.Map.map fst ctxt.structs;
+      ctx_enums = EnumName.Map.map fst ctxt.enums;
       ctx_scopes =
         ModuleName.Map.fold
           (fun _ -> ctx_scopes)
           ctxt.modules
           (ctx_scopes ctxt.local ScopeName.Map.empty);
-      ctx_topdefs = ctxt.topdef_types;
+      ctx_topdefs = TopdefName.Map.map fst ctxt.topdefs;
       ctx_struct_fields = ctxt.local.field_idmap;
       ctx_enum_constrs = ctxt.local.constructor_idmap;
       ctx_scope_index =
@@ -1748,25 +1773,29 @@ let translate_program (ctxt : Name_resolution.context) (surface : S.program) :
       ctx_modules;
     }
   in
+  let program_module_name =
+    surface.Surface.Ast.program_module
+    |> Option.map
+       @@ fun { Surface.Ast.module_name; module_external } ->
+       let mname = ModuleName.fresh module_name in
+       let hash_placeholder = Hash.raw 0 in
+       mname, { hash = hash_placeholder; is_external = module_external }
+  in
   let desugared =
     {
       Ast.program_lang = surface.program_lang;
-      Ast.program_module_name = surface.Surface.Ast.program_module_name;
-      Ast.program_modules;
+      Ast.program_module_name;
+      Ast.program_modules = ModuleName.Map.map fst program_modules;
       Ast.program_ctx;
-      Ast.program_root =
-        {
-          Ast.module_scopes = get_scopes ctxt.Name_resolution.local;
-          Ast.module_topdefs = TopdefName.Map.empty;
-        };
+      Ast.program_root;
     }
   in
-  let process_code_block ctxt prgm block =
+  let process_code_block ctxt prgm is_meta block =
     List.fold_left
       (fun prgm item ->
         match Mark.remove item with
         | S.ScopeUse use -> process_scope_use ctxt prgm use
-        | S.Topdef def -> process_topdef ctxt prgm def
+        | S.Topdef def -> process_topdef ctxt prgm is_meta def
         | S.ScopeDecl _ | S.StructDecl _ | S.EnumDecl _ -> prgm)
       prgm block
   in
@@ -1777,7 +1806,22 @@ let translate_program (ctxt : Name_resolution.context) (surface : S.program) :
       List.fold_left
         (fun prgm child -> process_structure prgm child)
         prgm children
-    | S.CodeBlock (block, _, _) -> process_code_block ctxt prgm block
+    | S.CodeBlock (block, _, is_meta) ->
+      process_code_block ctxt prgm is_meta block
     | S.ModuleDef _ | S.LawInclude _ | S.LawText _ | S.ModuleUse _ -> prgm
   in
-  List.fold_left process_structure desugared surface.S.program_items
+  let desugared =
+    List.fold_left process_structure desugared surface.S.program_items
+  in
+  {
+    desugared with
+    Ast.program_module_name =
+      (desugared.Ast.program_module_name
+      |> Option.map
+         @@ fun (mname, intf_id) ->
+         ( mname,
+           {
+             intf_id with
+             hash = Ast.Hash.module_binding mname desugared.Ast.program_root;
+           } ));
+  }
diff --git a/compiler/desugared/name_resolution.ml b/compiler/desugared/name_resolution.ml
index 4c758094..0352aafc 100644
--- a/compiler/desugared/name_resolution.ml
+++ b/compiler/desugared/name_resolution.ml
@@ -39,6 +39,7 @@ type scope_context = {
   scope_out_struct : StructName.t;
   sub_scopes : ScopeName.Set.t;
       (** Other scopes referred to by this scope. Used for dependency analysis *)
+  scope_visibility : visibility;
 }
 (** Inside a scope, we distinguish between the variables and the subscopes. *)
 
@@ -77,15 +78,17 @@ type module_context = {
           between different enums *)
   topdefs : TopdefName.t Ident.Map.t;  (** Global definitions *)
   used_modules : ModuleName.t Ident.Map.t;
+  is_external : bool;
 }
 (** Context for name resolution, valid within a given module *)
 
 type context = {
   scopes : scope_context ScopeName.Map.t;  (** For each scope, its context *)
-  topdef_types : typ TopdefName.Map.t;
-  structs : struct_context StructName.Map.t;
+  topdefs : (typ * visibility) TopdefName.Map.t;
+  structs : (struct_context * visibility) StructName.Map.t;
       (** For each struct, its context *)
-  enums : enum_context EnumName.Map.t;  (** For each enum, its context *)
+  enums : (enum_context * visibility) EnumName.Map.t;
+      (** For each enum, its context *)
   var_typs : var_sig ScopeVar.Map.t;
       (** The signatures of each scope variable declared *)
   modules : module_context ModuleName.Map.t;
@@ -426,8 +429,10 @@ let process_data_decl
     }
 
 (** Process a struct declaration *)
-let process_struct_decl (ctxt : context) (sdecl : Surface.Ast.struct_decl) :
-    context =
+let process_struct_decl
+    ?(visibility = Public)
+    (ctxt : context)
+    (sdecl : Surface.Ast.struct_decl) : context =
   let s_uid = get_struct ctxt sdecl.struct_decl_name in
   if sdecl.struct_decl_fields = [] then
     Message.error
@@ -454,25 +459,28 @@ let process_struct_decl (ctxt : context) (sdecl : Surface.Ast.struct_decl) :
       let ctxt = { ctxt with local } in
       let structs =
         StructName.Map.update s_uid
-          (fun fields ->
-            match fields with
+          (function
             | None ->
               Some
-                (StructField.Map.singleton f_uid
-                   (process_type ctxt fdecl.Surface.Ast.struct_decl_field_typ))
-            | Some fields ->
+                ( StructField.Map.singleton f_uid
+                    (process_type ctxt fdecl.Surface.Ast.struct_decl_field_typ),
+                  visibility )
+            | Some (fields, _) ->
               Some
-                (StructField.Map.add f_uid
-                   (process_type ctxt fdecl.Surface.Ast.struct_decl_field_typ)
-                   fields))
+                ( StructField.Map.add f_uid
+                    (process_type ctxt fdecl.Surface.Ast.struct_decl_field_typ)
+                    fields,
+                  visibility ))
           ctxt.structs
       in
       { ctxt with structs })
     ctxt sdecl.struct_decl_fields
 
 (** Process an enum declaration *)
-let process_enum_decl (ctxt : context) (edecl : Surface.Ast.enum_decl) : context
-    =
+let process_enum_decl
+    ?(visibility = Public)
+    (ctxt : context)
+    (edecl : Surface.Ast.enum_decl) : context =
   let e_uid = get_enum ctxt edecl.enum_decl_name in
   if List.length edecl.enum_decl_cases = 0 then
     Message.error
@@ -506,23 +514,24 @@ let process_enum_decl (ctxt : context) (edecl : Surface.Ast.enum_decl) : context
               | Some typ -> process_type ctxt typ
             in
             match cases with
-            | None -> Some (EnumConstructor.Map.singleton c_uid typ)
-            | Some fields -> Some (EnumConstructor.Map.add c_uid typ fields))
+            | None -> Some (EnumConstructor.Map.singleton c_uid typ, visibility)
+            | Some (fields, _) ->
+              Some (EnumConstructor.Map.add c_uid typ fields, visibility))
           ctxt.enums
       in
       { ctxt with enums })
     ctxt edecl.enum_decl_cases
 
-let process_topdef ctxt def =
+let process_topdef ?(visibility = Public) ctxt def =
   let uid =
     Ident.Map.find (Mark.remove def.Surface.Ast.topdef_name) ctxt.local.topdefs
   in
   {
     ctxt with
-    topdef_types =
+    topdefs =
       TopdefName.Map.add uid
-        (process_type ctxt def.Surface.Ast.topdef_type)
-        ctxt.topdef_types;
+        (process_type ctxt def.Surface.Ast.topdef_type, visibility)
+        ctxt.topdefs;
   }
 
 (** Process an item declaration *)
@@ -536,8 +545,10 @@ let process_item_decl
     process_subscope_decl scope ctxt sub_decl
 
 (** Process a scope declaration *)
-let process_scope_decl (ctxt : context) (decl : Surface.Ast.scope_decl) :
-    context =
+let process_scope_decl
+    ?(visibility = Public)
+    (ctxt : context)
+    (decl : Surface.Ast.scope_decl) : context =
   let scope_uid = get_scope ctxt decl.scope_decl_name in
   let ctxt =
     List.fold_left
@@ -588,11 +599,12 @@ let process_scope_decl (ctxt : context) (decl : Surface.Ast.scope_decl) :
       structs =
         StructName.Map.add
           (get_struct ctxt decl.scope_decl_name)
-          StructField.Map.empty ctxt.structs;
+          (StructField.Map.empty, visibility)
+          ctxt.structs;
     }
   else
     let ctxt =
-      process_struct_decl ctxt
+      process_struct_decl ~visibility ctxt
         {
           struct_decl_name = decl.scope_decl_name;
           struct_decl_fields = output_fields;
@@ -634,8 +646,10 @@ let typedef_info = function
   | TScope (s, _) -> ScopeName.get_info s
 
 (** Process the names of all declaration items *)
-let process_name_item (ctxt : context) (item : Surface.Ast.code_item Mark.pos) :
-    context =
+let process_name_item
+    ?(visibility = Public)
+    (ctxt : context)
+    (item : Surface.Ast.code_item Mark.pos) : context =
   let raise_already_defined_error (use : Uid.MarkedString.info) name pos msg =
     Message.error
       ~fmt_pos:
@@ -676,6 +690,7 @@ let process_name_item (ctxt : context) (item : Surface.Ast.code_item Mark.pos) :
           scope_in_struct = in_struct_name;
           scope_out_struct = out_struct_name;
           sub_scopes = ScopeName.Set.empty;
+          scope_visibility = visibility;
         }
         ctxt.scopes
     in
@@ -720,14 +735,16 @@ let process_name_item (ctxt : context) (item : Surface.Ast.code_item Mark.pos) :
     { ctxt with local = { ctxt.local with topdefs } }
 
 (** Process a code item that is a declaration *)
-let process_decl_item (ctxt : context) (item : Surface.Ast.code_item Mark.pos) :
-    context =
+let process_decl_item
+    ?visibility
+    (ctxt : context)
+    (item : Surface.Ast.code_item Mark.pos) : context =
   match Mark.remove item with
-  | ScopeDecl decl -> process_scope_decl ctxt decl
-  | StructDecl sdecl -> process_struct_decl ctxt sdecl
-  | EnumDecl edecl -> process_enum_decl ctxt edecl
+  | ScopeDecl decl -> process_scope_decl ?visibility ctxt decl
+  | StructDecl sdecl -> process_struct_decl ?visibility ctxt sdecl
+  | EnumDecl edecl -> process_enum_decl ?visibility ctxt edecl
   | ScopeUse _ -> ctxt
-  | Topdef def -> process_topdef ctxt def
+  | Topdef def -> process_topdef ?visibility ctxt def
 
 (** Process a code block *)
 let process_code_block
@@ -738,7 +755,11 @@ let process_code_block
 
 (** Process a law structure, only considering the code blocks *)
 let rec process_law_structure
-    (process_item : context -> Surface.Ast.code_item Mark.pos -> context)
+    (process_item :
+      ?visibility:visibility ->
+      context ->
+      Surface.Ast.code_item Mark.pos ->
+      context)
     (ctxt : context)
     (s : Surface.Ast.law_structure) : context =
   match s with
@@ -746,10 +767,14 @@ let rec process_law_structure
     List.fold_left
       (fun ctxt child -> process_law_structure process_item ctxt child)
       ctxt children
-  | Surface.Ast.CodeBlock (block, _, _) ->
-    process_code_block process_item ctxt block
+  | Surface.Ast.CodeBlock (block, _, is_meta) ->
+    process_code_block
+      (process_item ~visibility:(if is_meta then Public else Private))
+      ctxt block
+  | Surface.Ast.ModuleDef (_, is_external) ->
+    { ctxt with local = { ctxt.local with is_external } }
   | Surface.Ast.LawInclude _ | Surface.Ast.LawText _ -> ctxt
-  | Surface.Ast.ModuleDef _ | Surface.Ast.ModuleUse _ -> ctxt
+  | Surface.Ast.ModuleUse _ -> ctxt
 
 (** {1 Scope uses pass} *)
 
@@ -957,12 +982,13 @@ let empty_module_ctxt =
     constructor_idmap = Ident.Map.empty;
     topdefs = Ident.Map.empty;
     used_modules = Ident.Map.empty;
+    is_external = false;
   }
 
 let empty_ctxt =
   {
     scopes = ScopeName.Map.empty;
-    topdef_types = TopdefName.Map.empty;
+    topdefs = TopdefName.Map.empty;
     var_typs = ScopeVar.Map.empty;
     structs = StructName.Map.empty;
     enums = EnumName.Map.empty;
@@ -985,7 +1011,13 @@ let form_context (surface, mod_uses) surface_modules : context =
           let ctxt =
             {
               ctxt with
-              local = { ctxt.local with used_modules = mod_uses; path = [m] };
+              local =
+                {
+                  ctxt.local with
+                  used_modules = mod_uses;
+                  path = [m];
+                  is_external = intf.Surface.Ast.intf_modname.module_external;
+                };
             }
           in
           let ctxt =
@@ -1017,7 +1049,7 @@ let form_context (surface, mod_uses) surface_modules : context =
   in
   let ctxt =
     List.fold_left
-      (process_law_structure process_use_item)
+      (process_law_structure (fun ?visibility:_ -> process_use_item))
       ctxt surface.Surface.Ast.program_items
   in
   (* Gather struct fields and enum constrs from direct modules: this helps with
diff --git a/compiler/desugared/name_resolution.mli b/compiler/desugared/name_resolution.mli
index ce54061d..99356bd1 100644
--- a/compiler/desugared/name_resolution.mli
+++ b/compiler/desugared/name_resolution.mli
@@ -39,6 +39,7 @@ type scope_context = {
   scope_out_struct : StructName.t;
   sub_scopes : ScopeName.Set.t;
       (** Other scopes referred to by this scope. Used for dependency analysis *)
+  scope_visibility : visibility;
 }
 (** Inside a scope, we distinguish between the variables and the subscopes. *)
 
@@ -82,16 +83,18 @@ type module_context = {
   topdefs : TopdefName.t Ident.Map.t;  (** Global definitions *)
   used_modules : ModuleName.t Ident.Map.t;
       (** Module aliases and the modules they point to *)
+  is_external : bool;
 }
 (** Context for name resolution, valid within a given module *)
 
 type context = {
   scopes : scope_context ScopeName.Map.t;  (** For each scope, its context *)
-  topdef_types : typ TopdefName.Map.t;
+  topdefs : (typ * visibility) TopdefName.Map.t;
       (** Types associated with the global definitions *)
-  structs : struct_context StructName.Map.t;
+  structs : (struct_context * visibility) StructName.Map.t;
       (** For each struct, its context *)
-  enums : enum_context EnumName.Map.t;  (** For each enum, its context *)
+  enums : (enum_context * visibility) EnumName.Map.t;
+      (** For each enum, its context *)
   var_typs : var_sig ScopeVar.Map.t;
       (** The signatures of each scope variable declared *)
   modules : module_context ModuleName.Map.t;
diff --git a/compiler/driver.ml b/compiler/driver.ml
index adfc07ee..6e0cd14a 100644
--- a/compiler/driver.ml
+++ b/compiler/driver.ml
@@ -93,7 +93,7 @@ let load_module_interfaces
             Surface.Parser_driver.load_interface ?default_module_name
               (Global.FileName f)
           in
-          let modname = ModuleName.fresh intf.intf_modname in
+          let modname = ModuleName.fresh intf.intf_modname.module_name in
           let seen = File.Map.add f None seen in
           let seen, sub_use_map =
             aux
@@ -107,9 +107,9 @@ let load_module_interfaces
       (seen, Ident.Map.empty) uses
   in
   let seen =
-    match program.Surface.Ast.program_module_name with
+    match program.Surface.Ast.program_module with
     | Some m ->
-      let file = Pos.get_file (Mark.get m) in
+      let file = Pos.get_file (Mark.get m.module_name) in
       File.Map.singleton file None
     | None -> File.Map.empty
   in
@@ -712,7 +712,12 @@ module Commands = struct
     let prg, _ =
       Passes.dcalc options ~includes ~optimize ~check_invariants ~typed
     in
-    Interpreter.load_runtime_modules prg;
+    Interpreter.load_runtime_modules
+      ~hashf:
+        Hash.(
+          finalise ~avoid_exceptions:false ~closure_conversion:false
+            ~monomorphize_types:false)
+      prg;
     print_interpretation_results options Interpreter.interpret_program_dcalc prg
       (get_scopeopt_uid prg.decl_ctx ex_scope_opt)
 
@@ -781,7 +786,10 @@ module Commands = struct
       Passes.lcalc options ~includes ~optimize ~check_invariants
         ~avoid_exceptions ~closure_conversion ~monomorphize_types ~typed
     in
-    Interpreter.load_runtime_modules prg;
+    Interpreter.load_runtime_modules
+      ~hashf:
+        (Hash.finalise ~avoid_exceptions ~closure_conversion ~monomorphize_types)
+      prg;
     print_interpretation_results options Interpreter.interpret_program_lcalc prg
       (get_scopeopt_uid prg.decl_ctx ex_scope_opt)
 
@@ -844,7 +852,11 @@ module Commands = struct
     Message.debug "Writing to %s..."
       (Option.value ~default:"stdout" output_file);
     let exec_scope = Option.map (get_scope_uid prg.decl_ctx) ex_scope_opt in
-    Lcalc.To_ocaml.format_program fmt prg ?exec_scope type_ordering
+    let hashf =
+      Hash.finalise ~avoid_exceptions ~closure_conversion:false
+        ~monomorphize_types:false
+    in
+    Lcalc.To_ocaml.format_program fmt prg ?exec_scope ~hashf type_ordering
 
   let ocaml_cmd =
     Cmd.v
@@ -1010,7 +1022,7 @@ module Commands = struct
     let prg =
       Surface.Ast.
         {
-          program_module_name = None;
+          program_module = None;
           program_items = [];
           program_source_files = [];
           program_used_modules =
@@ -1038,7 +1050,7 @@ module Commands = struct
     in
     Format.open_hbox ();
     Format.pp_print_list ~pp_sep:Format.pp_print_space
-      (fun ppf m ->
+      (fun ppf (m, _) ->
         let f = Pos.get_file (Mark.get (ModuleName.get_info m)) in
         let f =
           match prefix with
diff --git a/compiler/lcalc/to_ocaml.ml b/compiler/lcalc/to_ocaml.ml
index 9b7a8e8b..e1c65611 100644
--- a/compiler/lcalc/to_ocaml.ml
+++ b/compiler/lcalc/to_ocaml.ml
@@ -716,9 +716,21 @@ let commands = if commands = [] then entry_scopes else commands
         name format_var var name)
     scopes_with_no_input
 
-let reexport_used_modules fmt modules =
+let check_and_reexport_used_modules fmt ~hashf modules =
   List.iter
-    (fun m ->
+    (fun (m, intf_id) ->
+      Format.fprintf fmt
+        "@[<hv 2>let () =@ @[<hov 2>match Runtime_ocaml.Runtime.check_module \
+         %S \"%a\"@ with@]@,\
+         | Ok () -> ()@,\
+         @[<hv 2>| Error h -> failwith \"Hash mismatch for module %a, it may \
+         need recompiling\"@]@]@,"
+        (ModuleName.to_string m)
+        (fun ppf h ->
+          if intf_id.is_external then
+            Format.pp_print_string ppf Hash.external_placeholder
+          else Hash.format ppf h)
+        (hashf intf_id.hash) ModuleName.format m;
       Format.fprintf fmt "@[<hv 2>module %a@ = %a@]@," ModuleName.format m
         ModuleName.format m)
     modules
@@ -726,7 +738,9 @@ let reexport_used_modules fmt modules =
 let format_module_registration
     fmt
     (bnd : ('m Ast.expr Var.t * _) String.Map.t)
-    modname =
+    modname
+    hash
+    is_external =
   Format.pp_open_vbox fmt 2;
   Format.pp_print_string fmt "let () =";
   Format.pp_print_space fmt ();
@@ -743,11 +757,17 @@ let format_module_registration
     (fun fmt (id, (var, _)) ->
       Format.fprintf fmt "@[<hov 2>%S,@ Obj.repr %a@]" id format_var var)
     fmt (String.Map.to_seq bnd);
+  (* TODO: pass the visibility info down from desugared, and filter what is
+     exported here *)
   Format.pp_close_box fmt ();
   Format.pp_print_char fmt ' ';
   Format.pp_print_string fmt "]";
   Format.pp_print_space fmt ();
-  Format.pp_print_string fmt "\"todo-module-hash\"";
+  Format.fprintf fmt "\"%a\""
+    (fun ppf h ->
+      if is_external then Format.pp_print_string ppf Hash.external_placeholder
+      else Hash.format ppf h)
+    hash;
   Format.pp_close_box fmt ();
   Format.pp_close_box fmt ();
   Format.pp_print_newline fmt ()
@@ -766,17 +786,21 @@ let format_program
     (fmt : Format.formatter)
     ?exec_scope
     ?(exec_args = true)
+    ~(hashf : Hash.t -> Hash.full)
     (p : 'm Ast.program)
     (type_ordering : Scopelang.Dependency.TVertex.t list) : unit =
   Format.pp_open_vbox fmt 0;
   Format.pp_print_string fmt header;
-  reexport_used_modules fmt (Program.modules_to_list p.decl_ctx.ctx_modules);
+  check_and_reexport_used_modules fmt ~hashf
+    (Program.modules_to_list p.decl_ctx.ctx_modules);
   format_ctx type_ordering fmt p.decl_ctx;
   let bnd = format_code_items p.decl_ctx fmt p.code_items in
   Format.pp_print_cut fmt ();
   let () =
     match p.module_name, exec_scope with
-    | Some modname, None -> format_module_registration fmt bnd modname
+    | Some (modname, intf_id), None ->
+      format_module_registration fmt bnd modname (hashf intf_id.hash)
+        intf_id.is_external
     | None, Some scope_name ->
       let scope_body = Program.get_scope_body p scope_name in
       format_scope_exec p.decl_ctx fmt bnd scope_name scope_body
diff --git a/compiler/lcalc/to_ocaml.mli b/compiler/lcalc/to_ocaml.mli
index b6f6a9f5..85f49490 100644
--- a/compiler/lcalc/to_ocaml.mli
+++ b/compiler/lcalc/to_ocaml.mli
@@ -14,6 +14,7 @@
    License for the specific language governing permissions and limitations under
    the License. *)
 
+open Catala_utils
 open Shared_ast
 
 (** Formats a lambda calculus program into a valid OCaml program *)
@@ -40,6 +41,7 @@ val format_program :
   Format.formatter ->
   ?exec_scope:ScopeName.t ->
   ?exec_args:bool ->
+  hashf:(Hash.t -> Hash.full) ->
   'm Ast.program ->
   Scopelang.Dependency.TVertex.t list ->
   unit
diff --git a/compiler/plugins/api_web.ml b/compiler/plugins/api_web.ml
index 63cf61fc..73d1a22a 100644
--- a/compiler/plugins/api_web.ml
+++ b/compiler/plugins/api_web.ml
@@ -489,7 +489,7 @@ let run
         (Option.value ~default:"stdout" jsoo_output_file);
       let modname =
         match prg.module_name with
-        | Some m -> ModuleName.to_string m
+        | Some (m, _) -> ModuleName.to_string m
         | None ->
           String.capitalize_ascii
             Filename.(
diff --git a/compiler/plugins/explain.ml b/compiler/plugins/explain.ml
index f059eb18..929f15f9 100644
--- a/compiler/plugins/explain.ml
+++ b/compiler/plugins/explain.ml
@@ -1381,7 +1381,10 @@ let run includes optimize ex_scope explain_options global_options =
     Driver.Passes.dcalc global_options ~includes ~optimize
       ~check_invariants:false ~typed:Expr.typed
   in
-  Interpreter.load_runtime_modules prg;
+  Interpreter.load_runtime_modules prg
+    ~hashf:
+      (Hash.finalise ~avoid_exceptions:false ~closure_conversion:false
+         ~monomorphize_types:false);
   let scope = Driver.Commands.get_scope_uid prg.decl_ctx ex_scope in
   (* let result_expr, env = interpret_program prg scope in *)
   let g, base_vars, env = program_to_graph explain_options prg scope in
diff --git a/compiler/plugins/lazy_interp.ml b/compiler/plugins/lazy_interp.ml
index 8ae7826e..7a2d4d02 100644
--- a/compiler/plugins/lazy_interp.ml
+++ b/compiler/plugins/lazy_interp.ml
@@ -271,7 +271,10 @@ let run includes optimize check_invariants ex_scope options =
     Driver.Passes.dcalc options ~includes ~optimize ~check_invariants
       ~typed:Expr.typed
   in
-  Interpreter.load_runtime_modules prg;
+  Interpreter.load_runtime_modules prg
+    ~hashf:
+      (Hash.finalise ~avoid_exceptions:false ~closure_conversion:false
+         ~monomorphize_types:false);
   let scope = Driver.Commands.get_scope_uid prg.decl_ctx ex_scope in
   let result_expr, _env = interpret_program prg scope in
   let fmt = Format.std_formatter in
diff --git a/compiler/scalc/ast.ml b/compiler/scalc/ast.ml
index 7beb09fe..48c2b2da 100644
--- a/compiler/scalc/ast.ml
+++ b/compiler/scalc/ast.ml
@@ -121,5 +121,5 @@ type ctx = { decl_ctx : decl_ctx; modules : VarName.t ModuleName.Map.t }
 type program = {
   ctx : ctx;
   code_items : code_item list;
-  module_name : ModuleName.t option;
+  module_name : (ModuleName.t * module_intf_id) option;
 }
diff --git a/compiler/scalc/from_lcalc.ml b/compiler/scalc/from_lcalc.ml
index c80024b7..ca4933eb 100644
--- a/compiler/scalc/from_lcalc.ml
+++ b/compiler/scalc/from_lcalc.ml
@@ -659,7 +659,7 @@ let translate_program ~(config : translation_config) (p : 'm L.program) :
     A.program =
   let modules =
     List.fold_left
-      (fun acc m ->
+      (fun acc (m, _) ->
         let vname = Mark.map (( ^ ) "Module_") (ModuleName.get_info m) in
         (* The "Module_" prefix is a workaround name clashes for same-name
            structs and modules, Python in particular mixes everything in one
diff --git a/compiler/scalc/print.ml b/compiler/scalc/print.ml
index 541cdbde..9cbf411b 100644
--- a/compiler/scalc/print.ml
+++ b/compiler/scalc/print.ml
@@ -21,10 +21,10 @@ open Ast
 let needs_parens (_e : expr) : bool = false
 
 let format_var_name (fmt : Format.formatter) (v : VarName.t) : unit =
-  Format.fprintf fmt "%a_%d" VarName.format v (VarName.hash v)
+  Format.fprintf fmt "%a_%d" VarName.format v (VarName.id v)
 
 let format_func_name (fmt : Format.formatter) (v : FuncName.t) : unit =
-  Format.fprintf fmt "@{<green>%a_%d@}" FuncName.format v (FuncName.hash v)
+  Format.fprintf fmt "@{<green>%a_%d@}" FuncName.format v (FuncName.id v)
 
 let rec format_expr
     (decl_ctx : decl_ctx)
diff --git a/compiler/scalc/to_c.ml b/compiler/scalc/to_c.ml
index de698df5..b46541da 100644
--- a/compiler/scalc/to_c.ml
+++ b/compiler/scalc/to_c.ml
@@ -96,11 +96,11 @@ let string_counter_map : int IntMap.t StringMap.t ref = ref StringMap.empty
 
 let format_var (fmt : Format.formatter) (v : VarName.t) : unit =
   let v_str = Mark.remove (VarName.get_info v) in
-  let hash = VarName.hash v in
+  let id = VarName.id v in
   let local_id =
     match StringMap.find_opt v_str !string_counter_map with
     | Some ids -> (
-      match IntMap.find_opt hash ids with
+      match IntMap.find_opt id ids with
       | None ->
         let max_id =
           snd
@@ -111,13 +111,13 @@ let format_var (fmt : Format.formatter) (v : VarName.t) : unit =
         in
         string_counter_map :=
           StringMap.add v_str
-            (IntMap.add hash (max_id + 1) ids)
+            (IntMap.add id (max_id + 1) ids)
             !string_counter_map;
         max_id + 1
       | Some local_id -> local_id)
     | None ->
       string_counter_map :=
-        StringMap.add v_str (IntMap.singleton hash 0) !string_counter_map;
+        StringMap.add v_str (IntMap.singleton id 0) !string_counter_map;
       0
   in
   if v_str = "_" then Format.fprintf fmt "dummy_var"
diff --git a/compiler/scalc/to_python.ml b/compiler/scalc/to_python.ml
index 640094fe..c76a26c2 100644
--- a/compiler/scalc/to_python.ml
+++ b/compiler/scalc/to_python.ml
@@ -152,20 +152,20 @@ let string_counter_map : int IntMap.t StringMap.t ref = ref StringMap.empty
 
 let format_var (fmt : Format.formatter) (v : VarName.t) : unit =
   let v_str = clean_name (Mark.remove (VarName.get_info v)) in
-  let hash = VarName.hash v in
+  let id = VarName.id v in
   let local_id =
     match StringMap.find_opt v_str !string_counter_map with
     | Some ids -> (
-      match IntMap.find_opt hash ids with
+      match IntMap.find_opt id ids with
       | None ->
-        let id = 1 + IntMap.fold (fun _ -> Int.max) ids 0 in
+        let local_id = 1 + IntMap.fold (fun _ -> Int.max) ids 0 in
         string_counter_map :=
-          StringMap.add v_str (IntMap.add hash id ids) !string_counter_map;
-        id
+          StringMap.add v_str (IntMap.add id local_id ids) !string_counter_map;
+        local_id
       | Some local_id -> local_id)
     | None ->
       string_counter_map :=
-        StringMap.add v_str (IntMap.singleton hash 0) !string_counter_map;
+        StringMap.add v_str (IntMap.singleton id 0) !string_counter_map;
       0
   in
   if v_str = "_" then Format.fprintf fmt "_"
diff --git a/compiler/scalc/to_r.ml b/compiler/scalc/to_r.ml
index 1a368b0f..eb1e82b9 100644
--- a/compiler/scalc/to_r.ml
+++ b/compiler/scalc/to_r.ml
@@ -220,11 +220,11 @@ let string_counter_map : int IntMap.t StringMap.t ref = ref StringMap.empty
 
 let format_var (fmt : Format.formatter) (v : VarName.t) : unit =
   let v_str = Mark.remove (VarName.get_info v) in
-  let hash = VarName.hash v in
+  let id = VarName.id v in
   let local_id =
     match StringMap.find_opt v_str !string_counter_map with
     | Some ids -> (
-      match IntMap.find_opt hash ids with
+      match IntMap.find_opt id ids with
       | None ->
         let max_id =
           snd
@@ -235,13 +235,13 @@ let format_var (fmt : Format.formatter) (v : VarName.t) : unit =
         in
         string_counter_map :=
           StringMap.add v_str
-            (IntMap.add hash (max_id + 1) ids)
+            (IntMap.add id (max_id + 1) ids)
             !string_counter_map;
         max_id + 1
       | Some local_id -> local_id)
     | None ->
       string_counter_map :=
-        StringMap.add v_str (IntMap.singleton hash 0) !string_counter_map;
+        StringMap.add v_str (IntMap.singleton id 0) !string_counter_map;
       0
   in
   if v_str = "_" then Format.fprintf fmt "dummy_var"
diff --git a/compiler/scopelang/ast.ml b/compiler/scopelang/ast.ml
index 100cfc10..07a0f6df 100644
--- a/compiler/scopelang/ast.ml
+++ b/compiler/scopelang/ast.ml
@@ -67,7 +67,7 @@ type 'm scope_decl = {
 }
 
 type 'm program = {
-  program_module_name : ModuleName.t option;
+  program_module_name : (ModuleName.t * module_intf_id) option;
   program_ctx : decl_ctx;
   program_modules : nil scope_decl Mark.pos ScopeName.Map.t ModuleName.Map.t;
   program_scopes : 'm scope_decl Mark.pos ScopeName.Map.t;
diff --git a/compiler/scopelang/ast.mli b/compiler/scopelang/ast.mli
index c47d1336..d44f6ef0 100644
--- a/compiler/scopelang/ast.mli
+++ b/compiler/scopelang/ast.mli
@@ -63,12 +63,13 @@ type 'm scope_decl = {
 }
 
 type 'm program = {
-  program_module_name : ModuleName.t option;
+  program_module_name : (ModuleName.t * module_intf_id) option;
   program_ctx : decl_ctx;
   program_modules : nil scope_decl Mark.pos ScopeName.Map.t ModuleName.Map.t;
   (* Using [nil] here ensure that program interfaces don't contain any
-     expressions. They won't contain any rules or topdefs, but will still have
-     the scope signatures needed to respect the call convention *)
+     expressions. They won't contain any rules or topdef implementations, but
+     will still have the scope signatures needed to respect the call
+     convention *)
   program_scopes : 'm scope_decl Mark.pos ScopeName.Map.t;
   program_topdefs : ('m expr * typ) TopdefName.Map.t;
   program_lang : Global.backend_lang;
diff --git a/compiler/scopelang/dependency.ml b/compiler/scopelang/dependency.ml
index de664409..fff9363d 100644
--- a/compiler/scopelang/dependency.ml
+++ b/compiler/scopelang/dependency.ml
@@ -42,9 +42,7 @@ module SVertex = struct
     | Topdef g1, Topdef g2 -> TopdefName.equal g1 g2
     | (Scope _ | Topdef _), _ -> false
 
-  let hash = function
-    | Scope s -> ScopeName.hash s
-    | Topdef g -> TopdefName.hash g
+  let hash = function Scope s -> ScopeName.id s | Topdef g -> TopdefName.id g
 
   let format ppf = function
     | Scope s -> ScopeName.format ppf s
@@ -206,7 +204,9 @@ module TVertex = struct
   type t = Struct of StructName.t | Enum of EnumName.t
 
   let hash x =
-    match x with Struct x -> StructName.hash x | Enum x -> EnumName.hash x
+    match x with
+    | Struct x -> StructName.id x
+    | Enum x -> Hashtbl.hash (`Enum (EnumName.id x))
 
   let compare x y =
     match x, y with
diff --git a/compiler/scopelang/from_desugared.ml b/compiler/scopelang/from_desugared.ml
index 03fa983a..630d6f1e 100644
--- a/compiler/scopelang/from_desugared.ml
+++ b/compiler/scopelang/from_desugared.ml
@@ -953,8 +953,9 @@ let translate_program
   let program_topdefs =
     TopdefName.Map.mapi
       (fun id -> function
-        | Some e, ty -> Expr.unbox (translate_expr ctx e), ty
-        | None, (_, pos) ->
+        | { D.topdef_expr = Some e; topdef_type = ty; topdef_visibility = _ } ->
+          Expr.unbox (translate_expr ctx e), ty
+        | { D.topdef_expr = None; topdef_type = _, pos; _ } ->
           Message.error ~pos "No definition found for %a" TopdefName.format id)
       desugared.program_root.module_topdefs
   in
@@ -964,8 +965,7 @@ let translate_program
       desugared.D.program_root.module_scopes
   in
   {
-    Ast.program_module_name =
-      Option.map ModuleName.fresh desugared.D.program_module_name;
+    Ast.program_module_name = desugared.D.program_module_name;
     Ast.program_topdefs;
     Ast.program_scopes;
     Ast.program_ctx = ctx.decl_ctx;
diff --git a/compiler/shared_ast/definitions.ml b/compiler/shared_ast/definitions.ml
index dcb80d0e..56dd2065 100644
--- a/compiler/shared_ast/definitions.ml
+++ b/compiler/shared_ast/definitions.ml
@@ -668,8 +668,14 @@ type scope_info = {
   out_struct_fields : StructField.t ScopeVar.Map.t;
 }
 
+type module_intf_id = { hash : Hash.t; is_external : bool }
+
+type module_tree_node = { deps : module_tree; intf_id : module_intf_id }
+
+and module_tree = module_tree_node ModuleName.Map.t
 (** In practice, this is a DAG: beware of repeated names *)
-type module_tree = M of module_tree ModuleName.Map.t [@@caml.unboxed]
+
+type visibility = Private | Public
 
 type decl_ctx = {
   ctx_enums : enum_ctx;
@@ -688,5 +694,5 @@ type 'e program = {
   decl_ctx : decl_ctx;
   code_items : 'e code_item_list;
   lang : Global.backend_lang;
-  module_name : ModuleName.t option;
+  module_name : (ModuleName.t * module_intf_id) option;
 }
diff --git a/compiler/shared_ast/interpreter.ml b/compiler/shared_ast/interpreter.ml
index 5e73afa3..88837758 100644
--- a/compiler/shared_ast/interpreter.ml
+++ b/compiler/shared_ast/interpreter.ml
@@ -1155,29 +1155,57 @@ let interpret_program_dcalc p s : (Uid.MarkedString.info * ('a, 'm) gexpr) list
    reflect that. *)
 let evaluate_expr ctx lang e = evaluate_expr ctx lang (addcustom e)
 
-let load_runtime_modules prg =
-  let load m =
+let load_runtime_modules ~hashf prg =
+  let load (mname, intf_id) =
+    let hash = hashf intf_id.hash in
+    let expect_hash =
+      if intf_id.is_external then Hash.external_placeholder
+      else Hash.to_string hash
+    in
     let obj_file =
       Dynlink.adapt_filename
-        File.(Pos.get_file (Mark.get (ModuleName.get_info m)) -.- "cmo")
+        File.(Pos.get_file (Mark.get (ModuleName.get_info mname)) -.- "cmo")
     in
-    if not (Sys.file_exists obj_file) then
+    (if not (Sys.file_exists obj_file) then
+       Message.error
+         ~pos_msg:(fun ppf -> Format.pp_print_string ppf "Module defined here")
+         ~pos:(Mark.get (ModuleName.get_info mname))
+         "Compiled OCaml object %a@ not@ found.@ Make sure it has been \
+          suitably compiled."
+         File.format obj_file
+     else
+       try Dynlink.loadfile obj_file
+       with Dynlink.Error dl_err ->
+         Message.error
+           "While loading compiled module from %a:@;<1 2>@[<hov>%a@]"
+           File.format obj_file Format.pp_print_text
+           (Dynlink.error_message dl_err));
+    match Runtime.check_module (ModuleName.to_string mname) expect_hash with
+    | Ok () -> ()
+    | Error bad_hash ->
+      Message.debug
+        "Module hash mismatch for %a:@ @[<v>Expected: %a@,Found:    %a@]"
+        ModuleName.format mname Hash.format hash
+        (fun ppf h ->
+          try Hash.format ppf (Hash.of_string h)
+          with Failure _ ->
+            if h = Hash.external_placeholder then
+              Format.fprintf ppf "@{<cyan>%s@}" Hash.external_placeholder
+            else Format.fprintf ppf "@{<red><invalid>@}")
+        bad_hash;
       Message.error
-        ~pos_msg:(fun ppf -> Format.pp_print_string ppf "Module defined here")
-        ~pos:(Mark.get (ModuleName.get_info m))
-        "Compiled OCaml object %a@ not@ found.@ Make sure it has been suitably \
-         compiled."
-        File.format obj_file
-    else
-      try Dynlink.loadfile obj_file
-      with Dynlink.Error dl_err ->
-        Message.error "Error loading compiled module from %a:@;<1 2>@[<hov>%a@]"
-          File.format obj_file Format.pp_print_text
-          (Dynlink.error_message dl_err)
+        "Module %a@ needs@ recompiling:@ %a@ was@ likely@ compiled@ from@ an@ \
+         older@ version@ or@ with@ incompatible@ flags."
+        ModuleName.format mname File.format obj_file
+    | exception Not_found ->
+      Message.error
+        "Module %a@ was loaded from file %a but did not register properly, \
+         there is something wrong in its code."
+        ModuleName.format mname File.format obj_file
   in
   let modules_list_topo = Program.modules_to_list prg.decl_ctx.ctx_modules in
   if modules_list_topo <> [] then
     Message.debug "Loading shared modules... %a"
       (Format.pp_print_list ~pp_sep:Format.pp_print_space ModuleName.format)
-      modules_list_topo;
+      (List.map (fun (m, _) -> m) modules_list_topo);
   List.iter load modules_list_topo
diff --git a/compiler/shared_ast/interpreter.mli b/compiler/shared_ast/interpreter.mli
index b6a21894..f89c494e 100644
--- a/compiler/shared_ast/interpreter.mli
+++ b/compiler/shared_ast/interpreter.mli
@@ -62,6 +62,6 @@ val delcustom :
 (** Runtime check that the term contains no custom terms (raises
     [Invalid_argument] if that is the case *)
 
-val load_runtime_modules : _ program -> unit
+val load_runtime_modules : hashf:(Hash.t -> Hash.full) -> _ program -> unit
 (** Dynlink the runtime modules required by the given program, in order to make
     them callable by the interpreter. *)
diff --git a/compiler/shared_ast/program.ml b/compiler/shared_ast/program.ml
index d3a7f1f1..753a1c74 100644
--- a/compiler/shared_ast/program.ml
+++ b/compiler/shared_ast/program.ml
@@ -58,7 +58,7 @@ let empty_ctx =
     ctx_struct_fields = Ident.Map.empty;
     ctx_enum_constrs = Ident.Map.empty;
     ctx_scope_index = Ident.Map.empty;
-    ctx_modules = M ModuleName.Map.empty;
+    ctx_modules = ModuleName.Map.empty;
   }
 
 let get_scope_body { code_items; _ } scope =
@@ -87,11 +87,11 @@ let to_expr p main_scope =
   res
 
 let modules_to_list (mt : module_tree) =
-  let rec aux acc (M mtree) =
+  let rec aux acc mtree =
     ModuleName.Map.fold
-      (fun mname sub acc ->
-        if List.exists (ModuleName.equal mname) acc then acc
-        else mname :: aux acc sub)
+      (fun mname mnode acc ->
+        if List.exists (fun (m, _) -> ModuleName.equal m mname) acc then acc
+        else (mname, mnode.intf_id) :: aux acc mnode.deps)
       mtree acc
   in
   List.rev (aux [] mt)
diff --git a/compiler/shared_ast/program.mli b/compiler/shared_ast/program.mli
index 54a95047..071b7873 100644
--- a/compiler/shared_ast/program.mli
+++ b/compiler/shared_ast/program.mli
@@ -53,5 +53,6 @@ val to_expr : ((_ any, _) gexpr as 'e) program -> ScopeName.t -> 'e boxed
 
 val find_scope : ScopeName.t -> 'e code_item_list -> 'e scope_body
 
-val modules_to_list : module_tree -> ModuleName.t list
-(** Returns a list of used modules, in topological order *)
+val modules_to_list : module_tree -> (ModuleName.t * module_intf_id) list
+(** Returns a list of used modules, in topological order ; the boolean indicates
+    if the module is external *)
diff --git a/compiler/shared_ast/type.ml b/compiler/shared_ast/type.ml
index 29274ab9..a792e489 100644
--- a/compiler/shared_ast/type.ml
+++ b/compiler/shared_ast/type.ml
@@ -93,6 +93,22 @@ let rec compare ty1 ty2 =
   | TClosureEnv, _ -> -1
   | _, TClosureEnv -> 1
 
+let rec hash ~strip ty =
+  let open Hash.Op in
+  match Mark.remove ty with
+  | TLit l -> !`TLit % !(l : typ_lit)
+  | TTuple tl -> List.fold_left (fun acc ty -> acc % hash ~strip ty) !`TTuple tl
+  | TStruct n -> !`TStruct % StructName.hash ~strip n
+  | TEnum n -> !`TEnum % EnumName.hash ~strip n
+  | TOption ty -> !`TOption % hash ~strip ty
+  | TArrow (tl, ty) ->
+    !`TArrow
+    % List.fold_left (fun acc ty -> acc % hash ~strip ty) (hash ~strip ty) tl
+  | TArray ty -> !`TArray % hash ~strip ty
+  | TDefault ty -> !`TDefault % hash ~strip ty
+  | TAny -> !`TAny
+  | TClosureEnv -> !`TClosureEnv
+
 let rec arrow_return = function TArrow (_, b), _ -> arrow_return b | t -> t
 let format = Print.typ_debug
 
diff --git a/compiler/shared_ast/type.mli b/compiler/shared_ast/type.mli
index ac2ec56d..5d026da7 100644
--- a/compiler/shared_ast/type.mli
+++ b/compiler/shared_ast/type.mli
@@ -14,6 +14,8 @@
    License for the specific language governing permissions and limitations under
    the License. *)
 
+open Catala_utils
+
 type t = Definitions.typ
 
 val format : Format.formatter -> t -> unit
@@ -23,6 +25,11 @@ module Map : Catala_utils.Map.S with type key = t
 val equal : t -> t -> bool
 val equal_list : t list -> t list -> bool
 val compare : t -> t -> int
+
+val hash : strip:Uid.Path.t -> t -> Hash.t
+(** The [strip] argument strips the given leading path components in included
+    identifiers before hashing *)
+
 val unifiable : t -> t -> bool
 
 val unifiable_list : t list -> t list -> bool
diff --git a/compiler/shared_ast/typing.ml b/compiler/shared_ast/typing.ml
index acaf4abb..88ca7433 100644
--- a/compiler/shared_ast/typing.ml
+++ b/compiler/shared_ast/typing.ml
@@ -31,6 +31,7 @@ module Any =
       let format fmt () = Format.fprintf fmt "any"
       let equal () () = true
       let compare () () = 0
+      let hash () = Hash.raw `Any
     end)
     (struct
       let style = Ocolor_types.(Fg (C4 hi_magenta))
@@ -166,7 +167,7 @@ let rec format_typ
     format_typ ~colors fmt t1;
     Format.pp_print_as fmt 1 "⟩"
   | TAny v ->
-    if Global.options.debug then Format.fprintf fmt "<a%d>" (Any.hash v)
+    if Global.options.debug then Format.fprintf fmt "<a%d>" (Any.id v)
     else Format.pp_print_string fmt "<any>"
   | TClosureEnv -> Format.fprintf fmt "closure_env"
 
diff --git a/compiler/surface/ast.ml b/compiler/surface/ast.ml
index 60f962ff..9a37fde1 100644
--- a/compiler/surface/ast.ml
+++ b/compiler/surface/ast.ml
@@ -318,7 +318,7 @@ and law_structure =
   | CodeBlock of code_block * source_repr * bool (* Metadata if true *)
 
 and interface = {
-  intf_modname : uident Mark.pos;
+  intf_modname : program_module;
   intf_code : code_block;
       (** Invariant: an interface shall only contain [*Decl] elements, or
           [Topdef] elements with [topdef_expr = None] *)
@@ -330,8 +330,10 @@ and module_use = {
   mod_use_alias : uident Mark.pos;
 }
 
+and program_module = { module_name : uident Mark.pos; module_external : bool }
+
 and program = {
-  program_module_name : uident Mark.pos option;
+  program_module : program_module option;
   program_items : law_structure list;
   program_source_files : (string[@opaque]) list;
   program_used_modules : module_use list;
diff --git a/compiler/surface/parser_driver.ml b/compiler/surface/parser_driver.ml
index b1474823..672749d3 100644
--- a/compiler/surface/parser_driver.ml
+++ b/compiler/surface/parser_driver.ml
@@ -259,18 +259,21 @@ and expand_includes (source_file : string) (commands : Ast.law_structure list) :
     List.fold_left
       (fun acc command ->
         let join_module_names name_opt =
-          match acc.Ast.program_module_name, name_opt with
+          match acc.Ast.program_module, name_opt with
           | opt, None | None, opt -> opt
           | Some id1, Some id2 ->
             Message.error
-              ~extra_pos:["", Mark.get id1; "", Mark.get id2]
+              ~extra_pos:
+                ["", Mark.get id1.module_name; "", Mark.get id2.module_name]
               "Multiple definitions of the module name"
         in
         match command with
-        | Ast.ModuleDef (id, _) ->
+        | Ast.ModuleDef (id, is_external) ->
           {
             acc with
-            Ast.program_module_name = join_module_names (Some id);
+            Ast.program_module =
+              join_module_names
+                (Some { module_name = id; module_external = is_external });
             Ast.program_items = command :: acc.Ast.program_items;
           }
         | Ast.ModuleUse (mod_use_name, alias) ->
@@ -288,22 +291,22 @@ and expand_includes (source_file : string) (commands : Ast.law_structure list) :
           @@ fun lexbuf ->
           let includ_program = parse_source lexbuf in
           let () =
-            includ_program.Ast.program_module_name
+            includ_program.Ast.program_module
             |> Option.iter
                @@ fun id ->
                Message.error
                  ~extra_pos:
                    [
                      "File include", Mark.get inc_file;
-                     "Module declaration", Mark.get id;
+                     "Module declaration", Mark.get id.Ast.module_name;
                    ]
                  "A file that declares a module cannot be used through the raw \
                   '@{<yellow>> Include@}'@ directive.@ You should use it as a \
                   module with@ '@{<yellow>> Use @{<blue>%s@}@}'@ instead."
-                 (Mark.remove id)
+                 (Mark.remove id.Ast.module_name)
           in
           {
-            Ast.program_module_name = acc.program_module_name;
+            Ast.program_module = acc.program_module;
             Ast.program_source_files =
               List.rev_append includ_program.program_source_files
                 acc.Ast.program_source_files;
@@ -316,7 +319,7 @@ and expand_includes (source_file : string) (commands : Ast.law_structure list) :
           }
         | Ast.LawHeading (heading, commands') ->
           let {
-            Ast.program_module_name;
+            Ast.program_module;
             Ast.program_items = commands';
             Ast.program_source_files = new_sources;
             Ast.program_used_modules = new_used_modules;
@@ -325,7 +328,7 @@ and expand_includes (source_file : string) (commands : Ast.law_structure list) :
             expand_includes source_file commands'
           in
           {
-            Ast.program_module_name = join_module_names program_module_name;
+            Ast.program_module = join_module_names program_module;
             Ast.program_source_files =
               List.rev_append new_sources acc.Ast.program_source_files;
             Ast.program_items =
@@ -336,7 +339,7 @@ and expand_includes (source_file : string) (commands : Ast.law_structure list) :
           }
         | i -> { acc with Ast.program_items = i :: acc.Ast.program_items })
       {
-        Ast.program_module_name = None;
+        Ast.program_module = None;
         Ast.program_source_files = [];
         Ast.program_items = [];
         Ast.program_used_modules = [];
@@ -346,7 +349,7 @@ and expand_includes (source_file : string) (commands : Ast.law_structure list) :
   in
   {
     Ast.program_lang = language;
-    Ast.program_module_name = rprg.Ast.program_module_name;
+    Ast.program_module = rprg.Ast.program_module;
     Ast.program_source_files = List.rev rprg.Ast.program_source_files;
     Ast.program_items = List.rev rprg.Ast.program_items;
     Ast.program_used_modules = List.rev rprg.Ast.program_used_modules;
@@ -396,8 +399,8 @@ let with_sedlex_source source_file f =
     f lexbuf
 
 let check_modname program source_file =
-  match program.Ast.program_module_name, source_file with
-  | ( Some (mname, pos),
+  match program.Ast.program_module, source_file with
+  | ( Some { module_name = mname, pos; _ },
       (Global.FileName file | Global.Contents (_, file) | Global.Stdin file) )
     when not File.(equal mname Filename.(remove_extension (basename file))) ->
     Message.error ~pos
@@ -413,10 +416,14 @@ let load_interface ?default_module_name source_file =
   let program = with_sedlex_source source_file parse_source in
   check_modname program source_file;
   let modname =
-    match program.Ast.program_module_name, default_module_name with
+    match program.Ast.program_module, default_module_name with
     | Some mname, _ -> mname
     | None, Some n ->
-      n, Pos.from_info (Global.input_src_file source_file) 0 0 0 0
+      {
+        module_name =
+          n, Pos.from_info (Global.input_src_file source_file) 0 0 0 0;
+        module_external = false;
+      }
     | None, None ->
       Message.error
         "%a doesn't define a module name. It should contain a '@{<cyan>> \
diff --git a/doc/devel/externals.md b/doc/devel/externals.md
index 6d837c09..28c7cfbe 100644
--- a/doc/devel/externals.md
+++ b/doc/devel/externals.md
@@ -31,10 +31,10 @@ catala implementation and compile to OCaml (removing the `external` directive):
     ```
 
 ```shell-session
-$ clerk build _build/.../Prorata_external.ml
+$ clerk build _build/.../prorata_external.ml
 ```
 
-(beware the `_build/`, and the capitalisation of the module name)
+(beware the `_build/`, it is required here)
 
 
 ## Write the OCaml implementation
@@ -44,9 +44,11 @@ capitalisation to match). Edit to replace the dummy implementation by your code.
 Refer to `runtimes/ocaml/runtime.mli` for what is available (especially the
 `Oper` module to manipulate the types).
 
-Keep the `register_module` at the end as is, it's needed for the toplevel to use
-the value (you would get `Failure("Could not resolve reference to Xxx")` during
-evaluation).
+Keep the `register_module` at the end, but replace the hash (which should be of
+the form `"CM0|XXXXXXXX|XXXXXXXX|XXXXXXXX"`) by the string `"*external*"`. This
+section is needed for the Catala interpreter to find the declared values --- the
+error `Failure("Could not resolve reference to Xxx")` during evaluation is a
+symptom that it is missing.
 
 ## Compile and test
 
diff --git a/runtimes/ocaml/runtime.ml b/runtimes/ocaml/runtime.ml
index 4d626efd..60d279c5 100644
--- a/runtimes/ocaml/runtime.ml
+++ b/runtimes/ocaml/runtime.ml
@@ -897,7 +897,9 @@ let register_module modname values hash =
   Hashtbl.add modules_table modname hash;
   List.iter (fun (id, v) -> Hashtbl.add values_table ([modname], id) v) values
 
-let check_module m h = String.equal (Hashtbl.find modules_table m) h
+let check_module m h =
+  let h1 = Hashtbl.find modules_table m in
+  if String.equal h h1 then Ok () else Error h1
 
 let lookup_value qid =
   try Hashtbl.find values_table qid
diff --git a/runtimes/ocaml/runtime.mli b/runtimes/ocaml/runtime.mli
index 9b9124b5..07b8550d 100644
--- a/runtimes/ocaml/runtime.mli
+++ b/runtimes/ocaml/runtime.mli
@@ -446,8 +446,8 @@ val register_module : string -> (string * Obj.t) list -> hash -> unit
     expected to be a hash of the source file and the Catala version, and will in
     time be used to ensure that the module and the interface are in sync *)
 
-val check_module : string -> hash -> bool
-(** Returns [true] if it has been registered with the correct hash, [false] if
+val check_module : string -> hash -> (unit, hash) result
+(** Returns [Ok] if it has been registered with the correct hash, [Error h] if
     there is a hash mismatch.
 
     @raise Not_found if the module does not exist at all *)
diff --git a/tests/modules/good/mod_def.catala_en b/tests/modules/good/mod_def.catala_en
index 4b0b1e84..65cd7131 100644
--- a/tests/modules/good/mod_def.catala_en
+++ b/tests/modules/good/mod_def.catala_en
@@ -19,12 +19,20 @@ declaration scope S:
 declaration half content decimal
   depends on x content integer
   equals x / 2
+
+declaration maybe content Enum1
+  depends on x content Enum1
 ```
 
 ```catala
 scope S:
   definition sr equals $1,000
   definition e1 equals Maybe
+
+
+declaration maybe content Enum1
+  depends on x content Enum1
+  equals Maybe
 ```
 
 
diff --git a/tests/modules/good/output/mod_def.ml b/tests/modules/good/output/mod_def.ml
index ebeabc56..d3ac9384 100644
--- a/tests/modules/good/output/mod_def.ml
+++ b/tests/modules/good/output/mod_def.ml
@@ -30,7 +30,7 @@ let s (s_in: S_in.t) : S.t =
     try
       (handle_default
          [|{filename="tests/modules/good/mod_def.catala_en";
-            start_line=26; start_column=24; end_line=26; end_column=30;
+            start_line=29; start_column=24; end_line=29; end_column=30;
             law_headings=["Test modules + inclusions 1"]}|]
          ([|(fun (_: unit) ->
                handle_default [||] ([||]) (fun (_: unit) -> true)
@@ -47,7 +47,7 @@ let s (s_in: S_in.t) : S.t =
     try
       (handle_default
          [|{filename="tests/modules/good/mod_def.catala_en";
-            start_line=27; start_column=24; end_line=27; end_column=29;
+            start_line=30; start_column=24; end_line=30; end_column=29;
             law_headings=["Test modules + inclusions 1"]}|]
          ([|(fun (_: unit) ->
                handle_default [||] ([||]) (fun (_: unit) -> true)
@@ -70,8 +70,12 @@ let half_ : integer -> decimal =
        law_headings=["Test modules + inclusions 1"]} x_ (integer_of_string
       "2")
 
+let maybe_ : Enum1.t -> Enum1.t =
+  fun (_: Enum1.t) -> Enum1.Maybe ()
+
 let () =
   Runtime_ocaml.Runtime.register_module "Mod_def"
     [ "S", Obj.repr s;
-      "half", Obj.repr half_ ]
-    "todo-module-hash"
+      "half", Obj.repr half_;
+      "maybe", Obj.repr maybe_ ]
+    "CMX|XXXXXXXX|XXXXXXXX|XXXXXXXX"
diff --git a/tests/modules/good/prorata_external.ml b/tests/modules/good/prorata_external.ml
index 528e1f16..97c8be44 100644
--- a/tests/modules/good/prorata_external.ml
+++ b/tests/modules/good/prorata_external.ml
@@ -37,4 +37,4 @@ let () =
   Runtime_ocaml.Runtime.register_module "Prorata_external"
     [ "prorata", Obj.repr prorata_;
       "prorata2", Obj.repr prorata2_ ]
-    "todo-module-hash"
+    "*external*"
diff --git a/tests/name_resolution/good/let_in2.catala_en b/tests/name_resolution/good/let_in2.catala_en
index 2253a9ff..826234bd 100644
--- a/tests/name_resolution/good/let_in2.catala_en
+++ b/tests/name_resolution/good/let_in2.catala_en
@@ -90,5 +90,5 @@ let s (s_in: S_in.t) : S.t =
 let () =
   Runtime_ocaml.Runtime.register_module "Let_in2"
     [ "S", Obj.repr s ]
-    "todo-module-hash"
+    "CMX|XXXXXXXX|XXXXXXXX|XXXXXXXX"
 ```