Replace module hashes for external modules

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.

compiler/catala_utils/hash.ml

@@ -106,3 +106,5 @@ let of_string s =
       (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*"

compiler/catala_utils/hash.mli

@@ -71,3 +71,11 @@ 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. *)

compiler/lcalc/to_ocaml.ml

@@ -725,8 +725,12 @@ let check_and_reexport_used_modules fmt ~hashf modules =
          | Ok () -> ()@,\
          @[<hv 2>| Error h -> failwith \"Hash mismatch for module %a, it may \
          need recompiling\"@]@]@,"
-        (ModuleName.to_string m) Hash.format (hashf intf_id.hash)
-        ModuleName.format m;
+        (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
@@ -735,7 +739,8 @@ let format_module_registration
     fmt
     (bnd : ('m Ast.expr Var.t * _) String.Map.t)
     modname
-    hash =
+    hash
+    is_external =
   Format.pp_open_vbox fmt 2;
   Format.pp_print_string fmt "let () =";
   Format.pp_print_space fmt ();
@@ -758,7 +763,11 @@ let format_module_registration
   Format.pp_print_char fmt ' ';
   Format.pp_print_string fmt "]";
   Format.pp_print_space fmt ();
-  Format.fprintf fmt "\"%a\"" Hash.format 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 ()
@@ -791,6 +800,7 @@ let format_program
     match p.module_name, exec_scope with
     | 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

compiler/shared_ast/interpreter.ml

@@ -1158,6 +1158,10 @@ let evaluate_expr ctx lang e = evaluate_expr ctx lang (addcustom e)
 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 mname)) -.- "cmo")
@@ -1176,9 +1180,7 @@ let load_runtime_modules ~hashf prg =
            "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) (Hash.to_string hash)
-    with
+    match Runtime.check_module (ModuleName.to_string mname) expect_hash with
     | Ok () -> ()
     | Error bad_hash ->
       Message.debug
@@ -1186,7 +1188,10 @@ let load_runtime_modules ~hashf prg =
         ModuleName.format mname Hash.format hash
         (fun ppf h ->
           try Hash.format ppf (Hash.of_string h)
-          with Failure _ -> Format.pp_print_string ppf "<invalid>")
+          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
         "Module %a@ needs@ recompiling:@ %a@ was@ likely@ compiled@ from@ an@ \

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
 

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*"