Support dolmen frontend in JS worker

[bclement-ocp]

          > I think Try Alt-Ergo is currently broken but I don't think that should block the release.

More precisely worker_js.ml does not seem to be compatible with dolmen.

Originally posted by @bclement-ocp in #1218 (comment)

[Halbaroth]

When I try to run the worker example as follows:
dune exec -- src/bin/js/worker_example.exe
I got:

Fatal error: Unimplemented Javascript primitive caml_js_expr!
fish: Job 1, 'dune exec -- src/bin/js/worker_…' terminated by signal SIGABRT (Abort)

Is it the issue you ran into?

[bclement-ocp]

I don't think calling src/bin/js/worker_example.exe is supposed to work. The way to run the js example is to build with make js-example and then run from the www directory.

The issue is that the worker always uses the legacy frontend, but tries to find a "dolmen" type. The easy fix is to force the use of the "legacy" frontend in the JS worker; the long-term fix is to modify the JS worker to use the dolmen frontend instead of the legacy frontend.

[bclement-ocp]

The way I run the worker is as follows (you can't simply open the www/index.html file; you run into restrictions in the browser that refuses to execute javascript from the file:// protocol):

$ make js-example
$ python -m http.server -d www

Sending a file with "Ask Alt-Ergo" returns an error in the dev console:

{ "worker_id": 42, "status": { "Error": "Unknown error" } }

and if I print the exception in src/bin/js/worker_js.ml:

diff --git a/src/bin/js/worker_js.ml b/src/bin/js/worker_js.ml
index 4d79920d..484d7812 100644
--- a/src/bin/js/worker_js.ml
+++ b/src/bin/js/worker_js.ml
@@ -259,11 +259,11 @@ let main worker_id content =
       Worker_interface.diagnostic =
         Some [Format.asprintf "%a" Errors.report e]
     }
-  | _ ->
+  | exn ->
     let res = Worker_interface.init_results () in
     { res with
       Worker_interface.worker_id = worker_id;
-      Worker_interface.status = Error "Unknown error";
+      Worker_interface.status = Error ("Unknown error: " ^ Printexc.to_string exn);
     }
 
 (** Worker initialisation

we get a more detailed error:

{ "worker_id": 42,
  "status":
    { "Error":
        "Unknown error: AltErgoLib.Input.Method_not_registered(\"dolmen\")" } }