How to use Lwt_unix with OCaml 5 effects?

[aantron]

The following code is a minimized version of camlworks/dream#297 that no longer calls into Dream at all:

type _ Effect.t += E : unit Effect.t

let () =
  Effect.Deep.try_with
    begin fun () ->
      Lwt_main.run begin
        Lwt.bind (Lwt_unix.sleep 1.) @@ fun () ->
        Effect.perform E;
        assert false
      end
    end
    ()
    {
      effc = fun (type a) (e : a Effect.t) ->
        match e with
        | E ->
          Option.some @@ fun (k : (a, _) Effect.Deep.continuation) ->
            prerr_endline "handling E";
            Effect.Deep.continue k ()
        | _ -> None
    }

Running this on OCaml 5.1.0 with Lwt 5.7.0 results in

Fatal error: exception Stdlib.Effect.Unhandled(Dune__exe__Json.E)

Replacing Lwt_unix.sleep 1. by, say, Lwt.pause () results in

handling E                           
Fatal error: exception File "dream/example/e-json/json.ml", line 9, characters 8-14: Assertion failed

which is the correct output, and what I would expect in both cases.

[raphael-proust]

related pointers:

• Lwt and OCaml 5 #897
• A prototype inclusion of effect handling into the Lwt_main.run function #961

[aantron]

I looked at the code linked in #961 and the comments, but I don't see why an outer effect handler would not work, and the comments in the examples suggest that it still should. So I'm surprised by the behavior of this particular example.

[aantron]

cc @ul

[Josef-Thorne-A]

Seconding aantron's comment here -- outer effect handlers do seem to have issues too -- not sure why that is.