Effect with a value

Learning
1

Hello,
I have been trying to create an effect like this to return a int. What types can I return like this ?

type _ Effect.t += Early_return : int -> int Effect.t

But didn’t manage to fix this error. Am I misunderstanding the type of the effect ?
I am aware that the syntax can be simplified when I use the new release.

type _ Effect.t += Early_return : int -> int Effect.t
  let rec  effective_request_vote mutex id term votes_received =
  match_with (fun () -> requestvote mutex id term votes_received)
    ()
  { effc = (fun (type c) (eff1: c Effect.t) ->
      match eff1 with
      | Early_return  v -> Some (fun (k:  (c,_) continuation) ->
              Printf.printf "Early Return" ;
              continue k v
          )
      | _ -> None
  );
  exnc = (function
        | e -> raise e
  );
  retc = (fun _ ->  ())

 }

The error is

perform (Early_return votes_received )
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Error: This constructor has type “int eff”
but an expression was expected of type “('a → 'b) eff”
Type “int” is not compatible with type “'a → 'b”

Update : I mistyped my code. Updated it now. This was the original error.
Thanks,
Mohan

2

Hi! I try to reproduce your case in ocaml 5.3.0 and I did’nt face to any errors

3

The issue probably lies outside the code you showed (the code you posted looks correct, but we don’t see the perform part in there).
This is likely a mismatch between the input and return types of perform: Early_return declares that performing it returns an integer, but the type-checker seems to assume the the result of the perform expression is a function, which could happen for instance if you passed an extra argument (perform (Early_return votes_received) foo) or if it occurs in a branch and the other branch was inferred to be a function (either correctly or because you forgot an argument to a function call).
You can add type annotations like (perform (Early_return votes_received) : int) to try to get a more useful type error.

4

As you mentioned it seems to be caused at the callsite. I am investigating.

Update :

I have code that works now.

type _ Effect.t += Early_return :  int -> int Effect.t

let rec effective_request_vote
  (mutex : Eio.Mutex.t)
  (id : int)
  (term : int)
  (votes_received : int) : int Lwt.t =

  match_with (fun () -> requestvote mutex id term votes_received)
    ()
  { effc = (fun (type c) (eff1: c Effect.t) ->
      match eff1 with
      | Early_return v  -> Some (fun (k:  (c,_) continuation) ->
              Printf.printf "Early Return" ;
              continue k v
          )
      | _ -> None
  );
  exnc = (function
        | e -> raise e
  );
  retc = (fun _ ->  Lwt.return votes_received)

 }

I had to change the retc and fix it like this in the function that produces that effect. Annotations helped really.

    Lwt.return (perform (Early_return votes_received));

I had to do this because the call to the function that produces that effect Early_return
expects Lwt.t since it mixes EIO and LWT. I believe I should use only EIO henceforth as I am not a seasoned OCaml dev.

The combination of effect handlers, LWT and EIO is hard

Thanks

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