API request: predicate to check whether we are running under an event loop

[dijkstracula]

Hi,

I'm a member of a team that is migrating a large legacy codebase from Parmap-based parallelism to Eio's fine-grained concurrency model. Two of the consequences of this refactoring is that we do not enter Eio's event loop until after some initial computation, and because it isn't feasible to move to Eio all in one go, there is a transitory period where our codebase uses both Parmap and Eio.

Certain functionality during this migration necessitates us knowing whether we are running under Eio or not. (I'll abstract away the gnarly particulars for you.). I, however, could not see anything in the public interface to do this check. So: at present, I've written the following predicate, which I think we can all agree is not ideal:

let has_eio_context () =
  (* XXX: This is an abuse of Fiber.check, which is really intended to verify
   * whether the running fiber has been canceled.  Here, we use it to ascertain
   * whether the "running fiber" actually exists (e.g. whether we are in the
   * synchronous world or the effects world). *)
  try
    Eio.Fiber.check ();
    true
  with
  | Stdlib.Effect.Unhandled Eio__core__Cancel.Get_context -> false
  | e -> raise e (* Do not swallow a cancelation notification, for example *)

Would the maintainers consider exposing this information without needing this sort of "spooky action" check? (I would also love to know whether I've missed a corner case in my above function...). Or, could you kindly point me to the existing check if it exists and I just missed it altogether?

Thanks for all your hard work building Eio.

Cheers,
Nathan

[talex5]

Adding this seems reasonable. Eio itself does something similar to make traceln work when the loop isn't running:

eio/lib_eio/core/debug.ml

Lines 28 to 32 in 8f7f82d

	let get () =
	match Fiber.get traceln_key with
	| Some traceln -> traceln
	| None
	| exception (Effect.Unhandled _) -> { traceln = default_traceln }

One potential problem with using Fiber.check (or Fiber.get) in application code for this is that if OCaml gets thread-local storage then Eio might stop using an effect for this.

A minor problem with putting it in the API: if OCaml ever gets typed effects then it won't make sense any longer.

(BTW, you don't need the | e -> raise e; OCaml does that automatically)

[dijkstracula]

Thanks for the notes - I do remember coming across some traceln - and for the potential issues with using Fiber.check - I'll document those for our internal awareness.