Deprecate Lwt_main.yield and Lwt_unix.yield?

[favonia]

This is a suggestion. After talking to @aantron on Discourse, I feel we should officially deprecate yield and related functions using [@@alert deprecate], in hope that one day we will merge the two pools into one.

[cyberhuman]

How do you yield then? 🤔

[favonia]

Lwt.pause () will do the job. There are currently two pools in the codebase due to the two almost identical mechanisms, but I believe they can be safely merged once every Lwt_unix.yield () and Lwt_main.yield () is replaced by Lwt.pause ().

[aantron]

See

lwt/src/unix/lwt_main.mli

Lines 45 to 52 in 9cbf3a9

	val yield : unit -> unit Lwt.t
	(** [yield ()] is a pending promise that is fulfilled after Lwt finishes
	calling all currently ready callbacks, i.e. it is fulfilled on the next
	“tick.”
	[yield] is similar to {!Lwt.pause} and it exists for historical reason.
	Prefer [pause] in order to stay compatible with other execution
	environments such as js_of_ocaml. *)

and https://discuss.ocaml.org/t/lwt-pause-versus-yield/7875.

[raphael-proust]

See #784 for some earlier discussion of this.

TL;DR:

• Lwt_main.run treats paused and yielded promises differently: it resolves paused promises twice as often as yielded promises.
• This is not an intended difference, but it's a significant difference and it may be significant for several binaries.

---

I think that, before we release this we should:

• run the tests of all the opam packages that depend on lwt with the modified lwt (I think this is possible through the opam CI in some way)
• notify packages that we break or even give them some patches
• advertise the possibility of the change on discuss (and maybe other channels)

Then if all of that goes well, we can release. It'll need a "breaking change" mention in the changelog.

Maybe we could leverage the opam infrastructure to test if we break some of the tests of the opam packages. I think we should do this (if possible), make upstream commits to all possibly broken packages, and we should make a special mention of this in the changelog (and advertise it in advance on discuss too).

[favonia]

I just updated CHANGES in the #858 but need help in all other actionable items.

[raphael-proust]

I'll ask some opam wizards about the general test of all dependencies.

[favonia]

Closing this as #917 will complete every actionable items in this issue.