GUI.listen: no need for a unit argument

[hannesm]

it is not clear why this was changed in https://github.com/mirage/mirage-qubes/commit/2393d8d5b937139fdf705d1defca067c9eb7eadc //cc @cfcs -- was there a good reason to introduce the unit argument?

I know this has been released since then (i.e. this PR breaks the API again), but that's fine. (see as well @reynir comment at https://github.com/mirage/mirage-qubes/commit/2393d8d5b937139fdf705d1defca067c9eb7eadc#r34791187) -- and with lwt 5.0.0 being released now (where Lwt.async requires unit -> unit Lwt.t), should the return type of val listen ("it never returns") be unit Lwt.t instead?

[cfcs]

It was changed because then it would fit better with Lwt.async indeed. I can't remember exactly where this was an issue, I believe it was somewhere in a hacky build script that injected strings into unikernel.ml through some fields that mirage failed to escape correctly.

I don't see the need to break it again, and if remove the unit argument, now all the consumers will invariably have to write Lwt.async (fun () -> GUI.listen t) instead of Lwt.async (GUI.listen t).

It would make sense to make the 'a a unit as you suggest:

val listen : t -> unit -> unit Lwt.t

[talex5]

Although I slightly prefer the original API, I don't think we should change it again. Breaking APIs just over style disagreements seems silly, and both APIs are fine.

However, GUI.listen should still return 'a Lwt.t to show that it doesn't return. It's just a coincidence that Lwt.async happened to have a similar-looking 'a in its type. They're not the same, because of the implicit forall. i.e. we really had:

val async : 'a. (unit -> 'a Lwt.t) -> unit
val listen : ('a. unit -> 'a Lwt.t)

async's (old) type said it doesn't matter what the function returns. listen's type says it doesn't return.

[cfcs]

I'm happy with 'a and @talex5's reasoning.

[hannesm]

since @talex5 prefers to not change the API, let's close this.