RFC / Feature request: cancelable

[raphael-proust]

Whilst coding I stumbled upon a situation where I needed a promise to be reliably cancelable. As in Lwt.cancel p needed to cause p to be rejected with Canceled if p was in the Sleep state. I came up with the following:

let cancelable p =
   match Lwt.state p with
   | Return _ | Fail _ -> p
   | Sleep -> Lwt.pick [p; fst (Lwt.task ())]

1. (RFC) This does what I need, but: have I missed some corner cases?

2. (Feature request) This would live nicely along protected and no_cancel in the Lwt module. I'm open to a change of name because cancelable could also be the name of a predicate to test whether a promise is cancelable. Maybe unprotected (but it suggests that this cancels out protected) or always_cancelable (but it's not if the argument is resolved)?

---

Considering protected, no_cancel, and cancelable suggests a fourth function: one that propagates the cancelation search backwards but doesn't propagate the cancelation error forwards. This completes the cross product of “carries cancelation backwards”/“doesn't” with “fails with Canceled when canceled”/“doesn't”.

Something along the lines of

let hide_cancelation p =
  Lwt.catch
    (fun () -> p)
    (function Lwt.Canceled -> fst @@ Lwt.wait () | e -> Lwt.fail e)

Although I can't find uses for it right now, and returning a never-resolving promise is a bit strange.

[aantron]

1. have I missed some corner cases?

None come to mind, though it does seem like a scary way to implement the function :) I would have probably used Lwt.task, Lwt.on_any, and Lwt.on_cancel to directly tie two promises together, rather than introduce a third promise with Lwt.pick.

I'd merge a PR for this function, but implemented in terms of the internals of Lwt (see protected and no_cancel). I can't readily think of a good name for it, either :)

hide_cancelation has both potentially dangerous behavior (the forever-pending promise) and known use case, so I suggest not to add it now.

[raphael-proust]

I will prepare an MR.