`Eio.Promise.map`

[patricoferris]

It would be nice to have a Eio.Promise.map function.

Currently, you can implement something close with:

  let map ~sw f v =
    let p, r = Eio.Promise.create ~label:"map" () in
    Eio.Fiber.fork ~sw (fun () ->
      Eio.Promise.await v |> f |> Eio.Promise.resolve r
    );
    p

But that essentially requires attaching a switch to your promise, and the promise API is switchless, it would be nice to keep it that way for mapping new promises. I don't quite understand the underlying Cell API enough to know if this is possible though.

[talex5]

We do have Fiber.fork_promise which simplifies it a bit, though that still needs a switch.

The problem is: where would f run? e.g.

1. Domain 1 creates a promise for a URL.
2. Domain 2 maps it to make a promise of the resource at that URL.
3. Domains 3 and 4 wait for the resource promise.

• Resolving a promise currently doesn't run any user code, and is even safe to do from a signal handler, so domain 1 shouldn't be running f.
• If domain 2 will do it, it needs a switch anyway for the download fiber.
• f shouldn't be run more than once. If it's run lazily then some locking is needed if 3 and 4 both try at the same time. And when does it run if no one is waiting for it?

So I think it's better to let the user decide what's best for their use. With that said, we do have Condition.register_immediate, but that's only really for integration with other IO libraries.

[patricoferris]

Thanks @talex5 -- that makes sense, my use case is for an immediate, non-yielding piece of code (something like Option.get), but the more general case as you point out is tricky.

I thought the first option would be possible, but as you point out it is probably better to keep that simplicity. In any case, there is a switch lying around anyway in my code and I can just use that :)