Allow functions to be used as promise inputs

[domenic]

See WICG/navigation-api#40 for background.

Basically, any time we convert an ES value to a Promise<T>, we should first check to see if it's a function. If so, we should call the function, and use its return value.

This allows code such as

event.waitUntil((async () => {
  // ...
})());

to become

event.waitUntil(async () => {
  // ...
});

This would be backward-compatible since there is no existing instances of Promise<T> where T is a callback function type, that are used as inputs.

[bakkot]

Re: waiting for async-do: I am no longer confident I'll get async-do to stage 3 within the next ~6 months. I'm still holding out hope for within a year, though. (There's some irritating complexity in the design of do-expressions which is slowing it down, and unfortunately for personal reasons I haven't been able to dedicate as much time as I'd like for the past few months.)

[annevk]

This would mean that something like Promise.resolve() cannot be expressed through Web IDL.

It does seem useful as it's the kind of code you would expect to work, but I worry about the mismatch with JavaScript in general becoming a problem for developers as they would have to know about this and the difference between Web IDL and non-Web IDL APIs.

[domenic]

So far the only promise-accepting APIs in JS are the promise combinators themselves, which I think should be exempt from this sort of special-casing. I do think if ES introduces new promise-accepting APIs (although it's hard to imagine what), they should follow the precedent we set here.

[ExE-Boss]

What about a function object that has a then method?

[domenic]

It would be called; the typeof check would happen first.

[ExE-Boss]

@domenic

It would be called; the typeof check would happen first.

That is the opposite of how Promise.resolve(), Promise.all(), Promise.prototype.then, etc. work, which call the then method.

[domenic]

Yes. That's fine.

[ExE-Boss]

APIs that want this should just be able to use:

callback PromiseCallback = Promise<undefined> ();
interface AppHistoryNavigateEvent : Event {
	undefined transitionWhile((Promise<undefined> | PromiseCallback) newNavigationAction);
};

[domenic]

The idea is to do it uniformly for the entire web platform, not as one-offs.