[v5] Awaitable actors

[davidkpiano]

This PR makes actors thenable; awaiting an actor will wait for output.

  const machine = createMachine({
    initial: 'a',
    states: {
      a: {
        after: {
          10: 'b'
        }
      },
      b: {
        type: 'final',
        output: { foo: 'bar' }
      }
    }
  });

  const output = await interpret(machine).start();

  expect(output).toEqual({ foo: 'bar' });

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[Andarist]

I don't "feel" this feature. If anything, I'd probably prefer smth like toPromise over this. Sure, we can make things "thenable" - should we though? The actor is not a promise and despite the fact that JS allows us to do this, it feels rather weird.

Since we already have waitFor I also don't necessarily feel the need for toPromise either. It's already simple to do this. Yet a different option would be to just have actor.waitFor.

Our ActorLogic interface starts to be fatter than a turkey on Thanksgiving 😬

[davidkpiano]

The current problem is there's no easy way to have "wait for actor to complete" for any actor logic, unless you're okay with an optional selector here:

// single argument - wait for actor to be done
const output = await waitFor(someActor);

// current behavior
// PROBLEM: not sure what should be returned
const output = await waitFor(someActor, state => /* ??? */);

What do you think would be a good workaround with waitFor?

[davidkpiano]

Ping @Andarist - need a good solution to this use-case:

// get actor output when the actor is completed
const output = await toPromise(actor);

Can't do this with waitFor().

[davidkpiano]

Superseded by #4198