Destructuring bluebird Promise.all([..])

Hi folks, I’m hoping someone can help with this. I have a fairly common pattern in my code of using array destructuring to name the results of await Promise.all([]). Example:

async function foo() {
  const [a, b] = await Promise.all([
    Promise.resolve(1),
    Promise.resolve('b'),
  ]);
}

(I’m using bluebird here). Running this through flow I get a bunch of errors that I don’t fully understand, but it looks like flow is not able to treat the awaited result of Promise.all([]) as a tuple?

foo.js:6
  6:   const [a, b] = await Promise.all([
                            ^ Bluebird$Promise. This type is incompatible with
606: declare function $await<T>(p: Promise<T> | T): T;
                                   ^^^^^^^^^^^^^^ union: type application of identifier `Promise` | type parameter `T` of await. See lib: /private/tmp/flow/flowlib_3ae7518b/core.js:606
  Member 1:
  606: declare function $await<T>(p: Promise<T> | T): T;
                                     ^^^^^^^^^^ type application of identifier `Promise`. See lib: /private/tmp/flow/flowlib_3ae7518b/core.js:606
  Error:
    6:   const [a, b] = await Promise.all([
                              ^ Bluebird$Promise. This type is incompatible with
  606: declare function $await<T>(p: Promise<T> | T): T;
                                     ^^^^^^^^^^ Promise. See lib: /private/tmp/flow/flowlib_3ae7518b/core.js:606
  Member 2:
    6:   const [a, b] = await Promise.all([
                        ^ type parameter `T` of await
  Error:
    6:   const [a, b] = await Promise.all([
                ^ element 0. Indexable signature not found in
    6:   const [a, b] = await Promise.all([
                        ^ Bluebird$Promise

Any help (or workarounds) would be much appreciated!

[SimenB]

facebook/flow#2171

I use:

// @flow

declare function $await<T>(p: Bluebird$Promise<T> | T): T;

[jcready]

The problem here is that an async function needs to be able to await any “thenable”. Here is an example of what should be accepted by flow:

/* @flow */

const thenable = { then: (cb) => cb('thenable') }

async function foo () {
  const a: string = await Promise.resolve('a')
  const b: string = await Promise.resolve(thenable)
  const c: string = await thenable
  return a + b + c
}

Here’s a basic “thenable” being awaited inside an async function on node 7 with the --harmony-async-await flag: