Promises returned from generators are not resolved automatically

[dstillman]

In Q, if an async generator returns a promise, that promise is resolved and the resolved value is passed on. In Bluebird, the promise itself is passed along as the next value:

Q v1.0.0:

Q.async(function* () {
    return Promise.resolve("")
})()
.then(function (str) {
    alert(typeof str); // 'string'
})
.done();

Bluebird v0.11.6-1:

Promise.spawn(function* () {
    return Promise.resolve("");
})
.then(function (str) {
    alert(typeof str); // 'object'
});

The workaround in Bluebird is to yield on the promise before returning it:

Promise.spawn(function* () {
    return yield Promise.resolve("");
})
.then(function (str) {
    alert(typeof str); // 'string'
});

But that seems like an unnecessary extra step that could be avoided, and I think it goes against expectations coming from Promise.try(). Is this behavior intentional?

[benjamingr]

To be fair, in C# you would write return await and not return to return from an async function.

The Bluebird behavior seems nice (and correct) but I see your point - this is inconsistent with the way promises unwrap. Definitely something we need to think about.

[petkaantonov]

Yea confirmed as bug.

[benjamingr]

@petkaantonov are you sure returning a promise acting like in thenables is the correct behavior?

[petkaantonov]

Yes it is always without any doubt a bug if you get a promise as a .then argument