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Make async iterator return() and next() more like async generators #891

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merged 3 commits into from
Jun 11, 2020
Merged

Make async iterator return() and next() more like async generators #891

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21ac10d
Make async iterator return() function after failed next
domenic Jun 2, 2020
f692e30
Port changes over from jsdom (plus minor editorial tweaks)
domenic Jun 8, 2020
48daab8
Fix per Timothy's review
domenic Jun 10, 2020
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63 changes: 38 additions & 25 deletions index.bs
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Original file line number Diff line number Diff line change
Expand Up @@ -12543,23 +12543,29 @@ The \[[Prototype]] [=internal slot=] of an [=asynchronous iterator prototype obj
1. Let |value| be |next|, [=converted to an ECMAScript value=].
1. Return [=!=] [$CreateIterResultObject$](|value|, <emu-val>false</emu-val>).
1. Let |onFulfilled| be [=!=] [$CreateBuiltinFunction$](|fulfillSteps|, « »).
1. Perform [=!=] [$PerformPromiseThen$](|nextPromise|, |onFulfilled|,
<emu-val>undefined</emu-val>, |nextPromiseCapability|).
1. Let |rejectSteps| be the following steps, given |reason|:
1. Set |object|'s [=default asynchronous iterator object/ongoing promise=] to
null.
1. Set |object|'s [=default asynchronous iterator object/is finished=] to true.
1. [=ECMAScript/Throw=] |reason|.
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A few more things to think about.

try {
  const a = await it.next();  // eventually rejected
} catch {
  const b = await it.next();
}

Async generators (and also sync generators) would set b to { value: undefined, done: true }. We could do the same by just setting the is finished flag to true, which would give us the invariant that getting the next iteration result will never be called after it has rejected a promise, which is something I think spec authors could come to expect.

Alternatively, we could consider a “one bad apple spoils the bunch” semantic, where b gets rejected. I think we should stick with the async generator behavior though.

Going further, consider this case:

const a = it.next();  // note: no await here
const b = it.next();

It seems like b should be settled in the same way as b above, as the timing of when it.next() is called shouldn't have any bearing on its result. But then this would require us to keep some unsettled promise set on object's target, which brings about additional complexity. Async generators has something analogous through the [[AsyncGeneratorQueue]] internal slot; see spec.

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I like these ideas, and I will work to align the spec with generators in the way you suggest. It's good that we're getting this review coverage now that these things are being used seriously in specs, and implemented (albeit only implemented in webidl2js so far).

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I've attempted to implement semantics that pass these test cases in the latest commit of jsdom/webidl2js#224. If you agree that the changes there are reasonable, I'll port them over to spec text form.

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@domenic domenic Jun 5, 2020 •
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Also feel free to peruse the tests added in web-platform-tests/wpt#22982 (commits)

1. Let |onRejected| be [=!=] [$CreateBuiltinFunction$](|rejectSteps|, « »).
1. Perform [=!=] [$PerformPromiseThen$](|nextPromise|, |onFulfilled|, |onRejected|,
|nextPromiseCapability|).
1. Return |nextPromiseCapability|.\[[Promise]].

1. Let |promise| be |object|'s [=default asynchronous iterator object/ongoing promise=].
1. Let |ongoingPromise| be |object|'s [=default asynchronous iterator object/ongoing promise=].

1. If |promise| is not null, then:
1. If |ongoingPromise| is not null, then:
1. Let |afterOngoingPromiseCapability| be [=!=] [$NewPromiseCapability$]({{%Promise%}}).
1. Let |onFulfilled| be [=!=] [$CreateBuiltinFunction$](|nextSteps|, « »).
1. Perform [=!=] [$PerformPromiseThen$](|promise|, |onFulfilled|,
<emu-val>undefined</emu-val>, |afterOngoingPromiseCapability|).
1. Let |onSettled| be [=!=] [$CreateBuiltinFunction$](|nextSteps|, « »).
1. Perform [=!=] [$PerformPromiseThen$](|ongoingPromise|, |onSettled|, |onSettled|,
|afterOngoingPromiseCapability|).
1. Set |object|'s [=default asynchronous iterator object/ongoing promise=] to
|afterOngoingPromiseCapability|.\[[Promise]].

1. Otherwise:
1. Run |nextSteps| and set |object|'s
[=default asynchronous iterator object/ongoing promise=] to the result.
1. Set |object|'s [=default asynchronous iterator object/ongoing promise=] to the result of
running |nextSteps|.

1. Return |object|'s [=default asynchronous iterator object/ongoing promise=].
</div>
Expand Down Expand Up @@ -12599,31 +12605,38 @@ The \[[Prototype]] [=internal slot=] of an [=asynchronous iterator prototype obj
<emu-val>undefined</emu-val>, « |error| »).
1. Return |returnPromiseCapability|.\[[Promise]].

1. If |object|'s [=default asynchronous iterator object/ongoing promise=] is not null, then:
1. Let |error| be a new {{ECMAScript/TypeError}}.
1. Perform [=!=] [$Call$](|returnPromiseCapability|.\[[Reject]],
<emu-val>undefined</emu-val>, « |error| »).
1. Return |returnPromiseCapability|.\[[Promise]].
1. Let |returnSteps| be the following steps:
1. Let |returnPromiseCapability| be [=!=] [$NewPromiseCapability$]({{%Promise%}}).
1. If |object|'s [=default asynchronous iterator object/is finished=] is true, then:
1. Let |result| be [$CreateIterResultObject$](|value|, <emu-val>true</emu-val>).
1. Perform [=!=] [$Call$](|returnPromiseCapability|.\[[Resolve]],
<emu-val>undefined</emu-val>, « |result| »).
1. Return |returnPromiseCapability|.\[[Promise]].
1. Set |object|'s [=default asynchronous iterator object/is finished=] to true.
1. Return the result of running the [=asynchronous iterator return=] algorithm for
|interface|, given |object|'s [=default asynchronous iterator object/target=], |object|,
and |value|.

1. If |object|'s [=default asynchronous iterator object/is finished=] is true, then:
1. Let |result| be [=!=] [$CreateIterResultObject$](|value|,
<emu-val>true</emu-val>).
1. Perform [=!=] [$Call$](|returnPromiseCapability|.\[[Resolve]],
<emu-val>undefined</emu-val>, « |result| »).
1. Return |returnPromiseCapability|.\[[Promise]].
1. Let |returnStepsPromise| be null.

1. Set |object|'s [=default asynchronous iterator object/is finished=] to true.
1. Let |ongoingPromise| be |object|'s [=default asynchronous iterator object/ongoing promise=].

1. Let |returnPromise| be the result of running the [=asynchronous iterator return=] algorithm
for |interface|, given |object|'s [=default asynchronous iterator object/target=],
|object|, and |value|.
1. If |ongoingPromise| is not null, then:
1. Let |afterOngoingPromiseCapability| be [=!=] [$NewPromiseCapability$]({{%Promise%}}).
1. Let |onSettled| be [=!=] [$CreateBuiltinFunction$](|returnSteps|, « »).
1. Perform [=!=] [$PerformPromiseThen$](|ongoingPromise|, |onSettled|, |onSettled|,
|afterOngoingPromiseCapability|).
1. Set |returnStepsPromise| to |afterOngoingPromiseCapability|.\[[Promise]].

1. Otherwise:
1. Set |returnStepsPromise| to the result of running |returnSteps|.

1. Let |fulfillSteps| be the following steps:
1. Return [=!=] [$CreateIterResultObject$](|value|, <emu-val>true</emu-val>).

1. Let |onFulfilled| be [=!=] [$CreateBuiltinFunction$](|fulfillSteps|, « »).

1. Perform [=!=] [$PerformPromiseThen$](|returnPromise|, |onFulfilled|,
1. Perform [=!=] [$PerformPromiseThen$](|returnStepsPromise|, |onFulfilled|,
<emu-val>undefined</emu-val>, |returnPromiseCapability|).

1. Return |returnPromiseCapability|.\[[Promise]].
Expand Down