Allow async iterators to specify return algorithms #805
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Reported @fallenAngelSpeaks2Me |
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Ms2ger
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| @@ -4307,6 +4307,21 @@ iteration, or resolves with a tuple containing two elements: | |||
| 1. a value of the first type given in the declaration; | |||
| 1. a value of the second type given in the declaration. | |||
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| The prose may also define an <dfn export>asynchronous iterator return</dfn> algorithm. This | |||
| algorithm receives the instance of the [=interface=] that is being iterated, the async iterator | |||
| itself, and a single argument value. This algorithm is invoked in the case of premature termination | |||
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And the single argument value is an {{any}} value?
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| <emu-val>undefined</emu-val>, « |error| »). | ||
| 1. Return |returnPromiseCapability|.\[[Promise]]. | ||
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| 1. If |object|'s [=default asynchronous iterator object/ongoing promise=] is not undefined, then: |
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Check if this undefined needs an emu-val? I don't remember which way this one goes.
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Mmm nice catch. Since we made it an IDL Promise, we should probably union it with null (another IDL value), not undefined (which is always <emu-val>undefined</emu-val>, a JS value). I switched all instances to do so.
Closes #804. Notably this allows consumers to not worry about resolving their promise with the correct iterator result.
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Closes #804. Notably this allows consumers to not worry about resolving their promise with the correct iterator result.
This is on top of the stage-setting commits in #802; only the last commit is worth reviewing.
One choice I made here where the right answer wasn't immediately obvious is that calling return() while a call to next() is outstanding will reject with no side effects. This mirrors what we've done in the Streams design, and seems simpler than e.g. waiting for 1+ outstanding next()s to settle. This will only occur when manually manipulating the async iterator (as opposed to using for-await-of) so it's not a very mainline scenario. /cc @ricea in case he has any thoughts.
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