[Webkit-unassigned] [Bug 166695] [ESNext] Async iteration - Implement Async Generator

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=166695

--- Comment #4 from Caitlin Potter (:caitp) <caitp at igalia.com> ---
Comment on attachment 298142
  --> https://bugs.webkit.org/attachment.cgi?id=298142
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=298142&action=review

>>> Source/JavaScriptCore/builtins/AsyncGeneratorPrototype.js:65
>>> +    if (generator. at asyncGeneratorState === @AsyncGeneratorStateAwait) {
>> 
>> the special generator continuation is a nice idea.
>> 
>> Though it doesn't affect JSC just yet due to not being implemented, note that this would _probably_ be observable via the function.sent metaproperty (or at least, it is in the v8 generator implementation without some additional work).
> 
> Oh Sorry, not sure that I got it. 
> Do you mean that in case of using chain of promises instead of queue, we will have visible side effects when function.sent would be implemented?
> Could you please provide some examples that you faced in v8? I'll try to emulate on current solution, possible current approach is not viable at all.
> 
> Also it seems that I need to replace this by: 
> 
>         let wrappedValue = @newPromiseCapability(@Promise);
>         wrappedValue. at resolve.@call(@undefined, value);
>         wrappedValue. at promise.@then(
>                                     function(result) { @asyncGeneratorResume(generator, promiseCapability, result, @GeneratorResumeModeNormal); },
>                                     function(error) { @asyncGeneratorResume(generator, promiseCapability, error, @GeneratorResumeModeThrow); });
> Because value is returned by await can be non promise value.

What I was getting at was about the `function.sent` metaproperty proposal (https://github.com/allenwb/ESideas/blob/master/Generator%20metaproperty.md), which allows the programmer to make `function.sent` an alias to the value the generator was resumed with (eg `generator.next("foo");` makes function.sent === "foo" inside the generator after resuming).

So, with Async Generators and await expressions, the awaited value would alias function.sent rather than the value sent via generator.next(), which observably breaks the metaproperty.

JSC does not implement function.sent yet, so it isn't really noticeable here, but it is something to pay attention to for the future, unless the proposal is withdrawn

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