Proposal: `await.all {...}` for parallelism

Jacob Bloom mr.jacob.bloom at gmail.com
Thu Nov 21 09:46:33 UTC 2019


>why not just `await` as already is, but supporting an
>iterable / array of promises, as `Promise.all` already does

`await` can already accept a non-promise, so I believe that'd be
breaking syntax if `Array.prototype.then` is set. It also requires
collecting the promises in an array, which is what the proposed syntax
is trying to avoid.

On Thu, Nov 21, 2019 at 2:41 AM Jacob Bloom <mr.jacob.bloom at gmail.com> wrote:
>
> >Just FYI, I previously suggested a couple things substantially more
> >flexible than this
>
> Ah, thank you for bringing those proposals to my attention. I looked
> through the archives for relevant discussions but I must've missed
> them.
>
> It seems like we converged on a similar syntax for what you called
> "merging," and the idea that there ought to be a separate syntax for
> iteration. I don't know whether that means that this is the right
> solution or just the most obvious one, but either way it's encouraging
> to know that other people have the same difficulties with the current
> syntax and are thinking about the problem.
>
> >from my experience, committee members are in general
> >very hesitant to add syntax for anything that doesn't pay for
> >itself well
>
> Yeah, I figured the bar would be high for new syntax. I've run into
> the awkwardness of dealing with distinct parallel tasks several times,
> and a few of the people I discussed it with were in the same boat, so
> I wrote up this proposal thinking it might have a wide appeal. The
> proposed syntax desugars via a relatively simple transformation but
> encourages developers to reason about the problem in a completely
> different way that I'd argue is more intuitive. Whether the committee
> agrees and thinks it justifies a new syntax remains to be seen, but
> either way I'm excited to see where this discussion goes (whether it
> leads to the proposed syntax, to some other syntax, or to somewhere
> else entirely).
>
> As a side note: thank you to everyone for the thoughtful questions and
> responses, I had no idea what to expect from this thread and it's gone
> better than I could've hoped for. Thank you for not immediately
> shooting down a proposal that looks similar to other proposals before
> it.
>
> On Wed, Nov 20, 2019 at 7:36 PM Isiah Meadows <contact at isiahmeadows.com> wrote:
> >
> > Just FYI, I previously suggested a couple things substantially more
> > flexible than this [1] [2] (originated from this [3]), and it mostly
> > fell flat due to being highly premature. Anything exclusive to
> > promises is unlikely to win as library methods exist for basically all
> > use cases and from my experience, committee members are in general
> > very hesitant to add syntax for anything that doesn't pay for itself
> > well. Similar questions have come up a few times in the past, too, and
> > I've commented on two of them. [4] [5]
> >
> > If anything, I don't feel we know the problem space well enough, and
> > the language lacks the primitives needed to really dig into it. (This
> > is why I came up with my generator forking strawman. [6])
> >
> > [1]: https://github.com/isiahmeadows/non-linear-proposal
> > [2]: https://github.com/isiahmeadows/lifted-pipeline-strawman
> > [3]: https://esdiscuss.org/topic/observable-promise-parallel-control-flow-proposal
> > [4]: https://esdiscuss.org/topic/stream-async-await
> > [5]: https://esdiscuss.org/topic/improved-syntax-for-observable-mapping-and-subscribing
> > [6]: https://github.com/isiahmeadows/proposal-generator-fork
> >
> > -----
> >
> > Isiah Meadows
> > contact at isiahmeadows.com
> > www.isiahmeadows.com
> >
> > On Wed, Nov 20, 2019 at 6:16 PM Jacob Bloom <mr.jacob.bloom at gmail.com> wrote:
> > >
> > > ...strike that, I misread the "but that still waits for the async
> > > functions to complete" part. So what you're proposing is that
> > > everything functions normally inside the curly braces, but execution
> > > doesn't continue until all promises have resolved? So your example
> > > would work essentially like this:
> > >
> > > ```javascript
> > > const x = doSomethingAsync();
> > > const y = doSomethingElseAsync();
> > > await x, await y;
> > > // all promises are resolved by now, but
> > > // still need to use await to unbox the values
> > > someFunction(await x, await y);
> > > ```
> > >
> > > On Wed, Nov 20, 2019 at 3:28 PM Jacob Bloom <mr.jacob.bloom at gmail.com> wrote:
> > > >
> > > > >Maybe if you drop the "await" in your example:
> > > > >
> > > > >```javascript
> > > > >await.all {
> > > > >    const x = doSomethingAsync();
> > > > >    //x is just the promise here
> > > > >}
> > > > >```
> > > > >
> > > > >...but that still waits for the async functions to complete, I think it would
> > > > >cause fewer bugs and would seem to still satisfy the motivation?
> > > >
> > > > It doesn't seem like the `await.all` block is doing anything in that
> > > > case. That code seems equivalent to this:
> > > >
> > > > ```javascript
> > > > const x = doSomethingAsync();
> > > > myFunction(await x)
> > > > ```
> > > >
> > > > >```javascript
> > > > >await.all {
> > > > >  const x = await doSomethingAsync();
> > > > >  //x is still undefined here!
> > > > >}
> > > > >```
> > > >
> > > > You bring up a good point about scoping and race conditions. It's a
> > > > little tricky since the curly braces create a block scope but none of
> > > > the parallel statements should be allowed to access each-other's
> > > > variables, it's almost like each statement should have its own scope.
> > > > Maybe it'd be better to have a syntax that ensures a set of curly
> > > > braces for each parallel task? Async do-expressions could be a good
> > > > solution (assuming they'd work kind of like an async IIFE):
> > > >
> > > > ```javascript
> > > > async function initialize() {
> > > >   let foo, bar, baz;
> > > >   await Promise.all([
> > > >     async do { foo = (await request('foo.json')).data },
> > > >     async do { bar = (await request('bar.json')).data },
> > > >     async do { baz = (await request('baz.json')).data },
> > > >   ]);
> > > >   render(foo, bar, baz);
> > > > }
> > > > ```
> > > >
> > > > (this is also a less drastic syntax change that piggybacks on an
> > > > existing proposal)
> > > >
> > > > On Wed, Nov 20, 2019 at 11:50 AM Bergi <a.d.bergi at web.de> wrote:
> > > > >
> > > > > Hello!
> > > > >
> > > > > > This [current] structure is also just fundamentally different from working
> > > > > > serially in async/await and it forces you to reason about the problem
> > > > > > in a specific way. This doesn't appear to be a conscious decision to
> > > > > > force good code practices
> > > > >
> > > > > Actually I'd argue that it is. Doing stuff concurrently *is*
> > > > > fundamentally different from doing it serially, and should be reasoned
> > > > > about every time you use it.
> > > > >
> > > > > kind regards,
> > > > >  Bergi
> > > > > _______________________________________________
> > > > > es-discuss mailing list
> > > > > es-discuss at mozilla.org
> > > > > https://mail.mozilla.org/listinfo/es-discuss
> > > _______________________________________________
> > > es-discuss mailing list
> > > es-discuss at mozilla.org
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