Stream + async await

[Naveen Chawla]

Jan-Ivar Bruaroey,

You are perhaps conflating asynchronicity with non-linearity.

Many asynchronous data flows are most definitely linear.

Hence the great benefit in code cleanliness afforded by `await` `async`.

I've concluded that a lot of stream-like functionality can be achieved
already with `await` `async` and promises alone, even without `for`
`await`, and in some cases `for` `await` gets in the way (as exemplified).

I just needed to think through it. Where a target value needs to change on
each iteration, it can simply be a wider scoped variable manipulated by the
stream. Understanding that a promise resolves to a single value just means
a new promise for each iteration.

So as things stand, I am very comfortable with just the current features,
and will be unlikely to use `for` `await` for what I was thinking of.

On Thu, 3 Aug 2017 at 07:15 Jan-Ivar Bruaroey <jib at mozilla.com> wrote:

> Right, the proposals went beyond that. In this context though I thought
> T.J. was looking for a control surface in promises. I shouldn't assume.
>
> In any case, the original challenge here seems solved by something like:
>
> ```javascript
>
> var wait = ms => new Promise(resolve => setTimeout(resolve, ms));
>
> async function consumeReadableStream(stream) {
>
>   async function read() {
>     try {
>
>       for await (const chunk of stream) {
>         // do work
>       }
>     } catch (e) {
>       if (e.name != "AbortError") throw e;
>     }
>   }
>   await Promise.race([read(), wait(30000).then(() => stream.destroy())]);
> }
>
> ```
>
> ...assuming .destroy() is overloaded to throw AbortError somehow, a detail
> IMHO. But aren't we digressing?
>
> For me, for-await still seems like a useful paradigm, and I'm not sure
> readability is helped by using linear code to represent non-linear code
> flow.
>
>
> .: Jan-Ivar :.
>
>
> On 8/1/17 4:29 PM, Domenic Denicola wrote:
>
> That is not why.
>
>
> ------------------------------
> *From:* Jan-Ivar Bruaroey <jib at mozilla.com> <jib at mozilla.com>
> *Sent:* Aug 1, 2017 3:47 PM
> *To:* es-discuss at mozilla.org
> *Subject:* Re: Stream + async await
>
> Because a promise is not a control surface of the asynchronous action
> fulfilling it; confuses owner with consumer.
> https://stackoverflow.com/a/41417429/918910
>
> .: Jan-Ivar :.
>
> On 7/31/17 7:35 AM, T.J. Crowder wrote:
>
> Related: https://esdiscuss.org/topic/how-about-awaiting-arrays
> (particularly the discussion of `await.race`), since effectively you're
> doing a race between a timeout and each chunk. Also relevant is the former
> work on cancelling promises, now withdrawn. (Can anyone point me at *why*
> it was withdrawn?)
>
> -- T.J. Crowder
>
> On Mon, Jul 31, 2017 at 6:10 AM, kai zhu <kaizhu256 at gmail.com> wrote:
>
>> the timeout handler will not work as advertised, e.g. what if io / db
>> issues causes a network stream to intermittently respond in intervals far
>> greater than 30000ms or not at all?
>>
>> > On Jul 31, 2017, at 7:26 AM, James Browning <thejamesernator at gmail.com>
>> wrote:
>> >
>> > It'll look something like this:
>> >
>> > ```javascript
>> >
>> > async function consumeReadableStream(stream) {
>> >     const start = Date.now()
>> >     for await (const chunk of stream) {
>> >
>> >        /* Do whatever you want with the chunk here e,g, await other
>> > async tasks with chunks
>> >            send them off to wherever, etc
>> >        */
>> >
>> >         if (Date.now() - start > 30000) {
>> >             throw new Error('30000 ms timeout')
>> >        }
>> >    }
>> >    /* Instead of callbackOnce the returned promise from this function
>> > itself can be used */
>> > }
>> >
>> > ```
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>>
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>
>
>
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>
> --
> .: Jan-Ivar :.
>
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