Sure, you can yield a promise in co, along with some other deferred types
of things. It's very flexible, but that's precisely the problem. You have
to know the type of every value you yield on, or at least know for
sure it's one of the deferred types co supports. In copromise you can just
`yield` anything, and if it's not a promise, it'll be lifted into one and
will resolve on the next turn, just like the `when` function of many
promise libs.
This has a number of uses, but I'm particularly interested in how this can
be used in combination with for-of syntax in es6 to create elegant lazy
streams (iterators that yield chunks as promise values). Here's a function
that frames an iterator stream yielding string chunks (lazily as promises
or not), and calls some `write` function with each line array it collects
up. Who knows if this `write` returns a promise, but if it does, yielding
on it automatically exerts backpressure:
copromise(function* () {
var buffer = '';
for (var chunk of stream) {
// split on newlines, awaiting chunk as it could be a promise
var lines = (buffer + (yield chunk)).split(/\r?\n/);
// keep the last partial line buffered
buffer = lines.pop();
// wait for write to finish before next read (applies
backpressure)
yield write(lines);
}
// Just handle any leftover
if (buffer) {
yield write([ buffer ]);
}
});
As far as transform streams go this is still more clunky than it has to be,
but it's just intended to show a few uses for being able to yield anything
(and act as a little teaser for how for-of and coroutines can be used to
completely node's wretched stream API).
On Tue, Sep 2, 2014 at 7:30 PM, Jonathan Ong <[email protected]>
wrote:
> i don't get it. co already allows yielding promises.
>
> Am Samstag, 30. August 2014 14:09:46 UTC-7 schrieb Dean Landolt:
>
>> I was trying to find a clean way to adapt co to be able to be able to
>> yield promise or non-promise values (e.g. use yield like the `when` method
>> of a lot of promise libs), so I started hacking out features. By the time I
>> was through I was pretty surprised to see that it essentially boiled down
>> to this little block of code (assuming es6 promises):
>>
>> function run(coroutine) {
>> return new Promise(function (resolve, reject) {
>> (function next(value, exception) {
>> var result;
>> try {
>> result = exception ? coroutine.throw(value) : coroutine.next(value);
>> }
>> catch (error) {
>> return reject(error);
>> }
>> if (result.done) return resolve(result.value);
>> Promise.resolve(result.value).then(next, function(error) {
>> next(error, true);
>> });
>> })();
>> });
>> }
>>
>> This allows you to yield any promise or non-promise value, and returns a
>> promise representing the result. You can also use `yield*` to delegate to
>> other coroutines correctly.
>>
>> If anyone's interested I wrapped this up and published it to npm as the
>> `copromise` module. Enjoy!
>>
> --
> Job board: http://jobs.nodejs.org/
> New group rules:
> https://gist.github.com/othiym23/9886289#file-moderation-policy-md
> Old group rules:
> https://github.com/joyent/node/wiki/Mailing-List-Posting-Guidelines
> ---
> You received this message because you are subscribed to the Google Groups
> "nodejs" group.
> To unsubscribe from this group and stop receiving emails from it, send an
> email to [email protected].
> To post to this group, send email to [email protected].
> To view this discussion on the web visit
> https://groups.google.com/d/msgid/nodejs/79c78a3d-b3a9-4e99-ab60-dded283dc132%40googlegroups.com
> <https://groups.google.com/d/msgid/nodejs/79c78a3d-b3a9-4e99-ab60-dded283dc132%40googlegroups.com?utm_medium=email&utm_source=footer>
> .
>
> For more options, visit https://groups.google.com/d/optout.
>
--
Job board: http://jobs.nodejs.org/
New group rules:
https://gist.github.com/othiym23/9886289#file-moderation-policy-md
Old group rules:
https://github.com/joyent/node/wiki/Mailing-List-Posting-Guidelines
---
You received this message because you are subscribed to the Google Groups
"nodejs" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to [email protected].
To post to this group, send email to [email protected].
To view this discussion on the web visit
https://groups.google.com/d/msgid/nodejs/CAPm8pjrYPhtoO5eeZ_8vUzR%2B1AcHfpaR2_XELPsUspMCNXqc-A%40mail.gmail.com.
For more options, visit https://groups.google.com/d/optout.
