First crack at a Streams proposal

Tab Atkins Jr. jackalmage at
Mon Apr 15 16:35:44 PDT 2013

On Mon, Apr 15, 2013 at 4:24 PM, Jake Verbaten <raynos2 at> wrote:
> Would it help to split the sugar and combinator from the actual stream
> interface?
> ```
> callback StreamInit = void (StreamResolver resolver);
> callback AnyCallback = any (optional any value);
> [Constructor(StreamInit init)]
> interface Stream {
>   Stream listen(optional AnyCallback? listenCB = null, optional
> AnyCallback? completeCB = null, optional AnyCallback? rejectCB =
> null);
> }
> interface StreamResolver {
>   void push(optional any value);
>   void complete(optional any value);
>   void reject(optional any value);
> };

Thanks, I should have done that originally!

> One basic thing missing from this API is the ability to stop listening to a
> stream.

Hm, yes, you need to be able to unlisten.  I'd forgotten about that.

> Other things missing are the ability to abort / cancel / close a Stream, the
> ability to pause or resume a stream.

I think all of these are only suited for single-consumer streams.  My
original proposal is explicitly for multi-consumer streams, as I
designed it explicitly for DOM use-cases that I ran into.

If you have a single-consumer stream, though, I agree that you should
be able to pause and cancel a stream.  Same with Futures, for that

> It should also be made clear how and when a stream may emit values. Whether
> it is at any arbitrary time and it will send to whomever is listening at
> that moment. Whether it is only allowed to emit values after a listen call.
> Does every call to listen get the entire history independently of other
> calls? (Doing so would buffer all data and defeat the point of a stream).
> What happens when you push a value into the resolved and no-one is
> listening?

By virtue of not defining this, I implicitly answered your questions.
^_^  It's a purely async data source, and cares not for whether you're
listening.  (Again, the use-cases I was trying to solve are
DOM-oriented.)  If you're not listening when one comes through, too
bad.  Of course, recall that updates are processed async, so you can't
miss any updates within any particular tick.

> Another decision that needs to be made is whether it makes sense for a
> stream to emit multiple errors? For a Future it doesn't make sense because
> it can only be fulfilled to a single value. For a stream it may make sense
> for multiple errors to occur.

Good question.  My design assumes that once something errors, it's
because it's now invalid, and won't be producing any more.  If you can
emit errors and still keep updating, they probably shouldn't reject
the stream, but just be a special update value.

> I personally like the `listen(onChunk, onEnd)` syntax as it matches a
> popular stream baseclass from the node community (
> ) in terms of simplicity.

That's my syntax, except that I allow you to distinguish between the
stream ending "normally" and with errors.  If you ignore the reject
callback, my listen() is *exactly* that syntax already.


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