Add a combinator for parallel composition of sinks

[iravid]

No description provided.

[iravid]

cc @simpadjo

[simpadjo]

Thank you, it would be very useful.

@jdegoes

The reason that probably doesn't exist now is because it's not obvious what to do with leftovers: each sink will, in general, have a different amount of leftover; and the leftovers will overlap. So in their composition, what do you do? Probably emit the shortest. All to say, possible but tricky to do correctly.

I had the same question in my head initially. Now I think that returning the shortest one looks pretty much consistent with respect to ZIIO#zipPar behavior. If a Stream is a timeline and Sinks are processes then ZSink#zipPar considered done at the latest point. Dual to ZSink#raceBoth.

[jdegoes]

I do think shortest is the best ("minimal leftover").

Albeit there will be some nice laws that it can't satisfy.

It'd be nice if you could take sink1 -> sink2 and sink3 -> sink4 and zipPar sink1 and sink3, and then zipPar sink2 and sink4, and then do (sink1 zipPar sink3) -> (sink2 zipPar sink4) and have that give you the same result. But Sink appears not to have the structure required to allow that, because it would require tuple leftovers and tuple inputs.

[simpadjo]

I'd like to try to implement it

[iravid]

Closed in #1344