Add EventStream.fromPublisher

[armanbilge]

And effectively implementing (part of) the Reactive Streams spec.

In practice, this makes it possible to convert an fs2.Stream to an EventStream via the Flow.Publisher interface. Monix and ZIO also interop with Reactive Streams, although this is not cross-published for Scala.js yet.

Edit: It turns out Monix also supports Reactive Streams on Scala.js!

[raquo]

Looks great, thank you! I should be able to include this in the next version, I just need to write some docs on how to use it with FS2.

[armanbilge]

I just need to write some docs on how to use it with FS2.

I've been thinking about this. After chatting with @BalmungSan we realized that we need to add one more API on the FS2 side. Interop is possible with the current APIs but I think it would require flatMapping EventStreams which is sub-optimal. We want to expose a new "unsafe" API that gives direct access to the Publisher from the fs2.Stream without wrapping it in Resource or IO.

[raquo]

Hm, I guess I'll hold off merging until that's cleared up. I'm not sure why flatMap would be necessary on the airstream side, but if you'll fix that need eventually, that's moot.

[armanbilge]

I'm not sure why flatMap would be necessary on the airstream side

Is there a way to get from a Future[Publisher[A]] to an EventStream[A] using fromFlowPublisher with the current signature and not using flatMap?

[raquo]

Is there a way to get from a Future[Publisher[A]] to an EventStream[A] using fromFlowPublisher with the current signature and not using flatMap?

No.

To clarify, unnecessary flatMap-s in Airstream are bad because they create unnecessary transactions (which can cause FRP glitches), but this particular flatMap, although unnecessary, does not increase the potential of FRP glitches, I think: since the events originate outside of Airstream (in FS2), Airstream will emit them in separate transactions anyway, even without flatMap.

Eliminating the need for a flatMap is still beneficial though: less boilerplate, and better efficiency.

[armanbilge]

Hm, I guess I'll hold off merging until that's cleared up.

FWIW I don't think it is necessary to hold this PR. The API here is solid, and interop is possible today. We can document as-is, and update the docs once FS2 adds the new API, without requiring a new Airstream release.

[armanbilge]

After typelevel/fs2#3346 is released, interop will be as simple as

import cats.effect.unsafe.implicits._ // implicit IORuntime
EventStream.fromPublisher(fs2Stream.unsafeToPublisher())

[armanbilge]

FS2 v3.9.4 was released with the new interop APIs.

[raquo]

@armanbilge Great, thanks, and for the docs too, I'll merge this PR for the next 17.x release

[raquo]

@armanbilge Merged now, thanks again!

I wanted to rebase this on latest master, add a minor commit to this, and squash it all, but I couldn't figure out how to do that while preserving this PR and without bothering yourself for such menial work, so I've done all that in #122. The resulting commit is here. No API changes, just moved things around a bit.