Reactive Streams implementation for Iterant

[alexandru]

Implements an Iterant conversion to org.reactivestreams.Publisher, compliant with the specification at reactive-streams.org.

sealed abstract class Iterant[F[_], A] {
  //...
  def toReactivePublisher(implicit F: Effect[F], ec: Scheduler): Publisher[A]
}

For this to work F[_] is constrained to have an Effect[F] implementation (type class from cats-effect).

Usage sample given in the ScalaDoc:

import monix.eval.Task
import monix.execution.rstreams.SingleAssignmentSubscription
import org.reactivestreams.{Publisher, Subscriber, Subscription}

def sum(source: Publisher[Int], requestSize: Int): Task[Long] =
  Task.create { (_, cb) =>
    val sub = SingleAssignmentSubscription()

    source.subscribe(new Subscriber[Int] {
      private[this] var requested = 0L
      private[this] var sum = 0L

      def onSubscribe(s: Subscription): Unit = {
        sub := s
        requested = requestSize
        s.request(requestSize)
      }

      def onNext(t: Int): Unit = {
        sum += t
        if (requestSize != Long.MaxValue) requested -= 1

        if (requested <= 0) {
          requested = requestSize
          sub.request(request)
        }
      }

      def onError(t: Throwable): Unit =
        cb.onError(t)
      def onComplete(): Unit =
        cb.onSuccess(sum)
    })

    // Cancelable that can be used by Task
    sub
  }

val pub: Publisher[Int] = Iterant[Task].of(1, 2, 3, 4).toReactivePublisher

// Yields 10
sum(pub, requestSize = 128)

Update

Since this PR, I've tuned the implementation and added comments in PR #418, view the latest implementation at IterantToReactivePublisher.

[codecov]

Codecov Report

Merging #417 into master will increase coverage by 0.16%.
The diff coverage is 96.59%.

@@            Coverage Diff             @@
##           master     #417      +/-   ##
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+ Coverage   88.49%   88.65%   +0.16%     
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  Files         339      340       +1     
  Lines        9297     9385      +88     
  Branches     1245     1267      +22     
==========================================
+ Hits         8227     8320      +93     
+ Misses       1070     1065       -5