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Proposal: make mapBoth always execute tasks in parallel #661

merged 1 commit into from Apr 29, 2018


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oleg-py commented Apr 22, 2018

Got caught up by surprise by the following behavior:

import cats._, implicits._, effect._, monix.eval._
import java.util.concurrent.TimeUnit
import scala.concurrent.duration._

// Imagine this is some blocking operation
val blockingOp = Task.eval { Thread.sleep(1000) }

val nowL = Timer[Task].clockMonotonic(TimeUnit.SECONDS)

val task = for {
  start <- nowL
  _ <- Task.zip4(blockingOp, blockingOp, blockingOp, blockingOp)
  end <- nowL
  _ <- Task.eval(println(s"${end - start}s passed")) // 4s passed, expected: 1s
} yield ()


This happens if Task is constructed by .eval as opposed to .apply (which I do all the time since this way I can control behavior later). In real example I have a flatMap-chain of Tasks built on Task.eval.

This affects Task.zipX, Task.parMapX and also Parallel instance. I think it makes sense to always fork inside mapBoth because it's more likely to be what user expects.

@alexandru alexandru merged commit 4b99f39 into monix:master Apr 29, 2018


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alexandru commented Apr 29, 2018

I agree with this change.

Had to force merge it because Travis is malfunctioning.

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