foreach in Scala parallel collections-CodePudding

I have this code evaluated with Ammonite:

$ amm
Welcome to the Ammonite Repl 2.5.1 (Scala 2.13.8 Java 17.0.1)
@ import $ivy.`org.scala-lang.modules::scala-parallel-collections:1.0.4`
@ import scala.collection.parallel.CollectionConverters._
@ Seq(1,2).foreach { x => Thread sleep x*1000; println(s"Fin $x") };println("Fin") 
Fin 1
Fin 2
Fin

It completes ok. If I add the par to parallelize, then it never finishes:

@ Seq(1,2).par.foreach { x => Thread sleep x*1000; println(s"Fin $x") };println("Fin")

Is this a bug? With Scala 2.12 I get the same behaviour.

CodePudding user response：

You're having this problem due this bug with Scala's lambda encoding that will also happen in the Scala REPL.

The bug on the Scala side: https://github.com/scala/scala-parallel-collections/issues/34

The corresponding ammonite bug report is here: https://github.com/com-lihaoyi/Ammonite/issues/556

You can work around this in two ways that I'm aware of. The first is to put your parallel work inside of an object e.g.

  object work {
    def execute() = Seq(1, 2).foreach { x =>
      Thread.sleep(x * 1000); println(s"Fin $x")
    }; println("Fin")
  }
  work.execute

I believe running Ammonite with amm --class-based should also do the trick, but I'm not able to test this right now.