parallel collections + object initialization = hang

[scabug]

This little poison pill:

val n = 0; for(i <- List(1).par) n

hangs the REPL in 2.9.3, 2.10.3, and 2.11.0-M7.

-Yrepl-sync doesn't help.

The collection type doesn't matter (List, Range, Vector, whatever).

Accessing n in the body is necessary.

Using semicolon to put it all on one line is necessary.

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-8119?orig=1
Reporter: @SethTisue
Affected Versions: 2.9.3, 2.10.3, 2.11.0-M7

[scabug]

@som-snytt said (edited on Jan 7, 2014 4:10:41 AM UTC):
This is not REPL-specific.

object Foreacher {
  val n = 0
  val m = List(1).par.foreach(_ => n)
  def main(args: Array[String]): Unit = println("Hello, all")
}

I guess the foreach is kicked off from the static initializer of the class and blocks on the par result, which is blocked in turn because it references the static module.

Of course, the REPL-specific part is the code wrapper. Put it in braces to localize it.

[scabug]

@SethTisue said:
Hmm. I'm not sure whether to remove "REPL" from the "Components" list, since one could imagine fixing this in the REPL without fixing it generally. Not sure how much hope there is for either kind of fix, as I see now that the issue is pretty fundamental.

For whatever it's worth, I encountered this while pasting in code from a blog post titled "More Benefits of Scala in CS1" (http://dynamicsofprogramming.blogspot.com/2013/12/more-benefits-of-scala-in-cs1.html). I wouldn't want to have to explain/defend this to a beginner.

[scabug]

@som-snytt said:

apm@mara:~$ scalam -Yrepl-class-based
Welcome to Scala version 2.11.0-M7 (OpenJDK 64-Bit Server VM, Java 1.7.0_25).
Type in expressions to have them evaluated.
Type :help for more information.

scala> val n = 0; for(i <- List(1).par) n
n: Int = 0

[scabug]

@SethTisue said:
Nice! I wasn't aware of scala/scala#3123 . Is there an issue or mailing list thread or something that explains the motivation for it...?

[scabug]

@som-snytt said:
"More details" link at the issue #7747.

[scabug]

@SethTisue said:
there is a nice writeup on this by @axel22 at http://stackoverflow.com/a/15176433/86485

[SethTisue]

I double-checked that it doesn't hang in the 2.13.2-bin-b1550dd REPL