Can views be used with parallel collections?

Question

The idiom for finding a result within a mapping of a collection goes something like this:

list.view.map(f).find(p)

where list is a List[A], f is an A => B, and p is a B => Boolean.

Is it possible to use view with parallel collections? I ask because I'm getting some very odd results:

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scala> val f : Int => Int = i => {println(i); i + 10}
f: Int => Int = <function1>

scala> val list = (1 to 10).toList
list: List[Int] = List(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)

scala> list.par.view.map(f).find(_ > 5)
1
res0: Option[Int] = Some(11)

scala> list.par.view.map(f).find(_ > 5)
res1: Option[Int] = None

Answer 1

See "A Generic Parallel Collection Framework", the paper by Martin Odersky et al that discusses the new parallel collections. Page 8 has a section "Parallel Views" that talks about how view and par can be used together, and how this can give the performance benefits of both views and parallel computation.

As for your specific example, that is definately a bug. The exists method also breaks, and having it break on one list breaks it for all other lists, so I think it is a problem where operations that may be aborted part way through (find and exists can stop once the have an answer) manage to break the thread pool in some way. It could be related to the bug with exceptions being thrown inside functions passed to parallel collections. If so, it should be fixed in 2.10.