Conversion of breakOut - use iterator or view?

Question

Scala 2.13 migration guide contains a note regarding how to port collection.breakOut:

collection.breakOut no longer exists, use .view and .to(Collection) instead.

and few paragraphs below in a overview table there is:

Description	Old Code	New Code	Automatic Migration Rule
collection.breakOut no longer exists	val xs: List[Int] = ys.map(f) (collection.breakOut)	val xs = ys.iterator.map(f).to(List)	Collection213Upgrade

The scala-collection-migration rewrite rule uses .iterator. What is the difference between the two? Is there a reason to prefer one to the other?

Answer 1

When used like that there is no real difference.

A View can be reused while an Iterator must be discarded after it's been used once.

val list = List(1,2,3,4,5)

val view = list.view
val viewPlus1 = view.map(_ + 1).toList
view.foreach(println) // works as expected

val it = list.iterator
val itPlus1 = it.map(_ + 1).toList
it.foreach(println) // undefined behavior

In its simplest form a View[A] is a wrapper around a function () => Iterator[A], so all its methods can create a fresh Iterator[A] and delegate to the appropriate method on that iterator.