Is foreach by-definition guaranteed to iterate the subject collection sequentially in Scala?

Question

Is foreach by-definition guaranteed to iterate the subject collection (if it defines order) sequentially from the very first to the very last (unless accidentally interrupted) element? Aren't there any compiler optimization switches which can brake it (shuffle the sequence) or plans to make the ordinary foreach parallel in future versions?

Answer 1

Foreach is guaranteed to be sequential for sequential collections (that is, the normal hierarchy, or for anything transformed by .seq). The parallel part of the collections library (which you get from a standard collection via .par or by explicitly using classes from collection.parallel) are pretty much guaranteed not to evaluate in order. If you want to be agnostic, you can use the GenX set of traits (e.g. GenSeq); these provide no guarantees either on execution order or that work will be done in parallel.

Answer 2

To complement the answer by Rex, foreach at GenTraversableOnce only guarantees all elements will be iterated through, unless you interrupt it. Traits lower on the hierarchy chain may provide additional guarantees, for example:

• TraversableOnce and descendants guarantee that only one element will be iterated through at a time (not guaranteed by GenTraversableOnce!).
• Seq and descendants guarantee that elements will be traversed on the order they are kept in the collection.

And since you asked about parallel foreach...

scala> (1 to 10).par foreach println
4
6
1
3
2
7
5
8
9
10