Is Queue.foreach properly ordered?

Question

Will the foreach method of a Scala immutable Queue always be processed in the order one expects for a queue or is there a method that guarantees the order? Or do I have to use a loop + dequeue?

Answer 1

scala.collection.immutable.Queue is scala.collection.Seq. See Seq documentation:

Sequences are special cases of iterable collections of class Iterable. Unlike iterables, sequences always have a defined order of elements.

So yes, you'll get the same elements order with foreach and with loop + dequeue.

If you don't trust documentation you could take a look at implementation:

Queue#foreach is inherited from IterableLike and implemented like this:

def foreach[U](f: A => U): Unit = iterator.foreach(f)

Queue#iterator is implemented like this:

override def iterator: Iterator[A] = (out ::: in.reverse).iterator

And Queue#dequeue returns the first element of out, if any, or the last element of in. So you'll get the same elements order.