Scala: Can I rely on the order of items in a Set?

Question

This was quite an unplesant surprise:

scala> Set(1, 2, 3, 4, 5)       
res18: scala.collection.immutable.Set[Int] = Set(4, 5, 1, 2, 3)
scala> Set(1, 2,         


        

Answer 1

ListSet will always return elements in the reverse order of insertion because it is backed by a List, and the optimal way of adding elements to a List is by prepending them.

Immutable data structures are problematic if you want first in, first out (a queue). You can get O(logn) or amortized O(1). Given the apparent need to build the set and then produce an iterator out of it (ie, you'll first put all elements, then you'll remove all elements), I don't see any way to amortize it.

You can rely that a ListSet will always return elements in last in, first out order (a stack). If that suffices, then go for it.