Scala Map.keys returns an Iterable that happens to be backed by a Set so if you map() over it you get Set-like behavior. Good times.

TrickyMapKeys.scala

// Map.keys returns an Iterable that happens to be backed by a Set so 
// if you map() over it you get Set-like behavior.  Good times.
//
// $ scalac TrickyMapKeys.scala
// $ scala TrickyMapKeys
// keys mod 2 (VERSION A): Set(0, 1)
// keys mod 2 (VERSION B): List(0, 0, 1, 0)
//
object TrickyMapKeys extends App {
 
  val keyValuePairs = Seq(2 -> "deux", 0 -> "al cifr", 5 -> "cinco", 4 -> "fore")

  val m = Map(keyValuePairs:_*)

  val keysModTwoA = m.keys.map(_ % 2)
  val keysModTwoB = m.map(_._1 % 2)

  println(s"keys mod 2 (VERSION A): ${keysModTwoA.toString}")
  println(s"keys mod 2 (VERSION B): ${keysModTwoB.toString}")
}