Why can't I pattern match on Stream.empty in Scala?

Question

The code below doesn't compile if I uncomment the line indicated. The compiler complains: "stable identifier required".

val Empty = Stream.empty     val a = Stream.range(0, 5) a match {   // case Stream.empty => println("nope") <-- does not work   case Empty => println("compiles") <-- works   case _ => println("ok") } 

If I assign Stream.empty to value Empty first, it works, but it feels strange that you can't pattern match on such a fundamental value without such a hack.

Am I missing something?

Answer 1

You can't pattern match on Stream.empty because it is a method (in object Stream) that always returns the empty stream (but the compiler doesn't know that).

Instead of assigning val empty = Stream.empty, you can match on Stream.Empty, which is an Object :

scala> a match {            case Stream.Empty => println("done")            case h #:: tl => println(h)        }