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         


        

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)
       }