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 a variable that is not a constant.
Stream.empty is not a "stable" identifier since it represents some method:

/** The empty stream */
  override def empty[A]: Stream[A] = Empty

that could potentially return any value at any time.
Compiler isn't aware that its returned value is always Empty, so it detects it as a potential changing variable.
Too deep for it to detect it.

However, when you assign the method's retult to a val (being a stable identifier since immutable), your code is able to process to pattern matching using it.

You might read this, evoking an hypothesis explaining why pattern matching expects a stable identifier.

Answer 2

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)
       }