Pattern Matching “case Nil” for Vector

Question

After reading this post on how to use pattern matching on Vector (or any collection that implements Seq), I tested pattern matching on this collection.

Answer 1

The comparison Vector[Int]() == Nil is possible because there is no constraint on the type level for what you compare; that allows the implementation of equals for collections, on the other hand, to perform an element by element comparison irrespective of the collection type:

Vector(1, 2, 3) == List(1, 2, 3)  // true!

In pattern matching, you cannot have a case for an empty list (Nil) when the type is not related to list (it's a Vector).

You can do this however:

val x = Vector(1, 2, 3)

x match {
  case y +: ys => println("head: " + y + "; tail: " + ys)
  case IndexedSeq() => println("empty vector")
}

But I would suggest just to use the default case here, because if x does not have a head element, it must be technically empty:

x match {
  case y +: ys => println("head: " + y + "; tail: " + ys)
  case _ => println("empty vector")
}