Matching with custom combinations/operators

Question

I know that you can do matching on lists in a way like

val list = List(1,2,3)
list match {
  case head::tail => head
  case _ => //whatever
}
         


        

Answer 1

Matching with case head :: tail uses an infix operation pattern of the form p1 op p2 which gets translated to op(p1, p2) before doing the actual matching. (See API for ::)

The problem with + is the following:

While it is easy to add an

object + { 
  def unapply(value: Int): Option[(Int, Int)] = // ...
}

object which would do the matching, you may only supply one result per value. E.g.

object + { 
  def unapply(value: Int): Option[(Int, Int)] = value match {
    case 0 => Some(0, 0)
    case 4 => Some(3, 1)
    case _ => None
}

Now this works:

0 match { case x + 0 => x } // returns 0

also this

4 match { case x + 1 => x } // returns 3

But this won’t and you cannot change it:

4 match { case x + 2 => x } // does not match

No problem for ::, though, because it is always defined what is head and what is tail of a list.