Using comparison operators in Scala's pattern matching system.

Question

Is it possible to match on a comparison using the pattern matching system in Scala? For example:

a match {
    case 10 => println("ten")
    case _ > 10 => println("greater than ten")
    case _ => println("less than ten")
}

The second case statement is illegal, but I would like to be able to specify "when a is greater than".

Answer 1

You can add a "pattern guard", i.e. an if and a boolean expression after the pattern:

a match {
    case 10 => println("ten")
    case x if x > 10 => println("greater than ten")
    case _ => println("less than ten")
}

Edit: Note that this is more than superficially different to putting an if after the =>, because a pattern won't match if the guard is not true.

Answer 2

As a non-answer to the question's spirit, which asked how to incorporate predicates into a match clause, in this case the predicate can be factored out before the match:

def assess(n: Int) {
  println(
    n compare 10 match {
      case 0 => "ten"
      case 1 => "greater than ten"
      case -1 => "less than ten"
    })
}

Now, the documentation for RichInt.compare(Int) promises only that the non-equal outcomes will be greater than or less than zero. Java's Comparable#compareTo(T) is specified similarly to Scala's. It happens to be conventional to use 1 and -1 for the positive and negative values, respectively, as Scala's current implementation does, but one can't make such an assumption without some risk of the implementation changing out from underneath.