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 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 scala.math.Ordering.compare(T, T) 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.

Answer 3

A solution that in my opinion is much more readable than adding guards:

(n compare 10).signum match {
    case -1 => "less than ten"
    case  0 => "ten"
    case  1 => "greater than ten"
}

Notes:

• Ordered.compare returns a negative integer if this is less than that, positive if greater, and 0 if equal.
• Int.signum compresses the output from compare to -1 for a negative number (less than 10), 1 for positive (greater than 10), or 0 for zero (equal to 10).

Answer 4

While all the above and bellow answers perfectly answer the original question, some additional information can be found in the documentation https://docs.scala-lang.org/tour/pattern-matching.html , they didn't fit in my case but because this stackoverflow answer is the first suggestion in Google I would like to post my answer which is a corner case of the question above.
My question is:

• How to use a guard in match expression with an argument of a function?

Which can be paraphrased:

• How to use an if statement in match expression with an argument of a function?

The answer is the code example below:

    def drop[A](l: List[A], n: Int): List[A] = l match {
      case Nil => sys.error("drop on empty list")
      case xs if n <= 0 => xs
      case _ :: xs => drop(xs, n-1)
    }

link to scala fiddle : https://scalafiddle.io/sf/G37THif/2 as you can see the case xs if n <= 0 => xs statement is able to use n(argument of a function) with the guard(if) statement.

I hope this helps someone like me.

Answer 5

Scala's pattern matching allows you to define your own extractor. In this case, you can simply define a new extractor:

class GreaterThan(n: Int) {
  def unapply(i: Int) = i > n
}

val GreaterThan10 = GreaterThan(10)

a match {
  case 10 => ???
  case GreaterThan10() => ???
  case _ => ???
}

or just use pattern guards.