A Tour of Scala: Pattern Matching

Scala has a built-in general pattern matching mechanism. It allows to match on any sort of data with a first-match policy. 

Here is a small example which shows how to match against an integer value:

object MatchTest1 extends Application {
  def matchTest(x: Int): String = x match {
    case 1 => "one"
    case 2 => "two"
    case _ => "many"
  }
  println(matchTest(3))
}

The block with the case statements defines a function which maps integers to strings. The match keyword provides a convenient way of applying a function (like the pattern matching function above) to an object.

Here is a second example which matches a value against patterns of different types:

object MatchTest2 extends Application {
  def matchTest(x: Any): Any = x match {
    case 1 => "one"
    case "two" => 2
    case y: Int => "scala.Int"
  }
  println(matchTest("two"))
}

The first case matches if x refers to the integer value 1. The second case matches if x is equal to the string"two". The third case consists of a typed pattern; it matches against any integer and binds the selector value xto the variable y of type integer.

Scala's pattern matching statement is most useful for matching on algebraic types expressed via case classes [1].

Scala also allows the definition of patterns independently of case classes, using unapply methods in extractor objects [2].