Scala Pattern Matching & Options

Pattern Matching

Pattern matching is used for decomposing data structures:

unknownObject match {
  case MyClass(n) => ...
  case MyClass2(a, b) => ...
}

Here are a few example patterns

(someList: List[T]) match {
  case Nil => ...          // empty list
  case x :: Nil => ...     // list with only one element
  case List(x) => ...      // same as above
  case x :: xs => ...      // a list with at least one element. x is bound to the head,
                           // xs to the tail. xs could be Nil or some other list.
  case 1 :: 2 :: cs => ... // lists that starts with 1 and then 2
  case (x, y) :: ps => ... // a list where the head element is a pair
  case _ => ...            // default case if none of the above matches
}

The last second example shows that every pattern consists of sub-patterns: it only matches lists with at least one element, where that element is a pair. x and y are again patterns that could match only specific types.

Options

Pattern matching can also be used for Option values. Some functions (like Map.get) return a value of typeOption[T] which is either a value of type Some[T] or the value None:

val myMap = Map("a" -> 42, "b" -> 43)
def getMapValue(s: String): String = {
  myMap get s match {
    case Some(nb) => "Value found: " + nb
    case None => "No value found"
  }
}
getMapValue("a")  // "Value found: 42"
getMapValue("c")  // "No value found"

Most of the times when you write a pattern match on an option value, the same expression can be written more concisely using combinator methods of the Option class. For example, the function getMapValue can be written as follows:

def getMapValue(s: String): String =
  myMap.get(s).map("Value found: " + _).getOrElse("No value found")

Pattern Matching in Anonymous Functions

Pattern matches are also used quite often in anonymous functions:

val pairs: List[(Char, Int)] = ('a', 2) :: ('b', 3) :: Nil
val chars: List[Char] = pairs.map(p => p match {
  case (ch, num) => ch
})

Instead of p => p match { case ... }, you can simply write {case ...}, so the above example becomes more concise:

val chars: List[Char] = pairs map {
  case (ch, num) => ch
}