Extractor Objects

In Scala, patterns can be defined independently of case classes. To this end, a method named unapply is defined to yield a so-called extractor. An extractor can be thought of as a special method that reverses the effect of applying a particular object on some inputs. Its purpose is to ‘extract’ the inputs that were present before the ‘apply’ operation. For instance, the following code defines an extractor object Twice.

object Twice {
  def apply(x: Int): Int = x * 2
  def unapply(z: Int): Option[Int] = if (z%2 == 0) Some(z/2) else None

object TwiceTest extends App {
  val x = Twice(21)
  x match { case Twice(n) => Console.println(n) } // prints 21

There are two syntactic conventions at work here:

The pattern case Twice(n) will cause an invocation of Twice.unapply, which is used to match any even number; the return value of the unapply signals whether the argument has matched or not, and any sub-values that can be used for further matching. Here, the sub-value is z/2

The apply method is not necessary for pattern matching. It is only used to mimick a constructor. val x = Twice(21) expands to val x = Twice.apply(21).

The return type of an unapply should be chosen as follows:

  • If it is just a test, return a Boolean. For instance case even()
  • If it returns a single sub-value of type T, return an Option[T]
  • If you want to return several sub-values T1,...,Tn, group them in an optional tuple Option[(T1,...,Tn)].

Sometimes, the number of sub-values is fixed and we would like to return a sequence. For this reason, you can also define patterns through unapplySeq. The last sub-value type Tn has to be Seq[S]. This mechanism is used for instance in pattern case List(x1, ..., xn).

Extractors can make code more maintainable. For details, read the paper “Matching Objects with Patterns” (see section 4) by Emir, Odersky and Williams (January 2007).

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