Extractor Objects

An extractor object is an object with an unapply method. Whereas the apply method is like a constructor which takes arguments and creates an object, the unapply takes an object and tries to give back the arguments. This is most often used in pattern matching and partial functions.

import scala.util.Random

object CustomerID {

  def apply(name: String) = s"$name--${Random.nextLong}"

  def unapply(customerID: String): Option[String] = {
    val name = customerID.split("--").head
    if (name.nonEmpty) Some(name) else None
  }
}

val customer1ID = CustomerID("Sukyoung")  // Sukyoung--23098234908
customer1ID match {
  case CustomerID(name) => println(name)  // prints Sukyoung
  case _ => println("Could not extract a CustomerID")
}

The apply method creates a CustomerID string from a name. The unapply does the inverse to get the name back. When we call CustomerID("Sukyoung"), this is shorthand syntax for calling CustomerID.apply("Sukyoung"). When we call case CustomerID(name) => customer1ID, we’re calling the unapply method.

The unapply method can also be used to assign a value.

val customer2ID = CustomerID("Nico")
val CustomerID(name) = customer2ID
println(name)  // prints Nico

This is equivalent to val name = CustomerID.unapply(customer2ID).get. If there is no match, a scala.MatchError is thrown:

val CustomerID(name2) = "--asdfasdfasdf"

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 which returns Option[Seq[T]] This mechanism is used for instance in pattern case List(x1, ..., xn).