Extractors

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.

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 Application {
  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 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 code in the preceding example would be expanded as follows:

object TwiceTest extends Application {
  val x = Twice.apply(21)
  Twice.unapply(x) match { case Some(n) => Console.println(n) } // prints 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 a 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).

When you create a case class, it automatically can be used with pattern matching since it has an extractor:

case class Employee(firstName: String, lastName: String)

val rob = new Employee("Robin", "Williams")
val result = rob match {
  case Employee("Robin", _) ⇒ "Where's Batman?"
  case _ ⇒ "No Batman Joke For You"
}

result should be(res0)

What's an extractor? In Scala it's a method in any object called unapply, and that method is used to disassemble the object given by returning a tuple wrapped in an option. Extractors can be used to assign values:

class Car(val make: String, val model: String, val year: Short, val topSpeed: Short)

object ChopShop {
  def unapply(x: Car) = Some(x.make, x.model, x.year, x.topSpeed)
}

val ChopShop(a, b, c, d) = new Car("Chevy", "Camaro", 1978, 120)

a should be(res0)
b should be(res1)
c should be(res2)
d should be(res3)

An extractor can also be used in pattern matching:

class Car(val make: String, val model: String, val year: Short, val topSpeed: Short)

object ChopShop {
  def unapply(x: Car) = Some(x.make, x.model, x.year, x.topSpeed)
}

val x = new Car("Chevy", "Camaro", 1978, 120) match {
  case ChopShop(s, t, u, v) ⇒ (s, t)
  case _ ⇒ ("Ford", "Edsel")
}

x._1 should be(res0)
x._2 should be(res1)

Since we aren't really using u and v in the previous pattern matching, they can be replaced with _:

class Car(val make: String, val model: String, val year: Short, val topSpeed: Short)

object ChopShop {
  def unapply(x: Car) = Some(x.make, x.model, x.year, x.topSpeed)
}

val x = new Car("Chevy", "Camaro", 1978, 120) match {
  case ChopShop(s, t, _, _) ⇒ (s, t)
  case _ ⇒ ("Ford", "Edsel")
}

x._1 should be(res0)
x._2 should be(res1)

As long as the method signatures aren't the same, you can have as many unapply methods as you want:

class Car(val make: String, val model: String, val year: Short, val topSpeed: Short)
class Employee(val firstName: String, val middleName: Option[String], val lastName: String)

object Tokenizer {
  def unapply(x: Car) = Some(x.make, x.model, x.year, x.topSpeed)

  def unapply(x: Employee) = Some(x.firstName, x.lastName)
}

val result = new Employee("Kurt", None, "Vonnegut") match {
  case Tokenizer(c, d) ⇒ "c: %s, d: %s".format(c, d)
  case _ ⇒ "Not found"
}

result should be(res0)

An extractor can be any stable object, including instantiated classes with an unapply method:

class Car(val make: String, val model: String, val year: Short, val topSpeed: Short) {
  def unapply(x: Car) = Some(x.make, x.model)
}

val camaro = new Car("Chevy", "Camaro", 1978, 122)

val result = camaro match {
  case camaro(make, model) ⇒ "make: %s, model: %s".format(make, model)
  case _ ⇒ "unknown"
}

result should be(res0)

A custom extractor is typically created in the companion object of the class. In this exercise, we use it as an assignment:

class Employee(
  val firstName: String,
  val middleName: Option[String],
  val lastName: String
)

object Employee {
  //factory methods, extractors, apply
  //Extractor: Create tokens that represent your object
  def unapply(x: Employee) =
    Some(x.lastName, x.middleName, x.firstName)
}

val singri = new Employee("Singri", None, "Keerthi")

val Employee(a, b, c) = singri

a should be(res0)
b should be(res1)
c should be(res2)

In this exercise we use unapply for pattern matching employee objects:

class Employee(
  val firstName: String,
  val middleName: Option[String],
  val lastName: String
)

object Employee {
  //factory methods, extractors, apply
  //Extractor: Create tokens that represent your object
  def unapply(x: Employee) =
    Some(x.lastName, x.middleName, x.firstName)
}

val singri = new Employee("Singri", None, "Keerthi")

val result = singri match {
  case Employee("Singri", None, x) ⇒
    "Yay, Singri %s! with no middle name!".format(x)
  case Employee("Singri", Some(x), _) ⇒
    "Yay, Singri with a middle name of %s".format(x)
  case _ ⇒ "I don't care, going on break"
}

result should be(res0)