Introduction to Scala Part3: Option & Pattern Matching


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Previous: Pattern Matching

Extractor Objects

In Scala, patterns can be defined using a method named unapply 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

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).

Extractor: unapply and unapplySeq

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).

Pattern in value definition

class Person(firstname: String, lastName: String)

object Person { 
    def apply(firstname: String, lastname: String): Person = …
    def  unapply(person: Person) : Option[(String, String)] = ...

val Person(firstname, lastname) = Person("A", "B")
// firstname : String = "A"
// lastname: String = "B"

val Person(_, toto) = Person("A", "B")
// toto : String = "B"

def kingOfTheNorth(): Person = Person("John", "Snow")
kingOfTheNorth // Person("John", "Snow")
apply Person
unapply Person
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