Scala LanguageType Parameterization (Generics)

The Option type

A nice example of a parameterized type is the Option type. It is essentially just the following definition (with several more methods defined on the type):

sealed abstract class Option[+A] {
  def isEmpty: Boolean
  def get: A

  final def fold[B](ifEmpty: => B)(f: A => B): B =
    if (isEmpty) ifEmpty else f(this.get)

  // lots of methods...
}

case class Some[A](value: A) extends Option[A] {
  def isEmpty = false
  def get = value
}

case object None extends Option[Nothing] {
  def isEmpty = true
  def get = throw new NoSuchElementException("None.get")
}

We can also see that this has a parameterized method, fold, which returns something of type B.

Parameterized Methods

The return type of a method can depend on the type of the parameter. In this example, x is the parameter, A is the type of x, which is known as the type parameter.

def f[A](x: A): A = x

f(1)         // 1
f("two")     // "two"
f[Float](3)  // 3.0F

Scala will use type inference to determine the return type, which constrains what methods may be called on the parameter. Thus, care must be taken: the following is a compile-time error because * is not defined for every type A:

def g[A](x: A): A = 2 * x  // Won't compile

Generic collection

Defining the list of Ints

trait IntList { ... }

class Cons(val head: Int, val tail: IntList) extends IntList { ... }

class Nil extends IntList { ... }

but what if we need to define the list of Boolean, Double etc?

Defining generic list

trait List[T] {
  def isEmpty: Boolean
  def head: T
  def tail: List[T]
}

class Cons[T](val head: [T], val tail: List[T]) extends List[T] {
  def isEmpty: Boolean = false
}

class Nil[T] extends List[T] {
  def isEmpty: Boolean = true

  def head: Nothing = throw NoSuchElementException("Nil.head")

  def tail: Nothing = throw NoSuchElementException("Nil.tail")
}