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Scala Type Parameterization


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")


In this page (written and validated by ) you learned about Scala Type Parameterization . What's Next? If you are interested in completing Scala tutorial, your next topic will be learning about: Scala Type Variance.

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