Type Classes

March 11, 2012

Scala supports ad-hoc polymorphism via traits, context bounds, and implicits.

The type class pattern

Start with a generic trait with a single type parameter:

trait Showable[S] {
  def show(s: S): String
}

Using a context bound, pull in an implicit instance of the type class:

implicit class ShowableOps[S: Showable](s: S) {
  def show: String =
    implicitly[Showable[S]].show(s)
}

Write a concrete, implicit instance of the type class:

implicit object IntShowable extends Showable[Int] {
  def show(s: Int): String =
    s"The number ${s}"
}

Use it:

println(42.show) // prints "The number 42"