How to code a type class in Scala

The aim of this post is to present the simplest type class example. A type class is a kind of design pattern that allows adding behaviour to a class without extending it (i.e. non intrusively). Let’s use the REPL :

A type class is just a regular scala trait

scala> trait Show[A] { def show(a: A) : String }
defined trait Show

Here is an instance of my type class for type Int

scala> implicit val IntShow = new Show[Int] { def show(i: Int) = s"'$i' is an int" }
IntShow: Show[Int] = $anon$1@14459d53

Last step : use the type class by passing it as an implicit parameter

scala> def f[A](a:A)(implicit s : Show[A]) = println(s.show(a))
f: [A](a: A)(implicit s: Show[A])Unit

Let’s call it

scala> f(1)
'1' is an int