Scala Design Patterns - Second Edition

Functors in real life

Before we show an example functor trait based on the laws we showed in the preceding section, you can conclude that standard Scala types such as List, Option, and others that define a map method are functors.

The map method in the built-in Scala types such as List has a different signature from the example we show here. In our examples, the first parameter is the functor and the second one is the transformation function we apply to it. In the standard Scala types, the first parameter doesn't need to be passed, as it is the actual object we're calling it on (this).

If we want to create our own types that follow the functor laws, we can create a base trait and make sure to implement it:

trait Functor[F[_]] {  def map[