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Functors

Introduction

Functors are useful when dealing with one effectful value

You may not be familiar with the term Functor, but if you are a proficient Scala programmer you are almost certainly familiar with the concept. Functor essentially expresses the ability to transform a wrapped value using the map combinator e.g. List.map, Option.map, Either.map, Future.map etc.

For example

Some(1).map(_ + 1) // Some(2)
List(1, 2, 3).map(_ + 1) // List(2, 3, 4)

functor

The definition of Functor looks like this. Note that this is a typeclass that acts on a type constructor F[_] where F[_] will be Maybe or CanFail in this workshop.

trait Functor[F[_]] {
  def map[A, B](fa: F[A])(fn: A => B): F[B]
}

Read The Code

  • In functors.scala the functor trait has been defined.

  • Notice that an implicit class has also been defined, this allows the map function from Functor to be called with dot syntax. This is following the same pattern we used when implementing the Monoid typeclass.

  • Notice that the way maybeFunctor and canFailFunctor are declared is slightly different. One is a val and the other is a def that takes a type parameter. This is because the Maybe data type has only one type parameter, Maybe[A], whereas the CanFail data type has two type parameters, CanFail[E, A]. Referring back to our definition of Functor, we can see that the higher kinded type parameter only matches type constructors with one type parameter, but we have two. So, to satisfy the signature of Functor we need to fix one of the parameters, so that there is only one "hole" left to fill.

    So, we want to say that F[A] => CanFail[E, A] where E is fixed and A is a parameter. We can express that with the ? syntax from a compiler plugin called kind-projector that conveniently solves this problem. e.g. CanFail[E, ?]

Exercises

Exercise 1 – Maybe functor

  • Implement a Functor instance for Maybe.
  • Run FunctorLaws using sbt 'testOnly *FunctorLaws' to check your implementation. Note, there will still be failing tests for the CanFail functor at this stage.

Exercise 2 – CanFail functor

  • Implement a Functor instance for CanFail
  • Run FunctorLaws using sbt 'testOnly *FunctorLaws' to check your implementation. All the tests should pass now.

Exercise 3 – Use functor instances

  • Use the functor instance for CanFail to define the greeting function This function should use getName to lookup the users name, and transform it into a greeting of the format Hello $name, and welcome to functor town
  • Run FunctorSpec using sbt 'testOnly *FunctorSpec' to check your implementation