Functor for scala.collection.Map[Int, T]

Question

I am trying to find information if it is possible to create a Functor for a Map type. The docs have information for the List, Option, but not for my case.

Can you please tell me if it is possible to create a Functor[Map[Int, T]]?

Below I will attach an implementation of a similar functor for a List.

trait Functor[F[_]]:
    def map[A, B](list: F[A])(f: A => B): F[B]

given Functor[List] with
    def map[A, B](list: List[A])(f: A => B): List[B] = ???

Answer 1

• Map[A, B] is almost the same as A => B, but with a finite domain.
• A => B is basically the Reader[A, B]
• Reader[A, B] is a monad
• All monads are functors

So, yes let's try to do the same as what Reader does: it simply post-composes the functions given to map:

given mapFunctor[K]: Functor[[V] =>> Map[K, V]] with
  def map[A, B](m: Map[K, A])(f: A => B): Map[K, B] = m.view.mapValues(f).toMap

The toMap is necessary, because mapValues is non-strict (it refuses to map all the values right away, and keeps the function composition in a symbolic MapView).