functor.scala

trait Functor[A, F[_]] { self =>
  def fmap[B](f: A => B): F[B]
}

trait ~>[-F[_], +G[_]]{
  def apply[A](fa: F[A]): G[A]
}

object Functor {

  implicit class list[A](val l: List[A]) extends Functor[A, List] {
    def fmap[B](f: A => B): List[B] = l map f
  }

  implicit class option[A](val o: Option[A]) extends Functor[A, Option]{
    def fmap[B](f: A => B): Option[B] = o map f
  }

  type FunctorConversion[F[_]] = (F[A] => Functor[A, F]) forSome {type A}
}

object Main {
  import Functor._

  def foo[F[_]: FunctorConversion, G[_]: FunctorConversion](fa: F ~> G): Unit = ???
  def bar[A, B, F[_], G[_]](fa: F ~> G)(implicit F: F[A] => Functor[A, F], G: G[B] => Functor[B, G]): Unit = ???

  val option2list = new (Option ~> List){
    def apply[A](fa: Option[A]) = fa.toList
  }

  // foo(option2list) // compile error. No implicit view available from Option => .
  // foo[Option, List](option2list) // compile error. No implicit view available from Option => .

  foo(option2list)(Functor.option _, Functor.list _)

  bar(option2list) // success
}