functors1.scala

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

val listFunctor = new Functor[List] {
  override def map[A, B](fa: List[A])(f: A => B): List[B] = fa match {
    case Nil => Nil
    case h :: t => f(h) :: map(t)(f)
  }
}

val ints = List(1, 2, 3, 4, 5, 6)
listFunctor.map(ints)(_.toString)