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

object FunctorImplicits {

  def fmap[F[_], A, B](a: F[A], f: A => B)(implicit functor: Functor[F]): F[B] =
    functor.fmap(functor, f)


  implicit object OptionFunctor extends Functor[Option] {
    def fmap[A, B](a:Option[A], f: A => B): Option[B]
      = a map f
  }