Scala Functor

Functor.scala

/*
  class Functor f where
    fmap :: (a -> b) -> f a -> f b

 */

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

trait Applicative[F[_]] extends Functor[F] {
  def pure[A] : F[A]
  def ap[A, B](f : F[A => B])(g : F[A]) : F[B]
}

object ABC extends App {
  /*
    instance Functor Maybe where
      fmap _ Nothing = Nothing
      fmap g (Just x) = Just (g x)
   */
  implicit val optionIsAFunctor : Functor[Option] = new Functor[Option] {
    override def fmap[A, B](f : Option[A])(g : (A) => B) : Option[B] =
      f match {
        case None => None
        case Some(x) => Some(g(x))
      }
  }

  /*
     ....
   */
  implicit val listIsAFunctor : Functor[List] = new Functor[List] {
    override def fmap[A, B](f : List[A])(g : (A) => B) : List[B] =
      f match {
        case Nil => Nil
        case (x :: xs) => g(x) :: fmap(xs)(g)
      }
  }

  val a : Option[Int] = Some(2)
  val b = List(1,2,3)

  def foo[F[_] : Functor, A, B](f : F[A])(g : A => B) : F[B] =
    implicitly[Functor[F]].fmap(f)(g)

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

  println(foo(a)(i => i + 2))
  println(bar(b)(i => i - 1))
}