Simple Scala Functor example, no cats

FunctorApp.scala

package example

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

  def lift[A, B](f: A => B): F[A] => F[B] = map(_)(f)
}

object Functor {
  def apply[F[_] : Functor]: Functor[F] = implicitly[Functor[F]]
}

object FunctorApp extends App {

  def mapAll[F[_] : Functor, A, B](fa: F[A])(f: A => B): F[B] = Functor[F].map(fa)(f)

  def liftAll[F[_] : Functor, A, B](f: A => B): F[A] => F[B] = Functor[F].lift(f)

  def mapAll2[F[_] : Functor, G[_] : Functor, A, B](fg: F[G[A]])(f: A => B): F[G[B]] = mapAll(fg)(liftAll(f))

  def liftAll2[F[_] : Functor, G[_] : Functor, A, B](f: A => B): F[G[A]] => F[G[B]] = liftAll(liftAll(f))

  implicit val functorForOption: Functor[Option] = new Functor[Option] {
    def map[A, B](fa: Option[A])(f: A => B): Option[B] = fa match {
      case None => None
      case Some(a) => Some(f(a))
    }
  }

  implicit val functorForList: Functor[List] = new Functor[List] {
    def map[A, B](fa: List[A])(f: A => B): List[B] = fa match {
      case head :: tail => f(head) :: map(tail)(f)
      case _ => Nil
    }
  }

  def increment(x: Int) = x + 1

  val option10 = Option(10)

  println(functorForOption.lift(increment)(option10))
  println(functorForOption.map(option10)(increment))
  println(mapAll(option10)(increment))

  println(functorForList.map(List(10, 20))(increment))
  println(mapAll(List(10, 20))(increment))

  val lo = List(option10, None, mapAll(option10)(_*2))
  println(mapAll2(lo)(increment))
  // more annoying to use as compiler cannot derive type for func
  val func = liftAll2[List, Option, Int, Int](increment)
  println(func(lo))

  val ol = Option(List(11, 32))
  println(mapAll2(ol)(increment))

  val ll = List(List(10, 22), List(30, 40))
  println(mapAll2(ll)(increment))
}

output.txt

Some(11)
Some(11)
Some(11)
List(11, 21)
List(11, 21)
List(Some(11), None, Some(21))
List(Some(11), None, Some(21))
Some(List(12, 33))
List(List(11, 23), List(31, 41))