Rank 2 polymorphism in Scala, from http://apocalisp.wordpress.com/2010/07/02/higher-rank-polymorphism-in-scala/

polymorphism.scala

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

type Id[A] = A

def apply[B](f: Id ~> List, b: B, s: String): (List[B], List[String]) = (f(b), f(s))