Functor.scala

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

trait Applicative[F[_]]
  def ap       [A, B](fa: F[A])(f: F[A  =>   B]): F[B]

trait Monad[F[_]]
  def bind     [A, B](fa: F[A])(f:   A  => F[B]): F[B]

trait Contravariant[F[_]]
  def contramap[A, B](fa: F[A])(f:   B  =>   A ): F[B]

trait Comonad[F[_]]
  def cobind   [A, B](fa: F[A])(f: F[A] =>   B ): F[B]