http://hackage.haskell.org/package/kan-extensions-3.7/docs/Control-Comonad-Density.html#t:Density

Density.scala

package scalaz

abstract class Density[F[_], A]{ self =>

  def apply[B](g: F[B] => A)(h: F[B]): Density[F, A]

  //  fmap f (Density g h) = Density (f . g) h
  def map[B](f: A => B): Density[F, B] =
    new Density[F, B] {
      def apply[C](x: F[C] => B)(fc: F[C]): Density[F, B] = {
        ???
      }
    }

  def cobind[B](f: Density[F, A] => B): Density[F, B] =
    ???
}

sealed abstract class DensityInstances2 {
  implicit def densityApply[F[_]: Apply]: Apply[({type λ[α] = Density[F, α]})#λ] =
    new DensityApply[F] {
      def F = implicitly
    }
}

sealed abstract class DensityInstances1 extends DensityInstances2{
  implicit def densityApplicative[F[_]: Applicative]: Applicative[({type λ[α] = Density[F, α]})#λ] =
    new DensityApplicative[F] {
      def F = implicitly
    }
}

sealed abstract class DensityInstances extends DensityInstances1 {

  implicit def densityInstance[F[_]]: Comonad[({type λ[α] = Density[F, α]})#λ] =
    new Comonad[({type λ[α] = Density[F, α]})#λ] with DensityFunctor[F] {
      def cobind[A, B](fa: Density[F, A])(f: Density[F, A] => B) =
        fa cobind f
      def copoint[A](fa: Density[F, A]): A =
        ???
    }

}

object Density extends DensityInstances{

}

private trait DensityFunctor[F[_]] extends Functor[({type λ[α] = Density[F, α]})#λ] {
  override final def map[A, B](fa: Density[F, A])(f: A => B) =
    fa map f
}

private trait DensityApply[F[_]]
  extends Apply[({type λ[α] = Density[F, α]})#λ] 
  with DensityFunctor[F] {
 
  implicit def F: Apply[F]

  // TODO 
  // Density kxf x <.> Density kya y =
  //   Density (\k -> kxf (fmap fst k) (kya (fmap snd k))) ((,) <$> x <.> y)
  def ap[A, B](fa: => Density[F, A])(f: => Density[F, A => B]): Density[F, B] =
    new Density[F, B] {
      def apply[C](f: F[C] => B)(fc: F[C]) =
        ???
    }
}

private trait DensityApplicative[F[_]]
  extends Applicative[({type λ[α] = Density[F, α]})#λ]
  with DensityApply[F] {

  def F: Applicative[F]

  override final def point[A](a: => A): Density[F, A] =
    new Density[F, A] {
      def apply[B](f: F[B] => A)(fb: F[B]): Density[F, A] = {
        // TODO
        // pure a = Density (const a) (pure ())
        ???
      }
    }
}