型引数の基本から学ぶ、FreeモナドとCoyoneda http://awekuit.hatenablog.com/entry/2014/12/04/091402

FreeMonad_blog_1.scala

import scala.language.higherKinds
import scala.language.implicitConversions
import scala.language.reflectiveCalls

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

implicit def functorOps[F[_] : Functor, A](self: F[A]) = new {
  def map[B](f: A => B): F[B] = implicitly[Functor[F]].map(self)(f)
}

abstract class HKFold[F[_] : Functor, A] {
  def point[B]  (x: B): HKFold[F, B] =
    F0[F,B](x)
  def map[B]    (f: A => B): HKFold[F, B] =
    flatMap(x => point(f(x)))
  def flatMap[B](f: A => HKFold[F, B]): HKFold[F, B]
}

case class F0[F[_] : Functor, A](x: A) extends HKFold[F, A] {
  def flatMap[B](f: A => HKFold[F, B]): HKFold[F, B] = 
    f(x)
}

case class F1[F[_] : Functor, A](x: F[A]) extends HKFold[F, A] {
  def flatMap[B](f: A => HKFold[F, B]): HKFold[F, B] = {
    val y = x.map(f)
    F3[F,B](y)
  }
}

case class F3[F[_] : Functor, A](x: F[HKFold[F, A]]) extends HKFold[F, A] {
  def flatMap[B](f: A => HKFold[F, B]): HKFold[F, B] = {
    val res = x.map{y =>
      y.flatMap(z => f(z))
    }
    F3(res)
  }
}

implicit val optionFunctor = new Functor[Option] {
  def map[A, B](m: Option[A])(f: A => B): Option[B] = m.map(f)
}

val res = for {
  a <- F1(Option(3))
  b <- F1(Option(5))
  c <- F1(Option(10))
} yield a * b + c