Functorの合成

ComposeFunctor.scala

import scala.language.higherKinds

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

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

implicit val seqFunctor = new Functor[Seq] {
  def map[A, B](a: Seq[A])(f: A => B): Seq[B] = a.map(f)
}

def composeFunctor[F[+_]: Functor, G[+_]: Functor] = new Functor[({type f[+x] = F[G[x]]})#f] {
  val F: Functor[F] = implicitly[Functor[F]]
  val G: Functor[G] = implicitly[Functor[G]]
  def map[A, B](a: F[G[A]])(f: (A) => B) = F.map(a)(G.map(_)(f))
}

implicit val cf = composeFunctor[Option, Seq]

val a: Option[Seq[Int]] = Some(Seq(1,2,3))

a.map(_.map(_ * 2)) // Some(Seq(2,4,6))

composeFunctor[Option, Seq].map(a)(_ * 2) // Some(Seq(2,4,6))