型クラスとimplicit conversion

functor.scala

import language.{implicitConversions, higherKinds}

trait Functor[A, Repr[_]] {

  def fmap[B](f: A => B): Repr[B]

  def fcompose[B, G[_]](implicit ev: A => Functor[B, G]): Functor[B, ({type λ[α] = Repr[G[α]]})#λ] =
    new Functor[B, ({type λ[α] = Repr[G[α]]})#λ] {
      def fmap[C](f: B => C): Repr[G[C]] = Functor.this.fmap(a => ev(a).fmap(f))
    }

}
object Functor {

  def lift[A, B, F[_]](f: A => B)(implicit ev: F[A] => Functor[A, F]): F[A] => F[B] = _.fmap(f)

}


class ListFunctor[A](underlying: List[A]) extends Functor[A, List] {
  def fmap[B](f: A => B): List[B] = underlying map f
}
object ListFunctor {
  implicit def listToFunctor[A](l: List[A]): Functor[A, List] = new ListFunctor(l)
}

class OptionFunctor[A](underlying: Option[A]) extends Functor[A, Option] {
  def fmap[B](f: A => B): Option[B] = underlying map f
}
object OptionFunctor {
  implicit def optionToFunctor[A](o: Option[A]): Functor[A, Option] = new OptionFunctor(o)
}

z_console.scala

> console
[info] Starting scala interpreter...
[info]
import ListFunctor._
import OptionFunctor._
import language._
Welcome to Scala version 2.10.2 (Java HotSpot(TM) 64-Bit Server VM, Java 1.7.0_45).
Type in expressions to have them evaluated.
Type :help for more information.

scala> List(1, 2, 3).fmap(_ * 2)
res0: List[Int] = List(2, 4, 6)

scala> List(Some(1), None, Some(3)).fcompose.fmap(_ * 2)
res1: List[Option[Int]] = List(Some(2), None, Some(6))

scala> Functor.lift[String, Int, List](_.toInt)
res2: List[String] => List[Int] = <function1>

scala>