Kleisli.scala

import scala.concurrent.Future
import scala.language.{implicitConversions, higherKinds, postfixOps}
import scala.concurrent.ExecutionContext.Implicits.global

trait Kleisli[M[+_], -A, +B] {
  def apply(a: A): M[B]
  def >=>[C](k: Kleisli[M, B, C]): Kleisli[M, A, C]
  def <=<[C](k: Kleisli[M, C, A]): Kleisli[M, C, B]
}

object futureKleisli {
  
  case class FutureKleisli[A, B](run: A ⇒ Future[B]) extends Kleisli[Future, A, B] { self =>
    def apply(a: A) = run(a)
    def >=>[C](k: Kleisli[Future, B, C]) =
      FutureKleisli[A, C](a => self(a).flatMap(b => k(b)))
    def <=<[C](k: Kleisli[Future, C, A]) =
      FutureKleisli[C, B](a => k(a).flatMap(b => self(b)))
  }
  
  implicit def futureKleisli[A, B](f: A => Future[B]): Kleisli[Future, A, B] =
    FutureKleisli(f)
}