Direct-Style Monads

m.scala

trait Monad[M[_]] {
  def unit[A](x : A) : M[A]

  def bind[A, B](m : M[A], f : A => M[B]) : M[B]
}

implicit object OptionMonad extends Monad[Option] {
  def unit[A](x : A) = Some(x)

  def bind[A, B](m : Option[A], f : A => Option[B]) = m.flatMap(f)
}

implicit object ListMonad extends Monad[List] {
  def unit[A](x : A) = List(x)

  def bind[A, B](m : List[A], f : A => List[B]) = m.flatMap(f)
}

import scala.util.continuations._

def reify[A, M[+_]](x : => A @cpsParam[M[A], M[A]])(implicit monad : Monad[M]) : M[A] =
  reset { monad.unit(x) }

class Reflective[+A, M[_]](m : M[A], monad : Monad[M]) {
  def reflect[B]() : A @cpsParam[M[B], M[B]] = {
    shift { (k : A => M[B]) =>
      monad.bind(m, k)
    }
  }
}

implicit def reflective[A](xs : Option[A])(implicit monad : Monad[Option]) =
  new Reflective[A, Option](xs, monad)

implicit def reflective[A](xs : List[A])(implicit monad : Monad[List]) =
  new Reflective[A, List](xs, monad)

reify {
  val left = List("x", "y", "z").reflect[(String, Int)]
  val right = List(4, 5, 6).reflect[(String, Int)]

  (left, right)
}