mymonad.scala

package mymonad

import scala.language.postfixOps
import scala.util.continuations._
import scala.util.control.NonFatal

sealed trait MyOption[+A]

case object MyNone extends MyOption[Nothing]

case class MySome[+A](value: A) extends MyOption[A]

object MyOption {
  def option[B](f: Monad[B] => MyOption[B] @ cps[MyOption[B]]) = reset(f(new Monad[B]))
  implicit class BindOps[A: Monad](a: MyOption[A]) {
    def bind = implicitly[Monad[A]] bind a
  }
  implicit class PureOps[A: Monad](a: A) {
    def pure = implicitly[Monad[A]] pure a
  }
  private class Monad[B] {
    def bind[A](m: MyOption[A]) = shift((k: A => MyOption[B]) =>
      m match {
        case MyNone => MyNone
        case MySome(a) => k(a)
      }
    )
    def pure(x: B) = MySome(x)
  }
}

object Main extends App {
  def toInt(s: String) =
    try MySome(s.toInt) catch { case NonFatal(_) => MyNone }
  import MyOption._
  val x = option[Int] { implicit m =>
    val a = toInt("1") bind
    val b = toInt("2") bind
    val c = toInt("3") bind;
    a + b + c pure
  }
  assert(x == MySome(6))
}