Generator.scala

//scalac -P:continuations:enable
import scala.util.continuations._
import scala.collection._

object Generator {
  type susp = cps[Any]
  def make[A](body: (A => Unit @susp) => Unit @susp): Iterator[A] = new Iterator[A] {
    val yields: A => Unit @susp = {v =>
      shift{k: (Unit => Any) => (k, v):Any }
    }
    var thunk: Unit => Any = {x: Unit =>
      reset {
        body(yields)
        None
      }
    }
    var fetched: Boolean = false
    var value: Option[A] = _
    def fetch(): Option[A] = {
      if(!fetched) {
        fetched = true
        value = (thunk(()) match {
          case (k, v) => 
            thunk = k.asInstanceOf[(Unit => Any)]
            Some(v.asInstanceOf[A])
          case None =>
            None
        })
      }
      value
    }
    def hasNext(): Boolean = !fetch().isEmpty
    def next: A = {
      fetch()
      fetched = false
      value getOrElse (throw new NoSuchElementException("next on empty iterator"))
    }
  }
}
object Control {
  import Generator._
  def upto[A](from: Int, to: Int)(f: Int => A @susp): Unit @susp = {
    if(from <= to) { f(from); upto(from + 1, to)(f) }
  }
  def each[A](iterable: Iterable[A])(f: A => (Unit @susp)): Unit @susp = {
    val it = iterable.iterator
    def loop: Unit @susp = {
      if(it.hasNext) {
        f(it.next())
        loop
      }
    }
    loop
  }
  def toIterator[A](f: (A => Unit @susp) => Unit @susp) = make[A]{yields =>
    f{a => yields(a)}
  }
}
object User {
  import Generator._
  import Control._
  def main(args: Array[String]) {
    val x = make[Int]{yields =>
      upto(1, 10){i => yields(i)}
    }
    val y = toIterator(upto(1, 20))
    println(x.toList)
    println(y.toList)
  }
}