Yet another "generator" library implementation in Scala.

Generator.scala

//To compile this, "-P:continuations:enable" option is needed.
import scala.util.continuations._
import scala.collection._

object Generator {
  abstract sealed class Computation[+A]
  case class Yielded[A](k: Unit => Computation[A], v: A) extends Computation[A]
  case object Done extends Computation[Nothing]
  type yieldable[A] = cps[Computation[A]]

  def make[A](body: (A => Unit @yieldable[A]) => Unit @yieldable[A]): Iterator[A] = new Iterator[A] {
    val yields: A => Unit @yieldable[A] = {v =>
      shift{k: (Unit => Computation[A]) => Yielded(k, v) }
    }

    var thunk: Unit => Computation[A] = {x: Unit =>
      reset {
        body(yields)
        Done
      }
    }

    var fetched: Boolean = false
    var value: Option[A] = _

    def fetch(): Option[A] = {
      if(!fetched) {
        fetched = true
        value = (thunk(()) match {
          case Yielded(k, v) => 
            thunk = k
            Some(v)
          case Done =>
            None
        })
      }
      value
    }

    def hasNext(): Boolean = !fetch().isEmpty

    def next: A = {
      fetch()
      fetched = false
      value getOrElse (throw new NoSuchElementException("next on empty iterator"))
    }
  }
}

GeneratorUsage.scala

//To compile this, "-P:continuations:enable" option is needed.
import scala.util.continuations._
import Generator._
object GeneratorUsage {
  def main(args: Array[String]) {
    val x1 = make[Int]{yields =>
      yields(1)
      println("X")
      yields(2)
      println("Y")
      yields(3)
    }
    while(x1.hasNext) {
      println("----")
      println(x1.next)
    }
    val x2 = make[Int]{yields =>
      var i = 0
      while(i < 4){
        println("i = " + i)
        yields(i)
        i += 1
      }
    }
    while(x2.hasNext) {
      println("****")
      println(x2.next)
    }
  }
}

result.txt

# Please see method callings which have side effects in GeneratorUsage.scala
----
1
X
----
2
Y
----
3
i = 0
****
0
i = 1
****
1
i = 2
****
2
i = 3
****
3