Continuations and for comprehensions — what's the incompatibility?

Question

I am new to Scala and trying to wrap my head around continuations I\'m trying to reproduce the yield return C# statement.

Following this post, I have wr

Answer 1

First let's look at what it will take to get gen2 to compile.

object CpsConversions {

  import scala.collection.IterableLike
  import scala.util.continuations._

  implicit def cpsIterable[A, Repr](xs: IterableLike[A, Repr]) = new {
    def cps = new {
      def foreach[B](f: A => Any@cpsParam[Unit, Unit]): Unit@cpsParam[Unit, Unit] = {
        val it = xs.iterator
        while(it.hasNext) f(it.next)
      }
    }
  }
}

object GenTest {

  import CpsConversions.cpsIterable
  val gen2 = new Generator[Int] {
    def produce = {
      var ints = List(1, 2, 3, 42)
      ints.cps.foreach((theInt) => yieldValue(theInt))
    }
  }

Now let's take a look at what's going on. The original gen2 fails to compile on the following line:

ints.foreach((theInt) => yieldValue(theInt))

Since the type of yieldValue includes an @cpsParam annotation, the continuations plugin transforms the function passed to the foreach method to one of type:

Int => Unit @cpsParam[Unit,Unit]

Way up in the hierarchy of List[Int], you'll see foreach defined as:

foreach [U] (f: (Int) ⇒ U): Unit

This is a problem, as the types do not match and Scala doesn't know how to get from Int => U to Int => Unit @cpsParam[Unit,Unit]. To fix it, I added the CPS version of foreach in an implicit conversion, which you can access by calling cps on any IterableLike.

It would be very nice if this implicit conversion could be done without the explicit cps call, but I have not found a way to make the Scala compiler recognize the applicability of such an implicit conversion to pimp the new foreach onto your list. This might have to do with the order in which the compiler uses the continuations plugin, but I know far too little about this process to be sure.

So that's all well and good for foreach. Your question mentions for comprehensions, which will require any of filter, map, or flatMap to be defined (depending on what goes on in your for comprehension). I have implemented these in the link in my above comment, which extends the CpsConversions object above to allow for general for comprehensions.