Scala implementation of C#-like yield with "for"

Question

I'm trying to use various scala implementations of C#-like yield return (i.e. link text) with "for" -constructions such as:

private def permutations[T](s: Vector[T]) = {
  def swap(i: Int, j: Int) {
    val tmp = s(i)
    s.set(i, s.get(j))
    s.set(j, tmp)
  }

  iterator[Vector[T]] {
    def generate(left: Int, right: Int): Unit @cps[Iteration[Vector[T]]] = {
      if (left >= right)
        yieldValue(s)

      else {
        generate(left, right)
        for (i <- left to right) {
          swap(left, i)
          generate(left+1, right)
          swap(left, i)
        }
      }
    }
  }

  generate(0, s.size-1)
  } 
}

But this code compiles with error: "error: no type parameters for method foreach: (f: (Int) => U)Unit exist so that it can be applied to arguments ((Int) => Unit @util.continuations.package.cps[ru.ispras.texterra.nlp.GHMMDisambiguator.Iteration[Vector[T]]]) --- because --- argument expression's type is not compatible with formal parameter type; found : (Int) => Unit @util.continuations.package.cps[ru.ispras.texterra.nlp.GHMMDisambiguator.Iteration[Vector[T]]] required: (Int) => ?U for (i <- left to right) {"

As I understand I have to make all code inside for to be type of () => Unit, not of () => Unit @with-annotations. How can I do that?

This problem seems to be very common, but I didn't found any mentions in the Internet.

Answer 1

If you're using the iterator type from the linked example, is it possible that your generate method needs to have the following return type rather than the one you have there?

Unit @cps[Iteration[Vector[T]],Iteration[Vector[T]]]

I'm afraid I haven't got much experience with this stuff but it looks a lot like the methods you call within iterator must have two (identical) type arguments on the annotation.