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

Question

I'm trying to use various Scala implementations of C#-like yield return (i.e. this one) 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.

Medieval

Maybe I'm missing something, but the braces on that example don't seem to match up. And how are you able to call generate there? It's in a nested scope. – Vanzandt

I solved both problems by removing an extra brace. – Rennes

This question is a duplicate of the following, which provides an answer: #8934726 – Seessel

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.

Foeticide

scala-lang.org/docu/files/api/scala/util/continuations/… : type cps = cpsParam[A, A] So I think scala team had renamed cps to cpsParam and added type cps Even if I use @cps[Iteration[Vector[T]],Iteration[Vector[T]]] the problem remains – Medieval

Oh well, sorry! I was just working from what I saw there. Good luck getting it working! – Foeticide