Problem Statement

Given non-negative integers   m   and   n,   generate all size   m   combinations   of the integers from   0   (zero)   to   n-1   in sorted order   (each combination is sorted and the entire table is sorted).

3   comb   5     is: If it is more "natural" in your language to start counting from   1   (unity) instead of   0   (zero), the combinations can be of the integers from   1   to   n.

Let's start with the solution:

def combinations(m, range) {
  return if (m <=> 0) { [[]] } else {
    def combGenerator {
      to iterate(f) {
        for i in range {
          for suffix in combinations(m.previous(), range & (int > i)) {
            f(null, [i] + suffix)
          }
        }
      }
    }
  }
}