Hamming.scala

def merge(a: Stream[Int], b: Stream[Int], c: Stream[Int]): Stream[Int] = {
  val m = a.head min b.head min c.head
  Stream.cons(m, merge(
    if (m < a.head) a else a.tail,
    if (m < b.head) b else b.tail,
    if (m < c.head) c else c.tail))
}

def hamming: Stream[Int] = Stream.cons(1, merge(
  hamming.map(_ * 2),
  hamming.map(_ * 3),
  hamming.map(_ * 5)))

hamming.take(20).print 

// Prints
// 1, 2, 3, 4, 5, 6, 8, 9, 10, 12, 15, 16, 18, 20, 24, 25, 27, 30, 32, 36, empty