Foo.sc

@ case class Cross[T](items: T*){
    def flatMap[V](f: T => Cross[V]): Cross[(T, V)] = {
      val flattened = for{
        i <- items
        k <- f(i).items
      } yield (i, k)
      Cross(flattened:_*)
    }
    def map[V](f: T => V): Cross[(T, V)] = {
      Cross(items.map(i => i -> f(i)):_*)
    }
    def withFilter(f: T => Boolean) = {
      Cross(items.filter(f):_*)
    }
  }
defined class Cross

@ for(a <- Cross(1, 2, 3)) yield a.toString
res15: Cross[(Int, String)] = Cross(ArrayBuffer((1, "1"), (2, "2"), (3, "3")))

@ for{
    a <- Cross(1, 2)
    b <- Cross("A", "B")
    if !(a == 2 && b == "B")
  } yield b * a
res16: Cross[(Int, (String, String))] = Cross(
  ArrayBuffer((1, ("A", "A")), (1, ("B", "B")), (2, ("A", "AA")))
)