cartesian product using anamorphism

cart.scala

object ex {
  def ana[A, B](start: B)(f: B => Option[(A, B)]): List[A] = f(start) match {
    case Some((a, b))  => a :: ana(b)(f)
    case _             => Nil
  }

  def cartesian[A](xss: List[List[A]]): List[List[(A, A)]] = ana(xss) {
    case xs ::  vs :: rest => 
      val action = for {
        x <- xs
        v <- vs
      } yield (x, v)

      Some((action, rest))
    case _                 => None
  }
}

object Cartesian {
  def yesfilter[A](a: A, b: A) = true

  def combine[A, B, C](a: Traversable[A], b: Traversable[B], f: (A, B) ⇒ Boolean = yesfilter _)(m: (A, B) ⇒ C): Seq[C] = {
    @annotation.tailrec
    def combine(c: Traversable[B], e: A, o: Seq[C]): Seq[C] =
      c match {
        case Nil ⇒ o
        case h :: t ⇒
          if (f(e, h)) combine(t, e, o :+ m(e, h))
          else combine(t, e, o)
      }

    @annotation.tailrec
    def go(x: Traversable[A], y: Traversable[B], o: Seq[C]): Seq[C] = x match {
      case Nil    ⇒ o
      case h :: t ⇒ go(t, y, combine(y, h, o))
    }

    go(a, b, Nil)
  }
}