[scala] 同じリスト同士の直積

product.scala

def product[A](list: Seq[A], length: Int): Iterator[Seq[A]] = {
  @tailrec
  def _product(list: Stream[A], length: Int,
    accum: Stream[Seq[A]]): Stream[Seq[A]] = {

    length match {
      case 0 => accum
      case _ =>
        _product(list, length - 1,
          list map {
            x => (x, accum)
          } flatMap {
            case (c, ls) => ls.map(y => x +: y)
          })
    }
  }

  val init = list.toStream
  _product(init, length - 1, init.map(_.toString)).toIterator
}

// > println(product(List(1, 2), 2).toList)
// List(List(1, 1), List(1, 2), List(2, 1), List(2, 2))