sameLength(4)

gistfile1.scala

object Test extends App {
  object Vec {
    def fromList[A](l: List[A]): Vec[_, A] = Vec(l)
  }

  case class Vec[Tag1, A] private (private val l: List[A]) {
    def toList = l
    
    def zipWith[Tag2, B, C](v: Vec[Tag2, B])(f: (A, B) => C)(implicit ev: Tag1 =:= Tag2) =
      Vec[Tag1, C](l.zip(v.l).map{case (a, b) => f(a, b)})
    
    def sameLength[B, Tag2](b: Vec[Tag2, B]) =
      if (l.length == b.l.length)
        Some((this, Vec[Tag1, B](b.l)))
      else
        None

    //こっちだと何故か動かない
    def badSameLength[B, Tag2](b: Vec[Tag2, B]) =
      if (l.length == b.l.length)
        Some(Vec[Tag1, B](b.l))
      else
        None
  }

  def test = {
    val v1 = Vec.fromList(List(1, 2, 3))
    val v2 = Vec.fromList(List(1, 2, 3))
    val v3 = Vec.fromList(List(1, 2, 3))
    for {
      (v1, v2) <- v1.sameLength(v2)
      (v1, v3) <- v1.sameLength(v3)
      //本当はこう書きたい
      //v2 <- v1.badSameLength(v2)
      //v3 <- v1.badSameLength(v3)
    } {
      val v4 = Vec.fromList(List(1, 2, 3))
      val v5 = v3.zipWith(v2)((_, _))
      //println(v4.zipWith(v5)((_, _))) //compile error
      println(v1.zipWith(v5)((_, _)))
    }
  }

  test
}