sameLength

gistfile1.scala

trait Forall3[X[_, _, _]] {
  def apply[U, V, R]: X[U, V, R]
}

type ZipWith[A[_]] = {
  type Apply[U, V, R] = ((U, V) => R) => (A[U], A[V]) => A[R]
}

type Result[T] = (A[T], A[T], Forall3[ZipWith[A]#Apply]) forSome { type A[_] }

def sameLength[T](xs: List[T], ys: List[T]): Option[Result[T]] = {
  if (xs.length == ys.length)
    Some((
      xs, ys,
      new Forall3[ZipWith[List]#Apply] {
        def apply[U, V, R] = f => _.zip(_).map{case (a, b) => f(a, b)}
      }
    ))
  else
    None
}

sameLength(List(1, 2), List(3, 4)).get match {
  case (a, b, zipWith) =>
    val x = zipWith[Int, Int, (Int,Int)]((_, _))(a, b)
    val y = zipWith[(Int, Int), Int, ((Int, Int), Int)]((_, _))(x, b)
    println(y)
}