rank-N polymorphism

gistfile1.scala

import scalaz.Forall

case class Record(x : List[Int], y : List[Double])

def lift(
  f: Forall[({type X[A] = (List[A], List[A]) => List[A]})#X],
  a: Record,
  b: Record
): Record =
  (a, b) match {
    case (Record(x1, y1), Record(x2, y2)) =>
      Record(f.apply(x1, x2), f.apply(y1, y2))
  }

def append(a: Record, b: Record): Record =
  lift(Forall[({type X[A] = (List[A], List[A]) => List[A]})#X](_(_:::_)), a, b)

val a = Record(List(1, 2, 3), List(1.0, 2.0, 3.0))
val b = Record(List(4, 5, 6), List(4.0, 5.0, 6.0))
println(append(a, b))