def drop[A](l: List[A], n: Int): List[A] =
  if (n <= 0) l
  else
    l match {
      case Nil => Nil
      case Cons(_, t) => drop(t, n - 1)
    }

drop(List(1, 2, 3), 0)

sealed trait List[+A]
case object Nil extends List[Nothing]
case class Cons[+A](head: A, tail: List[A]) extends List[A]