def init[A](l: List[A]): List[A] =
  l match {
    case Nil => sys.error("init of empty list")
    case Cons(_, Nil) => Nil
    case Cons(h, t) => Cons(h, init(t))
  }

init(List(1))

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