list.scala

trait List[T] {
 def isEmpty:Boolean
 def head: T
 def tail: List[T]
}

class Cons[T] (val head: T, val tail: List[T]) extends List[T] {
  def isEmpty = false;
}

class Nil[T] extends List[T] {
  def isEmpty = true
  def head = throw new NoSuchElementException("Empty")
  def tail = throw new NoSuchElementException("Empty")
}