title: Definition of Nil List in Scala
body: <p>I do not quite understand the definition of Nil List in a Scala textbook. The  List is defined as follows:</p>
  
  <pre><code>sealed trait List[+A]
    case object Nil extends List[Nothing]
    case class Cons[+A] (head: A, tail: List[A]) extends List[A]
  </code></pre>
  
  <ol>
  <li><p>Why Nil is not a class ? </p></li>
  <li><p>Why Nil does not extend List[A] ?</p></li>
  <li><p>Can I define the Nil case as: </p>
  
  <p>case class Nil extends List[A] ?</p></li>
  </ol>