bash-3.2$ scalac Stream.scala
Stream.scala:17: error: not found: type A
  def headOption: Option[A] = this match {
                         ^
Stream.scala:18: error: pattern type is incompatible with expected type;
 found   : Empty.type
 required: Stream.type
    case Empty => None
         ^
Stream.scala:19: error: constructor cannot be instantiated to expected type;
 found   : Cons[A]
 required: Stream.type
    case Cons(h, t) => Some(h())
         ^
three errors found