abstract class Expr { 
  def eval: Int = this match { 
    case Number(n) => n 
    case Sum(e1, e2) => e1.eval + e2.eval
  }
  def printExpr: Unit = this match { 
    case Number(n) => print(n)
    case Sum(e1, e2) => print("(") ; e1.printExpr ; print("+") ; e2.printExpr ; print(")")
  }
}

case class Number(n: Int) extends Expr 

case class Sum(e1: Expr, e2: Expr) extends Expr 


/* Case classes do a lot automatically
 * - implicit constructor function
 *      def Number(n: Int) = new Number(n)
 * - proper toString,  value-type equals  and  hashCode
 * - accessor for argument
 *      def n: Int
 * - allows for "Patterns" in match function:
*/


/*
 def eval(e: Expr): Int = e match {
 case Number(n) => n
 case Sum(l, r) => eval(l) + eval(r)
 } 
*/