object PatternMatching
{
  trait Expr {
    def eval: Int = this match {
      case Number(n) => n
      case Sum(e1, e2) => e1.eval + e2.eval
    }
    def show: String = this match {
      case Number(n) => n.toString
      case Sum(e1, e2) => e1.show + " + " + e2.show
    }
  }

  case class Number(n: Int) extends Expr
  case class Sum(e1: Expr, e2: Expr) extends Expr


  def eval1(e: Expr): Int = e match {
    case Number(n) => n
    case Sum(e1, e2) => eval1(e1) + eval1(e2)
  }

  def run = {
    println("PatternMatching")
    println( eval1(Sum(Number(1), Number(2))) )
    val e = Sum(Number(1), Number(2))
    println( e.show + " = " + e.eval )
  }
}