Case.scala

abstract class Atom
case class Var(value: int) extends Atom
case class Add(left:Atom, right:Atom) extends Atom
case class Mul(left:Atom, right:Atom) extends Atom

object Case {
    def main(args: Array[String]) = {
        val exp = Add(Var(1),Var(2))
        println(exp)
        println(eval(exp))
    }
    def eval(exp:Atom):int = {
        exp match {
            case Var(a) => a
            case Add(a,b) => eval(a) + eval(b)
            case Mul(a,b) => eval(a) * eval(b)
        }
    }

}