sealed trait Expr
case class Val(x: Int) extends Expr
case class Add(x: Expr, y: Expr) extends Expr
case class Mul(x: Expr, y: Expr) extends Expr

object HelloWorld {
  def eval(e: Expr) : Int =
    e match {
      case Val(x) => x
      case Add(x, y) => eval(x) + eval(y)
      case Mul(x, y) => eval(x) * eval(y)
    }

  def main(args: Array[String]): Unit = {
    val e : Expr = Mul(Val(2), Add(Val(3), Val(4)))
    println(eval(e))
  }
}