Example Expression Evaluator

trait Exp {
  def eval: Int
}

case class Oper(op: (Int, Int) => Int, left: Exp, right: Exp) extends Exp {
  override def eval: Int = op(left.eval, right.eval)
}

case class Num(value: Int) extends Exp {
  override def eval: Int = value
}

object Exp {
  def eval(exp: Exp): Int = {
    exp match {
      case Oper(op, l, r) =>
        op(eval(r), eval(l))
      case Num(v) => v
    }
  }
}