// expression structures as case classes:
abstract class Expr
case class Num(num: Int) extends Expr
case class BinOp(operator: String, left: Expr, right: Expr) extends Expr

// compute value of an expression:
def expValue(expr: Expr): Int = expr match {
  case Num(n) => n
  case BinOp("+",e1,e2) => expValue(e1) + expValue(e2)
  case BinOp("*",e1,e2) => expValue(e1) * expValue(e2)
}

val exp = BinOp("+", Num(2), BinOp("*", Num(2), Num(3)))