Simple calculator using dynamic dispatch.

DynamicDispatchCalculator.scala

package dynamic

abstract class Expression {
  def eval:Double = ???
}

case class Value(val value: Double) extends Expression {
  override def eval = value
}

case class Add(val exp1: Expression, val exp2: Expression) extends Expression {
  override def eval = exp1.eval + exp2.eval
}

case class Subtract(val exp1: Expression, val exp2: Expression) extends Expression {
  override def eval = exp1.eval - exp2.eval
}

case class Multiply(val exp1: Expression, val exp2: Expression) extends Expression {
  override def eval = exp1.eval * exp2.eval
}

case class Divide(val exp1: Expression, val exp2: Expression) extends Expression {
  override def eval = exp1.eval / exp2.eval
}

// third party users can extend 

object ScalaWithStyle {
  def main(args:Array[String]) {
    val result = Multiply(Add(Value(1), Value(2)), Add(Value(3), Value(4))).eval
    println(result)
  }
}