expression.scala

trait Expression {
  def +(right: Expression) = Addition(this, right)
  def *(right: Expression) = Multiplication(this, right)
  def **(right: Expression) = Exponential(this, right)
}

object Expression {
  implicit def intToConst(value: Int) = Const(value)
}

case class Var(name: String) extends Expression
case class Const(value: Int) extends Expression

case class Addition(left: Expression, right: Expression) extends Expression
case class Multiplication(left: Expression, right: Expression) extends Expression
case class Exponential(left: Expression, right: Expression) extends Expression

object Main {
  def main(string: Array[String]) {
    import Expression._

    val x = Var("x")
    val y = x ** 2 + x * 4

    println(y)
  }
}