object Book extends App {
  abstract class Expr { def eval: Double }

  class Op(name: String, op: (Double, Double) => (Double)) extends Expr {
    var left: Expr = null
    var right: Expr = null
    def apply(_left:Expr , _right: Expr) = {left = _left; right = _right; this}
    def eval = op(left.eval, right.eval)
    override def toString = "%s(%s, %s)".format(name, left, right)
  }

  case class Value(eval:Double) extends Expr {
    override def toString = eval.toString
  }

  implicit def MakeValue(d: Double) = new Value(d)

  def MakeOperator(name: String, op: (Double, Double) => (Double)): Op = new Op(name, op)

  def prod = MakeOperator("prod", _*_)
  def sum = MakeOperator("sum", _+_ )
  def div = MakeOperator("div", _/_)

  def test(x: Expr) { println(x + " = " + x.eval) }

  test(prod(sum(sum(2, 3), 1), 5))
  test(div(sum(28, 2), prod(5, 2)))
}