View difference between Paste ID: U39Z5UKG and Lt07zBJP

 object Book extends App {
   abstract class Expr { def eval: Double }
 
-  class Op(name: String, op: (Double, Double) => (Double)) extends Expr {
+  type op = (Double, Double) => (Double)
-    var left: Expr = null
+
-    var right: Expr = null
+  class Op(name: String, op: op, var left: Expr = null, var right: Expr = null) extends Expr {
     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 {
+  case class Value(eval:Double) extends Expr { override def toString = eval.toString }
-    override def toString = eval.toString
+
   implicit def MakeValue(d: Double) = new Value(d)
 
   def MakeOperator(name: String, op: op): Op = new Op(name, op)
 
-  def MakeOperator(name: String, op: (Double, Double) => (Double)): Op = new Op(name, op)
+
   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)))
 }