Simple parser combinator example

SimpleCombinatorExample.scala

import scala.util.parsing.combinator._

import scala.util.parsing.input.CharSequenceReader

object Simple {
  trait Expression
  case class Add(lhs: Expression, rhs: Expression) extends Expression
  case class Mul(lhs: Expression, rhs: Expression) extends Expression
  case class Num(v: Double) extends Expression

  object parseUtil extends JavaTokenParsers
  import parseUtil._

  def expr = term
  def term = factor.*("+" ^^^ ((l: Expression, r: Expression) => Add(l,r)))
  def factor = primary.*("*" ^^^ ((l: Expression, r: Expression) => Mul(l,r)))
  def primary: Parser[Expression] = floatingPointNumber ^^ (x => Num(x.toDouble))
}

val expression = Simple.expr(new CharSequenceReader("3 * 6 + 1 * 2"))