Calc2WithParserCombinator.scala

import scala.util.parsing.combinator.lexical.StdLexical
import scala.util.parsing.combinator.syntactical.StdTokenParsers

/**                                                                             
 * This program will parse the arithmetic expression defined as follows,        
 *   <exp>  ::= <term> | <exp> "+" <term> | <exp> "-" <term>                              
 *   <term> ::= <factor> | <term> "*" <factor> | <term> "/" <factor>                              
 *   <factor> ::= <n> | "(" <exp> ")"                                                   
 * , where n is a nonterminal symbol for numbers.                               
 *                                                                              
 * This program has expected to be compiled with Calc2.scala[1]                 
 * [1] http://gist.github.com/12517                                             
 */
object Calc2WithParserCombinator extends StdTokenParsers {
  type Tokens = StdLexical
  val lexical = new Tokens

  lexical.delimiters ++= List("(", ")", "+", "-", "*", "/")

  /**                                                                           
   * For conveniences(to eliminate the left recursion), we will transform the grammar as follows,                
   *   <exp>  ::= <term> { ("+" | "-" ) <term> }                                      
   *   <term> ::= <factor> { ("*" | "/" ) <factor> }                                      
   *   <factor> ::= <n> | "(" <exp> ")"                                                 
   */
  def exp:Parser[Atom] = term * ("+" ^^^ {Add(_, _)} | "-" ^^^ {Sub(_, _)})

  def term:Parser[Atom] = factor * ("*" ^^^ {Mul(_, _)} | "/" ^^^ {Div(_, _)})

  def factor:Parser[Atom] = numericLit ^^ {n => Var(n.toInt)} |
                          "(" ~> exp <~ ")"

  def main(arg:Array[String])={

    val a = phrase(exp)(new lexical.Scanner(arg(0))) match {
      case Success(result, _) => result
      case f@Failure(msg, _) => throw new Exception(f.toString)
      case e@Error(msg, _) => throw new Exception(e.toString)
    }
    println(a)

    import Calc2.{compile, eval}
    val code = compile(a)
    println(code)

    val result = eval(code)
    println(result)
  }
}