S式風前置四則演算.scala

import scala.util.parsing.combinator._

abstract class AST
case class Add(left: AST, right: AST) extends AST
case class Sub(left: AST, right: AST) extends AST
case class Multi(left: AST, right: AST) extends AST
case class Div(left: AST, right: AST) extends AST
case class Num(num: Int) extends AST

class OperationParsers extends RegexParsers {
  def expr: Parser[AST] = "(" ~> operator ~ (expr | num) ~ (expr | num)  <~ ")" ^^ { 
    case o ~ l ~ r => o match {
      case "+" => Add(l, r)
      case "-" => Sub(l, r)
      case "*" => Multi(l, r)
      case "/" => Div(l, r)
    }
  }
  def operator = "+" | "-" | "*" | "/"
  def num = """-?[0-9]+""".r ^^ {case n => Num(n.toInt)}
  
  def eval(ast: AST): Int = {
    ast match {
      case Num(n) => n
      case Add(l, r) => eval(l) + eval(r)
      case Sub(l, r) => eval(l) - eval(r)
      case Multi(l, r) => eval(l) * eval(r)
      case Div(l, r) => eval(l) / eval(r)
    }
  }
  
  def parser(str: String) = parseAll(expr, str) match {
    case Success(v, _) => println(eval(v))
    case Failure(msg, _) => println(msg)
    case Error(msg, _) => println(msg)
  }
}