very simple repl for math expressions(no variables)

math_repl.scala

package parsing

import util.parsing.combinator.RegexParsers

/**
 * User: andraz
 * Date: 8/25/12
 * Time: 4:05 PM
 */
object ExpressionParsing {

  object Tokens {

    sealed trait Expression

    case class Number(n: Double) extends Expression

    case class Add(left: Expression, right: Expression) extends Expression

    case class Subtract(left: Expression, right: Expression) extends Expression

    case class Multiply(left: Expression, right: Expression) extends Expression

    case class Divide(left: Expression, right: Expression) extends Expression

  }

  object Parser extends RegexParsers {

    import Tokens._

    private def number: Parser[Expression] = """\d+\.?\d*""".r ^^ {
      s => Number(s.toDouble)
    }

    private def factor: Parser[Expression] = number | ("(" ~> expr <~ ")")

    private def term: Parser[Expression] = factor ~ rep(("*" | "/") ~ factor) ^^ {
      case head ~ tail => {
        var tree: Expression = head
        tail.foreach {
          case "*" ~ e => tree = Multiply(tree, e)
          case "/" ~ e => tree = Divide(tree, e)
        }
        tree
      }
    }

    private def expr: Parser[Expression] = term ~ rep(("+" | "-") ~ term) ^^ {
      case head ~ tail => {
        var tree: Expression = head
        tail.foreach {
          case "+" ~ e => tree = Add(tree, e)
          case "-" ~ e => tree = Subtract(tree, e)
        }
        tree
      }
    }

    def apply(s: String) = parseAll(expr, s) match {
      case Success(tree, _) => Some(tree)
      case e: NoSuccess => None
    }
  }

  object Evaluator {

    import Tokens._

    def apply(e: Expression): Double = e match {
      case Number(n) => n
      case Add(l, r) => apply(l) + apply(r)
      case Subtract(l, r) => apply(l) - apply(r)
      case Multiply(l, r) => apply(l) * apply(r)
      case Divide(l, r) => apply(l) / apply(r)
    }
  }

  def apply(s: String) = Parser(s) map Evaluator.apply

  def repl = {
    def rep: Unit = {
      print(">> ")
      readLine match {
        case "exit" => return
        case "help" => println("enter an expression\nsupported: floats + - * / ( )")
        case exp => {
          println(
            apply(exp) match {
              case Some(res) => res
              case None => "invalid syntax"
            })
          rep
        }
      }
    }
    rep
  }
}