Sample Parser Combinator shown at Scala Lift Off London 2011 #scalalol

gistfile1.txt

import scala.util.parsing.combinator._

trait RunParser {
  this: RegexParsers =>
    type RootType
  def root: Parser[RootType]
  def run(in: String): ParseResult[RootType] = parseAll(root, in) 
}

val vars = Map("a" -> 4d, "pi" -> 3.1415d)


object Calc extends JavaTokenParsers with RunParser {

  lazy val sumExpr = prodExpr ~
  rep("+" ~> prodExpr ^^ (d => (x: Double) => x + d) |
      "-" ~> prodExpr ^^ (d => (x: Double) => x - d)) ^^ {
        case seed ~ fs => fs.foldLeft(seed)((a, f) => f(a))
      }

  lazy val prodExpr = factor ~
  rep("*" ~> factor ^^ (d => (x: Double) => x * d) |
      "/" ~> factor ^^ (d => (x: Double) => x / d)) ^^ {
        case seed ~ fs => fs.foldLeft(seed)((a, f) => f(a))
      }

  lazy val letters: Parser[String] = "[a-zA-Z]+".r

  lazy val variable: Parser[Double] = "$" ~> letters ^^ ((chars: String) => vars(chars))

  lazy val factor: Parser[Double] =
    variable |
    floatingPointNumber ^^ (_.toDouble) | "(" ~> sumExpr <~ ")"
  type RootType = Double
  def root = sumExpr 
}

println(Calc.run("1 + 5 * $pi"))