class ReversePolishCalculator extends JavaTokenParsers with Maths {
  def expr:   Parser[Float] = rep(term ~ operator) ^^ {
    case terms =>
      var stack  = List.empty[Float]
      var lastOp: (Float, Float) => Float = add
      terms.foreach(t =>
        t match {
          case nums ~ op => lastOp = op; stack = reduce(stack ++ nums, op)
        }
      )
      stack.reduceRight((x, y) => lastOp(y, x))
  }
  def term: Parser[List[Float]] = rep(factor)
  def factor: Parser[Float] = num | "(" ~> expr <~ ")" ^^ (_.toFloat)
  def num: Parser[Float] = floatingPointNumber ^^ (_.toFloat)
  def operator: Parser[(Float, Float) => Float] = ("*" | "/" | "+" | "-") ^^ {
    case "+" => add
    case "-" => sub
    case "*" => mul
    case "/" => div
  }
  def reduce(nums: List[Float], op: (Float, Float) => Float): List[Float] = {
    val result = nums.reverse match {
      case x :: y :: xs => xs ++ List(op(y, x))
      case List(x)      => List(x)
      case _            => List.empty[Float]
    }
    result
  }
}