FunctionalCalculator

package com.twitter.univ.calculator

object Operator {
  val all: Map[String, (Int, Int) => Int] = 
    Map("+" -> (_ + _),
        "-" -> (_ - _),
        "*" -> (_ * _),
        "/" -> (_ / _))
  def unapply(str: String) = all.get(str)
}

object FunctionalCalcApp extends App {
  val result = 
    args.foldLeft(Nil: List[Int]) { 
      case (_2 :: _1 :: tail, Operator(op)) => op(_1, _2) :: tail 
      case (_, Operator(op)) => throw new IllegalArgumentException // not enough operands
      case (opStack, num) => num.toInt :: opStack // NumberFormatException for invalid input
    }

  println(result.head) // not enough operators if result.length > 1
}