a simple program that evaluates a string and returns a result

implicit_classes.scala

package com.wausoft.main

import scala.collection.mutable.Stack

trait ImplicitClasses {
  implicit class RichStack[A](stack: Stack[A]) { 
    def operate(op: (A, A) => A): Unit = {
      val (b, a) = (stack.pop, stack.pop)
      stack push op(a,b)
    }
  }
}

postfix_parser.scala

package com.wausoft.main

import scala.collection.mutable.Stack

object Parser extends ImplicitClasses{
  def eval(str: String): Double = {
    val s = str.toLowerCase split ' '
    val stack = new Stack[Double]
    s.foreach(d => parse(d, stack))
    
    return if(stack.size > 0) stack.pop else 0
  }
  
  def parse(input: String, stack: Stack[Double]): Unit = input match {
    case "+" => stack.operate(_+_)
    case "-" => stack.operate(_-_)
    case "*" => stack.operate(_*_)
    case "/" => stack.operate(_/_)
    case "sqrt" => 
      val r = Math.sqrt(stack.pop)
      stack push r
    case n => stack push n.toDouble
  }
}

simple_postfix_calculator.scala

package com.wausoft.main

object Program {
  // testing it out
  def main(args: Array[String]): Unit = {
   val r = Parser eval "5 1 2 + 4 * + 3 -"
   // 5
   // 5 1
   // 5 1 2
   // 5 1 2 + => 5 3
   // 5 3 4
   // 5 3 4 * => 5 12
   // 5 12 + => 17
   // 17 3
   // 17 3 - => 14
   
   println(r) // prints 14.0
   println(Parser eval "2 sqrt") // prints 1.4142135623730951
  }
}