Exemplary ONP implementation in Scala

onp.scala

import scala.collection.mutable.Stack

val equation = "12 2 3 4 * 10 5 / + * +"
val ops = equation.split(" ")

val stack = new Stack[Int]()

def *(a: Int, b: Int) = a * b
def /(a: Int, b: Int) = a / b
def +(a: Int, b: Int) = a + b

val operators: Set[String] = Set("*", "/", "+")

def toOp: PartialFunction[String, (Int, Int) => Int] = {
  case "*" => *
  case "/" => /
  case "+" => +
}

ops.foreach { i =>
  if(operators.contains(i)) {
    val a      = stack.pop()
    val b      = stack.pop()
    val result = toOp(i)(b, a)
    stack.push(result)
  } else {
    stack.push(i.toInt)
  }
}

println("result: " + stack.head)