StackLanguage.scala

import scala.collection.immutable.List

object StackLanguage extends App {

  var program = List(Value(1), Value(2), Value(3), Operator(_ + _), Operator(_ - _), UnaryOperator(Math.sqrt(_).toInt))

  println("Tim & Matt lovin the Scalay goodness")

  def evaluate_stack(stack: List[Element]) = {
    stack match {
      case (head :Operator) :: tail => {
        head.eval(tail)
      }
      case (head :UnaryOperator) :: tail => {
        head.eval(tail)
      }
      case list => {
        list
      }
    }
  }

  def evaluate_program(program: List[Element]): List[Element] = {
      program.foldLeft(List[Element]())((stack, element) => {
        evaluate_stack(element :: stack)
      })

  }

  sealed trait Element {
  }

  case class Operator(func: (Int, Int) => Int) extends Element {

    def eval(stack : List[Element]) : List[Element] = stack match {
      case (a: Value) :: (b : Value) :: tail => {
        Value(func(a.n,b.n)) :: tail
      }
    }
  }

  case class UnaryOperator(func: Int => Int) extends Element {

    def eval(stack : List[Element]) : List[Element] = stack match {
      case (a: Value) :: tail => {
        Value(func(a.n)) :: tail
      }
    }
  }

  case class Value(n: Int) extends Element {
  }
  print(evaluate_program(program))
}