PostOrderCalc.scala

import cats.data.State
import cats.syntax.applicative._ // for pure

object PostOrderCalc {
  type CalcState[A] = State[List[Int], A]

  def evalInput(str: String): Int =
    evalAll(str.split(" ").toList).runA(Nil).value
  
  def evalAll(input: List[String]): CalcState[Int] = 
    input.foldLeft(0.pure[CalcState]) { (acc, str) =>
      acc.flatMap(_ => evalOne(str))
    }

  def evalOne(sym: String): CalcState[Int] = 
    State {
      sym match {
        case "+" => operator(_ + _)
        case "_" => operator(_ - _)
        case "*" => operator(_ * _)
        case "/" => operator(_ / _)
        case _ => stack =>
          val int = sym.toInt
          (int :: stack, int)
      }
    }
  
  private def operator(op: (Int, Int) => Int): List[Int] => (List[Int], Int) = {
    case a :: b :: rest =>
      val res = op(a, b)
      (res :: rest, res)
    case _ =>
      sys.error("Fail!")
  }
}