Functional solution for ReversePolishNotation

ReversePolishNotation.scala

object ReversePolishNotation {
  type Operand = Double
  type BinaryOp = (Operand, Operand) => Operand

  val parseOp: String => Double = _.toDouble

  val opMap: Map[String, BinaryOp] = Map(
    "+" -> (_ + _),
    "-" -> (_ - _),
    "*" -> (_ * _),
    "/" -> (_ / _)
  )

  val parseElem: String => Any = s => opMap.getOrElse(s, parseOp(s))

  val parse: String => Seq[Any] = _.split(" ").map(parseElem).toList

  val eval: Seq[Any] => Double = {
    case (x: Operand) :: Nil => x
    case (o1: Operand) :: (o2: Operand) :: (op: BinaryOp) :: xs => eval(op(o1, o2) +: xs)
  }

  def eval(s: String): Double = eval(parse(s))
}