open System

let doAdd (stack : double list) : double list =
    stack.Tail.Head + stack.Head :: stack.Tail.Tail

let doSub (stack : double list) : double list =
    stack.Tail.Head - stack.Head :: stack.Tail.Tail

let doDiv (stack : double list) : double list =
    stack.Tail.Head / stack.Head :: stack.Tail.Tail

let doMul (stack : double list) : double list =
    stack.Tail.Head * stack.Head :: stack.Tail.Tail


let rec parseAndCalcCore (tokens : string list) (stack : double list) : double =
    match tokens with
        | [] -> stack.Head
        | h::t -> match h with
            | "+" -> doAdd stack |> parseAndCalcCore t
            | "-" -> doSub stack |> parseAndCalcCore t
            | "/" -> doDiv stack |> parseAndCalcCore t
            | "*" -> doMul stack |> parseAndCalcCore t
            | _ -> ((h |> double) :: stack) |> parseAndCalcCore t


let parseAndCalc (expr : string) : string =
    let tokens = (expr.Split ' ' |> Array.toList)
    parseAndCalcCore tokens [] |> string

[<EntryPoint>]
let main argv =
    Console.WriteLine (parseAndCalc "1 1 +")
    Console.WriteLine (parseAndCalc "2 -3 *")
    Console.WriteLine (parseAndCalc "1 1 + 2 3 * -")
    Console.WriteLine (parseAndCalc "1 3 / 2 *")
    Console.WriteLine (parseAndCalc "10 1 2 + 2 + 5 + +")
    0