F# RPN calculator

rpn.fs

type Atom =
  | Num of int
  | Op of char

let opToProc = function
  | '+' -> (+)
  | '*' -> (*)
  | _ -> failwith "invalid operation"

let rec evalRPN (expr : Atom list) (stack : int list) =
  match expr with
  | [] -> stack
  | Num a :: rst -> evalRPN rst (a :: stack)
  | Op op :: rst ->
    let rhs = stack.Head
    let lhs = stack.Tail.Head
    let result = ((opToProc op) lhs rhs)
    evalRPN rst (result :: stack.Tail.Tail)

let main =
  let r = evalRPN [Num 3; Num 2; Op '+'; Num 2; Op '*'; Num 2; Op '*'] []
  printf "result: %d" (r.Head)
  System.Console.ReadKey true |> ignore