// postfix evaluation
open System

// without error handling:

printf "Enter Postfix Expression: "
let raw_input = System.Console.ReadLine();

let tokens = Array.toList (raw_input.Split()); // get list of tokens

let rec eval1(stack,input) =
  match (stack, input) with
    | ([x], []) -> x
    | (a::b::c, "+"::s) -> eval1 ((b+a)::c, s)
    | (a::b::c, "*"::s) -> eval1 ((b*a)::c, s)
    | (a::b::c, "-"::s) -> eval1 ((b-a)::c, s)
    | (a::b::c, "/"::s) -> eval1 ((b/a)::c, s)
    | (_, x::s) -> eval1 ((int x)::stack, s)
    | _ -> raise(Exception(sprintf "ERROR, stack=%A, input=%A" stack input));;

//printfn "Result = %d" (eval1([],tokens))

let parse_int x =
  try Ok(int(x)) with | _ -> Error(sprintf "invalid sym %s" x);;

// with error handling using the Result monad

let rec eval(stack,input) =
  match (stack, input) with
    | ([Ok(x)], []) -> Ok(x)
    | (Ok(a)::Ok(b)::c, "+"::s) -> eval (Ok(b+a)::c, s)
    | (Ok(a)::Ok(b)::c, "-"::s) -> eval (Ok(b-a)::c, s)
    | (Ok(a)::Ok(b)::c, "*"::s) -> eval (Ok(b*a)::c, s)
    | (Ok(0)::Ok(b)::c, "/"::s) -> eval (Error("div by 0")::c, s)
    | (Ok(a)::Ok(b)::c, "/"::s) -> eval (Ok(b/a)::c, s)
//    | (Error(_)::rest, s) -> eval (rest,s)  // skip errors
    | (_, x::s) -> eval ((parse_int x)::stack, s)
    | (s,i) -> Error(sprintf "stack=%A" s);;

let result = eval([],tokens)

//open Result;

let rmap = Result.map
let rbind = Result.bind
let ifelse okfun errfun result =
  match result with
    | Ok(x) -> okfun x
    | Error(e) -> errfun e

result |>
  ifelse (fun x -> printfn "result = %d" x)
         (fun e -> printfn "failure: %s" e)