type Result =
    | Result of int
    | Error

let range modulo x : Result =
    match x with
    | a when a < 0 -> Error
    | a when a >= modulo -> Error
    | a -> Result a

let rec join1 modulo (x:Result) rest : Result =
    match x with
    | Error -> Error
    | Result a ->
        match rest with
        | [] -> x
        | y::xy -> match range modulo y with
                   | Error -> Error
                   | Result b -> join1 modulo (Result (a * modulo + b)) xy

let join modulo list : Result =
    match modulo with
    | m when m < 2 -> Error
    | m -> match list with
           | [] -> Result 0
           | [x] -> (range m x)
           | x::xs -> join1 modulo (range m x) xs

let expect a b =
    printf "Value '%A', expected '%A' => %A\n" a b (a = b)

[<EntryPoint>]
let main argv =
    join 10 [] |>expect<| Result 0
    join 10 [1] |>expect<| Result 1
    join 10 [1; 2] |>expect<| Result 12
    join 10 [1; 2; 3] |>expect<| Result 123
    join 10 [1; 2; 3; 4] |>expect<| Result 1234
    join 10 [-1; 2; 3; 4] |>expect<| Error
    join 10 [1; 2; 30; 4] |>expect<| Error
    0