parser skeleton for dice notation

module randall.Parser

open System

type Token = AddT | SubT | MulT | DivT | ExpT | DT | GT | LT | OpT | CpT | NumT of int

let rec lex s =
    if String.length s = 0 then [] else
    match s.[0] with
    | '+' -> AddT :: lex s.[1..]
    | '-' -> SubT :: lex s.[1..]
    | '*' -> MulT :: lex s.[1..]
    | '/' -> DivT :: lex s.[1..]
    | '^' -> ExpT :: lex s.[1..]
    | 'd' -> DT :: lex s.[1..]
    | 'g' -> GT :: lex s.[1..]
    | 'l' -> LT :: lex s.[1..]
    | '(' -> OpT :: lex s.[1..]
    | ')' -> CpT :: lex s.[1..]
    | d when d > '/' && d < ':' -> tokenize_num s
    | c when c < '!' -> lex s.[1..]
    | c -> failwith ("Illegal character: " + Char.ToString c)
and tokenize_num s =
    let rec select (s : string) n len =
        if len = 0 then n else
        if s.[0] > '/' && s.[0] < ':' then
            select s.[1..] (n + 1) (len - 1)
        else n
    in
    let fin = select s 0 (String.length s) in
    NumT (int s.[..fin - 1]) :: lex s.[fin..]

type Rpg0  = Single of Rpg1 * Rpg01
           | Debug
and Rpg01  = Add of Rpg1 * Rpg01
           | Sub of Rpg1 * Rpg01
           | Nil
           | Debug
and Rpg1   = Single of Rpg2 * Rpg11
           | Debug
and Rpg11  = Mul of Rpg2 * Rpg11
           | Div of Rpg2 * Rpg11
           | Nil
           | Debug
and Rpg2   = Single of All * Rpg21
           | Debug
and Rpg21  = Exp of All * Rpg21
           | Nil
           | Debug
and All    = A of Loop
           | B of Loop * Loop
           | Next of G_Dice
           | Debug
and G_Dice = A of Loop
           | B of Loop * Loop
           | C of Loop * Loop
           | D of Loop * Loop * Loop
           | Next of L_Dice
           | Debug
and L_Dice = A of Loop
           | B of Loop * Loop
           | C of Loop * Loop
           | D of Loop * Loop * Loop
           | Next of Loop
           | Debug
and Loop   = Num of int
           | Closed of Rpg0
           | Debug

let rpg0 (t : Token list ref) : Rpg0 = Rpg0.Debug
let rpg01 (t : Token list ref) : Rpg01 = Rpg01.Debug
let rpg1 (t : Token list ref) : Rpg1 = Rpg1.Debug
let rpg11 (t : Token list ref) : Rpg11 = Rpg11.Debug
let rpg2 (t : Token list ref) : Rpg2 = Rpg2.Debug
let rpg21 (t : Token list ref) : Rpg21 = Rpg21.Debug
let all (t : Token list ref) : All = All.Debug
let g_dice (t : Token list ref) : G_Dice = G_Dice.Debug
let l_dice (t : Token list ref) : L_Dice = L_Dice.Debug
let loop (t : Token list ref) : Loop = Loop.Debug