(* Copyright (C) 2017--2024  Petter A. Urkedal <paurkedal@gmail.com>
 *
 * This library is free software; you can redistribute it and/or modify it
 * under the terms of the GNU Lesser General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or (at your
 * option) any later version, with the LGPL-3.0 Linking Exception.
 *
 * This library is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
 * License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public License
 * and the LGPL-3.0 Linking Exception along with this library.  If not, see
 * <http://www.gnu.org/licenses/> and <https://spdx.org>, respectively.
 *)

(** Multiplicities of zero, one, and many. *)

type +'m t = (* not GADT due to variance *)
 | Zero
 | One
 | Zero_or_one
 | Zero_or_more
constraint 'm = [< `Zero | `One | `Many]

type zero = [`Zero]
type one = [`One]
type zero_or_one = [`Zero | `One]
type zero_or_more = [`Zero | `One | `Many]

let zero : [> `Zero] t = Zero
let one : [> `One] t = One
let zero_or_one : [> `Zero | `One] t = Zero_or_one
let zero_or_more : ([> `Zero | `One | `Many] as 'a) t = Zero_or_more

let only_zero : [< `Zero] t -> unit =
  function Zero -> () | _ -> assert false
let only_one : [< `One] t -> unit =
  function One -> () | _ -> assert false
let only_zero_or_one : [< `Zero | `One] t -> unit =
  function Zero | One -> () | _ -> assert false

let expose = function
 | Zero -> `Zero
 | One -> `One
 | Zero_or_one -> `Zero_or_one
 | Zero_or_more -> `Zero_or_more

let can_be_zero = function
 | One -> false
 | Zero | Zero_or_one | Zero_or_more -> true

let can_be_many = function
 | Zero | One | Zero_or_one -> false
 | Zero_or_more -> true