(*
Copyright (c) 2013, Simon Cruanes
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(** {1 Universal type} *)

(** This is largely inspired by https://ocaml.janestreet.com/?q=node/18 . *)

type t = { 
  mutable id : unit ref;
  mutable store : unit -> unit;
} (** The universal type *)

type 'a embedding = {
  pack : 'a -> t;           (** Pack a 'a into a univ value *)
  unpack : t -> 'a option;  (** Try to unpack the univ value into an 'a *)
  set : t -> 'a -> unit;    (** Change, in-place, the content of the univ value *)
  compatible : t -> bool;   (** Check whether the univ value can be unpacked *)
} (** Conversion between the universal type and 'a *)

(** Create a new embedding. Values packed by a given embedding can
    only be unpacked by the same embedding. *)
let embed () = 
  let id = ref () in  (* unique ID of the embedding *)
  let r = ref None in (* place to store values *)
  let pack a =        (* pack the 'a value into a new univ cell *)
    let o = Some a in
    { id = id; store = (fun () -> r := o); }
  in
  let unpack t =      (* try to extract the content of a univ cell *)
    if id == t.id then begin
      t.store ();
      let a = !r in
      r := None;
      a
    end else None
  in
  let set t a =       (* change, in place, the embedding and content of the cell *)
    t.id <- id;
    let o = Some a in
    t.store <- (fun () -> r := o)
  in
  let compatible t =  (* check whether the univ cell is from this embedding *)
    id == t.id
  in
  { pack; unpack; compatible; set; }

let pack emb x = emb.pack x

let unpack emb t = emb.unpack t

let compatible emb t = emb.compatible t

let set emb t x = emb.set t x