univ.ml

module Univ : sig
  type t
  val embed : unit -> ('a -> t) * (t -> 'a option)
end = struct
  type t = unit ref * (unit -> unit)
  let embed () =
    let id = ref () in
    let loc = ref None in
    let put x = id, (fun () -> loc := Some x) in
    let get (rid, rstore) =
      if id != rid then None else begin
        rstore ();
        let r = !loc in
        loc := None; r
      end in
    put, get
end