pt.ml

(** ---------- ACL phantom tags (no runtime values) ---------- *)
type readonly      (* no constructors – a compile-time marker *)
type readwrite     (* idem *)

(** ---------- Generic driver ---------- *)
type _ driver = { mutable value : int }   (* 'acl is phantom *)

(** Universally available *)
let read (d : _ driver) = d.value

(** Only for read-write drivers *)
let write (d : readwrite driver) v = d.value <- v

(** Constructors (factories) *)
let new_readonly  () : readonly  driver = { value = 15 }
let new_readwrite () : readwrite driver = { value = 15 }

let read_more (d : _ driver) = d.value + 10

(* ---------------- Demo ---------------- *)
let () =
  let drw = new_readwrite () in
  Printf.printf "%d\n" (read drw);   (* 15 *)
  write drw 10;
  Printf.printf "%d\n" (read drw);   (* 10 *)

  let dro = new_readonly () in
  Printf.printf "%d\n" (read dro);   (* 15 *)
  (* write dro 10 *)                  (* - : compile-time error *)

  Printf.printf "%d\n" (read_more dro); (* 25 *)
  Printf.printf "%d\n" (read_more drw); (* 20 *)