First_class_module_and_identity.ml

type _ data_cstr = ..

module type t = sig
  type a
  type _ data_cstr += Fmt: a data_cstr
end

type 'a data_format = (module t with type a = 'a)

let mk_data_format : type a. unit -> a data_format = fun () ->
  (module struct type nonrec a = a type _ data_cstr += Fmt : a data_cstr end)

let data_format_iseq : type a b. a data_format -> b data_format -> (a, b) eq option =
  fun (module M) (module N) ->
  match M.Fmt, N.Fmt with
  | M.Fmt, M.Fmt -> Some Eq
  | _ -> None