Error handling with Macaque (Ocsigen)

gistfile1.ml

module Lwt_thread = struct
  include Lwt
  include Lwt_chan
end
module Lwt_PGOCaml = PGOCaml_generic.Make Lwt_thread
module Lwt_Query = Query.Make_with_Db Lwt_thread Lwt_PGOCaml

let get_db : unit -> unit Lwt_PGOCaml.t Lwt.t =
  let db_handler = ref None in
  fun () ->
    match !db_handler with
      | Some h -> Lwt.return h
      | None -> Lwt_PGOCaml.connect ~user:login
        ~password:password ~database:dbname ()

(** Return the result of a macaque query *)
let query q =
  lwt dbh = get_db () in
  Lwt_Query.query dbh ~log:stderr q

let create_user =
[...]
    try (ignore (query q); Success ())
    with _ -> Failure [...]