type (_,_) query =
  | Exec  : ('req, unit, [`Zero]) Caqti_request.t -> ('req, unit) query
  | Find  : ('req, 'resp, [`One | `Zero]) Caqti_request.t -> ('req, 'resp option) query
  | List  : ('req, 'resp, [`Many | `One | `Zero]) Caqti_request.t -> ('req, 'resp list) query
  | Trans : ('req, 'resp) query * 'acc * ('acc -> 'resp -> 'acc) ->
            ('req list, 'acc) query

let rec request (type req resp) (module Db : Caqti_lwt.CONNECTION) (q : (req, resp) query) (args : req) : resp Lwt.t =
    match q with
    | Exec q -> Db.exec q args >>= fail_if
    | Find q -> Db.find_opt q args >>= fail_if
    | List q -> Db.rev_collect_list q args >>= fail_if
    | Trans (ql, acc, f) ->
       Db.start () >>= fail_if >>= fun () ->
       List.fold_left (fun acc arg ->
           acc >>= function
           | Error _ as e -> Lwt.return e
           | Ok v -> request (module Db : Caqti_lwt.CONNECTION) ql arg >>= (fun r -> Lwt.return_ok (f v r)))
         (Lwt.return_ok acc) args
       >>= fail_if >>= Db.commit >>= fail_if