proposal for https://github.com/ocsigen/lwt/issues/919

lwt.ml

(* just some type to work in one file *)
module Lwt = struct
  type 'a t = 'a ref
end
type process_in = int


type ('handler, 'return) onTimeout =
  | Catch: float -> (exn -> process_in -> 'return Lwt.t, 'return) onTimeout
  | Fail: float -> (process_in -> 'return Lwt.t, 'return) onTimeout
  | NoTimeout: (process_in -> 'return Lwt.t, 'return) onTimeout (* for the default value *)
                                                                
                                                                
let with_process_in (type handler return) ?(timeout: (handler, return) onTimeout option) (f: handler): return Lwt.t =
  let process_in = 13 in
  match timeout with
    Some (Catch t) ->
      Printf.printf "Sleep %f\n" t;
      f (Invalid_argument "timeout") process_in
  | None -> assert false (* to be typed *)
  | Some (Fail t) -> 
      Printf.printf "Sleep %f\n" t;
      if true then
        f process_in
      else
        raise (Invalid_argument "timeout")
  | Some NoTimeout -> f process_in