no need for objects

option_parse_gadt.ml

type 'a option_parser = string list -> 'a list
type 'a handler = 'a list -> unit

type subcommand = 
  | Subcommand : 'a option_parser * 'a handler -> subcommand

let handle_run options =
  let refresh = ref false in
  let wrapper = ref None in
  ListLabels.iter options ~f:(function
    | `Refresh -> refresh := true
    | `Wrapper w -> wrapper := Some w
  ); print_endline "handle_run..."

let handle_download options =
  let refresh = ref false in
  let show = ref false in
  ListLabels.iter options ~f:(function
    | `Refresh -> refresh := true
    | `Show -> show := true
  ); print_endline "handle_download..."

let parse_run : [> `Refresh | `Wrapper of int] option_parser = 
  fun _ -> [`Refresh; `Wrapper 5]

let parse_download : [> `Refresh | `Show] option_parser =
  fun _ -> [`Refresh; `Show]

let subcommands = [
  ("run", Subcommand (parse_run, handle_run));
  ("download", Subcommand (parse_download, handle_download));
]

let (|>) f x = x f (* in case you're not on 4.01 yet *)

let run_command args (Subcommand (parse, handler)) =
  args |> parse |> handler

let () = subcommands
  |> List.map snd
  |> List.iter (run_command ["test";"me"])