extensible_functions.ml

type msg = ..

type query_answer =
| Accepted
| Unknown

let printer_registry: (Format.formatter -> msg -> query_answer) list ref = ref []
let register_printer x = printer_registry := x :: !printer_registry

let print ppf msg =
  let rec find msg = function
  | [] -> Format.fprintf ppf "[unknown message]"
  | p :: q -> match p ppf msg with
    | Accepted -> ()
    | Unknown -> find msg q
  in
  find msg !printer_registry

 type msg += Int of int
 type msg += Float of float

 let int_printer ppf = function
 | Int x -> Format.fprintf ppf "[%d]" x; Accepted
 | _ -> Unknown

 let float_printer ppf = function
 | Float x -> Format.fprintf ppf "[%g]" x; Accepted
 | _ -> Unknown


 let () = List.iter register_printer [int_printer; float_printer]

 let () =
   let newline ppf _  = Format.pp_print_cut ppf () in
   Format.printf "@[<v>%a@]@."
   (Format.pp_print_list ~pp_sep:newline print) [Int 1; Float 2.5]