first_plugin.ml

open Containers
open Intf

type state_class +=
  | Post_hello

let rec hello_world_transform ic oc state =
  if List.mem ~eq:(Equal.physical) Post_hello state then
    None
  else
    Lwt_io.write_line "hello";
    Some Post_hello

let () =
  transforms := hello_world_transform :: transforms

intf.ml

type state_class = ..

type state_data = state_class list

type transform = (Lwt_io.input_channel -> Lwt_io.output_channel -> state_data -> state_class option)

let transforms : (transform list) ref = ref []

jbuild

(jbuild_version 1)

(executable
 ((name loader)
  (libraries (containers sqlite3 lwt lwt.unix dynlink))))

(library
 ((name first_plugin)
  (libraries (containers lwt lwt.unix))))

loader.ml

open Container
open Lwt
open Intf

exception No_plugins_loaded

let () =
  let open Dynlink in
  Sys.readdir extdir
  |> Array.filter Sys.is_directory
  |> Array.iter Dynlink.loadfile;
  if List.length !transforms = 0 then raise No_plugins_loaded else ()

(* …cut… *)