gistfile1.ml

module type Subscriber = sig
  module Message : sig
    type t = ..
  end

  type msgs

  val filter_message : msgs -> Message.t option
  val callback : Message.t -> msgs list
end

module type Bus = sig
  include Subscriber
  val components : (module Subscriber) list
end

module Add (A : Subscriber) (B : Subscriber) : Bus with module Message = (struct (* Error: syntax error *)
      type t = ..
      type t += A.Message.t
      type t += B.Message.t
end) =
struct
  let components = (module A) :: [ (module B) ]
  let filter_message = ident
  let callback msg = A.callback (A.filter_message msg) @ B.callback (B.filter_message msg)
end