Dear Serge,

If I understand correctly what you are asking, then you can make it work by
wrapping the definition of topicf in an existential:

  module R = struct
    type topicf = TF : ((module I with type message = 'a) -> 'a -> 'a) ->
topicf
    and topic = { func: topicf; name: string }
    ...
  end

then, given test, you can create an R.topic by doing

  { R.func = TF test; name = "test" }

Best wishes,
Nicolás


On Thu, Nov 23, 2017 at 1:52 PM, Serge Sivkov <ssp.mryau@gmail.com> wrote:

> Hello,
>
> is there way to define module with similar interface:
>
> module R = struct
>         type topicf = (module F : I) -> F.message -> F.message
>         and topic = { func: topicf; name: string }
>         ...
> end
>
> it is possible in case I want to define function with similar signature I
> can declare used types by:
> let test (type message') (module M : I with type message = message') (v:
> message') : message' = ...
>
> But how should I create such type in module?
>
> WBR, ssp
>