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From: Joel Reymont <joelr1@gmail.com>
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Subject: [Caml-list] Re: module typing issue

It appears that I spoke to early as I'm still having trouble with the implementation of my concept. 

What is it?

I'm working with ZeroMQ and Thrift. The latter generates messages using OCaml classes. 

I have request and reply messages in my system and replies are created (dispatched) based on their request pair.

A server should not have to know what messages it's dealing with.

I would like to wrap each message class in a variant, particularly helpful since some message classes are unions in the original Thrift DSL but the Thrift OCaml code generator implements them poorly.

Is my intent clear in the code below and if so how do I make it work?

	Thanks, Joel

--- error

File "x.ml", line 56, characters 26-35:
Error: Signature mismatch:
       Modules do not match:
         sig
           type obj = MyRequest.obj
           type variant = MyRequest.variant = A
           val make : unit -> int
           val convert : 'a -> variant
           val print : 'a -> unit
         end
       is not included in
         Request
       Values do not match:
         val make : unit -> int
       is not included in
         val make : unit -> obj

--- x.ml

module type Message = 
sig 
  type obj 
  type variant
  val make : unit -> obj
  (* encapsulate object in a variant type *)
  val convert : obj -> variant
  val print : obj -> unit
end

module type Request = 
sig 
  include Message
end

(* dispatches based on a request *)

module type Reply = functor (Req : Request) ->
sig 
  include Message
  val dispatch : Req.variant -> obj
end

module Server (Req : Request) (REP : Reply) =
struct 
  module Rep = REP(Req)
  let server = 
    while true do
      let req = Req.make () in
      let var = Req.convert req in
      let rep = Rep.dispatch var in
      Req.print req;
      Rep.print rep
    done
end

module MyRequest =
struct
  type obj = private int
  type variant = A
  let make () = 1
  let convert o = A
  let print o = ()
end

module MyReply = functor (Req: Request) ->
struct
  type obj = private int
  type variant = B
  let make () = 2
  let convert o = B
  let print o = ()
  let dispatch _ = B
end

module MyServer = Server (MyRequest) (MyReply)

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