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From: Joel Reymont <joelr1@gmail.com>
In-Reply-To: <63968844-AD95-4359-80F0-E9E071CF6518@mpi-sws.org>
Date: Fri, 25 Mar 2011 15:57:53 +0000
Cc: caml-list <caml-list@inria.fr>
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References: <89643D7F-B0F9-4452-BBAF-0445D9D3CA4E@gmail.com> <C2959F12-C0AA-49F2-931F-3BEAD90493A9@gmail.com> <639CCD2C-402F-4754-B942-B460FD908A95@gmail.com> <63968844-AD95-4359-80F0-E9E071CF6518@mpi-sws.org>
To: Andreas Rossberg <rossberg@mpi-sws.org>
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Subject: Re: [Caml-list] Re: module typing issue


On Mar 25, 2011, at 3:46 PM, Andreas Rossberg wrote:

> I don't think it ever makes sense to define private type abbreviations in a structure - they are intended for use in signatures. Just drop the private from your example.

This compiles but fails as soon as I try to replace dispatch in MyReply (bottom) with 

  let dispatch = function | A -> 1

This is obvious conceptually as MyReply doesn't know about the variant type defined in MyRequest. 

How do I make let MyReply know about it?

	Thanks, Joel

--- 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 : Request =
struct
 type obj = int
 type variant = A
 let make () = 1
 let convert o = A
 let print o = ()
end

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

module MyServer = Server (MyRequest) (MyReply)


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