Hi,

This is not exactly what you asked, but you can use GADTs to constrain the
phantom type (_, _) socket:

type client
type server

type (_, _) socket =
  | ClientServer : mysocket -> (client, server) socket
  | ServerClient : mysocket -> (server, client) socket

Best wishes,
Nicolas


On Fri, Jul 24, 2015 at 1:01 PM, Markus Weißmann <markus.weissmann@in.tum.de
> wrote:

> Hello OCaml list,
>
> I'm trying to do something clever regarding the interface for a
> communication library:
> There is a server and a client which can only send "client" (client to
> server) and "server" (server to client) messages.
> The current idea is to use a phantom type to annotate the socket as either
> being "client to server" or "server to client";
>
> type client
> type server
>
> type ('a, 'b) socket
> type 'a message
>
> I've got a bunch of functions that only work on either "client messages",
> "server messages" or some on both. Something like:
>
> val p1 : 'a message -> int
> val p2 : server message -> float
> val p3 : client message -> char
>
> You can only send "client messages" on the "client socket" and "server
> messages" on the "server socket".
>
> val send : ('a, _) socket -> 'a message -> unit
>
> You can get these messages only on the respective other side.
>
> val recv : (_, 'b) socket -> 'b message
>
> but is there some clever way to only have the socket annotated with one
> type while keeping only one send and one recv function?
> Something in the spirit of this:
>
> type 'a socket
> val send : 'a socket -> 'a message -> unit
> val recv : [server socket -> client message | client socket -> server
> message]
>
> there is no "(client, client) socket" or "(server, server) socket";
>
>
> regards
> -Markus
>
> PS: Under the hood its basically
>
> type 'a message = bytes
> type ('a, 'b) socket = mysocket
>
> But the underlying system allows me to add filters that guarantee me the
> aforementioned properties of send/receive.
>
> regards
> Markus
>
> --
> Markus Weißmann, M.Sc.
> Technische Universität München
> Institut für Informatik
> Boltzmannstr. 3
> D-85748 Garching
> Germany
> http://wwwknoll.in.tum.de/
>
>
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