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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Thinking of using Ada for a job at home. Couple of questions
 first.
Newsgroups: comp.lang.ada
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Date: Wed, 3 Mar 2010 23:20:34 +0100
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Date: 2010-03-03T23:20:33+01:00
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On Wed, 03 Mar 2010 20:38:01 +0000, John McCabe wrote:

> As for the tagged Connection type, I'm not sure what you mean. Would
> you mind elaborating on that please?

I meant something like this:

type Abstract_Connection is tagged ...;
procedure On_XYZ (Link : in out Connection) is abstract;
...

task type Driver (Link : not null access Abstract_Connection'Class);

task body Driver is
begin
   loop
       -- read a message (could be polymorphic or not)
       ...
       -- process the message (usually polymorphic)
       case ... is
           when XYZ => -- XYZ is here
                Link.On_XYZ; -- Do what has to be done
           when ... =>
               ...
       end case;
   end loop;
exception
   when Shut_Down_Exception =>
        null;
   when Error : others =>
        Put ("I am dead! " & Exception_Information (Error));
end Driver;

type My_Fancy_MIDI_Connection is new Abstract_Connection with private;
overriding procedure On_XYZ (Link : in out My_Fancy_MIDI_Connection);
...

I usually have a driver task which deals with a class-wide Connection
object that implements the lower-level communication protocol (the part
that reads messages) and the higher-level semantic call-backs (the part
that reacts to the messages read).

If the communication library is built on callbacks (i.e. already has a
hidden Driver task), then I pass the connection object as user parameter to
the callback, and the loop body above is what to be done in the callback
routine.

The advantage of having Connection tagged is that I can implement layered
protocols and semantic actions incrementally. In complex cases I add
further mix-ins as access discrimnants to Connection, usually when I need
to make lower layers pluggable. But that would be over the top in your
case.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de