Re: OO problem: Performing actions on messages (very long, sorry)

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On 5 Jan 2005 07:59:05 -0800, per wrote:

>>OK, I see. But then the question to answer what purpose serves message
>>apart from being just a container for unrelated data? The difficulties of
>>the design come from that missing pupose. If there is nothing in common
>>then nothing can be programmed for that nothing. (:-))
> 
> Well, each message *is* a simple container, more or less. The contents
> are related in the sense they are to be sent to/from a certain device
> that expects a defined set of information. The program we discuss
> doesn't really care at all of the contents of each message, but shall
> provide means to the user to override any message content (among other
> things).

How do you plan to maintain data consistency of a message if the user may
arbitrarily override its fields? I would try to do it more OO. To think
about what can be done with a message. And consider action as an
implementation detail of a concrete type of messages. Then actions could
vanish. (Just a guess, I still can't tolerate both messages and actions in
one bottle. (:-))

>>Hmm, what about this mix-in:
>>
>>package Interfaces is
>>  type Message is abstract tagged ...
>>  type Action is abstract tagged limited ...;
>>  procedure Execute (This : in out Action) is abstract;
>>end Interfaces;
>>
>>package M1_Thinds is
>>  type M1 is new Message with record -- Message M1
>>     A : Integer;
>>     B : Float;
>>  end record;
>>
>>  type M1_Action (Data : access M1) is -- Abstract actions on M1
>>     abstract new Action with null record;
>>
>>  type Override_A is new M1_Action with record
>>     A : Integer;
>>  end record;
>>  procedure Execute (This : in out Override_A);
>>
>>  type Override_B is new M1_Action with record
>>     B : Float;
>>  end record;
>>  procedure Execute (This : in out Override_B);
>>  ...
>>
>>package body M1_Things
>>  procedure Execute (This : in out Override_A) is
>>  begin
>>     This.Data.A := This.A;
>>  end Execute;
>>
>>  procedure Execute (This : in out Override_B) is
>>  begin
>>     This.Data.B := This.B;
>>  end Execute;
>>
>>Here all actions on some type of messages have a "pointer" to the
>>corresponding message object. The scope of an action should be nested in
>>the scope of the message.
>>
>>Here in fact action is a pointer to message extended with some additional
>>data. When scopes get different, then action could be a handle to message.
>>Messages will be collected using reference counting.
> 
> I kind of see what this aims at (although "mix-in" is new to me).
> 
> As far as I see we're back to defining one action type for each message
> and each field in the message, although this time without the generics,
> right?

Yes, generics do not help here much because the field name cannot be a
formal parameter.

> Can an action, eg Override_A, exist without an M1 instance to "point"
> at?

It cannot, this is the whole idea of access discriminants. They are never
null and cannot be changed. Otherwise one should use access types
(pointers).

> Action has a different life-cycle than Message.

That's no problem as long as its life in Message's one. For exampe:

declare
   M : aliased M1;
begin
   ...
   declare
      A : Override_A (M'Access);
   begin
      ... -- The scope of the action A is nested
   end;
   ...
end;

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