Re: Seeking for papers about tagged types vs access to subprograms

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

On Mon, 06 May 2013 15:16:21 +0200, Yannick Duchêne (Hibou57) wrote:

> Le Mon, 06 May 2013 14:11:32 +0200, Dmitry A. Kazakov  
> <mailbox@dmitry-kazakov.de> a écrit:
>>> I feel the status distinction between tagged and untagged types as an
>>> issue in some ways, this breaks something in the big picture, and break
>>> Ada, the language, in two too much distinct and foreign parts.
>>
>> Yes it does. That was a flaw of Ada 95 design. I guess it was the maximum
>> of what Tucker Taft could press through fierce opposition.
> 
> Fierce opposition from who? For what reasons? What did Tucker initially  
> wanted?

I wasn't there.

>> There is nothing non-deterministic in tagged types. All calls with  
>> specific objects are statically resolved. Dynamic are only dispatching  
>> calls involving class-wide objects.
> 
> Theoretically. In practice, GNAT don't agree (*). It has a pragma for  
> that, `pragma Restrictions (No_Dispatching_Calls)` which should even  
> enforce it in a whole application, but seems even with this active, it  
> still requires the whole part of the runtime dedicated to it, a part which  
> depends on this part,

I suppose it would be difficult to implement this so that the code used to
maintain dispatching tables and tags could be removed. It may require two
versions of the compiler. Furthermore there are packages defining
operations on tags which could require some of the stuff.

Why do you bother? Especially, because dispatching calls are deterministic
and could be done time bounded, unless you manipulate tags. (Not everything
in people write in their guidelines for RT projects makes sense.)

> However and in the large, I  
> agree with you and share the same understanding of the topic, at least  
> theoretically.

There are far reaching consequences of having classes for all types.
 
> (*) Don't know if SPARK agree and believe something can be proved about  
> such a system, it's a too long time I didn't use it. A quick search on the  
> web seems to show it may be possible.

Yes, through global analysis. Though I would prefer a much finer approach
when time constraints could be imposed on the interface rather than checked
afterwards. What is in the interface cannot be decided before you have
interfaces properly articulated. Thus type system overhaul is the first
step to make. Without doing that you get only mess. See Ada 2012 dynamic
pre-/postconditions as an example.

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