Re: Ada vs Eiffel - Ada programmer approach

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

On Mon, 8 Jun 2009 17:52:27 -0500, Randy Brukardt wrote:

> "Robert A Duff"  writes:
> ...
>> An invariant (whether it be a pragma, or some special syntax) would be
>> attached to a type, and would mean "all objects of this type have
>> this property, always" (except that maybe objects can temporarily
>> violate their invariant, so long as the code puts it back).
>>
>> A related concept that has been discussed is "user-defined constraints".
>> I'm not sure what the difference is, exactly -- I think with user-defined
>> constraints, you can't "temporarily violate".
> 
> A user-defined constraint applies to a view (usually via a subtype), while 
> an invariant applies to a type (always).

I do not see any difference. Invariant is merely a private constraint.
User-defined constraint is a [public] part of the invariant.

> For instance, a user-defined 
> constraint can apply to a formal parameter but not to the actual parameter 
> object (just as any other constraint).

Hmm, I think this confuses constraints put on a type/subtype with
discriminants of an object. Obviously, any combination of discriminants'
values determine some anonymous subtype with a corresponding invariant.
That is an invariant of the object only in the sense that the object has
this anonymous subtype. Objects do not have invariants.

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