Re: Subverting 'Access for Sub-programs

["Jean-Pierre Rosen" <rosen.adalog@wanadoo.fr>]

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Ray Blaak <blaak@infomatch.com> a �crit dans le message :
m3iu6tjw2d.fsf@vault83.infomatch.bc.ca...
> Robert Dewar <robert_dewar@my-deja.com> writes:
> > But it is pretty fundamental and obvious that a tagged type
> > cannot be extended in a temporary inner scope, no?
>
> It is not fundamental and obvious to me. Perhaps you could explain.
>
> I am aware of implementation concerns with allowing such a thing, but the
> restriction seems is an exception to the generality of declaring Ada
> constructs that is usually allowed.
>
> One can declare types, exceptions, variables, and routines anywhere,
except for
> tagged types. This restriction prevents a completely localized programming
> style when doing OO stuff.
>
Remember that tagged types basically include an array of pointers to
subprograms, so the restrictions on tagged types are the same as those on
pointers to subprograms.

I find it a bit unfair to blame Ada for these restrictions. C++, Eiffel,
Java have no embedded subprograms. Turbo-Pascal with objects (I gave up with
Turbo-Pascal before it became Delphi, but I guess it is the same for Delphi)
allows classes only at the top level. In all these case, there is no issue
since everything MUST be global. Ada's restriction is that derivations must
occur at the same level as the parent type, while other languages force
derivations only at level 1. So Ada is actually more liberal than other OO
languages I know of. You may regret that Ada is not even more liberal, but
please consider that the situation is better than other languages.

--
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           J-P. Rosen (Rosen.Adalog@wanadoo.fr)
Visit Adalog's web site at http://perso.wanadoo.fr/adalog