Re: Extending discriminant types

[Adam Beneschan <adam@irvine.com>]

On Nov 20, 3:45 am, Georg Bauhaus <rm.dash-bauh...@futureapps.de>
wrote:
> Ludovic Brenta schrieb:
>
> > On Nov 20, 9:30 am, christoph.gr...@eurocopter.com wrote:
> >> On 20 Nov., 08:21, a...@anon.org (anon) wrote:
>
> >>> Since, the orginal post suggest that the poster was using GNAT Ada 95
> >>> compiler. Because the poster states that he can compiler the code. So,
> >>> I used Gnat 3.15p using Ada 95 specs.
>
> > One can also try GNAT GPL Edition 2008 or GCC 4.3 (or even 4.4) and
> > pass it the -gnat95 option which is documented in the Secret GNAT
> > User's Guide.
>
> GNAT has -gnat83 as well, but rejects, too, because there are
> neither tagged nor abstract types in Ada 83.
>
> Just out of curiosity, given
>
>    type T is tagged ...;
>    type D is new T with ...;
>    type E is new D with ...;
>
> what would E'Base be?

Type declarations define both "types" and "subtypes".  Only subtypes
have constraints; types don't.  Subtypes and types are different kinds
of entities---a subtype isn't a special case of a type.  So the
declaration of T defines both a type T, and a "first subtype" T.  The
declaration of D defines a derived type D whose parent type is the
*type* T; and it also defines a "first subtype" D of the *type* D..
Similarly for E: it defines a type E whose parent type is D, and a
first subtype E of the type E.  If E'Base were defined the same way
it's defined for scalars, then it would refer to an unconstrained
subtype of the *type* E.  As Bob pointed out, that would be
essentially the same as the "first subtype" E, since there are no
discriminants on that type.  But one could imagine something like

   type U (Disc : Integer) is record ... end record;
   type F is new U(7);
   type G is new F;

The *types* U, F, and G aren't constrained, so if G'Base were allowed,
it would be an unconstrained subtype of G---i.e. Disc could be
anything.  I don't really know why E'Base or G'Base isn't allowed.  As
far as I can tell, it wouldn't cause any semantic problems to allow
it, but it might prevent some useful optimizations in variant record
cases.  But I'm just guessing.


> Assuming that D is privately tagged, would E'Base
> have different effects depending on whether [D's] tag
> is visible or not?  [corrected]

I don't think this question makes sense unless you mistakenly think
that 'Base has something to do with the parent type of a type.  But as
we've pointed out, it doesn't.  By the way, I believe there was a
proposal for a 'Parent attribute or something like that to refer to a
derived type's parent (or its first subtype), but it didn't make it
into the language.

                                 -- Adam