Re: Half Constrained Array Types and Slices

["Randy Brukardt" <randy@rrsoftware.com>]

"Robert A Duff" <bobduff@shell01.TheWorld.com> wrote in message
news:wccmzfyurm5.fsf@shell01.TheWorld.com...
> "Jeffrey R. Carter" <spam.not.jrcarter@acm.not.spam.org> writes:
> > I think the index type should be discrete. But surely it doesn't need to
> > be an integer:
> >
> > with Ada.Text_IO;
> > procedure Null_Range is
> >     type A is (B, C);
> >
> >     type D is array (A range <>) of Integer;
> >
> >     E : D (C .. B);
> > begin -- Null_Range
> >     Ada.Text_IO.Put_Line (Item => A'Image (E'Last) );
> > end Null_Range;
> >
> > This is valid Ada.
>
> Indeed it is.  I don't think it's very useful, however.
> My claim is that if you want empty arrays, you almost
> certainly want to index them by (non-modular) integers.
> Which is why I'm happy with the restriction in the Containers
> library that Vector indices must be signed integers.

I don't buy this, especially in the case of modular integers, but even in
the case of large enumeration sets (like Character). I don't see any reason
to think that integers are special; the whole reason for allowing
non-integer array indexes is for greater abstraction. Empty arrays are never
very useful, but I don't think there is much reason to say that they'll
useful in the one case but not the other. That's essentially that same as
saying that integers are more useful as array indexes; in that case, why
bother allowing anything else as array indexes? Of course, I was very
against the restriction in the Containers library, too...

                                     Randy.