Re: Generic Zero Length Array

[Robert A Duff <bobduff@shell01.TheWorld.com>]

Adam Beneschan <adam@irvine.com> writes:

> Way back before Ada 95, someone made a proposed language array to add
> a "null array" aggregate to the language, so that you could write an
> aggregate to represent a zero-element array without needing a fake
> value for the elements.  I thought this was a great idea and could
> never understand why it wasn't adopted.  It would have solved the
> problem here.  But the <> syntax of Ada 2005 obviates the need for
> something like that.

I still think it's a language design flaw that you can't write a
zero-element or one-element positional aggregate.

>...I don't think we need "null record" (i.e. in
> aggregates) any more either, although certainly nobody is going to
> suggest removing it.

Why don't we need that?

> [Actually, there's one case where you still can't specify a null array
> aggregate, and that's where the index type is either "mod 1" or an
> enumeration type with one value:
>
>          type Enum is (This_One);
>          type Arr is array (Enum range <>) of Anything;
>
> Now go ahead and try to specify a zero-element array aggregate of type
> Arr.  You can't.

Right.  Not just aggregates -- you can't create any zero-element array
of type Arr.

>...But this is a pretty pathological case so it's not
> worth trying to change the language for it.]

If it were worth it, what would be the best change?
I suppose ranges could be characterized by 'First/'Length,
instead of 'First/'Last.  And eliminate 'Last from the language.
Or else say 'Last raises Constraint_Error if 'First = 'Base'First
and 'Length = 0.

- Bob