Re: Half Constrained Array Types and Slices

[Stefan Lucks <lucks@th.informatik.uni-mannheim.de>]

On Mon, 6 Mar 2006, Jeff Carter wrote:

> Suppose we could say something like
>
> V : Degenerate (null range); -- V'Length = 0
>
> Now we can do things like
>
> for I in V'range loop -- loop is not entered
>
> if X in V'range -- always False
>
> Y : Degenerate (V'range); -- Y'Length = 0
>
> More importantly, we do these kinds of things in subprograms with
> Degenerate parameters.
>
> If Degenerate is a string type, "" is meaningful.
>
> We seem to be fine as long as we don't do V'Last. I can't figure out
> how such a language should handle 'First and 'Last for such a
> value/object. Probably a failure of imagination on my part.

Don't use V'Last at all ... but a new attribute V'Beyond instead.

Informally, V'Beyond should be the "next" value beyond V'last.

To avod unneccessary runtime errors, such a (non-Ada-)language should
rather not provide V'Last at all ...

-- 
Stefan Lucks      Th. Informatik, Univ. Mannheim, 68131 Mannheim, Germany
            e-mail: lucks@th.informatik.uni-mannheim.de
            home: http://th.informatik.uni-mannheim.de/people/lucks/
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