Re: Half Constrained Array Types and Slices

["Jeff Carter" <jcgoogle@earthlink.net>]

Robert A Duff wrote:
>
> I don't know of any change between 83 and 95, here.
> But I could be forgetting something; please remind me.

I just checked my copy of the green book, and you're right. I seem to
recall some problem with slices of constrained types in Ada 83, but I
guess I'm mistaken.

> I'd also be happy to eliminate slices as l-values.  They cause a lot of
> implementation difficulty for not much benefit.  Or else go the whole
> hog, and allow X(5..10) := "xx", which does not work in Ada, because
> that would require changing the length of the whole thing.

What would that do?

> Yes, but this is all very not-Ada in several ways.
> There are probably some semantic anomalies I have not considered.

Of course, it's not-Ada by definition. But it's fun to talk about.

I've been thinking about a language that allows null arrays, even for
degenerate types like

type Single is (One);

type Degenerate is array (Single range <>) of Whatever;

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.

--
Jeff Carter

jrcarter commercial-at acm [period | full stop] org