Re: Overlapping ranges

[Richard Riehle <richard@adaworks.com>]

AG wrote:

> "Richard Riehle" <richard@adaworks.com> wrote in message
> news:3EFE3491.F968568D@adaworks.com...
> > AG wrote:
> >
> > > Assuming I have a package declaration like this:
> > >
> > > package range is
> > >   type x1 is range 0..255;
> > >   type x2 is range 127..2**15-1;
> > > end ranges;
> > >
> > > Is there a way to declare a function
> > > using that package that will accept
> > > values of either x1 or x2 (and nothing
> > > else) restricted to the range 127..255 ?
> >
> > Perhaps I don't understand why this is a problem for
> > you.  Why not something such as,
>
> Because I don't want to allow an illegal
> value to make it inside the function
> (not even to raise an exception)
>
> What I was after was some implementation
> that won't need custom exceptions/error
> codes or so.

This is a good argument for the addition of pre-conditions,
post-conditions, and invariants to the language.  I like the
designations from Eiffel.  So the function might look like,

          function Fudge(A, B : Number'Base) return Number'Base;
          require A in X2'First..X1'Last;         -- pre-condition
          require B in X2'First..X1'Last;         -- pre-condition
          ensure return in X2'First..X2'Last;   -- post-condition

The only problem now is what the program does with a pre-
or post-condition violation.   The answer to this question is
not as trivial as it might at first seem.

Richard Riehle