Re: Dimensionality Checking (Ada 20XX)

["Mark Lundquist" <mlundquist2@attbi.com>]

"Wes Groleau" <wwgrol@sparc01.ftw.rsc.raytheon.com> wrote in message
news:3C14EF19.76C85864@sparc01.ftw.rsc.raytheon.com...
> Mark Lundquist wrote:
>
> > 2) There's a bad problem with generics.  Unit-safe programming totally
> > destroys the generic contract model.  For example... given this:
> >
> >     generic
> >         type T is digits <>;
> >     function F (X, Y : T) return T;
> >
> > How can you tell if this is legal?:
>
> I've seen generics that were legal (in the sense of compiling)
> but where the compiler was capable of rejecting particular
> instantiations.

Oh sure, that happens... it's as common as rocks, actually.

But the compiler only has to look at the generic *spec* to determine this.
What you *never* see today, and what we'd really like to avoid in any
language revision, is this: you make a change to the generic *body*, the
change is legal (the generic body compiles), but suddenly one or more
*instantiations* become illegal as a result.  That is the problem I'm
talking about with units.

Obviously, a solution would be to bring the unit specifications into the
contract, i.e. the generic formal part.

>  I've also seen instantiations where the illegality
> was detected at run-time.  My memory is fuzzy on the details, though.

Not by a conforming implementation :-).  RM 1.1.5(3) prohibits an illegality
from being detected at "run-time", i.e. by the execution (as defined in
10.2) itself.  Certain illegalities can be detected after compilation but
before execution (again in the strict sense), but only if they are covered
by "post-compilation rules" (1.1.5(4)).  But none of the "instatiation
errors" you usually see are due to post-compilation rules (there really are
not very many post-compilation rules in the standard).

-- mark