Re: Mutually dependent private types

[Matthew Heaney <matthew_heaney@acm.org>]

John Volan <johnv@ac3i.dseg.ti.com> writes:

> > I don't know what to do really.  Maybe a pragma be better:
> > 
> > with P;
> > pragma Mutual (P);
> > package Q is
> 
> It seems to me that if there's a mutual spec dependency going on between
> P and Q, then the "with" clause here is already illegal according to
> Ada95's language rules.  So what exactly would this "Mutual" pragma
> mean?  "I know the with clause is illegal, but ignore that and pretend
> to be a different language"? :-)  What about Ada95's run-time
> elaboration mechanism?  When would the elaboration code for the specs of
> packages P and Q get executed? In what order?

The pragma idea is more or less equivalent to "with P.T".

Thinking about it more, you could name it thusly:

with P;
pragma Elaborate_None (P);
package Q is ...;

So the answer to your question is that the spec for P is not elaborated
prior to its use (with certain constraints) by Q.

Yes, the "with type" idea was Tuck's, but I'd be willing to bet this
problem will be solved using a new set of pragmas.  We already have a
precedent for using pragmas for elaboration control, and since this is
really an elaboration issue, an additional pragma seems like a more
natural fit.