Re: 'withing' problem

[Simon Wright <simon@pushface.org>]

"Matthew Heaney" <mheaney@on2.com> writes:

> "Simon Wright" <simon@pushface.org> wrote in message
> news:x7vy9lbxih0.fsf@smaug.pushface.org...
> > I think that people looking at this model, perhaps for the purpose of
> > maintaining code written to implement it, need to have a very clear
> > idea of how the code will look.
> 
> My philosophy is, if you want to have a good idea how the code will look,
> then you should look at the code...  In any case, you should just ignore the
> forward declaration package, as it's just syntactic overhead that has no
> bearing on your model of the problem.
> 
> > It's a Bad Thing if you need to make arbitrary decisions to break
> > cycles.
> 
> You haven't explained why arbitrary decisions are bad, so this isn't a very
> cogent argument, which just reduces to "I don't like it, so it's bad."

Well, "arbitrary" translates roughly to "unpredictable", "surprising"
both of which are definitely bad (I think so, anyway).

> And besides, this solution is no different from other areas of the language.
> For example, if you want to declare a recursive pair of subprograms, then
> you have to forward declare one of them (and it's arbitrary which one):

Here, you can "arbitrarily" (or following the warnings given by
-gnaty) forward declare _both_ (ie, _all_) subprograms. Symmetric.

> > What would be even worse would be to have a framework code generator
> > that applied heuristics to break cycles. I imagine that it wouldn't
> > take much for such heuristics to make the opposite decision for minor
> > model changes (remember that very few models are as simple as this
> > one!)
> 
> Let's concentrate on real problems.  John Volan couldn't figure out
> how to declare two mutually dependent abstractions in two separate
> packages.  I showed how to do this.  Problem solved.

This topic is a real problem for me.


Does your Patient_Forward solution extends to declaring Doctor_Forward
similarly? I get trouble at

   procedure Visit_Doctor
     (Patient : in out Patient_Type;
      Doctor  : in out Root_Doctor'Class) is
   begin
      Treat_Patient (Doctor, Patient);
patients.adb:9:22: expected type "Doctor_Type" defined at doctors.ads:6
patients.adb:9:22: found type "Root_Doctor'Class" defined at doctors_forward.ads:3
   end;