Re: Problem With Self-Referential Access-to-Subprogram Type

[taashlo@cyberdude.com (Tad Ashlock)]

Marius Amado Alves <amado.alves@netcabo.pt> wrote in message news:<mailman.285.1068058551.25614.comp.lang.ada@ada-france.org>...
> On Wed, 2003-11-05 at 17:49, Adam Beneschan wrote:
> > ... The feeling at the time was that
> > the additional work needed to change the language and the compilers
> > shouldn't be required if no one has run into the problem in practice. 
> > I've let them know that someone now has run into it.
> 
> Did you verify that the problem was "real"? The example does not seem to
> show enough. Can be just an experimentation of the general construction,
> with no specific application in mind. Some newbies do that. (In which
> case the 2002 decision should probably stand. Private types solve the
> general problem.)

Well, as the OP I can't really say how "real" the problem is.  But I
can describe what it is I'm trying to accomplish.

What I'm trying to do is implement (in Ada) the ideas in the book
"Practical Statecharts in C/C++" by Miro Samek, CMP Books, 2002, ISBN
1-57820-110-1 <http://www.quantum-leaps.com/writings/book.htm>.  The
(quite ingenious) idea is that the current state of a hierarchical
state machine can be represented by nothing more than a reference (a
function pointer in C++) to the state handling function.  And to
implement the hierarchical aspect of statecharts, each state handling
function returns a reference to its parent state if it doesn't handle
the passed-in event itself.

Of course there are other ways of getting the required information
back from the state handler, but I am trying to keep the Ada
implementation similar to the original C++ implementation.  At least
where it makes sense to do so.  That's what started me exploring the
possibility of an access-to-function type being defined as returning a
value of its own type.  I then stumbled across the inability to
convert my state handler functions to procedures.  And that prompted
my original posting.  I now see that my original access-to-function
type was erroneous and therefore so is my entire line of reasoning. 
Oops!

Whether or not this situation justifies changing Ada is well beyond
me.  I certainly wouldn't like to see this capability added just
because C++ can do it (besides, it's a hack in C++ anyway).  But if
the Ada-Powers-That-Be decide that this capatility is worth adding to
Ada, I could certainly make use of it.

Thanks again,
Tad