Re: Run-Time Type Assignment

[Robert A Duff <bobduff@shell01.TheWorld.com>]

"Frank J. Lhota" <NOSPAM.lhota.adarose@verizon.net> writes:

> "Robert A Duff" <bobduff@shell01.TheWorld.com> wrote in message
> news:wccn0r7hx5y.fsf@shell01.TheWorld.com...
> > Kilgallen@SpamCop.net (Larry Kilgallen) writes:
> >
> > > What is a downward closure ?
> >
> > The ability to pass a procedure as a parameter to a procedure.
> 
> <snip>
> 
> > GNAT supports downward closures via the 'Unrestricted_Access attribute,
> > but that's not Ada (unfortunately).
> 
> What about using just the plain old 'Access attribute to pass a pointer to
> the procedure as a parameter? That is supported by all Ada 95 compilers.

No, 'Access doesn't work.  Typically, the loop body (the procedure to be
passed as a parameter) is more nested than the iterator, which makes
'Access illegal.

function Count_Items(X: Container) return Natural is
    Result: Natural := 0;
    
    procedure Incr(...) is
    begin
        Result := Result + 1;
    end Incr;
begin
    Iterate(X, Incr'Access); -- Wrong!
    return Result;
end Count_Items;

That's illegal, because Incr is more nested than the access-to-procedure
type passed to the iterator (which is defined in some package
elsewhere).

Usually, the loop body (e.g. Incr) needs to be more nested so it can see
some local variables.

GNAT's 'Unrestricted_Access attribute works in this case, but it is
nonportable and dangerous.  The question was whether Pascal supports
features not supported by Ada: in this case, it does; the feature is
portable and safe (compared to 'Unrestricted_Access).  It's more
readable, too, since you can just pass the procedure, not a pointer to
it.

- Bob