Re: Language lawyer question: Equality on 'Access attributes

[adam@irvine.com (Adam Beneschan)]

Robert A Duff <bobduff@shell01.TheWorld.com> wrote in message news:<wcc1xqa1fmj.fsf@shell01.TheWorld.com>...

> >...  This makes X'Access rather unique in this
> > regard.  I believe that for any other construct that returns a value,
> > one can determine what the type of the construct is (once it has been
> > determined what declarations all the usage names pertain to) without
> > knowing anything about the context in which the construct occurs.
> 
> Well, there are many cases where *context* is required to resolve
> overloading.  For example:
> 
>     type Color is (Red);
>     type Light is (Red, Amber, Green);
> 
>     procedure P(X: Light);
> 
>     P(Red); -- Same as P(Light'(Red));

That's why I included the parenthesized phrase, "once it has been
determined what declarations the usage names pertain to".  Once you've
determined that the name "Red" pertains to the second declaration of
Red, and not the first one, you then know what the type of "Red" is. 
(Of course, you need the context to determine which declaration is the
correct one.)  This isn't the case for X'Access: even if you know what
declaration X refers to, you still don't know waht type X'Access is.

However, I was wrong when I thought that 'Access was unique here. 
There are other cases where the type of a construct cannot be
determined without context: namely, "null", character literals, and
string literals.


> You said above, "once it has been determined what declarations all the
> usage names pertain to".  But that's sort of backward, since you can't
> determine that without feeding information down from context (namely,
> the type of the parameter X of procedure P).

I have to admit that while I think my assertion is true, it's pretty
abstract.  It probably requires one to think backwards from how a
practical programmer, or a compiler, would think when trying to
determine the meaning of an Ada statement like that.

                                      -- Adam