Re: Bus error

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

"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:

> On Sat, 30 Jun 2007 11:34:16 -0400, Robert A Duff wrote:
>
>> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:
>> 
>>> On Sat, 30 Jun 2007 15:14:39 +0200, Georg Bauhaus wrote:
>>>
>>>> Dmitry A. Kazakov wrote:
>>>>> Note also that Ada's renaming has conceptually little to do
>>>>> with objects. Consider:
>>>>> 
>>>>>    X : T;
>>>>>    X : T renames X; -- Illegal
>>>>> 
>>>>> though the "object" is obviously same. Same object, same name, what was
>>>>> wrong?
>>>> 
>>>> A programmer construing strange assumptions about the possibility of
>>>> duplicate names in a declarative part of an Ada program, I'd say.
>>>
>>> Where do you see duplicated names?  Is bare name an object? Observe that X
>>> referred to exactly same object, if the theory were correct. But apparently
>>> the theory is wrong, and in fact X : T renames X; produces something else.
>> 
>> I must say, I'm mystified as to what the above example is supposed
>> illustrate.  You can't have two declarations with the same name
>> (unless they are both overloadable).  The fact that they are both
>> names for the same object is irrelevant to THAT rule.
>
> Declarations of what?

Well, the RM doesn't seem to care:

8 {homograph} Two declarations are homographs if they have the same
defining name, and, if both are overloadable, their profiles are type
conformant. {type conformance [partial]} [An inner declaration hides any
outer homograph from direct visibility.]

9/1 {8652/0025} [Two homographs are not generally allowed immediately
within the same declarative region unless one overrides the other (see
Legality Rules below).]...

>... That is the question. Allegedly, as the name of the
> thing let suggest, "rename" does not declare anything [new].

A renaming declaration is syntactically a declaration,
and it declares a name.  It does not declare a new entity.

>... Does "use"
> declare something?

No.  And it's not a declaration.

>... Or is name an object?

No.  But:

5 {Declaration} [Glossary Entry]A declaration is a language construct
that associates a name with (a view of) an entity. ...

7 {Definition} [Glossary Entry]{view} All declarations contain a
definition for a view of an entity. A view consists of an identification
of the entity (the entity of the view), plus view-specific
characteristics that affect the use of the entity through that view
(such as mode of access to an object, formal parameter names and
defaults for a subprogram, or visibility to components of a type). In
most cases, a declaration also contains the definition for the entity
itself (a renaming_declaration is an example of a declaration that does
not define a new entity, but instead defines a view of an existing
entity (see 8.5)).

> If I designed rename, I would certainly allow X renames X; similarly to
> "use" or "with" A, A;

OK, I think I see what you're getting at.  You would then say that
referring to X is legal (not ambiguous) because both of those
declarations of the name X refer to the same entity.  Right?

To make this work, you need to distinguish static and dynamic.

    X : Integer renames Func(...).all.Component(Func2(...));
    X : Integer renames Func(...).all.Component(Func2(...));

    X := X + 1; -- legal?

I don't think you want a run-time check that the two
names X denote the same entity!

FWIW, I would simply delete the no-duplicate-homographs rule.
And weaken the overload resolution rules.

- Bob