Re: pragma Pure (Ada)

[Adam Beneschan <adam@irvine.com>]

On Sep 30, 7:11 pm, Yannick Duchêne (Hibou57)
<yannick_duch...@yahoo.fr> wrote:
> Le Sat, 01 Oct 2011 03:50:36 +0200, Yannick Duchêne (Hibou57)  
> <yannick_duch...@yahoo.fr> a écrit:
>
> > In “A Brief Introduction to Ada 2012” (a great paper from John Barnes) ->
> >http://www2.adacore.com/wp-content/uploads/2006/03/Ada2012_Rational_I...
>
> On page 13, there's something I don't feel to understand
>
> > Programmers have always moaned about the need for many
> > explicit conversions in Ada. Accordingly, implicit
> > conversions from anonymous access types to named access
> > types are now permitted provided the explicit conversion is
> > legal. The idea is that the need for an explicit conversion
> > with access types should only arise if the conversion could
> > fail. A curious consequence of this change is that a
> > preference rule is needed for the equality of anonymous
> > access types.
>
> “a preference rule is needed for the equality of anonymous access types” ?  
> What does that mean ?

If you have two objects of an anonymous access type that point to the
same type, in Ada 2005 you can test them for equality:

   X : access My_Record;
   Y : access My_Record;

   if X = Y then ...

This calls a function "=" that is defined in the language and takes
"universal access types" as parameters.

If you define a named access type:

   type My_Record_Acc is access My_Record;

this defines an implicit "=" operator:

   function "=" (Left, Right : My_Record_Acc) return Boolean ...

[and you can override it with your own operator if you really want
to].  In Ada 2005, this didn't pose a problem.  But in Ada 2012, if an
anonymous access type (access My_Record) could be implicitly converted
to a named access type (My_Record_Acc), then in this:

   if X = Y then ...

the compiler couldn't tell whether "=" means the function on
"universal access", or the function on My_Record_Acc (since X and Y
can now be implicitly converted to My_Record_Acc).  This is ambiguous,
and ambiguous function calls are normally an error, but making it an
error would make some legal Ada 2005 programs illegal in Ada 2012.  So
a special rule had to be added to make the language *prefer* the
"universal access" equality function over any other function.

Hope this helps,

                       -- Adam