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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: ANN: Ada 2005 Math Extensions, 20100810 release
Newsgroups: comp.lang.ada
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Date: Fri, 13 Aug 2010 10:40:57 +0200
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Date: 2010-08-13T10:40:56+02:00
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On Fri, 13 Aug 2010 09:42:15 +0200, Georg Bauhaus wrote:

> On 8/13/10 8:27 AM, Dmitry A. Kazakov wrote:
> 
>> Another rule of thumb (I don't know if this is going to be fixed in Ada
>> 2012), is that a child should always rename formal generic arguments in its
>> specifications. Under certain circumstances, usually in children, they
>> might become unavailable. I mean this:
> 
> Would you mind showing an example of a name that is needed
> but unavailable?

>> generic
>>     with package P is new Generic_P (<>);
>> package Generic_S is
>>     package P_Of renames P; -- You will need this!

Do you mean the use case or the offending code? I cannot say for the latter
because I have no idea when it happens and if the code is legal [*]. The
simplest possible use case is the diamond diagram of generic packages [**]:

  A
 / \
B  C
 \ /
  D

In the specification of D you have to constrain B and C to the same A. For
this you will need to refer the parameters of B and C:

   generic
   package A is end A;

   generic
      with package AA is new A (<>);
   package B is end B;

   generic
      with package AA is new A (<>);
   package C is end C;

   generic
      with package BB is new B (<>);
      with package CC is new C (BB.AA);
         -- Somewhere, somehow this ceases to work
   package D is end D;

Note that Simon's problem is the same use case (just one shoulder of the
diagram) but of different shape [*]:

   generic
   package A is end A;

   generic
   package A.B is end A.B;

   generic
   package A.C is end A.C;

   with A.B, A.C;
   generic
      with package AB is new A.B (my own A); -- No way!
      with package AC is new A.C (my own A); -- No way!
   package A.D is end D;

-----------
* Generics is a mess, remember?
** We were told how dreadful the diamond is, so bad that Ada may not allow
to have MI, but for sacred generics anything is perfectly OK!

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de