Re: Diamond diagram for 'with'

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On 22/02/2018 02:38, Randy Brukardt wrote:
> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message
> news:p6k68f$9li$1@gioia.aioe.org...
>> I would ask language lawyers regarding multiple with.
>>
>> Consider this:
>>
>>     limited with Root.A;
>>     package Root is
>>     end Root;
>>
>>     package Root.A is
>>        type T is ...;
>>     end Root.A;
>>
>>     with Root.A;
>>     package Root.B is
>>     end Root.B;
>>
>> Now Root.B has both limited (inherited from Root) and full "with" of
>> Root.A. So, may Root.B use Root.A.T? It cannot according to "limited with"
>> and it can due to full "with". Which one to win?
> 
> Off the top of my head, it should be the "full with". Generally, Ada allows
> one to open more visibility, but you can't remove it. Deriving this formally
> would be somewhat painful, but it has to be true, since the motivating use
> for
> limited with is something like:
> 
>      limited with P;
>      package Q is
>           ...
>      end Q;
> 
>      limited with Q;
>      package P is
>          ...
>      end P;
> 
>      with Q;
>      package body P is
>          -- Q is normally visible here.
>      end P;
> 
> If the limited with "won" in the body, one could never access the full
> package in the body, which would make actually implementing any mutually
> dependent package hard. (You may want to call some primitive routine
> declared in Q in the body of P, but that isn't possible for a "limited
> with").

That is logical. A more permissive access should consume the less 
permissive one.

> Since all withs work basically the same way, the same has to be true for a
> child package.
> 
> BTW, Root.A.T would be legal either way. But if it came from the limited
> with, it would be an incomplete type (which has its own limitations).

Yes, this is how I came to this, GNAT complained about incomplete types. 
So it is possible a compiler bug, however I worked it around anyway.

I am keeping on avoiding "limited with", and the only time I lowered my 
guard, it bite me! (:-))

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