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From: Victor Porton <porton@narod.ru>
Newsgroups: comp.lang.ada
Subject: Re: Rationale for this not to compile?
Date: Fri, 25 Jul 2014 15:19:39 +0300
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Date: 2014-07-25T15:19:39+03:00
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Randy Brukardt wrote:

> 
> "Adam Beneschan" <adambeneschan@gmail.com> wrote in message
> news:1cb91e6a-9e49-4bc8-bf4a-67a16a00e3a0@googlegroups.com...
>>On Thursday, July 24, 2014 1:53:32 PM UTC-7, Victor Porton wrote:
>>> Is there any rationale why this does not compile?
>>
>>A generic instantiation is equivalent to "the instance declaration ...
>>immediately followed
>>by the instance body, both at the place of the instantiation."  (12.3(12))
>>Since the body
>>could contain code that depends on the representation of X already being
>>determined,
>>but it hasn't yet, the language makes this illegal.  At least I think
>>that's the rationale.
>>AARM section 13.14, which discusses the freezing rules, explains in more
>>detail why
>>types have to be completely defined before they're used in certain cases
>>(see
>> http://www.ada-auth.org/standards/12aarm/html/AA-13-14.html).
> 
> That's a pretty good explanation.
> 
> We (the ARG) have spent dozens of hours attempting to find a way to allow
> something like the OPs example. (There's something like 10 AIs associated
> with that.) Each solution had some sort of fatal flaw (implementability,
> maintenance problems, too hard to use, etc.)
> 
> The best solution is the put the instance into a child package; one of the
> reasons that we didn't adopt any of the ugly solutions is that using a
> child package often works fine and an ugly solution doesn't seem worth it
> in that case.
> 
> In the special case where the generic has little implementation of it's
> own (a signature package), one can use a formal incomplete type as the
> formal type; it does allow an instantiation like the one the OP wrote.
> Specifically:
> 
> package OK_Test is
> 
>    generic
>       type T;
>    package Gen is
>    end Gen;
> 
>    type X is private;
> 
>    package Instance is new Gen(X);
> 
> private
> 
>    type X is null record;
> 
> end OK_Test;
> 
> is legal. But the use of the formal incomplete type strongly restricts
> what you can do in the generic unit (because it's an incomplete type, and
> those allow very limited uses).
> 
> One of my pet projects (that I don't have any time to follow up on!) is to
> create a useful container where the element type is a formal incomplete
> type. I think that the container would have to manage elements that live
> elsewhere (unlike Ada.Containers) -- since element creation isn't
> possible, but this would allow a container of limited elements, and of
> course would allow instantiation by a private type as here.
> 
>                                           Randy.
> 
> 
> 
> 
> 
> 
>> If it compiled, it would
>> be probably useful in practice with a real example similar to the below
>> toy
>> example (as to make the type T not private, I must create an *.Internal.*
>> package to serve as a "private repository" for types like T, as real
>> private
>> types cannot be used). It is called a hack.
>>
>> gnatmake -q -c -gnatc -u -P/home/porton/t/default.gpr test.ads
>> test.ads:10:32: premature use of private type
>> gnatmake: "/home/porton/t/test.ads" compilation error
>>
>> package Test is
>>
>>    generic
>>       type T is limited private;
>>    package Gen is
>>    end Gen;
>>
>>    type X is private;
>>
>>    package Instance is new Gen(X);
>>
>> private
>>    type X is null record;
>> end Test;

Sorry for a stupid question:

What is the difference of:

generic
    type T
package Gen is
end Gen;

and

generic
    type T is private
package Gen is
end Gen;


-- 
Victor Porton - http://portonvictor.org