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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: another way to shoot yourself in the foot?
Newsgroups: comp.lang.ada
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Date: Tue, 24 Jun 2008 09:30:17 +0200
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Date: 2008-06-24T09:30:17+02:00
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On Mon, 23 Jun 2008 21:02:29 -0700 (PDT), george.priv@gmail.com wrote:

> On Jun 23, 3:49 am, "Dmitry A. Kazakov" <mail...@dmitry-kazakov.de>
> wrote:
>> On Sun, 22 Jun 2008 16:44:08 -0400, Robert A Duff wrote:
>>> I don't see that as a problem, and anyway, the same thing can happen
>>> with the existing syntax:
>>
>>> function Foo (...) return String is
>>> begin
>>>    return Result : String (1..200) do  -- Going to return 200 characters
>>>       Result (1) := 'a'; -- Here the result has to be allocated
>>>       if HALT (x) then
>>>          goto Label;
>>>       end if;
>>>    end;  -- Drop already allocated and initialized result
>>>    <<Label>>
>>>    return "Hallo!";  -- Return a completely different thing
>>> end Foo;
>>
>>> The implementation of this is "interesting" (if we replace String by a
>>> build-in-place type).  ;-)
>>
>> How much interesting is it? (:-))
>>
>> Unfortunately the following crazy sample crashes GNAT, but I wonder what
>> could be the result if it compiled:
>>
>> procedure Return_Task is
>>    Just_For_Fun : exception;
>>
>>    task type T is
>>       entry A;
>>    end T;
>>
>>    task body T is
>>    begin
>>       accept A;
>>    end T;
>>
>>    function Interesting return T is
>>    begin
>>       return X : T do
>>          raise Just_For_Fun;
>>       end return;
>>    exception
>>       when Just_For_Fun =>
>>          return X : T;
>>    end Interesting;
>>
>>    X : T := Interesting;
>> begin
>>    X.A;
>> end Return_Task;
>>
>> Should it block?
>>
>> Another question is about the semantics such stuff. In which sense in-place
>> is it? It seems that an in-place initialization might result in n
>> finalizations and n+1 initializations (n>=0) of the same thing.
> 
> Aren't tasks limited types by definitions?

Yes, they are.

> In which case compiler
> should be giving error message..

Ada 2005 introduced "Pickwickian" functions "returning" limited objects,
in-place, upon initialization. In the example, the result task of
Interesting, being constructed in the object X, is first initialized, then
finalized upon exception propagation and, finally, re-initialized again. I
suggest that the example (if legal) should hang forever, because to
finalize T one has to have a rendezvous with its entry A.

All this is confusing and counterintuitive, at least. I mean both
"Pickwickian" functions and the semantics return statements.

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