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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: another way to shoot yourself in the foot?
Date: Tue, 24 Jun 2008 13:16:06 -0400
Organization: The World Public Access UNIX, Brookline, MA
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Date: 2008-06-24T13:16:06-04:00
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"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:

> 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.

No, that's not the right way to look at it.  There is no single X
that is being [re]initialized twice.

X is initialized (but never activated).  The exception causes it to be
terminated -- this part is from Ada 83.

Then a different object (also called X) is created.
On successful return, this object becomes the result
object of the function -- which is also called X (the
one declared in Return_Task.  This task is activated
when Return_Task reaches its "begin".

>... 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.

Well, I don't think so, but anyway, this is the only way
return of limited types can make sense.  The Ada 83 way
(return is by copy, even for limited) is just wrong.
And the Ada 95 way (return by reference) is confusing
and nearly useless.

- Bob