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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: Sun, 22 Jun 2008 16:44:08 -0400
Organization: The World Public Access UNIX, Brookline, MA
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Date: 2008-06-22T16:44:08-04:00
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"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:

> One problem with your proposal is:
>
> function Foo (...) return String is
> begin
>    declare
>       Result : return String (1..200);  -- Going to return 200 characters
>    begin
>       Result (1) := 'a'; -- Here the result has to be allocated
>       if HALT (x) then
>          return;  -- Return from Foo with Result
>       end if;
>    end;  -- Drop already allocated and initialized result
>    return "Hallo!";  -- Return a completely different thing
> end Foo;

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).  ;-)

Instead of goto, you could cause the same issue with an exit statement,
or with an exception raised inside the return, and handled outside it
(but still inside the function).  In fact that exception case can
happen with the old-fashioned syntax:

    function Bar (...) return Limited_Unconstrained_Array is
    begin
        return (1..10 => F(...));
    exception
        when Some_Error =>
            return ...;
    end Bar;

And suppose F raises an exception on the 5th call.

- Bob