From: Simon Wright <simon@pushface.org>
Subject: Re: Constraints in extended return
Date: Tue, 22 Mar 2011 08:28:59 +0000
Date: 2011-03-22T08:28:59+00:00 [thread overview]
Message-ID: <m2r59zftis.fsf@pushface.org> (raw)
In-Reply-To: im8vvs$6v0$1@munin.nbi.dk
"Randy Brukardt" <randy@rrsoftware.com> writes:
> "Simon Wright" <simon@pushface.org> wrote in message
> news:m2d3lngzmi.fsf@pushface.org...
>> This code transposes a matrix:
>>
>> function Transpose (M : Complex_Matrix) return Complex_Matrix
>> is
>> begin
>> return Result : Complex_Matrix (M'Range (2), M'Range (1)) do
>> for J in M'Range (1) loop
>> for K in M'Range (2) loop
>> Result (K, J) := M (J, K);
>> end loop;
>> end loop;
>> end return;
>> end Transpose;
>>
>> This is all very well for providing a value where no constraint is
>> otherwise imposed, for example as an actual in a subprogram call, but
>> what about the case where there is a prior constraint?
>>
>> Input : Complex_Matrix (1 .. 2, 11 .. 12);
>> Output : Complex_Matrix (1 .. 2, 1 .. 2);
>> begin
>> Input := (...);
>> Output := Transpose (Input);
>>
>> Is there any way for the extended return to determine the constraints of
>> the 'target'? I suspect not, but the language in RM6.5 is deep.
>
> Why do we care what the bounds are? Assignments "slide" bounds; all that is
> required is that the lengths of the array dimensions match. This is true
> whether this is an extended return or a regular return (at least it had
> better be; it would be awful to have the semantics change depending on the
> form of the return statement).
>
> For instance,
> Output := Input;
> is both a legal assignment and does not raise Constraint_Error (both
> dimensions have length 2).
>
> Now, if you get the lengths wrong, you have a problem, but not the actual
> bounds.
Thanks for that (which I can confirm works just fine with GNAT). Unable
to see the wood for the trees ...
prev parent reply other threads:[~2011-03-22 8:28 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-03-19 10:42 Constraints in extended return Simon Wright
2011-03-19 13:30 ` Robert A Duff
2011-03-19 13:51 ` Dmitry A. Kazakov
2011-03-19 15:55 ` Simon Wright
2011-03-22 2:02 ` Randy Brukardt
2011-03-22 8:28 ` Simon Wright [this message]
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