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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Constraints in extended return
Date: Mon, 21 Mar 2011 21:02:32 -0500
Organization: Jacob Sparre Andersen Research & Innovation
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Date: 2011-03-21T21:02:32-05:00
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"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.

                                 Randy.