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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Constraints in extended return
Date: Sat, 19 Mar 2011 10:42:45 +0000
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Date: 2011-03-19T10:42:45+00:00
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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.