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From: Ludovic Brenta <ludovic@ludovic-brenta.org>
Newsgroups: comp.lang.ada
Subject: Re: Is this a bug in Ada.Numerics?
Date: Sun, 21 Oct 2012 05:39:09 +0200
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Date: 2012-10-21T05:39:09+02:00
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Leo Brewin <leo.brewin@internode.on.net> writes:
> But having digested your answer I'm still a bit confused (sorry). Here
> is the "procedure" version of the above package,
>
>    with Ada.Numerics.Generic_Complex_Types;
>    with Ada.Numerics.Generic_Complex_Elementary_Functions;
>
>    procedure foo is
>
>       type Real is digits 18;
> 		
>       package Complex_Types is new 
>          Ada.Numerics.Generic_Complex_Types (Real);
>
>       use foo.Complex_Types;
>       subtype Complex is foo.Complex_Types.Complex;
>       
>       procedure bar (z : in out Complex);
>    
>       package Complex_Maths is new 
>          Ada.Numerics.Generic_Complex_Elementary_Functions (Complex_Types);
>    
>       procedure bar (z : in out Complex) is
>       begin
>          z := Complex'(Re(z),0.0);
>       end bar;
>
>    begin
>       null;
>    end foo;
>
> This does compile and yet (by my reading of your reply) the "use foo"
> and "subtype complex" lines should introduce two distinct versions of
> Complex and thus should produce a compiler error.

The difference is that, in the procedure, the subtype Foo.Complex is
declared in the immediate scope where Complex_Maths is declared, so it
hides Foo.Complex_Types.Complex, so there is no ambiguity anymore.  You
get the same effect with the package if you move the declaration of
Complex_Maths to the package spec.

These rules are quite subtle and the error messages from GNAT less than
helpful.  Maybe that's why Ada has a reputation for being "difficult to
learn".

-- 
Ludovic Brenta.