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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Is this a bug in my code or the compiler?
Date: Fri, 12 Jun 2015 18:15:59 +0200
Organization: cbb software GmbH
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Date: 2015-06-12T18:15:59+02:00
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On Fri, 12 Jun 2015 08:56:23 -0700 (PDT), David Botton wrote:

> Given:
> 
>    function Token_Start (Source : in out Awesome.Source.Source_Type'Class)
>                          return Character;
>    function Token_End (Source : in out Awesome.Source.Source_Type'Class)
>                        return String;
> 
> The following works:
> 
>    function Get_Token_Text (Source : in out Awesome.Source.Source_Type'Class)
>                             return String
>    is
>       N : Character := Token_Start (Source);
>    begin
>       return N & Token_End (Source);
>    end Get_Token_Text;
> 
> The following does not work:
> 
>    function Get_Token_Text (Source : in out Awesome.Source.Source_Type'Class)
>                             return String
>    is
>    begin
>       return Token_Start (Source) & Token_End (Source);
>    end Get_Token_Text;
> 
> Token_End is never called and only the value of Token_Start.

That is one of the reasons why in-out's were not allowed for functions in
Ada 83. The code is clearly erroneous because the computation order is not
defined. Possibly Token_End is called before Token_Start!

P.S. When in-out's for functions were discussed I proposed not to allow
them as operands in operations that do not state the evaluation order. E.g.

   function Foo (X : in out T) return Boolean;

Foo (A) and Foo (A) -- Illegal
Foo (A) and then Foo (A) -- Legal

P.P.S. It would become even worse with fine-grained parallelism, as Georg
keep on suggesting. If Token_Start (Source) and Token_End (Source) were
running in parallel on different cores, that would be great fun!

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de