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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: spec/body/rep (Was: Compilation error (GNAT bug?))
Date: Tue, 27 May 2014 18:41:46 +0200
Organization: cbb software GmbH
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Date: 2014-05-27T18:41:46+02:00
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On Tue, 27 May 2014 17:45:07 +0200, G.B. wrote:

> On 27.05.14 10:55, Dmitry A. Kazakov wrote:

>> I have issues with this approach in general. The problem is that there is
>> no any check that the interfaces of the target-specific packages are same
>> in the sense that all target-independent clients were compilable with any
>> "implementation."
> 
> Assuming that a universal expression of "compilability" in any
> configuration is nice, but likely impossible,

It is surely possible at the same level of scrutiny the language mandates
certain programs legal <=> compilable.

> would you still think
> that what Randy has called a semantics-denying fiction could
> be preserved for some aspects? For example,
> 
>     procedure Doubled (X : in out Natural)
>     with Post => (if
>                     X'Old <= Natural'Last / 2
>                   then
>                     X = X'Old * 2
>                   else
>                     False),
>       Import,
>       External_Name => "d2";
> 
> If the External_Name aspect of Doubled were moved to
> a representation unit tied to whatever Doubled lives in,

That is what I suggested. I don't care about aspects, there is no such
thing in real-life at all. There are specifications and implementations,
nothing else.

External_Name certainly belongs to the implementation as any definition of
a body does. No difference.

So if you moved it to the implementation and then switched implementations
with the targets, that will be all OK to me. Some implementations could
have a proper body of Doubled.

> then developing the program that calls Doubled should work
> up to a point, namely when Natural's representation finally
> matters.

It never matters if post-condition you wrote was meant proper
post-condition = checked at compile time.

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