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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: Question about circular elaboration order error (GNAT).
Date: Mon, 14 Apr 2008 09:56:03 -0400
Organization: The World Public Access UNIX, Brookline, MA
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Date: 2008-04-14T09:56:03-04:00
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"Peter C. Chapin" <pchapin@sover.net> writes:

> Thanks to you and Samuel Tardieu for your replies.

You're welcome.

>...They do help to
> clarify things for me somewhat. It is easy for me to understand how
> Parent.Child's body would have an elaboration dependency on Parent's
> spec, but I did not expect Parent.Child's body to necessarily have an
> elaboration dependency on Parent's body. Are you saying that it doesn't
> (necessarily) have such a dependency in standard Ada, but that I'm
> experiencing an example of GNAT's stricter rules?

Yes.  Your original example (with no pragmas added) is legal, and a
valid Ada compiler (such as GNAT with -gnatE) will compile and link it
just fine.  However, it might fail at run time.  Worst case: it works
fine, but 2 years later, you port it to a new compiler (new machine, or
even a new version of the same compiler), and you get a mysterious
Program_Error.

If you get an elab cycle when you modify your program (e.g., you add a
with_clause), it's easy to deal with -- your change is small, and you
know it caused the cycle.  But if you get a Program_Error when porting a
giant program to a new compiler, it can easily take several hours to
figure out the problem.

- Bob