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From: Robert Dewar <dewar@gnat.com>
Subject: Re: generic package dilemma
Date: 1999/12/01
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In article <wccvh6l3wva.fsf@world.std.com>,
  Robert A Duff <bobduff@world.std.com> wrote:
> So the ACVC isn't interesting.  What's interesting is real
programs with
> hundreds (or more) of library packages, all trying to
initialize
> themselves at elaboration time in the most natural way.


As you know, the GNAT static model is the default model. The
GNAT approach is in practice upwards compatible with the RM,
in that you can always change from this default model to the
dynamic model if you want.

What this means, making it the default, is that people naturally
use the static model until they run into trouble.

So far, with almost no exceptions (maybe none, but I hate to
say none without a very careful check), the only people who
have run into trouble are those porting large Ada 83 codes
where in many cases the elaboration is a huge mess.

In about half these cases, people have decided to clean things
up so that the static model can be used, on the grounds that it
is better to eliminate problems in the future.

In the other half of cases, people use -gnatE and go to the
dynamic model.

But for new code, people seem to have no trouble with the
static model, and get themselves into FAR less trouble.

In fact I think the mistake in the Ada 95 design process was
trying to do too much in the elaboration area, and consequently
ending up doing nothing (well we have elaborate_all, but I mean
nothing in terms of making the model more static). I think it
would have been a good idea to bless something like the
GNAT static model, which after all simply corresponds to good
coding practices in the first place.

The elaboration order is of course deterministic in GNAT.
However we don't think any programs should depend on the
knowledge of the order, so we don't document the determinisitc
algorithm.

Robert Dewar
Ada Core Technologiesb


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