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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: Operation can be dispatching in only one type
Date: Thu, 19 Nov 2009 14:39:14 -0500
Organization: The World Public Access UNIX, Brookline, MA
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Date: 2009-11-19T14:39:14-05:00
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Adam Beneschan <adam@irvine.com> writes:

> On Nov 18, 6:11�pm, Robert A Duff <bobd...@shell01.TheWorld.com>
> wrote:
>
>> My objection to run-time accessibility checks is that you can't pin the
>> blame on a particular line of code:
>>
>> � � procedure P (X : access T);
>>
>> P either stores the access value in a long-lived global data structure
>> (in which case P(Local'Access) is a bad idea in clients)
>> or P does not do that
>> (in which case P(Local'Access) is just fine).
>> The author of P knows which one is the case,
>> but has no way (other than comments) to communicate this to
>> the author of the client.
>> I've never seen a case where P decides at run time which it is.
>
> Since I'm too lazy to look, I'm hoping you know this off the top of
> your head: is there anything in the AI's about preconditions that
> would allow something to be caught when P is called rather than later
> in the body of P?  I thought there were also some proposed attributes
> that could test accessibility levels.

Such things have been discussed.  We're probably going to have
preconditions, and I seem to recall discussing some way to query the
necessary accessibility-level info.

But I think perhaps the best solution is to use a named access type,
or an 'in out' parameter, in most cases.  Functions are going to allow
'in out' parameters (probably), so the need for access parameters
is much diminished.

- Bob