From: Nikolaj Schumacher <me@nschum.de>
To: pjb@informatimago.com (Pascal J. Bourguignon)
Cc: help-gnu-emacs@gnu.org
Subject: Re: Working with constansts
Date: Tue, 12 May 2009 12:06:24 +0200 [thread overview]
Message-ID: <m24ovqfxen.fsf@nschum.de> (raw)
In-Reply-To: <87zldkxicx.fsf@galatea.local> (Pascal J. Bourguignon's message of "Mon, 11 May 2009 08:29:34 +0200")
pjb@informatimago.com (Pascal J. Bourguignon) wrote:
> Richard Riley <rileyrgdev@googlemail.com> writes:
>
>> Of course I understand if the answer is "history and that's the way it
>> is" but I would sympathise with a new programmer to Lisp that is
>> surprised he can modify a "const" especially if he came from a C/C++
>> background where we all fully understand WHY consts are useful for the
>> programmer but the compiler also enforced it.
>
> The important point is that a lisp system is being programmed at the
> same time it is executed. Therefore redefining a constant may be
> taken into account, because it may be what the _programmer_ really
> means.
>
> In C, you would have to recompile the program before a change to a
> constant is taken into account, but it would be very possible to
> modify a constant: nothing prevents you to edit the C sources,
> recompile and relaunch.
Certainly const in C doesn't mean the value is determined at compile
time. It just means: "This variable shouldn't be modified after its
initialization."
And in fact, you can cast constness away in C++, so it really has
nothing to do with execution versus compile time. It's just a helper
for the developer to prevent side-effects.
There really is no pressing requirement for the current behavior, the
run-time just doesn't verify it. It does one thing, though:
(setq xxx 'user)
(defvar xxx 'library)
xxx => 'user
(setq xxx 'user)
(defconst xxx 'library)
xxx => 'library
A tiny step towards enforcing the value.
regards,
Nikolaj Schumacher
next prev parent reply other threads:[~2009-05-12 10:06 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-05-10 15:25 Working with constansts Decebal
2009-05-10 16:19 ` Pascal J. Bourguignon
2009-05-10 16:20 ` Richard Riley
2009-05-10 16:33 ` Pascal J. Bourguignon
2009-05-12 10:34 ` Nikolaj Schumacher
2009-05-10 17:02 ` Drew Adams
2009-05-10 17:28 ` Richard Riley
2009-05-11 7:39 ` Tassilo Horn
[not found] ` <mailman.6953.1241976532.31690.help-gnu-emacs@gnu.org>
2009-05-10 18:17 ` Pascal J. Bourguignon
2009-05-11 1:36 ` Richard Riley
2009-05-11 6:29 ` Pascal J. Bourguignon
2009-05-12 10:06 ` Nikolaj Schumacher [this message]
[not found] ` <mailman.7056.1242122790.31690.help-gnu-emacs@gnu.org>
2009-05-12 11:54 ` Pascal J. Bourguignon
2009-05-18 10:55 ` Nikolaj Schumacher
[not found] ` <7ceiuuczad.fsf@pbourguignon.informatimago.com>
[not found] ` <mailman.7379.1242644154.31690.help-gnu-emacs@gnu.org>
2009-05-18 12:20 ` Pascal J. Bourguignon
2009-05-18 19:19 ` Nikolaj Schumacher
2009-05-10 18:59 ` Barry Margolin
2009-05-11 1:38 ` Richard Riley
2009-05-12 9:44 ` Nikolaj Schumacher
[not found] ` <mailman.7052.1242121473.31690.help-gnu-emacs@gnu.org>
2009-05-12 11:43 ` Pascal J. Bourguignon
2009-05-13 4:59 ` Barry Margolin
2009-05-13 13:41 ` Ralf Wachinger
2009-05-13 21:23 ` Barry Margolin
2009-05-11 9:58 ` Thien-Thi Nguyen
[not found] ` <mailman.6988.1242036217.31690.help-gnu-emacs@gnu.org>
2009-05-12 1:31 ` Barry Margolin
2009-05-11 8:27 ` Decebal
2009-05-12 9:46 ` Nikolaj Schumacher
2009-05-10 16:31 ` Drew Adams
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