Re: Working with constansts

[Nikolaj Schumacher <me@nschum.de>]

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