From: Denis Bueno <dbueno@gmail.com>
Subject: Re: Error: Symbol's value as variable is void
Date: Fri, 25 Mar 2005 14:26:59 -0500 [thread overview]
Message-ID: <6dbd4d0005032511264f58d3a5@mail.gmail.com> (raw)
In-Reply-To: <E1DEtXy-0005uB-00@lab45.ma.utexas.edu>
On Fri, 25 Mar 2005 12:28:38 -0600, Joe Corneli
<jcorneli@math.utexas.edu> wrote:
>
> I wrote some code that contains the following forms:
>
> (defvar nero-link-regexp "\\[\\([0-9]+\\)\\]"
> "Regular expression that tells nero what links look like.
> The first parenthesized subexpression is the unique string
> denoting the webpage to load, which will sought among the
> references.")
I'm guessing that if you wrap the above defvar in an
(eval-when-compile ...) and leave the other eval-when-compile present,
all will be fine.
>
> (defvar nero-font-lock-keywords
> (eval-when-compile
> (list `(,nero-link-regexp . font-lock-keyword-face)))
> "Font lock for `nero-mode'.
> Currently, only numbered links are fontified.")
>
> These work fine for me, but another person using the code reports the
> following error upon byte-compiling:
>
> Compiling file /stor/garray/src/nero.el at Fri Mar 25 08:21:48 2005
> nero.el:295:33:Error: Symbol's value as variable is void: nero-link-regexp
>
> I don't see any such error when I byte compile. I'm just curious to
> know if using `eval-when-compile' here is bad form, or what. I think
> I've seen it being used in other packages in a similar context, but I
> don't understand it well enough to know whether I should be using it
> here.
>
> Removing it did make the other user's error go away. I told him to
> create a bug report, but if its just the fault of my bad code, I'd
> like to know. More generally, as a point of style, when is it good to
> use `eval-when-compile'
Seems silly, but I think the answer is "whenever you need to evaluate
something at compile time". I once needed to use a similar form
(albeit I was programming in Common Lisp at the time, but
nevertheless), because I needed to evaluate a constant at
compile-time. In emacs lisp, the code would have looked something like
this:
(defvar c1 1)
(defvar c2 2)
(defvar c3 3)
(defun foo (x ...)
(case x
((eval-when-compile c1) (do-something))
((eval-when-compile c2) (do-something-else))))
The reason I need the eval-when-compile was that `case' by default
doesn't evaluate its cases; so if eval-when-compile weren't there,
`case' would compare the value of x to the symbol c1, then c2, etc.
Usu. the case described above doesn't come up because you do things like:
(case x
(3 (do-something))
(4 (do-something-else)))
i.e., you have literal values as your cases. I didn't in my case.
(Note: The `case' form might work differently in emacs lisp; but
that's the way it works in common lisp. I'm just trying to give an
example of what I think is a proper usage of `eval-when-compile'.)
(Just to avoid befuddlement, the common lisp code looked like:
(defconstant c1 1)
(defconstant c2 2)
(defun foo (x ...)
(case x
(#.c1 (do-something))
(#.c2 (do-something-else))))
In CL #. is a read-macro which evaluates its argument at read-time.)
Hope I haven't confused you. =]
--
Denis Bueno
PGP: http://pgp.mit.edu:11371/pks/lookup?search=0xA1B51B4B&op=index
next prev parent reply other threads:[~2005-03-25 19:26 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2005-03-25 18:28 Error: Symbol's value as variable is void Joe Corneli
2005-03-25 19:26 ` Denis Bueno [this message]
[not found] <mailman.227.1111777231.28103.help-gnu-emacs@gnu.org>
2005-03-26 0:00 ` Thien-Thi Nguyen
2005-03-26 2:48 ` Joe Corneli
[not found] ` <mailman.255.1111806378.28103.help-gnu-emacs@gnu.org>
2005-03-27 0:12 ` Stefan Monnier
2005-03-27 0:45 ` Joe Corneli
2005-03-28 20:18 ` Kevin Rodgers
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