Re: Meta-code confusion

[Barry Margolin]

In article <mailman.6744.1344634688.855.help-gnu-emacs@gnu.org>,
 Doug Lewan <dougl@shubertticketing.com> wrote:

> Well, after a few more hours I have a work-around. I think the original 
> problem was because (defvar) is a special form, but I'm still not sure.
> 
> The current work-around is to build a string that looks right and then (eval 
> (read THAT-STRING)) and let the results of that be the return value. I can't 
> see the interesting difference, but it does what I want.

That's almost always the wrong thing.  Rather than that, you should 
build a list (probably using backquote to fill in a template) and then 
(eval THAT-LIST).

But more likely, you probably need to define your own macro rather than 
a function.

> 
> ,Doug
> 
> > -----Original Message-----
> > From: help-gnu-emacs-bounces+dougl=shubertticketing.com@gnu.org
> > [mailto:help-gnu-emacs-bounces+dougl=shubertticketing.com@gnu.org] On
> > Behalf Of Doug Lewan
> > Sent: Friday, 2012 August 10 15:12
> > To: help-gnu-emacs@gnu.org
> > Subject: Meta-code confusion
> > 
> > I have development environment that has a handful of files with
> > isomorphic (but not identical) structures.
> > I'd like to use the same code to define modes for all of them, but
> > clearly there are things I don't understand about writing meta-code in
> > emacs lisp.
> > 
> > As a starting point I'm failing with the following:
> > 
> > (defvar *pp-symbol-re* "\\(?:\\s_\\|\\sw\\)")
> > (defun ppm-define-vars (name abbreviation)
> >   "Build a list of vars for the new mode."
> >   (let* ((start-var-name (format "*pp-%s-start-re*" abbreviation))
> >      (start-var (make-symbol start-var-name))
> >      (start-re (list 'concat (format "^%s\\s-+" name) '*pp-symbol-
> > re*)))
> >     (list (list 'defvar start-var start-re))))
> > 
> > I want all such functions to return a list of definitions of the type
> > of interest.
> > This one happens to return a list of one element, [I would like] a
> > variable definition.
> > It certainly looks like it's returning the right kind of thing:
> > 
> > (car (ppm-define-vars "douglas" "dug"))
> > => (defvar *pp-dug-start-re* (concat "^douglas\\s-+" *pp-symbol-re*))
> > 
> > And (eval) looks like it does the right thing:
> > (eval (car (ppm-define-vars "douglas" "dug")))
> > => *pp-dug-start-re*
> > 
> > (defvar) returns a symbol and (symbolp) confirms that that is a symbol.
> > 
> > However, using *pp-dug-start-re* gets a void-variable error.
> > On the other hand
> > (boundp (eval (car (ppm-define-vars "douglas" "dug"))))
> > => t
> > which would seem to be a contradiction.
> > 
> > I'm using
> > GNU Emacs 24.1.1 (i686-pc-cygwin, GTK+ Version 2.24.10) of 2012-07-17
> > on fiona    and
> > GNU Emacs 24.1.1 (powerpc-ibm-aix6.1.0.0, X toolkit, Xaw scroll bars)
> > of 2012-07-25 on devlpar1.
> > 
> > I would gladly pay with a beer¹ to understand this better.
> > 
> > ,Douglas
> > Douglas Lewan
> > Shubert Ticketing
> > (201) 489-8600 ext 224
> > 
> > There is no national science just as there is no national
> > multiplication table; what is national is no longer science. - Anton
> > Checov
> > _____________
> > ¹ "beer" is a variable, replace it with an appropriate value should
> > beer be distasteful to you or otherwise inappropriate.
> >

-- 
Barry Margolin, barmar@alum.mit.edu
Arlington, MA
*** PLEASE post questions in newsgroups, not directly to me ***