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Re: emacs lisp confusion using append
From: |
David Kastrup |
Subject: |
Re: emacs lisp confusion using append |
Date: |
05 Jun 2003 08:53:13 +0200 |
User-agent: |
Gnus/5.09 (Gnus v5.9.0) Emacs/21.3.50 |
Balaji Venkataraman <bvenkata+nospam@sm.intel.com> writes:
> I was trying to write some lisp code, but am completely boggled by this.
>
> Here's what I originally had in my ~/.emacs:
> --WAS WORKING--
> (defun append-path (elisp-path)
> "Appends ELISP-PATH path to the load-path.
> Emacs will look for additional .el files there"
> (setq load-path (append load-path (list (expand-file-name elisp-path)))))
>
> (append-path "~/usr/elisp")
> --WAS WORKING--
For some definition of "working". This will not hesitate to add more
and more identical copies to the path each time you call it.
> Then I changed it to this:
> --NOT WORKING--
> (defun append-path (orig-path path)
> "Appends PATH to ORIG-PATH. PATH is a string and ORIG-PATH is a variable.
> If ORIG-PATH is not defined, returns nil."
> (if (boundp 'orig-path)
> (setq orig-path (append orig-path (list (expand-file-name path))))
> nil))
>
> (append-path load-path "~/usr/elisp")
> --NOT WORKING--
>
> Now "~/usr/elisp" is *missing* from the load-path.
Well, but it _is_ in orig-path (before append-path exits). Lisp is
call-by-value, not call-by-reference.
You need to use a macro to achieve call-by-reference. Like
(defmacro append-path (orig-path path)
`(and (boundp ',orig-path)
(add-to-list ',orig-path ,path t)))
> I don't understand why the first one works and the second doesn't. I
> no lisp expert and I realise there are many deficiencies in my
> code. Is listp a better substitute for boundp?
listp is something completely different. boundp checks whether a given
name has a value. listp checks whether a given value is a list.
--
David Kastrup, Kriemhildstr. 15, 44793 Bochum