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bug#1406: backward-up-list reports scan error incorrectly?

From: Alan Mackenzie
Subject: bug#1406: backward-up-list reports scan error incorrectly?
Date: Sat, 22 Nov 2008 12:36:45 +0000
User-agent: Mutt/1.5.9i

Hi, Xah!

On Fri, Nov 21, 2008 at 03:26:03PM -0800, xah lee wrote:
> Hi Alan,

> it works if you switch to text mode though.

> It doesn't work while in emacs-lisp-mode and fundamental mode.

> In any case, normally it works when inside string too. Just not in  
> this particular case.

OK, I think half of the problem here is your prolixity, the other half
is my prolixity.  :-)

Try instead the following example:

(defun foo (arg)
  "foo does nothing."
  (interactive "P"))

Put point inside "P", and do C-M-u `backward-up-list'.  You get the
error.  Now modify the above function by inserting an open paren into
the doc string,

(defun foo (arg)
  "foo does nothing. ("
    (interactive "P"))

, and do the same again.  It finds that paren.  Why?  Because
backward-up-list assumes its starting point is NOT in a string.  It
jumps backwards over (what it thinks is) the string

    "\n(interactive "

, and then finds the ?\(. 

This is exactly what is happening in your somewhat larger example.

In text mode, presumably the syntax table doesn't define ?\" as a string
quote.  In fundamental mode, presumably it does.  "(syntax-after
(point))" is your friend here.

>   Xah

Alan Mackenzie (Nuremberg, Germany).

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