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Advice on troubleshooting function to "unscroll"
From: |
Will Monroe |
Subject: |
Advice on troubleshooting function to "unscroll" |
Date: |
Sun, 25 Jan 2015 14:52:13 -0600 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.4.0 |
Hello,
I'm studying Emacs Lisp so that I can become more proficient at
customizing GNU Emacs for purposes, mainly using org-mode. I'm studying
alone using Bob Glickstein's *Writing GNU Emacs Extensions*. So far,
the book has been a terrific guide to basic concepts. But I've run into
a problem that I can't seem to troubleshoot any further and I'd
appreciate advice on: 1) the best way to troubleshoot this problem
(e.g., other forums, listservs, or IRC channels that might be more
appropriate) and 2) troubleshooting the problem itself. I apologize in
advance if this query has failed to take into account any basics or if
it would be better posed elsewhere. I welcome your on the best way to
proceed.
So, following Glickstein's advice in Chapter 3, I'm trying to create a
function that will allow me to create an "unscroll" function that will
allow me to return to a prior buffer position after (inadvertently)
pressing C-v a few times. The code to do this defines three new
variables, defines advice for an extant Emacs function, scroll-up, and
then defines a new function, unscroll. The code is below.
#+BEGIN_SRC emacs-lisp
(defvar unscroll-point nil
"Cursor position for next call to 'unscroll'.")
(defvar unscroll-window-start nil
"Window start for next call to 'unscroll'.")
(defvar unscroll-hscroll nil
"Hscroll start for next call to 'unscroll'.")
(defadvice scroll-up (before remember-for-unscroll
activate compile)
"Remember where we started from, for 'unscroll'."
(if (not (eq last-command 'scroll-up))
(setq unscroll-point (point)
unscroll-window-start (window-start)
unscroll-hscroll (window-hscroll))))
(defun unscroll ()
"Revert to 'unscroll-point' and 'unscroll-window-start'."
(interactive)
(goto-char unscroll-point)
(set-window-start nil unscroll-window-start)
(set-window-hscroll nil unscroll-hscroll))
#+END_SRC
I've loaded this code in Emacs under two different conditions: 1) by
firing up a vanilla Emacs instance without any configuration file, and
evaluating the code in scratch and 2) by saving the above code as the
sole contents in a configuration file (~/.emacs.d/init.el) and then
starting up Emacs. I've done this on two different machines both
running versions of Emacs 24.
The test was just opening a lengthy file, usually an info page or an
existing org-mode file, pressing C-v a few times, and then using M-x
unscroll. In all cases, I found that M-x unscroll would return to the
position just before the last C-v but not to the original position. In
other words, if I pressed C-v two times and then pressed M-x unscroll,
in would only go back one C-v. My intent, and that of the example in
the book, is to return the point the position before any C-v key
sequences were pressed.
I could move one but I suspect that the mistake I've made will come up
again. I'd really welcome your (gentle) feedback!
All the best,
Will