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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


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'."
  (goto-char unscroll-point)
  (set-window-start nil unscroll-window-start)
  (set-window-hscroll nil unscroll-hscroll))

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,


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