Writing GNU Emacs Extensions

Generalizing the Solution

It's easy to press C-v when meaning to press C-b. That's what led us to devise the unscroll function. Now observe that it's just as easy to press M-v (scroll-down) when meaning to press M-b (backward-word). It's the same problem, but in the other direction, sort of. It would be nice if we could generalize unscroll to undo scrolling in any direction.

The obvious way to generalize unscroll is to advise scroll-down in the same way that we advised scroll-up:

(defadvice scroll-down (before remember-for-unscroll
                        activate compile)
  "Remember where we started from, for 'unscroll'."
  (if (not (eq last-command 'scroll-down))
      (setq unscroll-point (point)
            unscroll-window-start (window-start)
            unscroll-hscroll (window-hscroll))))

(Note that two functions, such as scroll-up and scroll-down, may have identically named pieces of advice, such as remember-for-unscroll, without conflict.)

Now we must decide how we want unscroll to behave in the case where we mingle erroneous C-vs with erroneous M-vs. In other words, suppose you mistakenly press C-v C-v M-v. Should unscroll revert to the position before the M-v, or should it revert all the way back to the position before the first C-v?

I prefer the latter behavior. But this means that in the advice for scroll-up, where we now test whether the last command was scroll-up, we must now test whether it was either scroll-up or scroll-down, and do the same in scroll-down.

(defadvice scroll-up (before remember-for-unscroll activate compile) ...