Emacs Registers

Over at the Emacs subreddit, kosakgroove has just learned about Emacs registers and considers it a game changer. His post is little more than a headline so all the action is in the comments. Surprisingly—at least to me—many of the commenters were unaware that registers even existed.

For me, registers are useful in two situations:

1. I want to save my current window configuration so that I can run some function—eshell, for example—in full screen, and reinstate my previous window configuration when I’m done.
2. Saving values or counters in keyboard macros.

For example, here’s how I run eshell:

(global-set-key (kbd "H-e")
                (lambda (exit)
                  "Bring up a full-screen eshell or restore previous config.
With a prefix argument, exit eshell before restoring previous config."
                  (interactive "P")
                  (if (string= "eshell-mode" major-mode)
                      (progn
                        (when exit
                          (insert "exit")
                          (eshell-send-input))
                        (jump-to-register :eshell-fullscreen))
                    (progn
                      (window-configuration-to-register :eshell-fullscreen)
                      (eshell)
                      (delete-other-windows)))))

The important thing to notice in this example is the use of :eshell-fullscreen as the register key. That’s a trick I learned from Magnar Sveen. In interactive use, the key has to be a single character but because of an implementation artifact, any symbol will do from within Elisp. That’s nice because you don’t have to worry about having a user write something over your saved window configuration. I use this same trick a lot in my init.el.

Here’s an example of using a counter register in a keyboard macro. Both of these example are a bit specialized. You can simply save some text or a position in a register and insert it later when needed. Registers really are an almost magical power and it pays to get familiar with them.