[Now largely historical. See ETA for achieving the same effect with bookmarks, and ETA 2 for achieving the same effect with Hydra, which feels better still.]

Yesterday’s Emacs coaching session with Sacha Chua included a sidebar on jumping to and setting default directories. Sacha’s .emacs.d uses registers for this, and her code sets defaults and org-refile targets.

I found later that I had six shortcuts I’d clean forgotten about, that set the default directory and open either dired or a specific file:

(global-set-key (kbd "C-c C-g C-c")
(lambda ()
(interactive)
(setq default-directory "~/.emacs.d/")
(dired ".")))

(global-set-key (kbd "C-c C-g C-h")
(lambda ()
(interactive)
(setq default-directory "~/Dropbox/gesta/")
(find-file "2015.org")))

But a forgotten solution doesn’t count, and having to remember triple-decker key bindings doesn’t help either.

A general solution for the second problem is that an incomplete key binding followed by ? opens a help buffer with all the bindings starting with sequence. C-c C-g ? on my machine yielded:

Global Bindings Starting With C-c C-g:
key binding
— ——-

C-c C-g C-c ??
C-c C-g C-d ??
C-c C-g C-e to-english
C-c C-g C-f to-french
C-c C-g C-h ??
C-c C-g C-p ??
C-c C-g C-r ??
C-c C-g C-u ??
C-c C-g C-w ??

We can make this more useful by switching from defining the key binding functions anonymously inline to moving them out to their own named functions, thus:

(defun my/to-emacs ()
(interactive)
(setq default-directory "~/.emacs.d/")
(dired "."))

(defun my/to-today ()
(interactive)
(setq default-directory "~/Dropbox/gesta/")
(find-file "2015.org"))

(global-set-key (kbd "C-c C-g C-c") ‘my/to-emacs)
(global-set-key (kbd "C-c C-g C-h") ‘my/to-today)

Which fixes those two ?? entries:

C-c C-g C-c my/to-emacs
C-c C-g C-h my/to-today

Another niggle is that I’ve overloaded C-c C-g across three different kinds of commands. Let’s make C-x j the shortcut specifically and only for jumping to a new default directory, and extract some duplicate methods while we’re at it. After:

(defun my/to-file (dir file)
(interactive)
(setq default-directory dir)
(find-file file))

(defun my/to-dir (dir)
(interactive)
(setq default-directory dir)
(dired "."))

(defun my/to-gesta-file (file)
(interactive)
(my/to-file "~/Dropbox/gesta/" file))

(defun my/to-emacs-config ()
(interactive)
(my/to-file "~/.emacs.d/" "sean.org"))

(defun my/to-autrui ()
(interactive)
(my/to-dir "~/code/autrui/"))

(defun my/to-gesta ()
(interactive)
(my/to-dir "~/Dropbox/gesta/"))

(defun my/to-today ()
(interactive)
(my/to-gesta-file "2015.org"))

(defun my/to-readings ()
(interactive)
(my/to-gesta-file "readings.org"))

(defun my/to-writings ()
(interactive)
(my/to-gesta-file "writings.org"))

(defun my/to-twc ()
(interactive)
(my/to-dir "~/Dropbox/gesta/twc/"))

(global-set-key (kbd "C-x j e") ‘my/to-emacs-config)
(global-set-key (kbd "C-x j a") ‘my/to-autrui)
(global-set-key (kbd "C-x j g") ‘my/to-gesta)
(global-set-key (kbd "C-x j h") ‘my/to-today)
(global-set-key (kbd "C-x j r") ‘my/to-readings)
(global-set-key (kbd "C-x j w") ‘my/to-writings)
(global-set-key (kbd "C-x j t") ‘my/to-twc)

C-x j ? then yields:

Global Bindings Starting With C-x j:
key binding
— ——-

C-x j a my/to-autrui
C-x j e my/to-emacs-config
C-x j g my/to-gesta
C-x j h my/to-today
C-x j r my/to-readings
C-x j t my/to-twc
C-x j w my/to-writings

Which is an improvement, but the “Bindings Starting With” help menu is not interactive: we still have to either remember the triple-decker key binding, or type C-x j ? to find the list and then type C-x j h (say) to get the specific shortcut we want. It would be nice if we could over-ride C-x j ? and have it prompt us for one more character to select which of the seven jumps we wanted.

Something like this, in fact:

(defun my/pick-destination (pick)
(interactive "ce = ~/.emacs.d/sean.org a = ~/code/autrui/ g = ~/Dropbox/gesta/ h = …/2015.org r = …/readings.org w = …/writings.org t = …/twc/ ? ")
(case pick
(?e (my/to-emacs-config))
(?a (my/to-autrui))
(?g (my/to-gesta))
(?h (my/to-today))
(?r (my/to-readings))
(?w (my/to-writings))
(?t (my/to-twc))))

(global-set-key (kbd "C-x j ?") ‘my/pick-destination)

interactive "c… takes a single character (see further options here), so the conditional is on a character, which in Emacs Lisp is represented by a ? followed by the character {n}. If there were a single conditional we might use (when (char-equal ?e pick) {n}, but since there are seven of them, we look instead for Emacs Lisp’s equivalent of a switch statement, and find it in case (an alias for cl-case).

So what this does, when you type C-x j ?, is provide a prompt of options in the echo area at the bottom of the screen, and if you type one of the significant letters, it uses that shortcut to default.

Purists would probably prefer to bind ’my/pick-destination to something other than C-x j ? (C-x j j say), so that if we ever bound other commands to something starting with C-x j we would still be able to discover them with C-x j ?. It’s also easier to type because it doesn’t need the shift key for the third element. Having demonstrated that we could over-ride C-x j ? if we wanted to, I’m probably going to side with the purists on this one.

And we’re done. Commit.