Hidden features of Emacs Lisp?

Question

What are some features of Emacs Lisp that you use to solve real problems?

• One feature per answer
• Give an example and short description of the feature, not just a link to documentation
• Label the feature using bold title as the first line

See also:

1. Hidden features of Python
2. Hidden features of Ruby
3. Hidden features of Perl
4. Hidden features of Java

Answer 1

emacs --script; # Emacs scripting

Since this is a Community question, I'll start off with my new favorite thing. Writing scripts with Emacs Lisp instead of bash.

#!/usr/bin/emacs --script

(defun surf-news ()
  (interactive)
  (progn
    (browse-url "http://news.ycombinator.com")
    (browse-url "http://stackoverflow.com")
      ))

(surf-news)

Answer 2

M-x sunrise-sunset

Takes a latitude and longitude, and tells you what time sunrise and sunset are. I use it when I need to be able to, with just a few keystrokes, know at a glance what time sunrise is. I like to imagine it'll come in handy.

Answer 3

Advice. It lets you customize behavior without modifying the original source.

Examples are:

• prompt to view buffer changes when killing a modified buffer
• changing M-x find-file history
• change M-x list-buffer behavior in emacsclient
• add unsupported indentation to a cc-mode derived mode

What's nice about advice is that you don't have to touch the original code, so when you get a new version of Emacs or whatever package you're modifying, you don't have to re-merge your changes with the new code.

The best way to learn advice is to go over the tutorial found in the advice package itself.

M-x find-library advice RET

and search for Foo games: An advice tutorial

The documentation found in that file is also very good (at least as good as the info pages).

Answer 4

(run-at-time time repeat function &rest args)

Perform an action at time time. Repeat the action every repeat seconds

todochiku for example uses this to 'growl' a set number of minutes in the future. I use that feature to set a reminder say 20 minutes into the future. See function todochiku-in, and my blog post about using growl from emacs.

The time can be specified in a flexible number of ways for example:

  (run-at-time "2:30pm" nil 'todochiku-message "Todochiku Timer" "Make coffee"
     (todochiku-icon 'bell)))

  (run-at-time "30 seconds" nil 'todochiku-message "Todochiku Timer" "Do some work"
     (todochiku-icon 'bell))

Change the nil to a number of seconds to have it repeat. Then when you want to disable:

(cancel-function-timers 'todochiku-message)

Answer 5

Fuzzy matching on M-x (and some other places)

M-x em-li-moTAB ... or M-x em*li*moTAB 

expands to:

M-x emacs-lisp-mode