Some Emacs advice

The concept of advice, as far as I know, comes from the Common Lisp Object System (CLOS) and was briefly popular in the Java world under the term “Aspect Oriented Programming”. It is a mechanism for not just overriding, but adding some additional functionality before, after, or around an existing function. In Emacs terms, you could almost think of it as every existing function not only having a “before” and “after” hook, but with functions in those hooks being able to change the arguments or return value, or even not call the original function.

There are two different ways of using advice in Emacs – defadvice, from the original advice.el (written in 1993) and the newer advice-add from nadvice.el (added in 2012). The newer library is much simpler in both functionality and implementation (advice.el is 3262 lines long, while nadvice.el is less than a fifth as long, at a compact 592 lines) and is generally recommended unless you need some of the advanced features of defadvice.

We’ll take a quick like at the older defadvice first; just the most basic features, since it has a lot of flexibility, but I never use it.

The basic usage looks like:

(defadvice function-to-be-advised (type-of-advice name-of-this-advice)
  ;; code
  )

Where type-of-advice is before, after, or around. There are many other options and ways of using defadvice that you can learn about by reading the documentation.

For example, let’s say you want to make it easier to create new entries in org-mode lists, so you want to be able to hit M-RET anywhere in the current line and have it create a new entry below, instead of splitting the current entry. One way to do this with advice would be as follows:

(defadvice org-meta-return (before eol-before)
  (org-end-of-line))
(ad-activate 'org-meta-return)

;; or, equivalently

(defadvice org-meta-return (before eol-before-2 activate)
  (org-end-of-line))

Here’s the equivalent using advice-add, something I actually have in my emacs configuration:

(advice-add 'org-meta-return :before #'org-end-of-line)

advice-add is a bit simpler and has fewer options. It has some optional arguments, but you mostly just need the three required ones here: A symbol naming the function to advise, the type of advice, and the advising function. There are actualy ten different types of advice you can use (see the help for add-function for a list and an explanation of what they all are), but in practice one mainly just uses four: :before, :after, :override and :around.

:before and :after are pretty straight-forward – they get called either before or after the original function is called and will recieve the same arguments as the original function.

:override simply replaces the original function with the advising one.

:around is more flexible – it recieves as argument the original function itself and all the arguments it was called with, so you can essentially do whatever you want: Do things before, then call the function but change some arguments, modify the return value, and do something after. Sky’s the limit!

The other options are essentially convenience wrappers to make some simple patterns of :around advice simpler to implement.

The inverse of advice-add is advice-remove. You can either use it to remove some advice you’ve added to a function, making it a much cleaner process if you want to only temporarily or conditionally change a function’s definition.

Finally, let’s have a look at some practical uses of advice. I have a couple uses of advice in my Emacs config. All of them are using the new advice-add – I haven’t had anything arise thus far that would require defadvice.