Re: replacing a function with another one

[Florian Beck]

> >    (lambda (&rest r) (apply FUNCTION OLDFUN r))

This line tells you what the ADVICE does (in this case the around 
advice). But what does it mean?

1. Once you define an advice, instead of OLDFUN, emacs calls
(lambda (&rest r) (apply FUNCTION OLDFUN r)), which is a function that 
has any arguments and only calls (apply FUNCTION OLDFUN r)

2. &rest r means all (arbitrarily many) arguments are collected in the 
variable r.

3. apply then calls your FUNCTION with OLDFUN and the rest as arguments

For example, define a function:

(defun my-fun (a b)
  (list a b))

and a function we will add as an advice:

(defun my-advice (&rest args)
  args)

Add it:

(advice-add 'my-fun :around 'my-advice)

Now call (my-fun 'a 'b)

This returns ((lambda (a b) (list a b)) 1 2)

As you can see, your advice function gets called with three arguments of 
which the first is the original definition.

With that knowledge, we can redefine our advice function:

(defun my-advice (fun a b)
  (funcall fun a 'X))

using the first argument to call the original function.

And that is really all you have to know when advising a function

 - define a new function that takes the old function as an additional 
first argument

 - add the advice (advice-add 'my-fun :around 'my-advice)

Things are even easier with a :before advice, where you can do anything 
you want with the arguments:

(defun my-fun2 (a b)
  (list a b))

(defun my-advice2 (x y)
  (message "List contains %s and %s." x y))

(advice-add 'my-fun2 :before 'my-advice2)


--
Florian Beck