A common use of advice is for named functions and macros.
You could just use
add-function as in:
(add-function :around (symbol-function 'fun) #'his-tracing-function)
But you should use
advice-remove for that
instead. This separate set of functions to manipulate pieces of advice applied
to named functions, offers the following extra features compared to
add-function: they know how to deal with macros and autoloaded
functions, they let
describe-function preserve the original docstring as
well as document the added advice, and they let you add and remove advice
before a function is even defined.
advice-add can be useful for altering the behavior of existing calls
to an existing function without having to redefine the whole function.
However, it can be a source of bugs, since existing callers to the function may
assume the old behavior, and work incorrectly when the behavior is changed by
advice. Advice can also cause confusion in debugging, if the person doing the
debugging does not notice or remember that the function has been modified
For these reasons, advice should be reserved for the cases where you cannot modify a function's behavior in any other way. If it is possible to do the same thing via a hook, that is preferable (see Hooks). If you simply want to change what a particular key does, it may be better to write a new command, and remap the old command's key bindings to the new one (see Remapping Commands). In particular, Emacs's own source files should not put advice on functions in Emacs. (There are currently a few exceptions to this convention, but we aim to correct them.)
Special forms (see Special Forms) cannot be advised, however macros can be advised, in much the same way as functions. Of course, this will not affect code that has already been macro-expanded, so you need to make sure the advice is installed before the macro is expanded.
It is possible to advise a primitive (see What Is a Function), but one should typically not do so, for two reasons. Firstly, some primitives are used by the advice mechanism, and advising them could cause an infinite recursion. Secondly, many primitives are called directly from C, and such calls ignore advice; hence, one ends up in a confusing situation where some calls (occurring from Lisp code) obey the advice and other calls (from C code) do not.
This macro defines a piece of advice and adds it to the function named symbol. The advice is an anonymous function if name is
nilor a function named
advice-addfor explanation of other arguments.
Add the advice function to the named function symbol. where and props have the same meaning as for
add-function(see Core Advising Primitives).
Remove the advice function from the named function symbol. function can also be the
nameof a piece of advice.
nilif the advice function is already in the named function symbol. function can also be the
nameof a piece of advice.