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13.11.1 Primitives to manipulate advices

macro add-function where place function \&optional props​

This macro is the handy way to add the advice function to the function stored in place (see Generalized Variables).

where determines how function is composed with the existing function, e.g., whether function should be called before, or after the original function. See Advice Combinators, for the list of available ways to compose the two functions.

When modifying a variable (whose name will usually end with -function), you can choose whether function is used globally or only in the current buffer: if place is just a symbol, then function is added to the global value of place. Whereas if place is of the form (local symbol), where symbol is an expression which returns the variable name, then function will only be added in the current buffer. Finally, if you want to modify a lexical variable, you will have to use (var variable).

Every function added with add-function can be accompanied by an association list of properties props. Currently only two of those properties have a special meaning:

name​

This gives a name to the advice, which remove-function can use to identify which function to remove. Typically used when function is an anonymous function.

depth​

This specifies how to order the advice, should several pieces of advice be present. By default, the depth is 0. A depth of 100 indicates that this piece of advice should be kept as deep as possible, whereas a depth of -100 indicates that it should stay as the outermost piece. When two pieces of advice specify the same depth, the most recently added one will be outermost.

For :before advice, being outermost means that this advice will be run first, before any other advice, whereas being innermost means that it will run right before the original function, with no other advice run between itself and the original function. Similarly, for :after advice innermost means that it will run right after the original function, with no other advice run in between, whereas outermost means that it will be run right at the end after all other advice. An innermost :override piece of advice will only override the original function and other pieces of advice will apply to it, whereas an outermost :override piece of advice will override not only the original function but all other advice applied to it as well.

If function is not interactive, then the combined function will inherit the interactive spec, if any, of the original function. Else, the combined function will be interactive and will use the interactive spec of function. One exception: if the interactive spec of function is a function (i.e., a lambda expression or an fbound symbol rather than an expression or a string), then the interactive spec of the combined function will be a call to that function with as sole argument the interactive spec of the original function. To interpret the spec received as argument, use advice-eval-interactive-spec.

Note: The interactive spec of function will apply to the combined function and should hence obey the calling convention of the combined function rather than that of function. In many cases, it makes no difference since they are identical, but it does matter for :around, :filter-args, and :filter-return, where function receives different arguments than the original function stored in place.

macro remove-function place function​

This macro removes function from the function stored in place. This only works if function was added to place using add-function.

function is compared with functions added to place using equal, to try and make it work also with lambda expressions. It is additionally compared also with the name property of the functions added to place, which can be more reliable than comparing lambda expressions using equal.

function advice-function-member-p advice function-def​

Return non-nil if advice is already in function-def. Like for remove-function above, instead of advice being the actual function, it can also be the name of the piece of advice.

function advice-function-mapc f function-def​

Call the function f for every piece of advice that was added to function-def. f is called with two arguments: the advice function and its properties.

function advice-eval-interactive-spec spec​

Evaluate the interactive spec just like an interactive call to a function with such a spec would, and then return the corresponding list of arguments that was built. E.g., (advice-eval-interactive-spec "r\nP") will return a list of three elements, containing the boundaries of the region and the current prefix argument.

For instance, if you want to make the C-x m (compose-mail) command prompt for a ‘From:’ header, you could say something like this:

(defun my-compose-mail-advice (orig &rest args)
  "Read From: address interactively."
  (interactive
   (lambda (spec)
     (let* ((user-mail-address
             (completing-read "From: "
                              '("one.address@example.net"
                                "alternative.address@example.net")))
            (from (message-make-from user-full-name
                                     user-mail-address))
            (spec (advice-eval-interactive-spec spec)))
       ;; Put the From header into the OTHER-HEADERS argument.
       (push (cons 'From from) (nth 2 spec))
       spec)))
  (apply orig args))

(advice-add 'compose-mail :around #'my-compose-mail-advice)