23.1.2 Setting Hooks

Here’s an example that adds a function to a mode hook to turn on Auto Fill mode when in Lisp Interaction mode:

(add-hook 'lisp-interaction-mode-hook 'auto-fill-mode)

The value of a hook variable should be a list of functions. You can manipulate that list using the normal Lisp facilities, but the modular way is to use the functions add-hook and remove-hook, defined below. They take care to handle some unusual situations and avoid problems.

It works to put a lambda-expression function on a hook, but we recommend avoiding this because it can lead to confusion. If you add the same lambda-expression a second time but write it slightly differently, you will get two equivalent but distinct functions on the hook. If you then remove one of them, the other will still be on it.

function add-hook hook function \&optional depth local

This function is the handy way to add function function to hook variable hook. You can use it for abnormal hooks as well as for normal hooks. function can be any Lisp function that can accept the proper number of arguments for hook. For example,

(add-hook 'text-mode-hook 'my-text-hook-function)

adds my-text-hook-function to the hook called text-mode-hook.

If function is already present in hook (comparing using equal), then add-hook does not add it a second time.

If function has a non-nil property permanent-local-hook, then kill-all-local-variables (or changing major modes) won’t delete it from the hook variable’s local value.

For a normal hook, hook functions should be designed so that the order in which they are executed does not matter. Any dependence on the order is asking for trouble. However, the order is predictable: normally, function goes at the front of the hook list, so it is executed first (barring another add-hook call).

In some cases, it is important to control the relative ordering of functions on the hook. The optional argument depth lets you indicate where the function should be inserted in the list: it should then be a number between -100 and 100 where the higher the value, the closer to the end of the list the function should go. The depth defaults to 0 and for backward compatibility when depth is a non-nil symbol it is interpreted as a depth of 90. Furthermore, when depth is strictly greater than 0 the function is added after rather than before functions of the same depth. One should never use a depth of 100 (or -100), because one can never be sure that no other function will ever need to come before (or after) us.

add-hook can handle the cases where hook is void or its value is a single function; it sets or changes the value to a list of functions.

If local is non-nil, that says to add function to the buffer-local hook list instead of to the global hook list. This makes the hook buffer-local and adds t to the buffer-local value. The latter acts as a flag to run the hook functions in the default value as well as in the local value.

function remove-hook hook function \&optional local

This function removes function from the hook variable hook. It compares function with elements of hook using equal, so it works for both symbols and lambda expressions.

If local is non-nil, that says to remove function from the buffer-local hook list instead of from the global hook list.