23.3.1 Conventions for Writing Minor Modes

There are conventions for writing minor modes just as there are for major modes (see Major Modes). These conventions are described below. The easiest way to follow them is to use the macro define-minor-mode. See Defining Minor Modes.

Define a variable whose name ends in ‘-mode’. We call this the mode variable. The minor mode command should set this variable. The value will be nil if the mode is disabled, and non-nil if the mode is enabled. The variable should be buffer-local if the minor mode is buffer-local.
This variable is used in conjunction with the minor-mode-alist to display the minor mode name in the mode line. It also determines whether the minor mode keymap is active, via minor-mode-map-alist (see Controlling Active Maps). Individual commands or hooks can also check its value.
Define a command, called the mode command, whose name is the same as the mode variable. Its job is to set the value of the mode variable, plus anything else that needs to be done to actually enable or disable the mode’s features.
The mode command should accept one optional argument. If called interactively with no prefix argument, it should toggle the mode (i.e., enable if it is disabled, and disable if it is enabled). If called interactively with a prefix argument, it should enable the mode if the argument is positive and disable it otherwise.
If the mode command is called from Lisp (i.e., non-interactively), it should enable the mode if the argument is omitted or nil; it should toggle the mode if the argument is the symbol toggle; otherwise it should treat the argument in the same way as for an interactive call with a numeric prefix argument, as described above.
The following example shows how to implement this behavior (it is similar to the code generated by the define-minor-mode macro):
```
(interactive (list (or current-prefix-arg 'toggle)))
(let ((enable
       (if (eq arg 'toggle)
           (not foo-mode) ; this is the mode’s mode variable
         (> (prefix-numeric-value arg) 0))))
  (if enable
      do-enable
    do-disable))
```
The reason for this somewhat complex behavior is that it lets users easily toggle the minor mode interactively, and also lets the minor mode be easily enabled in a mode hook, like this:
```
(add-hook 'text-mode-hook 'foo-mode)
```
This behaves correctly whether or not foo-mode was already enabled, since the foo-mode mode command unconditionally enables the minor mode when it is called from Lisp with no argument. Disabling a minor mode in a mode hook is a little uglier:
```
(add-hook 'text-mode-hook (lambda () (foo-mode -1)))
```
However, this is not very commonly done.
Enabling or disabling a minor mode twice in direct succession should not fail and should do the same thing as enabling or disabling it only once. In other words, the minor mode command should be idempotent.
Add an element to minor-mode-alist for each minor mode (see Definition of minor-mode-alist), if you want to indicate the minor mode in the mode line. This element should be a list of the following form:
```
(mode-variable string)
```
Here mode-variable is the variable that controls enabling of the minor mode, and string is a short string, starting with a space, to represent the mode in the mode line. These strings must be short so that there is room for several of them at once.
When you add an element to minor-mode-alist, use assq to check for an existing element, to avoid duplication. For example:
```
(unless (assq 'leif-mode minor-mode-alist)
  (push '(leif-mode " Leif") minor-mode-alist))
```
or like this, using add-to-list (see List Variables):
```
(add-to-list 'minor-mode-alist '(leif-mode " Leif"))
```

In addition, several major mode conventions (see Major Mode Conventions) apply to minor modes as well: those regarding the names of global symbols, the use of a hook at the end of the initialization function, and the use of keymaps and other tables.

The minor mode should, if possible, support enabling and disabling via Custom (see Customization). To do this, the mode variable should be defined with defcustom, usually with :type 'boolean. If just setting the variable is not sufficient to enable the mode, you should also specify a :set method which enables the mode by invoking the mode command. Note in the variable’s documentation string that setting the variable other than via Custom may not take effect. Also, mark the definition with an autoload cookie (see autoload cookie), and specify a :require so that customizing the variable will load the library that defines the mode. For example:

;;;###autoload
(defcustom msb-mode nil
  "Toggle msb-mode.
Setting this variable directly does not take effect;
use either \\[customize] or the function `msb-mode'."
  :set 'custom-set-minor-mode
  :initialize 'custom-initialize-default
  :version "20.4"
  :type    'boolean
  :group   'msb
  :require 'msb)