23.2.1 Conventions for Writing Minor Modes

There are conventions for writing minor modes just as there are for major modes. Several of the major mode conventions apply to minor modes as well: those regarding the name of the mode initialization function, the names of global symbols, and the use of keymaps and other tables.

In addition, there are several conventions that are specific to minor modes.

• Make a variable whose name ends in ‘-mode’ to control the minor mode. We call this the mode variable. The minor mode command should set this variable (nil to disable; anything else to enable).

  If possible, implement the mode so that setting the variable automatically enables or disables the mode. Then the minor mode command does not need to do anything except set the variable.

  This variable is used in conjunction with the minor-mode-alist to display the minor mode name in the mode line. It can also enable or disable a minor mode keymap. Individual commands or hooks can also check the variable's value.

  If you want the minor mode to be enabled separately in each buffer, make the variable buffer-local.

• Define a command whose name is the same as the mode variable. Its job is to enable and disable the mode by setting the variable.

  The command should accept one optional argument. If the argument is nil, it should toggle the mode (turn it on if it is off, and off if it is on). Otherwise, it should turn the mode on if the argument is a positive integer, a symbol other than nil or -, or a list whose car is such an integer or symbol; it should turn the mode off otherwise.

  Here is an example taken from the definition of transient-mark-mode. It shows the use of transient-mark-mode as a variable that enables or disables the mode's behavior, and also shows the proper way to toggle, enable or disable the minor mode based on the raw prefix argument value.

            (setq transient-mark-mode
                  (if (null arg) (not transient-mark-mode)
                    (> (prefix-numeric-value arg) 0)))
       

• Add an element to minor-mode-alist for each minor mode (see Mode Line Variables), 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)
              (setq minor-mode-alist
                    (cons '(leif-mode " Leif") minor-mode-alist)))
       

  or like this, using add-to-list (see Setting Variables):

            (add-to-list 'minor-mode-alist '(leif-mode " Leif"))
       

Global minor modes distributed with Emacs should if possible support enabling and disabling via Custom (see Customization). To do this, the first step is to define the mode variable with defcustom, and specify :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 invoke 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), and specify a :require so that customizing the variable will load the library that defines the mode. This will copy suitable definitions into loaddefs.el so that users can use customize-option to enable 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 (lambda (symbol value)
     	 (msb-mode (or value 0)))
       :initialize 'custom-initialize-default
       :version "20.4"
       :type    'boolean
       :group   'msb
       :require 'msb)