(lispref.info)Minor Mode Conventions

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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

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

   * Make a variable whose name ends in `-mode' to represent the minor
     mode.  Its value should enable or disable the mode (`nil' to
     disable; anything else to enable.)  We call this the "mode

     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 `overwrite-mode'.
     It shows the use of `overwrite-mode' as a variable which enables or
     disables the mode's behavior.

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

   * Add an element to `minor-mode-alist' for each minor mode (*note
     Mode Line Variables::.).  This element should be a list of the
     following form:


     Here MODE-VARIABLE is the variable that controls enablement 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:

          (or (assq 'leif-mode minor-mode-alist)
              (setq minor-mode-alist
                    (cons '(leif-mode " Leif") minor-mode-alist)))

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