Sync with r21-2-33 and r21-2-33-utf-2000.

[chise/xemacs-chise.git-] / info / lispref.info-21

This is ../info/lispref.info, produced by makeinfo version 4.0 from
lispref/lispref.texi.

INFO-DIR-SECTION XEmacs Editor
START-INFO-DIR-ENTRY
* Lispref: (lispref).           XEmacs Lisp Reference Manual.
END-INFO-DIR-ENTRY

   Edition History:

   GNU Emacs Lisp Reference Manual Second Edition (v2.01), May 1993 GNU
Emacs Lisp Reference Manual Further Revised (v2.02), August 1993 Lucid
Emacs Lisp Reference Manual (for 19.10) First Edition, March 1994
XEmacs Lisp Programmer's Manual (for 19.12) Second Edition, April 1995
GNU Emacs Lisp Reference Manual v2.4, June 1995 XEmacs Lisp
Programmer's Manual (for 19.13) Third Edition, July 1995 XEmacs Lisp
Reference Manual (for 19.14 and 20.0) v3.1, March 1996 XEmacs Lisp
Reference Manual (for 19.15 and 20.1, 20.2, 20.3) v3.2, April, May,
November 1997 XEmacs Lisp Reference Manual (for 21.0) v3.3, April 1998

   Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995 Free Software
Foundation, Inc.  Copyright (C) 1994, 1995 Sun Microsystems, Inc.
Copyright (C) 1995, 1996 Ben Wing.

   Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

   Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided that the
entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

   Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions, except that this permission notice may be stated in a
translation approved by the Foundation.

   Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided also
that the section entitled "GNU General Public License" is included
exactly as in the original, and provided that the entire resulting
derived work is distributed under the terms of a permission notice
identical to this one.

   Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions, except that the section entitled "GNU General Public License"
may be included in a translation approved by the Free Software
Foundation instead of in the original English.

\1f
File: lispref.info,  Node: Major Modes,  Next: Minor Modes,  Up: Modes

Major Modes
===========

   Major modes specialize XEmacs for editing particular kinds of text.
Each buffer has only one major mode at a time.

   The least specialized major mode is called "Fundamental mode".  This
mode has no mode-specific definitions or variable settings, so each
XEmacs command behaves in its default manner, and each option is in its
default state.  All other major modes redefine various keys and options.
For example, Lisp Interaction mode provides special key bindings for
<LFD> (`eval-print-last-sexp'), <TAB> (`lisp-indent-line'), and other
keys.

   When you need to write several editing commands to help you perform a
specialized editing task, creating a new major mode is usually a good
idea.  In practice, writing a major mode is easy (in contrast to
writing a minor mode, which is often difficult).

   If the new mode is similar to an old one, it is often unwise to
modify the old one to serve two purposes, since it may become harder to
use and maintain.  Instead, copy and rename an existing major mode
definition and alter the copy--or define a "derived mode" (*note
Derived Modes::).  For example, Rmail Edit mode, which is in
`emacs/lisp/rmailedit.el', is a major mode that is very similar to Text
mode except that it provides three additional commands.  Its definition
is distinct from that of Text mode, but was derived from it.

   Rmail Edit mode is an example of a case where one piece of text is
put temporarily into a different major mode so it can be edited in a
different way (with ordinary XEmacs commands rather than Rmail).  In
such cases, the temporary major mode usually has a command to switch
back to the buffer's usual mode (Rmail mode, in this case).  You might
be tempted to present the temporary redefinitions inside a recursive
edit and restore the usual ones when the user exits; but this is a bad
idea because it constrains the user's options when it is done in more
than one buffer: recursive edits must be exited most-recently-entered
first.  Using alternative major modes avoids this limitation.  *Note
Recursive Editing::.

   The standard XEmacs Lisp library directory contains the code for
several major modes, in files including `text-mode.el', `texinfo.el',
`lisp-mode.el', `c-mode.el', and `rmail.el'.  You can look at these
libraries to see how modes are written.  Text mode is perhaps the
simplest major mode aside from Fundamental mode.  Rmail mode is a
complicated and specialized mode.

* Menu:

* Major Mode Conventions::  Coding conventions for keymaps, etc.
* Example Major Modes::     Text mode and Lisp modes.
* Auto Major Mode::         How XEmacs chooses the major mode automatically.
* Mode Help::               Finding out how to use a mode.
* Derived Modes::           Defining a new major mode based on another major
                              mode.

\1f
File: lispref.info,  Node: Major Mode Conventions,  Next: Example Major Modes,  Up: Major Modes

Major Mode Conventions
----------------------

   The code for existing major modes follows various coding conventions,
including conventions for local keymap and syntax table initialization,
global names, and hooks.  Please follow these conventions when you
define a new major mode:

   * Define a command whose name ends in `-mode', with no arguments,
     that switches to the new mode in the current buffer.  This command
     should set up the keymap, syntax table, and local variables in an
     existing buffer without changing the buffer's text.

   * Write a documentation string for this command that describes the
     special commands available in this mode.  `C-h m'
     (`describe-mode') in your mode will display this string.

     The documentation string may include the special documentation
     substrings, `\[COMMAND]', `\{KEYMAP}', and `\<KEYMAP>', that
     enable the documentation to adapt automatically to the user's own
     key bindings.  *Note Keys in Documentation::.

   * The major mode command should start by calling
     `kill-all-local-variables'.  This is what gets rid of the local
     variables of the major mode previously in effect.

   * The major mode command should set the variable `major-mode' to the
     major mode command symbol.  This is how `describe-mode' discovers
     which documentation to print.

   * The major mode command should set the variable `mode-name' to the
     "pretty" name of the mode, as a string.  This appears in the mode
     line.

   * Since all global names are in the same name space, all the global
     variables, constants, and functions that are part of the mode
     should have names that start with the major mode name (or with an
     abbreviation of it if the name is long).  *Note Style Tips::.

   * The major mode should usually have its own keymap, which is used
     as the local keymap in all buffers in that mode.  The major mode
     function should call `use-local-map' to install this local map.
     *Note Active Keymaps::, for more information.

     This keymap should be kept in a global variable named
     `MODENAME-mode-map'.  Normally the library that defines the mode
     sets this variable.

   * The mode may have its own syntax table or may share one with other
     related modes.  If it has its own syntax table, it should store
     this in a variable named `MODENAME-mode-syntax-table'.  *Note
     Syntax Tables::.

   * The mode may have its own abbrev table or may share one with other
     related modes.  If it has its own abbrev table, it should store
     this in a variable named `MODENAME-mode-abbrev-table'.  *Note
     Abbrev Tables::.

   * Use `defvar' to set mode-related variables, so that they are not
     reinitialized if they already have a value.  (Such reinitialization
     could discard customizations made by the user.)

   * To make a buffer-local binding for an Emacs customization
     variable, use `make-local-variable' in the major mode command, not
     `make-variable-buffer-local'.  The latter function would make the
     variable local to every buffer in which it is subsequently set,
     which would affect buffers that do not use this mode.  It is
     undesirable for a mode to have such global effects.  *Note
     Buffer-Local Variables::.

     It's ok to use `make-variable-buffer-local', if you wish, for a
     variable used only within a single Lisp package.

   * Each major mode should have a "mode hook" named
     `MODENAME-mode-hook'.  The major mode command should run that
     hook, with `run-hooks', as the very last thing it does. *Note
     Hooks::.

   * The major mode command may also run the hooks of some more basic
     modes.  For example, `indented-text-mode' runs `text-mode-hook' as
     well as `indented-text-mode-hook'.  It may run these other hooks
     immediately before the mode's own hook (that is, after everything
     else), or it may run them earlier.

   * If something special should be done if the user switches a buffer
     from this mode to any other major mode, the mode can set a local
     value for `change-major-mode-hook'.

   * If this mode is appropriate only for specially-prepared text, then
     the major mode command symbol should have a property named
     `mode-class' with value `special', put on as follows:

          (put 'funny-mode 'mode-class 'special)

     This tells XEmacs that new buffers created while the current
     buffer has Funny mode should not inherit Funny mode.  Modes such
     as Dired, Rmail, and Buffer List use this feature.

   * If you want to make the new mode the default for files with certain
     recognizable names, add an element to `auto-mode-alist' to select
     the mode for those file names.  If you define the mode command to
     autoload, you should add this element in the same file that calls
     `autoload'.  Otherwise, it is sufficient to add the element in the
     file that contains the mode definition.  *Note Auto Major Mode::.

   * In the documentation, you should provide a sample `autoload' form
     and an example of how to add to `auto-mode-alist', that users can
     include in their `.emacs' files.

   * The top-level forms in the file defining the mode should be
     written so that they may be evaluated more than once without
     adverse consequences.  Even if you never load the file more than
     once, someone else will.

 - Variable: change-major-mode-hook
     This normal hook is run by `kill-all-local-variables' before it
     does anything else.  This gives major modes a way to arrange for
     something special to be done if the user switches to a different
     major mode.  For best results, make this variable buffer-local, so
     that it will disappear after doing its job and will not interfere
     with the subsequent major mode.  *Note Hooks::.

\1f
File: lispref.info,  Node: Example Major Modes,  Next: Auto Major Mode,  Prev: Major Mode Conventions,  Up: Major Modes

Major Mode Examples
-------------------

   Text mode is perhaps the simplest mode besides Fundamental mode.
Here are excerpts from  `text-mode.el' that illustrate many of the
conventions listed above:

     ;; Create mode-specific tables.
     (defvar text-mode-syntax-table nil
       "Syntax table used while in text mode.")
     
     (if text-mode-syntax-table
         ()              ; Do not change the table if it is already set up.
       (setq text-mode-syntax-table (make-syntax-table))
       (modify-syntax-entry ?\" ".   " text-mode-syntax-table)
       (modify-syntax-entry ?\\ ".   " text-mode-syntax-table)
       (modify-syntax-entry ?' "w   " text-mode-syntax-table))
     
     (defvar text-mode-abbrev-table nil
       "Abbrev table used while in text mode.")
     (define-abbrev-table 'text-mode-abbrev-table ())
     
     (defvar text-mode-map nil)   ; Create a mode-specific keymap.
     
     (if text-mode-map
         ()              ; Do not change the keymap if it is already set up.
       (setq text-mode-map (make-sparse-keymap))
       (define-key text-mode-map "\t" 'tab-to-tab-stop)
       (define-key text-mode-map "\es" 'center-line)
       (define-key text-mode-map "\eS" 'center-paragraph))

   Here is the complete major mode function definition for Text mode:

     (defun text-mode ()
       "Major mode for editing text intended for humans to read.
      Special commands: \\{text-mode-map}
     Turning on text-mode runs the hook `text-mode-hook'."
       (interactive)
       (kill-all-local-variables)
       (use-local-map text-mode-map)     ; This provides the local keymap.
       (setq mode-name "Text")           ; This name goes into the modeline.
       (setq major-mode 'text-mode)      ; This is how `describe-mode'
                                         ;   finds the doc string to print.
       (setq local-abbrev-table text-mode-abbrev-table)
       (set-syntax-table text-mode-syntax-table)
       (run-hooks 'text-mode-hook))      ; Finally, this permits the user to
                                         ;   customize the mode with a hook.

   The three Lisp modes (Lisp mode, Emacs Lisp mode, and Lisp
Interaction mode) have more features than Text mode and the code is
correspondingly more complicated.  Here are excerpts from
`lisp-mode.el' that illustrate how these modes are written.

     ;; Create mode-specific table variables.
     (defvar lisp-mode-syntax-table nil "")
     (defvar emacs-lisp-mode-syntax-table nil "")
     (defvar lisp-mode-abbrev-table nil "")
     
     (if (not emacs-lisp-mode-syntax-table) ; Do not change the table
                                            ;   if it is already set.
         (let ((i 0))
           (setq emacs-lisp-mode-syntax-table (make-syntax-table))
     
           ;; Set syntax of chars up to 0 to class of chars that are
           ;;   part of symbol names but not words.
           ;;   (The number 0 is `48' in the ASCII character set.)
           (while (< i ?0)
             (modify-syntax-entry i "_   " emacs-lisp-mode-syntax-table)
             (setq i (1+ i)))
           ...
           ;; Set the syntax for other characters.
           (modify-syntax-entry ?  "    " emacs-lisp-mode-syntax-table)
           (modify-syntax-entry ?\t "    " emacs-lisp-mode-syntax-table)
           ...
           (modify-syntax-entry ?\( "()  " emacs-lisp-mode-syntax-table)
           (modify-syntax-entry ?\) ")(  " emacs-lisp-mode-syntax-table)
           ...))
     ;; Create an abbrev table for lisp-mode.
     (define-abbrev-table 'lisp-mode-abbrev-table ())

   Much code is shared among the three Lisp modes.  The following
function sets various variables; it is called by each of the major Lisp
mode functions:

     (defun lisp-mode-variables (lisp-syntax)
       ;; The `lisp-syntax' argument is `nil' in Emacs Lisp mode,
       ;;   and `t' in the other two Lisp modes.
       (cond (lisp-syntax
              (if (not lisp-mode-syntax-table)
                  ;; The Emacs Lisp mode syntax table always exists, but
                  ;;   the Lisp Mode syntax table is created the first time a
                  ;;   mode that needs it is called.  This is to save space.
                  (progn (setq lisp-mode-syntax-table
                            (copy-syntax-table emacs-lisp-mode-syntax-table))
                         ;; Change some entries for Lisp mode.
                         (modify-syntax-entry ?\| "\"   "
                                              lisp-mode-syntax-table)
                         (modify-syntax-entry ?\[ "_   "
                                              lisp-mode-syntax-table)
                         (modify-syntax-entry ?\] "_   "
                                              lisp-mode-syntax-table)))
               (set-syntax-table lisp-mode-syntax-table)))
       (setq local-abbrev-table lisp-mode-abbrev-table)
       ...)

   Functions such as `forward-paragraph' use the value of the
`paragraph-start' variable.  Since Lisp code is different from ordinary
text, the `paragraph-start' variable needs to be set specially to
handle Lisp.  Also, comments are indented in a special fashion in Lisp
and the Lisp modes need their own mode-specific
`comment-indent-function'.  The code to set these variables is the rest
of `lisp-mode-variables'.

       (make-local-variable 'paragraph-start)
       ;; Having `^' is not clean, but `page-delimiter'
       ;; has them too, and removing those is a pain.
       (setq paragraph-start (concat "^$\\|" page-delimiter))
       ...
       (make-local-variable 'comment-indent-function)
       (setq comment-indent-function 'lisp-comment-indent))

   Each of the different Lisp modes has a slightly different keymap.
For example, Lisp mode binds `C-c C-l' to `run-lisp', but the other
Lisp modes do not.  However, all Lisp modes have some commands in
common.  The following function adds these common commands to a given
keymap.

     (defun lisp-mode-commands (map)
       (define-key map "\e\C-q" 'indent-sexp)
       (define-key map "\177" 'backward-delete-char-untabify)
       (define-key map "\t" 'lisp-indent-line))

   Here is an example of using `lisp-mode-commands' to initialize a
keymap, as part of the code for Emacs Lisp mode.  First we declare a
variable with `defvar' to hold the mode-specific keymap.  When this
`defvar' executes, it sets the variable to `nil' if it was void.  Then
we set up the keymap if the variable is `nil'.

   This code avoids changing the keymap or the variable if it is already
set up.  This lets the user customize the keymap.

     (defvar emacs-lisp-mode-map () "")
     (if emacs-lisp-mode-map
         ()
       (setq emacs-lisp-mode-map (make-sparse-keymap))
       (define-key emacs-lisp-mode-map "\e\C-x" 'eval-defun)
       (lisp-mode-commands emacs-lisp-mode-map))

   Finally, here is the complete major mode function definition for
Emacs Lisp mode.

     (defun emacs-lisp-mode ()
       "Major mode for editing Lisp code to run in XEmacs.
     Commands:
     Delete converts tabs to spaces as it moves back.
     Blank lines separate paragraphs.  Semicolons start comments.
     \\{emacs-lisp-mode-map}
     Entry to this mode runs the hook `emacs-lisp-mode-hook'."
       (interactive)
       (kill-all-local-variables)
       (use-local-map emacs-lisp-mode-map)    ; This provides the local keymap.
       (set-syntax-table emacs-lisp-mode-syntax-table)
       (setq major-mode 'emacs-lisp-mode)     ; This is how `describe-mode'
                                              ;   finds out what to describe.
       (setq mode-name "Emacs-Lisp")          ; This goes into the modeline.
       (lisp-mode-variables nil)              ; This defines various variables.
       (run-hooks 'emacs-lisp-mode-hook))     ; This permits the user to use a
                                              ;   hook to customize the mode.

\1f
File: lispref.info,  Node: Auto Major Mode,  Next: Mode Help,  Prev: Example Major Modes,  Up: Major Modes

How XEmacs Chooses a Major Mode
-------------------------------

   Based on information in the file name or in the file itself, XEmacs
automatically selects a major mode for the new buffer when a file is
visited.

 - Command: fundamental-mode
     Fundamental mode is a major mode that is not specialized for
     anything in particular.  Other major modes are defined in effect
     by comparison with this one--their definitions say what to change,
     starting from Fundamental mode.  The `fundamental-mode' function
     does _not_ run any hooks; you're not supposed to customize it.
     (If you want Emacs to behave differently in Fundamental mode,
     change the _global_ state of Emacs.)

 - Command: normal-mode &optional find-file
     This function establishes the proper major mode and local variable
     bindings for the current buffer.  First it calls `set-auto-mode',
     then it runs `hack-local-variables' to parse, and bind or evaluate
     as appropriate, any local variables.

     If the FIND-FILE argument to `normal-mode' is non-`nil',
     `normal-mode' assumes that the `find-file' function is calling it.
     In this case, it may process a local variables list at the end of
     the file and in the `-*-' line.  The variable
     `enable-local-variables' controls whether to do so.

     If you run `normal-mode' interactively, the argument FIND-FILE is
     normally `nil'.  In this case, `normal-mode' unconditionally
     processes any local variables list.  *Note Local Variables in
     Files: (emacs)File variables, for the syntax of the local
     variables section of a file.

     `normal-mode' uses `condition-case' around the call to the major
     mode function, so errors are caught and reported as a `File mode
     specification error',  followed by the original error message.

 - User Option: enable-local-variables
     This variable controls processing of local variables lists in files
     being visited.  A value of `t' means process the local variables
     lists unconditionally; `nil' means ignore them; anything else means
     ask the user what to do for each file.  The default value is `t'.

 - Variable: ignored-local-variables
     This variable holds a list of variables that should not be set by
     a local variables list.  Any value specified for one of these
     variables is ignored.

   In addition to this list, any variable whose name has a non-`nil'
`risky-local-variable' property is also ignored.

 - User Option: enable-local-eval
     This variable controls processing of `Eval:' in local variables
     lists in files being visited.  A value of `t' means process them
     unconditionally; `nil' means ignore them; anything else means ask
     the user what to do for each file.  The default value is `maybe'.

 - Function: set-auto-mode
     This function selects the major mode that is appropriate for the
     current buffer.  It may base its decision on the value of the `-*-'
     line, on the visited file name (using `auto-mode-alist'), or on the
     value of a local variable.  However, this function does not look
     for the `mode:' local variable near the end of a file; the
     `hack-local-variables' function does that.  *Note How Major Modes
     are Chosen: (emacs)Choosing Modes.

 - User Option: default-major-mode
     This variable holds the default major mode for new buffers.  The
     standard value is `fundamental-mode'.

     If the value of `default-major-mode' is `nil', XEmacs uses the
     (previously) current buffer's major mode for the major mode of a
     new buffer.  However, if the major mode symbol has a `mode-class'
     property with value `special', then it is not used for new buffers;
     Fundamental mode is used instead.  The modes that have this
     property are those such as Dired and Rmail that are useful only
     with text that has been specially prepared.

 - Function: set-buffer-major-mode buffer
     This function sets the major mode of BUFFER to the value of
     `default-major-mode'.  If that variable is `nil', it uses the
     current buffer's major mode (if that is suitable).

     The low-level primitives for creating buffers do not use this
     function, but medium-level commands such as `switch-to-buffer' and
     `find-file-noselect' use it whenever they create buffers.

 - Variable: initial-major-mode
     The value of this variable determines the major mode of the initial
     `*scratch*' buffer.  The value should be a symbol that is a major
     mode command name.  The default value is `lisp-interaction-mode'.

 - Variable: auto-mode-alist
     This variable contains an association list of file name patterns
     (regular expressions; *note Regular Expressions::) and
     corresponding major mode functions.  Usually, the file name
     patterns test for suffixes, such as `.el' and `.c', but this need
     not be the case.  An ordinary element of the alist looks like
     `(REGEXP .  MODE-FUNCTION)'.

     For example,

          (("^/tmp/fol/" . text-mode)
           ("\\.texinfo\\'" . texinfo-mode)
           ("\\.texi\\'" . texinfo-mode)
           ("\\.el\\'" . emacs-lisp-mode)
           ("\\.c\\'" . c-mode)
           ("\\.h\\'" . c-mode)
           ...)

     When you visit a file whose expanded file name (*note File Name
     Expansion::) matches a REGEXP, `set-auto-mode' calls the
     corresponding MODE-FUNCTION.  This feature enables XEmacs to select
     the proper major mode for most files.

     If an element of `auto-mode-alist' has the form `(REGEXP FUNCTION
     t)', then after calling FUNCTION, XEmacs searches
     `auto-mode-alist' again for a match against the portion of the file
     name that did not match before.

     This match-again feature is useful for uncompression packages: an
     entry of the form `("\\.gz\\'" . FUNCTION)' can uncompress the file
     and then put the uncompressed file in the proper mode according to
     the name sans `.gz'.

     Here is an example of how to prepend several pattern pairs to
     `auto-mode-alist'.  (You might use this sort of expression in your
     `.emacs' file.)

          (setq auto-mode-alist
            (append
             ;; File name starts with a dot.
             '(("/\\.[^/]*\\'" . fundamental-mode)
               ;; File name has no dot.
               ("[^\\./]*\\'" . fundamental-mode)
               ;; File name ends in `.C'.
               ("\\.C\\'" . c++-mode))
             auto-mode-alist))

 - Variable: interpreter-mode-alist
     This variable specifies major modes to use for scripts that
     specify a command interpreter in an `#!' line.  Its value is a
     list of elements of the form `(INTERPRETER . MODE)'; for example,
     `("perl" . perl-mode)' is one element present by default.  The
     element says to use mode MODE if the file specifies INTERPRETER.

     This variable is applicable only when the `auto-mode-alist' does
     not indicate which major mode to use.

 - Function: hack-local-variables &optional force
     This function parses, and binds or evaluates as appropriate, any
     local variables for the current buffer.

     The handling of `enable-local-variables' documented for
     `normal-mode' actually takes place here.  The argument FORCE
     usually comes from the argument FIND-FILE given to `normal-mode'.

\1f
File: lispref.info,  Node: Mode Help,  Next: Derived Modes,  Prev: Auto Major Mode,  Up: Major Modes

Getting Help about a Major Mode
-------------------------------

   The `describe-mode' function is used to provide information about
major modes.  It is normally called with `C-h m'.  The `describe-mode'
function uses the value of `major-mode', which is why every major mode
function needs to set the `major-mode' variable.

 - Command: describe-mode
     This function displays the documentation of the current major mode.

     The `describe-mode' function calls the `documentation' function
     using the value of `major-mode' as an argument.  Thus, it displays
     the documentation string of the major mode function.  (*Note
     Accessing Documentation::.)

 - Variable: major-mode
     This variable holds the symbol for the current buffer's major mode.
     This symbol should have a function definition that is the command
     to switch to that major mode.  The `describe-mode' function uses
     the documentation string of the function as the documentation of
     the major mode.

\1f
File: lispref.info,  Node: Derived Modes,  Prev: Mode Help,  Up: Major Modes

Defining Derived Modes
----------------------

   It's often useful to define a new major mode in terms of an existing
one.  An easy way to do this is to use `define-derived-mode'.

 - Macro: define-derived-mode variant parent name docstring body...
     This construct defines VARIANT as a major mode command, using NAME
     as the string form of the mode name.

     The new command VARIANT is defined to call the function PARENT,
     then override certain aspects of that parent mode:

        * The new mode has its own keymap, named `VARIANT-map'.
          `define-derived-mode' initializes this map to inherit from
          `PARENT-map', if it is not already set.

        * The new mode has its own syntax table, kept in the variable
          `VARIANT-syntax-table'.  `define-derived-mode' initializes
          this variable by copying `PARENT-syntax-table', if it is not
          already set.

        * The new mode has its own abbrev table, kept in the variable
          `VARIANT-abbrev-table'.  `define-derived-mode' initializes
          this variable by copying `PARENT-abbrev-table', if it is not
          already set.

        * The new mode has its own mode hook, `VARIANT-hook', which it
          runs in standard fashion as the very last thing that it does.
          (The new mode also runs the mode hook of PARENT as part of
          calling PARENT.)

     In addition, you can specify how to override other aspects of
     PARENT with BODY.  The command VARIANT evaluates the forms in BODY
     after setting up all its usual overrides, just before running
     `VARIANT-hook'.

     The argument DOCSTRING specifies the documentation string for the
     new mode.  If you omit DOCSTRING, `define-derived-mode' generates
     a documentation string.

     Here is a hypothetical example:

          (define-derived-mode hypertext-mode
            text-mode "Hypertext"
            "Major mode for hypertext.
          \\{hypertext-mode-map}"
            (setq case-fold-search nil))
          
          (define-key hypertext-mode-map
            [down-mouse-3] 'do-hyper-link)

\1f
File: lispref.info,  Node: Minor Modes,  Next: Modeline Format,  Prev: Major Modes,  Up: Modes

Minor Modes
===========

   A "minor mode" provides features that users may enable or disable
independently of the choice of major mode.  Minor modes can be enabled
individually or in combination.  Minor modes would be better named
"Generally available, optional feature modes" except that such a name is
unwieldy.

   A minor mode is not usually a modification of single major mode.  For
example, Auto Fill mode may be used in any major mode that permits text
insertion.  To be general, a minor mode must be effectively independent
of the things major modes do.

   A minor mode is often much more difficult to implement than a major
mode.  One reason is that you should be able to activate and deactivate
minor modes in any order.  A minor mode should be able to have its
desired effect regardless of the major mode and regardless of the other
minor modes in effect.

   Often the biggest problem in implementing a minor mode is finding a
way to insert the necessary hook into the rest of XEmacs.  Minor mode
keymaps make this easier than it used to be.

* Menu:

* Minor Mode Conventions::      Tips for writing a minor mode.
* Keymaps and Minor Modes::     How a minor mode can have its own keymap.

\1f
File: lispref.info,  Node: Minor Mode Conventions,  Next: Keymaps and Minor Modes,  Up: Minor Modes

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 represent the minor
     mode.  Its value should enable or disable the mode (`nil' to
     disable; anything else to enable.)  We call this the "mode
     variable".

     This variable is used in conjunction with the `minor-mode-alist' to
     display the minor mode name in the modeline.  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 (*note
     Modeline Variables::).  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 modeline.  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)))

\1f
File: lispref.info,  Node: Keymaps and Minor Modes,  Prev: Minor Mode Conventions,  Up: Minor Modes

Keymaps and Minor Modes
-----------------------

   Each minor mode can have its own keymap, which is active when the
mode is enabled.  To set up a keymap for a minor mode, add an element
to the alist `minor-mode-map-alist'.  *Note Active Keymaps::.

   One use of minor mode keymaps is to modify the behavior of certain
self-inserting characters so that they do something else as well as
self-insert.  In general, this is the only way to do that, since the
facilities for customizing `self-insert-command' are limited to special
cases (designed for abbrevs and Auto Fill mode).  (Do not try
substituting your own definition of `self-insert-command' for the
standard one.  The editor command loop handles this function specially.)

\1f
File: lispref.info,  Node: Modeline Format,  Next: Hooks,  Prev: Minor Modes,  Up: Modes

Modeline Format
===============

   Each Emacs window (aside from minibuffer windows) includes a
modeline, which displays status information about the buffer displayed
in the window.  The modeline contains information about the buffer,
such as its name, associated file, depth of recursive editing, and the
major and minor modes.

   This section describes how the contents of the modeline are
controlled.  It is in the chapter on modes because much of the
information displayed in the modeline relates to the enabled major and
minor modes.

   `modeline-format' is a buffer-local variable that holds a template
used to display the modeline of the current buffer.  All windows for
the same buffer use the same `modeline-format' and their modelines
appear the same (except for scrolling percentages and line numbers).

   The modeline of a window is normally updated whenever a different
buffer is shown in the window, or when the buffer's modified-status
changes from `nil' to `t' or vice-versa.  If you modify any of the
variables referenced by `modeline-format' (*note Modeline Variables::),
you may want to force an update of the modeline so as to display the
new information.

 - Function: redraw-modeline &optional all
     Force redisplay of the current buffer's modeline.  If ALL is
     non-`nil', then force redisplay of all modelines.

   The modeline is usually displayed in inverse video.  This is
controlled using the `modeline' face.  *Note Faces::.

* Menu:

* Modeline Data::         The data structure that controls the modeline.
* Modeline Variables::    Variables used in that data structure.
* %-Constructs::          Putting information into a modeline.

\1f
File: lispref.info,  Node: Modeline Data,  Next: Modeline Variables,  Up: Modeline Format

The Data Structure of the Modeline
----------------------------------

   The modeline contents are controlled by a data structure of lists,
strings, symbols, and numbers kept in the buffer-local variable
`mode-line-format'.  The data structure is called a "modeline
construct", and it is built in recursive fashion out of simpler modeline
constructs.  The same data structure is used for constructing frame
titles (*note Frame Titles::).

 - Variable: modeline-format
     The value of this variable is a modeline construct with overall
     responsibility for the modeline format.  The value of this variable
     controls which other variables are used to form the modeline text,
     and where they appear.

   A modeline construct may be as simple as a fixed string of text, but
it usually specifies how to use other variables to construct the text.
Many of these variables are themselves defined to have modeline
constructs as their values.

   The default value of `modeline-format' incorporates the values of
variables such as `mode-name' and `minor-mode-alist'.  Because of this,
very few modes need to alter `modeline-format'.  For most purposes, it
is sufficient to alter the variables referenced by `modeline-format'.

   A modeline construct may be a list, a symbol, or a string.  If the
value is a list, each element may be a list, a symbol, or a string.

`STRING'
     A string as a modeline construct is displayed verbatim in the mode
     line except for "`%'-constructs".  Decimal digits after the `%'
     specify the field width for space filling on the right (i.e., the
     data is left justified).  *Note %-Constructs::.

`SYMBOL'
     A symbol as a modeline construct stands for its value.  The value
     of SYMBOL is used as a modeline construct, in place of SYMBOL.
     However, the symbols `t' and `nil' are ignored; so is any symbol
     whose value is void.

     There is one exception: if the value of SYMBOL is a string, it is
     displayed verbatim: the `%'-constructs are not recognized.

`(STRING REST...) or (LIST REST...)'
     A list whose first element is a string or list means to process
     all the elements recursively and concatenate the results.  This is
     the most common form of mode line construct.

`(SYMBOL THEN ELSE)'
     A list whose first element is a symbol is a conditional.  Its
     meaning depends on the value of SYMBOL.  If the value is non-`nil',
     the second element, THEN, is processed recursively as a modeline
     element.  But if the value of SYMBOL is `nil', the third element,
     ELSE, is processed recursively.  You may omit ELSE; then the mode
     line element displays nothing if the value of SYMBOL is `nil'.

`(WIDTH REST...)'
     A list whose first element is an integer specifies truncation or
     padding of the results of REST.  The remaining elements REST are
     processed recursively as modeline constructs and concatenated
     together.  Then the result is space filled (if WIDTH is positive)
     or truncated (to -WIDTH columns, if WIDTH is negative) on the
     right.

     For example, the usual way to show what percentage of a buffer is
     above the top of the window is to use a list like this: `(-3
     "%p")'.

   If you do alter `modeline-format' itself, the new value should use
the same variables that appear in the default value (*note Modeline
Variables::), rather than duplicating their contents or displaying the
information in another fashion.  This way, customizations made by the
user or by Lisp programs (such as `display-time' and major modes) via
changes to those variables remain effective.

   Here is an example of a `modeline-format' that might be useful for
`shell-mode', since it contains the hostname and default directory.

     (setq modeline-format
       (list ""
        'modeline-modified
        "%b--"
        (getenv "HOST")      ; One element is not constant.
        ":"
        'default-directory
        "   "
        'global-mode-string
        "   %[("
        'mode-name
        'modeline-process
        'minor-mode-alist
        "%n"
        ")%]----"
        '(line-number-mode "L%l--")
        '(-3 . "%p")
        "-%-"))

\1f
File: lispref.info,  Node: Modeline Variables,  Next: %-Constructs,  Prev: Modeline Data,  Up: Modeline Format

Variables Used in the Modeline
------------------------------

   This section describes variables incorporated by the standard value
of `modeline-format' into the text of the mode line.  There is nothing
inherently special about these variables; any other variables could
have the same effects on the modeline if `modeline-format' were changed
to use them.

 - Variable: modeline-modified
     This variable holds the value of the modeline construct that
     displays whether the current buffer is modified.

     The default value of `modeline-modified' is `("--%1*%1+-")'.  This
     means that the modeline displays `--**-' if the buffer is
     modified, `-----' if the buffer is not modified, `--%%-' if the
     buffer is read only, and `--%*--' if the buffer is read only and
     modified.

     Changing this variable does not force an update of the modeline.

 - Variable: modeline-buffer-identification
     This variable identifies the buffer being displayed in the window.
     Its default value is `("%F: %17b")', which means that it usually
     displays `Emacs:' followed by seventeen characters of the buffer
     name.  (In a terminal frame, it displays the frame name instead of
     `Emacs'; this has the effect of showing the frame number.)  You may
     want to change this in modes such as Rmail that do not behave like
     a "normal" XEmacs.

 - Variable: global-mode-string
     This variable holds a modeline spec that appears in the mode line
     by default, just after the buffer name.  The command `display-time'
     sets `global-mode-string' to refer to the variable
     `display-time-string', which holds a string containing the time and
     load information.

     The `%M' construct substitutes the value of `global-mode-string',
     but this is obsolete, since the variable is included directly in
     the modeline.

 - Variable: mode-name
     This buffer-local variable holds the "pretty" name of the current
     buffer's major mode.  Each major mode should set this variable so
     that the mode name will appear in the modeline.

 - Variable: minor-mode-alist
     This variable holds an association list whose elements specify how
     the modeline should indicate that a minor mode is active.  Each
     element of the `minor-mode-alist' should be a two-element list:

          (MINOR-MODE-VARIABLE MODELINE-STRING)

     More generally, MODELINE-STRING can be any mode line spec.  It
     appears in the mode line when the value of MINOR-MODE-VARIABLE is
     non-`nil', and not otherwise.  These strings should begin with
     spaces so that they don't run together.  Conventionally, the
     MINOR-MODE-VARIABLE for a specific mode is set to a non-`nil'
     value when that minor mode is activated.

     The default value of `minor-mode-alist' is:

          minor-mode-alist
          => ((vc-mode vc-mode)
              (abbrev-mode " Abbrev")
              (overwrite-mode overwrite-mode)
              (auto-fill-function " Fill")
              (defining-kbd-macro " Def")
              (isearch-mode isearch-mode))

     `minor-mode-alist' is not buffer-local.  The variables mentioned
     in the alist should be buffer-local if the minor mode can be
     enabled separately in each buffer.

 - Variable: modeline-process
     This buffer-local variable contains the modeline information on
     process status in modes used for communicating with subprocesses.
     It is displayed immediately following the major mode name, with no
     intervening space.  For example, its value in the `*shell*' buffer
     is `(": %s")', which allows the shell to display its status along
     with the major mode as: `(Shell: run)'.  Normally this variable is
     `nil'.

 - Variable: default-modeline-format
     This variable holds the default `modeline-format' for buffers that
     do not override it.  This is the same as `(default-value
     'modeline-format)'.

     The default value of `default-modeline-format' is:

          (""
           modeline-modified
           modeline-buffer-identification
           "   "
           global-mode-string
           "   %[("
           mode-name
           modeline-process
           minor-mode-alist
           "%n"
           ")%]----"
           (line-number-mode "L%l--")
           (-3 . "%p")
           "-%-")

 - Variable: vc-mode
     The variable `vc-mode', local in each buffer, records whether the
     buffer's visited file is maintained with version control, and, if
     so, which kind.  Its value is `nil' for no version control, or a
     string that appears in the mode line.

\1f
File: lispref.info,  Node: %-Constructs,  Prev: Modeline Variables,  Up: Modeline Format

`%'-Constructs in the ModeLine
------------------------------

   The following table lists the recognized `%'-constructs and what
they mean.  In any construct except `%%', you can add a decimal integer
after the `%' to specify how many characters to display.

`%b'
     The current buffer name, obtained with the `buffer-name' function.
     *Note Buffer Names::.

`%f'
     The visited file name, obtained with the `buffer-file-name'
     function.  *Note Buffer File Name::.

`%F'
     The name of the selected frame.

`%c'
     The current column number of point.

`%l'
     The current line number of point.

`%*'
     `%' if the buffer is read only (see `buffer-read-only');
     `*' if the buffer is modified (see `buffer-modified-p');
     `-' otherwise.  *Note Buffer Modification::.

`%+'
     `*' if the buffer is modified (see `buffer-modified-p');
     `%' if the buffer is read only (see `buffer-read-only');
     `-' otherwise.  This differs from `%*' only for a modified
     read-only buffer.  *Note Buffer Modification::.

`%&'
     `*' if the buffer is modified, and `-' otherwise.

`%s'
     The status of the subprocess belonging to the current buffer,
     obtained with `process-status'.  *Note Process Information::.

`%l'
     the current line number.

`%S'
     the name of the selected frame; this is only meaningful under the
     X Window System.  *Note Frame Name::.

`%t'
     Whether the visited file is a text file or a binary file.  (This
     is a meaningful distinction only on certain operating systems.)

`%p'
     The percentage of the buffer text above the *top* of window, or
     `Top', `Bottom' or `All'.

`%P'
     The percentage of the buffer text that is above the *bottom* of
     the window (which includes the text visible in the window, as well
     as the text above the top), plus `Top' if the top of the buffer is
     visible on screen; or `Bottom' or `All'.

`%n'
     `Narrow' when narrowing is in effect; nothing otherwise (see
     `narrow-to-region' in *Note Narrowing::).

`%['
     An indication of the depth of recursive editing levels (not
     counting minibuffer levels): one `[' for each editing level.
     *Note Recursive Editing::.

`%]'
     One `]' for each recursive editing level (not counting minibuffer
     levels).

`%%'
     The character `%'--this is how to include a literal `%' in a
     string in which `%'-constructs are allowed.

`%-'
     Dashes sufficient to fill the remainder of the modeline.

   The following two `%'-constructs are still supported, but they are
obsolete, since you can get the same results with the variables
`mode-name' and `global-mode-string'.

`%m'
     The value of `mode-name'.

`%M'
     The value of `global-mode-string'.  Currently, only `display-time'
     modifies the value of `global-mode-string'.