emacs/lispref/modes.texi

@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990, 1991, 1992, 1993, 1994 Free Software Foundation, Inc. 
@c See the file elisp.texi for copying conditions.
@setfilename ../info/modes
@node Modes, Documentation,  Keymaps, Top
@chapter Major and Minor Modes
@cindex mode

  A @dfn{mode} is a set of definitions that customize Emacs and can be
turned on and off while you edit.  There are two varieties of modes:
@dfn{major modes}, which are mutually exclusive and used for editing
particular kinds of text, and @dfn{minor modes}, which provide features
that users can enable individually.

  This chapter describes how to write both major and minor modes, how to
indicate them in the mode line, and how they run hooks supplied by the
user.  For related topics such as keymaps and syntax tables, see
@ref{Keymaps}, and @ref{Syntax Tables}.

@menu
* Major Modes::        Defining major modes.
* Minor Modes::        Defining minor modes.
* Mode Line Format::   Customizing the text that appears in the mode line.
* Hooks::              How to use hooks; how to write code that provides hooks.
@end menu

@node Major Modes
@section Major Modes
@cindex major mode
@cindex Fundamental mode

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

  The least specialized major mode is called @dfn{Fundamental mode}.
This mode has no mode-specific definitions or variable settings, so each
Emacs 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
@key{LFD} (@code{eval-print-last-sexp}), @key{TAB}
(@code{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 @dfn{derived mode} (@pxref{Derived
Modes}).  For example, Rmail Edit mode, which is in
@file{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 Emacs 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.  @xref{Recursive
Editing}.

  The standard GNU Emacs Lisp library directory contains the code for
several major modes, in files including @file{text-mode.el},
@file{texinfo.el}, @file{lisp-mode.el}, @file{c-mode.el}, and
@file{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 Emacs 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.
@end menu

@node Major Mode Conventions
@subsection 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:

@itemize @bullet
@item
Define a command whose name ends in @samp{-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.

@item
Write a documentation string for this command that describes the
special commands available in this mode.  @kbd{C-h m}
(@code{describe-mode}) in your mode will display this string.

The documentation string may include the special documentation
substrings, @samp{\[@var{command}]}, @samp{\@{@var{keymap}@}}, and
@samp{\<@var{keymap}>}, that enable the documentation to adapt
automatically to the user's own key bindings.  @xref{Keys in
Documentation}.

@item
The major mode command should start by calling
@code{kill-all-local-variables}.  This is what gets rid of the local
variables of the major mode previously in effect.

@item
The major mode command should set the variable @code{major-mode} to the
major mode command symbol.  This is how @code{describe-mode} discovers
which documentation to print.

@item
The major mode command should set the variable @code{mode-name} to the
``pretty'' name of the mode, as a string.  This appears in the mode
line.

@item
@cindex functions in modes
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).  @xref{Style Tips}.

@item
@cindex keymaps in modes
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 @code{use-local-map} to install this local map.
@xref{Active Keymaps}, for more information.

This keymap should be kept in a global variable named
@code{@var{modename}-mode-map}.  Normally the library that defines the
mode sets this variable.

@xref{Tips for Defining}, for advice about how to write the code to set
up the mode's keymap variable.

@item
@cindex syntax tables in modes
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 @code{@var{modename}-mode-syntax-table}.  @xref{Syntax
Tables}.

@item
@cindex abbrev tables in modes
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 @code{@var{modename}-mode-abbrev-table}.  @xref{Abbrev
Tables}.

@item
Use @code{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.)

@item
@cindex buffer-local variables in modes
To make a buffer-local binding for an Emacs customization variable, use
@code{make-local-variable} in the major mode command, not
@code{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.  @xref{Buffer-Local Variables}.

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

@item
@cindex mode hook
@cindex major mode hook
Each major mode should have a @dfn{mode hook} named
@code{@var{modename}-mode-hook}.  The major mode command should run that
hook, with @code{run-hooks}, as the very last thing it
does. @xref{Hooks}.

@item
The major mode command may also run the hooks of some more basic modes.
For example, @code{indented-text-mode} runs @code{text-mode-hook} as
well as @code{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.

@item
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
@code{change-major-mode-hook}.

@item
If this mode is appropriate only for specially-prepared text, then the
major mode command symbol should have a property named @code{mode-class}
with value @code{special}, put on as follows:

@cindex @code{mode-class} property
@cindex @code{special}
@example
(put 'funny-mode 'mode-class 'special)
@end example

@noindent
This tells Emacs 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.

@item
If you want to make the new mode the default for files with certain
recognizable names, add an element to @code{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
@code{autoload}.  Otherwise, it is sufficient to add the element in the
file that contains the mode definition.  @xref{Auto Major Mode}.

@item
@cindex @file{.emacs} customization
In the documentation, you should provide a sample @code{autoload} form
and an example of how to add to @code{auto-mode-alist}, that users can
include in their @file{.emacs} files.

@item
@cindex mode loading
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.
@end itemize

@defvar change-major-mode-hook
This normal hook is run by @code{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.  @xref{Hooks}.
@end defvar

@node Example Major Modes
@subsection Major Mode Examples

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

@smallexample
@group
;; @r{Create mode-specific tables.}
(defvar text-mode-syntax-table nil 
  "Syntax table used while in text mode.")
@end group

@group
(if text-mode-syntax-table
    ()              ; @r{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))
@end group

@group
(defvar text-mode-abbrev-table nil
  "Abbrev table used while in text mode.")
(define-abbrev-table 'text-mode-abbrev-table ())
@end group

@group
(defvar text-mode-map nil)   ; @r{Create a mode-specific keymap.}

(if text-mode-map
    ()              ; @r{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))
@end group
@end smallexample

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

@smallexample
@group
(defun text-mode ()
  "Major mode for editing text intended for humans to read. 
 Special commands: \\@{text-mode-map@}
@end group
@group
Turning on text-mode runs the hook `text-mode-hook'."
  (interactive)
  (kill-all-local-variables)
@end group
@group
  (use-local-map text-mode-map)     ; @r{This provides the local keymap.}
  (setq mode-name "Text")           ; @r{This name goes into the mode line.}
  (setq major-mode 'text-mode)      ; @r{This is how @code{describe-mode}}
                                    ;   @r{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))      ; @r{Finally, this permits the user to}
                                    ;   @r{customize the mode with a hook.}
@end group
@end smallexample

@cindex @file{lisp-mode.el}
  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
@file{lisp-mode.el} that illustrate how these modes are written.

@cindex syntax table example
@smallexample
@group
;; @r{Create mode-specific table variables.}
(defvar lisp-mode-syntax-table nil "")  
(defvar emacs-lisp-mode-syntax-table nil "")
(defvar lisp-mode-abbrev-table nil "")
@end group

@group
(if (not emacs-lisp-mode-syntax-table) ; @r{Do not change the table}
                                       ;   @r{if it is already set.}
    (let ((i 0))
      (setq emacs-lisp-mode-syntax-table (make-syntax-table))
@end group

@group
      ;; @r{Set syntax of chars up to 0 to class of chars that are}
      ;;   @r{part of symbol names but not words.}
      ;;   @r{(The number 0 is @code{48} in the @sc{ASCII} character set.)}
      (while (< i ?0) 
        (modify-syntax-entry i "_   " emacs-lisp-mode-syntax-table)
        (setq i (1+ i)))
      @dots{}
@end group
@group
      ;; @r{Set the syntax for other characters.}
      (modify-syntax-entry ?  "    " emacs-lisp-mode-syntax-table)
      (modify-syntax-entry ?\t "    " emacs-lisp-mode-syntax-table)
      @dots{}
@end group
@group
      (modify-syntax-entry ?\( "()  " emacs-lisp-mode-syntax-table)
      (modify-syntax-entry ?\) ")(  " emacs-lisp-mode-syntax-table)
      @dots{}))
;; @r{Create an abbrev table for lisp-mode.}
(define-abbrev-table 'lisp-mode-abbrev-table ())
@end group
@end smallexample

  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:

@smallexample
@group
(defun lisp-mode-variables (lisp-syntax)
  ;; @r{The @code{lisp-syntax} argument is @code{nil} in Emacs Lisp mode,}
  ;;   @r{and @code{t} in the other two Lisp modes.}
  (cond (lisp-syntax
         (if (not lisp-mode-syntax-table)
             ;; @r{The Emacs Lisp mode syntax table always exists, but}
             ;;   @r{the Lisp Mode syntax table is created the first time a}
             ;;   @r{mode that needs it is called.  This is to save space.}
@end group
@group
             (progn (setq lisp-mode-syntax-table
                       (copy-syntax-table emacs-lisp-mode-syntax-table))
                    ;; @r{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)))
@end group
@group
          (set-syntax-table lisp-mode-syntax-table)))
  (setq local-abbrev-table lisp-mode-abbrev-table)
  @dots{})
@end group
@end smallexample

  Functions such as @code{forward-paragraph} use the value of the
@code{paragraph-start} variable.  Since Lisp code is different from
ordinary text, the @code{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
@code{comment-indent-function}.  The code to set these variables is the
rest of @code{lisp-mode-variables}.

@smallexample
@group
  (make-local-variable 'paragraph-start)
  ;; @r{Having @samp{^} is not clean, but @code{page-delimiter}}
  ;; @r{has them too, and removing those is a pain.}
  (setq paragraph-start (concat "^$\\|" page-delimiter))
  @dots{}
@end group
@group
  (make-local-variable 'comment-indent-function)
  (setq comment-indent-function 'lisp-comment-indent))
@end group
@end smallexample

  Each of the different Lisp modes has a slightly different keymap.  For
example, Lisp mode binds @kbd{C-c C-l} to @code{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.

@smallexample
@group
(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))
@end group
@end smallexample

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

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

@smallexample
@group
(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))
@end group
@end smallexample

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

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

@node Auto Major Mode
@subsection How Emacs Chooses a Major Mode

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

@deffn 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 @code{fundamental-mode} function does @emph{not}
run any hooks; you're not supposed to customize it.  (If you want Emacs
to behave differently in Fundamental mode, change the @emph{global}
state of Emacs.)
@end deffn

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

If the @var{find-file} argument to @code{normal-mode} is
non-@code{nil}, @code{normal-mode} assumes that the @code{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 @samp{-*-} line.  The variable
@code{enable-local-variables} controls whether to do so.

If you run @code{normal-mode} interactively, the argument
@var{find-file} is normally @code{nil}.  In this case,
@code{normal-mode} unconditionally processes any local variables list.
@xref{File variables, , Local Variables in Files, emacs, The GNU Emacs
Manual}, for the syntax of the local variables section of a file.

@cindex file mode specification error
@code{normal-mode} uses @code{condition-case} around the call to the
major mode function, so errors are caught and reported as a @samp{File
mode specification error},  followed by the original error message.
@end deffn

@defopt enable-local-variables
This variable controls processing of local variables lists in files
being visited.  A value of @code{t} means process the local variables
lists unconditionally; @code{nil} means ignore them; anything else means
ask the user what to do for each file.  The default value is @code{t}.
@end defopt

@defvar 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.
@end defvar

In addition to this list, any variable whose name has a non-@code{nil}
@code{risky-local-variable} property is also ignored.

@defopt enable-local-eval
This variable controls processing of @samp{Eval:} in local variables
lists in files being visited.  A value of @code{t} means process them
unconditionally; @code{nil} means ignore them; anything else means ask
the user what to do for each file.  The default value is @code{maybe}.
@end defopt

@defun set-auto-mode
@cindex visited file mode
  This function selects the major mode that is appropriate for the
current buffer.  It may base its decision on the value of the @w{@samp{-*-}}
line, on the visited file name (using @code{auto-mode-alist}), on the
@w{@samp{#!}} line (using @code{interpreter-mode-alist}), or on the
value of a local variable.  However, this function does not look for
the @samp{mode:} local variable near the end of a file; the
@code{hack-local-variables} function does that.  @xref{Choosing Modes, ,
How Major Modes are Chosen, emacs, The GNU Emacs Manual}.
@end defun

@defopt default-major-mode 
  This variable holds the default major mode for new buffers.  The
standard value is @code{fundamental-mode}.

  If the value of @code{default-major-mode} is @code{nil}, Emacs uses
the (previously) current buffer's major mode for the major mode of a new
buffer.  However, if the major mode symbol has a @code{mode-class}
property with value @code{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.
@end defopt

@defun set-buffer-major-mode buffer
This function sets the major mode of @var{buffer} to the value of
@code{default-major-mode}.  If that variable is @code{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 @code{switch-to-buffer} and
@code{find-file-noselect} use it whenever they create buffers.
@end defun

@defvar initial-major-mode
@cindex @samp{*scratch*}
The value of this variable determines the major mode of the initial
@samp{*scratch*} buffer.  The value should be a symbol that is a major
mode command name.  The default value is @code{lisp-interaction-mode}.
@end defvar

@defvar auto-mode-alist
This variable contains an association list of file name patterns
(regular expressions; @pxref{Regular Expressions}) and corresponding
major mode functions.  Usually, the file name patterns test for
suffixes, such as @samp{.el} and @samp{.c}, but this need not be the
case.  An ordinary element of the alist looks like @code{(@var{regexp} .
@var{mode-function})}.

For example,

@smallexample
@group
(("^/tmp/fol/" . text-mode)
 ("\\.texinfo\\'" . texinfo-mode)
 ("\\.texi\\'" . texinfo-mode)
@end group
@group
 ("\\.el\\'" . emacs-lisp-mode)
 ("\\.c\\'" . c-mode) 
 ("\\.h\\'" . c-mode)
 @dots{})
@end group
@end smallexample

When you visit a file whose expanded file name (@pxref{File Name
Expansion}) matches a @var{regexp}, @code{set-auto-mode} calls the
corresponding @var{mode-function}.  This feature enables Emacs to select
the proper major mode for most files.

If an element of @code{auto-mode-alist} has the form @code{(@var{regexp}
@var{function} t)}, then after calling @var{function}, Emacs searches
@code{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 @code{("\\.gz\\'" . @var{function})} can uncompress the file
and then put the uncompressed file in the proper mode according to the
name sans @samp{.gz}.

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

@smallexample
@group
(setq auto-mode-alist
  (append 
   ;; @r{File name starts with a dot.}
   '(("/\\.[^/]*\\'" . fundamental-mode)  
     ;; @r{File name has no dot.}
     ("[^\\./]*\\'" . fundamental-mode)   
     ;; @r{File name ends in @samp{.C}.}
     ("\\.C\\'" . c++-mode))
   auto-mode-alist))
@end group
@end smallexample
@end defvar

@defvar interpreter-mode-alist
This variable specifes major modes to use for scripts that specify a
command interpreter in an @samp{#!} line.  Its value is a list of
elements of the form @code{(@var{interpreter} . @var{mode})}; for
example, @code{("perl" . perl-mode)} is one element present by default.
The element says to use mode @var{mode} if the file specifies
@var{interpreter}.

This variable is applicable only when the @code{auto-mode-alist} does
not indicate which major mode to use.
@end defvar

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

  The handling of @code{enable-local-variables} documented for
@code{normal-mode} actually takes place here.  The argument @var{force}
usually comes from the argument @var{find-file} given to
@code{normal-mode}.
@end defun

@node Mode Help
@subsection Getting Help about a Major Mode
@cindex mode help
@cindex help for major mode
@cindex documentation for major mode

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

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

The @code{describe-mode} function calls the @code{documentation}
function using the value of @code{major-mode} as an argument.  Thus, it
displays the documentation string of the major mode function.
(@xref{Accessing Documentation}.)
@end deffn

@defvar 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 @code{describe-mode} function uses the
documentation string of the function as the documentation of the major
mode.
@end defvar

@node Derived Modes
@subsection 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 @code{define-derived-mode}.

@defmac define-derived-mode variant parent name docstring body@dots{}
This construct defines @var{variant} as a major mode command, using
@var{name} as the string form of the mode name.

The new command @var{variant} is defined to call the function
@var{parent}, then override certain aspects of that parent mode:

@itemize @bullet 
@item
The new mode has its own keymap, named @code{@var{variant}-map}.
@code{define-derived-mode} initializes this map to inherit from
@code{@var{parent}-map}, if it is not already set.

@item
The new mode has its own syntax table, kept in the variable
@code{@var{variant}-syntax-table}.
@code{define-derived-mode} initializes this variable by copying 
@code{@var{parent}-syntax-table}, if it is not already set.

@item
The new mode has its own abbrev table, kept in the variable
@code{@var{variant}-abbrev-table}.
@code{define-derived-mode} initializes this variable by copying 
@code{@var{parent}-abbrev-table}, if it is not already set.

@item
The new mode has its own mode hook, @code{@var{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 @var{parent} as part 
of calling @var{parent}.)
@end itemize

In addition, you can specify how to override other aspects of
@var{parent} with @var{body}.  The command @var{variant}
evaluates the forms in @var{body} after setting up all its usual 
overrides, just before running @code{@var{variant}-hook}.

The argument @var{docstring} specifies the documentation string for the
new mode.  If you omit @var{docstring}, @code{define-derived-mode}
generates a documentation string.

Here is a hypothetical example:

@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)
@end example
@end defmac

@node Minor Modes
@section Minor Modes
@cindex minor mode

  A @dfn{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 Emacs.  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.
@end menu

@node Minor Mode Conventions
@subsection Conventions for Writing Minor Modes
@cindex minor mode conventions
@cindex 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.

@itemize @bullet
@item
@cindex mode variable
Make a variable whose name ends in @samp{-mode} to represent the minor
mode.  Its value should enable or disable the mode (@code{nil} to
disable; anything else to enable.)  We call this the @dfn{mode
variable}.

This variable is used in conjunction with the @code{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.

@item
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
@code{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 @code{nil} or @code{-}, or a
list whose @sc{car} is such an integer or symbol; it should turn the
mode off otherwise.

Here is an example taken from the definition of @code{transient-mark-mode}.
It shows the use of @code{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.

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

@item
Add an element to @code{minor-mode-alist} for each minor mode
(@pxref{Mode Line Variables}).  This element should be a list of the
following form:

@smallexample
(@var{mode-variable} @var{string})
@end smallexample

Here @var{mode-variable} is the variable that controls enabling of the
minor mode, and @var{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 @code{minor-mode-alist}, use @code{assq} to
check for an existing element, to avoid duplication.  For example:

@smallexample
@group
(or (assq 'leif-mode minor-mode-alist)
    (setq minor-mode-alist
          (cons '(leif-mode " Leif") minor-mode-alist)))
@end group
@end smallexample
@end itemize

@node Keymaps and Minor Modes
@subsection 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 @code{minor-mode-map-alist}.  @xref{Active Keymaps}.

@cindex @code{self-insert-command}, minor modes
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 @code{self-insert-command} are limited to
special cases (designed for abbrevs and Auto Fill mode).  (Do not try
substituting your own definition of @code{self-insert-command} for the
standard one.  The editor command loop handles this function specially.)

@node Mode Line Format
@section Mode Line Format
@cindex mode line

  Each Emacs window (aside from minibuffer windows) includes a mode line,
which displays status information about the buffer displayed in the
window.  The mode line 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 mode line are
controlled.  It is in the chapter on modes because much of the
information displayed in the mode line relates to the enabled major and
minor modes.

  @code{mode-line-format} is a buffer-local variable that holds a
template used to display the mode line of the current buffer.  All
windows for the same buffer use the same @code{mode-line-format} and
their mode lines appear the same (except for scrolling percentages and
line numbers).

  The mode line of a window is normally updated whenever a different
buffer is shown in the window, or when the buffer's modified-status
changes from @code{nil} to @code{t} or vice-versa.  If you modify any of
the variables referenced by @code{mode-line-format} (@pxref{Mode Line
Variables}), you may want to force an update of the mode line so as to
display the new information.

@c Emacs 19 feature
@defun force-mode-line-update
Force redisplay of the current buffer's mode line.
@end defun

  The mode line is usually displayed in inverse video; see
@code{mode-line-inverse-video} in @ref{Inverse Video}.

@menu
* Mode Line Data::        The data structure that controls the mode line.
* Mode Line Variables::   Variables used in that data structure.
* %-Constructs::          Putting information into a mode line.
@end menu

@node Mode Line Data
@subsection The Data Structure of the Mode Line
@cindex mode line construct

  The mode line contents are controlled by a data structure of lists,
strings, symbols, and numbers kept in the buffer-local variable
@code{mode-line-format}.  The data structure is called a @dfn{mode line
construct}, and it is built in recursive fashion out of simpler mode line
constructs.  The same data structure is used for constructing
frame titles (@pxref{Frame Titles}).

@defvar mode-line-format
The value of this variable is a mode line construct with overall
responsibility for the mode line format.  The value of this variable
controls which other variables are used to form the mode line text, and
where they appear.
@end defvar

  A mode line 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 mode line
constructs as their values.

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

  A mode line 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.

@table @code
@cindex percent symbol in mode line
@item @var{string}
A string as a mode line construct is displayed verbatim in the mode line
except for @dfn{@code{%}-constructs}.  Decimal digits after the @samp{%}
specify the field width for space filling on the right (i.e., the data
is left justified).  @xref{%-Constructs}.

@item @var{symbol}
A symbol as a mode line construct stands for its value.  The value of
@var{symbol} is used as a mode line construct, in place of @var{symbol}.
However, the symbols @code{t} and @code{nil} are ignored; so is any
symbol whose value is void.

There is one exception: if the value of @var{symbol} is a string, it is
displayed verbatim: the @code{%}-constructs are not recognized.

@item (@var{string} @var{rest}@dots{}) @r{or} (@var{list} @var{rest}@dots{})
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.

@item (@var{symbol} @var{then} @var{else})
A list whose first element is a symbol is a conditional.  Its meaning
depends on the value of @var{symbol}.  If the value is non-@code{nil},
the second element, @var{then}, is processed recursively as a mode line
element.  But if the value of @var{symbol} is @code{nil}, the third
element, @var{else}, is processed recursively.  You may omit @var{else};
then the mode line element displays nothing if the value of @var{symbol}
is @code{nil}.

@item (@var{width} @var{rest}@dots{})
A list whose first element is an integer specifies truncation or
padding of the results of @var{rest}.  The remaining elements
@var{rest} are processed recursively as mode line constructs and
concatenated together.  Then the result is space filled (if
@var{width} is positive) or truncated (to @minus{}@var{width} columns,
if @var{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: @code{(-3 "%p")}.
@end table

  If you do alter @code{mode-line-format} itself, the new value should
use the same variables that appear in the default value (@pxref{Mode
Line 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 @code{display-time} and major
modes) via changes to those variables remain effective.

@cindex Shell mode @code{mode-line-format}
  Here is an example of a @code{mode-line-format} that might be
useful for @code{shell-mode}, since it contains the hostname and default
directory.

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

@node Mode Line Variables
@subsection Variables Used in the Mode Line

  This section describes variables incorporated by the
standard value of @code{mode-line-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 mode line if
@code{mode-line-format} were changed to use them.

@defvar mode-line-modified
This variable holds the value of the mode-line construct that displays
whether the current buffer is modified.

The default value of @code{mode-line-modified} is @code{("--%1*%1+-")}.
This means that the mode line displays @samp{--**-} if the buffer is
modified, @samp{-----} if the buffer is not modified, @samp{--%%-} if
the buffer is read only, and @samp{--%*--} if the buffer is read only
and modified.

Changing this variable does not force an update of the mode line.
@end defvar

@defvar mode-line-buffer-identification
This variable identifies the buffer being displayed in the window.  Its
default value is @code{("%F: %17b")}, which means that it usually
displays @samp{Emacs:} followed by seventeen characters of the buffer
name.  (In a terminal frame, it displays the frame name instead of
@samp{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'' Emacs.
@end defvar

@defvar global-mode-string
This variable holds a mode line spec that appears in the mode line by
default, just after the buffer name.  The command @code{display-time}
sets @code{global-mode-string} to refer to the variable
@code{display-time-string}, which holds a string containing the time and
load information.

The @samp{%M} construct substitutes the value of
@code{global-mode-string}, but this is obsolete, since the variable is
included directly in the mode line.
@end defvar

@defvar 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 mode line.
@end defvar

@defvar minor-mode-alist
This variable holds an association list whose elements specify how the
mode line should indicate that a minor mode is active.  Each element of
the @code{minor-mode-alist} should be a two-element list:

@example
(@var{minor-mode-variable} @var{mode-line-string})
@end example

More generally, @var{mode-line-string} can be any mode line spec.  It
appears in the mode line when the value of @var{minor-mode-variable} is
non-@code{nil}, and not otherwise.  These strings should begin with
spaces so that they don't run together.  Conventionally, the
@var{minor-mode-variable} for a specific mode is set to a non-@code{nil}
value when that minor mode is activated.

The default value of @code{minor-mode-alist} is:

@example
@group
minor-mode-alist
@result{} ((vc-mode vc-mode)
    (abbrev-mode " Abbrev") 
    (overwrite-mode overwrite-mode) 
    (auto-fill-function " Fill")         
    (defining-kbd-macro " Def")
    (isearch-mode isearch-mode))
@end group
@end example

@code{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.
@end defvar

@defvar mode-line-process
This buffer-local variable contains the mode line 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 @samp{*shell*} buffer is
@code{(":@: %s")}, which allows the shell to display its status along
with the major mode as: @samp{(Shell:@: run)}.  Normally this variable
is @code{nil}.
@end defvar

@defvar default-mode-line-format
This variable holds the default @code{mode-line-format} for buffers
that do not override it.  This is the same as @code{(default-value
'mode-line-format)}.

The default value of @code{default-mode-line-format} is:

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

@defvar vc-mode
The variable @code{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 @code{nil} for no version control, or a string
that appears in the mode line.
@end defvar

@node %-Constructs
@subsection @code{%}-Constructs in the Mode Line

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

@table @code
@item %b
The current buffer name, obtained with the @code{buffer-name} function.
@xref{Buffer Names}.

@item %f
The visited file name, obtained with the @code{buffer-file-name}
function.  @xref{Buffer File Name}.

@item %F
The name of the selected frame.

@item %c
The current column number of point.

@item %l
The current line number of point.

@item %*
@samp{%} if the buffer is read only (see @code{buffer-read-only}); @*
@samp{*} if the buffer is modified (see @code{buffer-modified-p}); @*
@samp{-} otherwise.  @xref{Buffer Modification}.

@item %+
@samp{*} if the buffer is modified (see @code{buffer-modified-p}); @*
@samp{%} if the buffer is read only (see @code{buffer-read-only}); @*
@samp{-} otherwise.  This differs from @samp{%*} only for a modified
read-only buffer.  @xref{Buffer Modification}.

@item %&
@samp{*} if the buffer is modified, and @samp{-} otherwise.

@item %s
The status of the subprocess belonging to the current buffer, obtained with
@code{process-status}.  @xref{Process Information}.

@item %t
Whether the visited file is a text file or a binary file.  (This is a
meaningful distinction only on certain operating systems.)

@item %p
The percentage of the buffer text above the @strong{top} of window, or
@samp{Top}, @samp{Bottom} or @samp{All}.

@item %P
The percentage of the buffer text that is above the @strong{bottom} of
the window (which includes the text visible in the window, as well as
the text above the top), plus @samp{Top} if the top of the buffer is
visible on screen; or @samp{Bottom} or @samp{All}.

@item %n
@samp{Narrow} when narrowing is in effect; nothing otherwise (see
@code{narrow-to-region} in @ref{Narrowing}).

@item %[
An indication of the depth of recursive editing levels (not counting
minibuffer levels): one @samp{[} for each editing level.
@xref{Recursive Editing}.

@item %]
One @samp{]} for each recursive editing level (not counting minibuffer
levels).

@item %%
The character @samp{%}---this is how to include a literal @samp{%} in a
string in which @code{%}-constructs are allowed.

@item %-
Dashes sufficient to fill the remainder of the mode line.
@end table

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

@table @code
@item %m
The value of @code{mode-name}.

@item %M
The value of @code{global-mode-string}.  Currently, only
@code{display-time} modifies the value of @code{global-mode-string}.
@end table

@node Hooks
@section Hooks
@cindex hooks

  A @dfn{hook} is a variable where you can store a function or functions
to be called on a particular occasion by an existing program.  Emacs
provides hooks for the sake of customization.  Most often, hooks are set
up in the @file{.emacs} file, but Lisp programs can set them also.
@xref{Standard Hooks}, for a list of standard hook variables.

  Most of the hooks in Emacs are @dfn{normal hooks}.  These variables
contain lists of functions to be called with no arguments.  The reason
most hooks are normal hooks is so that you can use them in a uniform
way.  You can always tell when a hook is a normal hook, because its 
name ends in @samp{-hook}.

  The recommended way to add a hook function to a normal hook is by
calling @code{add-hook} (see below).  The hook functions may be any of
the valid kinds of functions that @code{funcall} accepts (@pxref{What Is
a Function}).  Most normal hook variables are initially void;
@code{add-hook} knows how to deal with this.

  As for abnormal hooks, those whose names end in @samp{-function} have
a value that is a single function.  Those whose names end in
@samp{-hooks} have a value that is a list of functions.  Any hook that
is abnormal is abnormal because a normal hook won't do the job; either
the functions are called with arguments, or their values are meaningful.
The name shows you that the hook is abnormal and that you should look at
its documentation string to see how to use it properly.

  Major mode functions are supposed to run a hook called the @dfn{mode
hook} as the last step of initialization.  This makes it easy for a user
to customize the behavior of the mode, by overriding the local variable
assignments already made by the mode.  But hooks are used in other
contexts too.  For example, the hook @code{suspend-hook} runs just
before Emacs suspends itself (@pxref{Suspending Emacs}).

  Here's an expression that uses a mode hook to turn on Auto Fill mode
when in Lisp Interaction mode:

@example
(add-hook 'lisp-interaction-mode-hook 'turn-on-auto-fill)
@end example

  The next example shows how to use a hook to customize the way Emacs
formats C code.  (People often have strong personal preferences for one
format or another.)  Here the hook function is an anonymous lambda
expression.

@cindex lambda expression in hook
@example
@group
(add-hook 'c-mode-hook 
  (function (lambda ()
              (setq c-indent-level 4
                    c-argdecl-indent 0
                    c-label-offset -4
@end group
@group
                    c-continued-statement-indent 0
                    c-brace-offset 0
                    comment-column 40))))

(setq c++-mode-hook c-mode-hook)
@end group
@end example

  At the appropriate time, Emacs uses the @code{run-hooks} function to
run particular hooks.  This function calls the hook functions that have
been added with @code{add-hook}.

@defun run-hooks &rest hookvar
This function takes one or more hook variable names as arguments, and
runs each hook in turn.  Each @var{hookvar} argument should be a symbol
that is a hook variable.  These arguments are processed in the order
specified.

If a hook variable has a non-@code{nil} value, that value may be a
function or a list of functions.  If the value is a function (either a
lambda expression or a symbol with a function definition), it is
called.  If it is a list, the elements are called, in order.
The hook functions are called with no arguments.

For example, here's how @code{emacs-lisp-mode} runs its mode hook:

@example
(run-hooks 'emacs-lisp-mode-hook)
@end example
@end defun

@defun add-hook hook function &optional append local
This function is the handy way to add function @var{function} to hook
variable @var{hook}.  The argument @var{function} may be any valid Lisp
function with the proper number of arguments.  For example,

@example
(add-hook 'text-mode-hook 'my-text-hook-function)
@end example

@noindent
adds @code{my-text-hook-function} to the hook called @code{text-mode-hook}.

You can use @code{add-hook} for abnormal hooks as well as for normal
hooks.

It is best to design your hook functions 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,
@var{function} goes at the front of the hook list, so it will be
executed first (barring another @code{add-hook} call).

If the optional argument @var{append} is non-@code{nil}, the new hook
function goes at the end of the hook list and will be executed last.

If @var{local} is non-@code{nil}, that says to make the new hook
function local to the current buffer.  Before you can do this, you must
make the hook itself buffer-local by calling @code{make-local-hook}
(@strong{not} @code{make-local-variable}).  If the hook itself is not
buffer-local, then the value of @var{local} makes no difference---the
hook function is always global.
@end defun

@defun remove-hook hook function &optional local
This function removes @var{function} from the hook variable @var{hook}.

If @var{local} is non-@code{nil}, that says to remove @var{function}
from the local hook list instead of from the global hook list.  If the
hook itself is not buffer-local, then the value of @var{local} makes no
difference.
@end defun

@defun make-local-hook hook
This function makes the hook variable @code{hook} local to the current
buffer.  When a hook variable is local, it can have local and global
hook functions, and @code{run-hooks} runs all of them.

This function works by making @code{t} an element of the buffer-local
value.  That serves as a flag to use the hook functions in the default
value of the hook variable as well as those in the local value.  Since
@code{run-hooks} understands this flag, @code{make-local-hook} works
with all normal hooks.  It works for only some non-normal hooks---those
whose callers have been updated to understand this meaning of @code{t}.

Do not use @code{make-local-variable} directly for hook variables; it is
not sufficient.
@end defun