A 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: major modes, which are mutually exclusive and used for editing particular kinds of text, and 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 section Keymaps, and section Syntax Tables.
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 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 C-j
(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 (see section Defining 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 offers an example of changing the major mode temporarily for a buffer, so it can be edited in a different way (with ordinary Emacs commands rather than Rmail commands). 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 an alternative major mode avoids this limitation. See section Recursive Editing.
The standard GNU Emacs Lisp library directory contains the code for several major modes, in files such as `text-mode.el', `texinfo.el', `lisp-mode.el', `c-mode.el', and `rmail.el'. You can study 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.
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:
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. See section Substituting Key
Bindings in Documentation.
kill-all-local-variables. This is what gets rid of the
buffer-local variables of the major mode previously in effect.
major-mode to the major mode command symbol. This is
how describe-mode discovers which documentation to
print.
mode-name to the "pretty" name of the mode, as a
string. This string appears in the mode line.
use-local-map to install this local map. See section
Active Keymaps, for more
information. This keymap should be stored permanently in a global
variable named modename-mode-map. Normally
the library that defines the mode sets this variable. See section
Tips for Defining Variables
Robustly, for advice about how to write the code to set up the
mode's keymap variable.
modename-mode-syntax-table. See section Syntax Tables.
modename-mode-abbrev-table. See section Abbrev Tables.
font-lock-defaults (see section Font Lock Mode).
imenu-generic-expression or
imenu-create-index-function (see section Imenu).
defvar or defcustom 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.)
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. See section 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.
modename-mode-hook. The
major mode command should run that hook, with
run-hooks, as the very last thing it does. See section
Hooks.
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.
change-major-mode-hook (see
section Creating and Deleting
Buffer-Local Bindings).
mode-class with value special, put on as
follows:
(put 'funny-mode 'mode-class 'special)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.
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. See section How Emacs Chooses a Major Mode.
autoload form and an example
of how to add to auto-mode-alist, that users can
include in their `.emacs' files.
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" 'indent-relative)
(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@enddots{}
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)
(setq local-abbrev-table text-mode-abbrev-table)
(set-syntax-table text-mode-syntax-table)
(make-local-variable 'paragraph-start)
(setq paragraph-start (concat "[ \t]*$\\|" page-delimiter))
(make-local-variable 'paragraph-separate)
(setq paragraph-separate paragraph-start)
(setq mode-name "Text")
(setq major-mode 'text-mode)
(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)
(cond (lisp-syntax
(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) (setq paragraph-start (concat page-delimiter "\\|$" )) (make-local-variable 'paragraph-separate) (setq paragraph-separate paragraph-start) ... (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-z to
run-lisp, but the other Lisp modes do not. However,
all Lisp modes have some commands in common. The following code
sets up the common commands:
(defvar shared-lisp-mode-map ()
"Keymap for commands shared by all sorts of Lisp modes.")
(if shared-lisp-mode-map
()
(setq shared-lisp-mode-map (make-sparse-keymap))
(define-key shared-lisp-mode-map "\e\C-q" 'indent-sexp)
(define-key shared-lisp-mode-map "\177"
'backward-delete-char-untabify))
And here is the code to set up the keymap for Lisp mode:
(defvar lisp-mode-map ()
"Keymap for ordinary Lisp mode@enddots{}")
(if lisp-mode-map
()
(setq lisp-mode-map (make-sparse-keymap))
(set-keymap-parent lisp-mode-map shared-lisp-mode-map)
(define-key lisp-mode-map "\e\C-x" 'lisp-eval-defun)
(define-key lisp-mode-map "\C-c\C-z" 'run-lisp))
Finally, here is the complete major mode function definition for Emacs Lisp mode.
(defun lisp-mode ()
"Major mode for editing Lisp code for Lisps other than GNU Emacs Lisp.
Commands:
Delete converts tabs to spaces as it moves back.
Blank lines separate paragraphs. Semicolons start comments.
\\{lisp-mode-map}
Note that `run-lisp' may be used either to start an inferior Lisp job
or to switch back to an existing one.
Entry to this mode calls the value of `lisp-mode-hook'
if that value is non-nil."
(interactive)
(kill-all-local-variables)
(use-local-map lisp-mode-map) ; Select the mode's keymap.
(setq major-mode 'lisp-mode) ; This is how describe-mode
; finds out what to describe.
(setq mode-name "Lisp") ; This goes into the mode line.
(lisp-mode-variables t) ; This defines various variables.
(setq imenu-case-fold-search t)
(set-syntax-table lisp-mode-syntax-table)
(run-hooks 'lisp-mode-hook)) ; This permits the user to use a
; hook to customize the 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. It also processes local variables specified in the file text.
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.)
set-auto-mode, then it
runs hack-local-variables to parse, and bind or
evaluate as appropriate, the file's 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. See
section `Local Variables in Files' in The GNU Emacs
Manual, for the syntax of the local variables section of a
file.
If you run normal-mode interactively, the argument
find-file is normally nil. In this case,
normal-mode unconditionally processes any local
variables list.
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.
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.
In addition to this list, any variable whose name has a
non-nil risky-local-variable property is
also ignored.
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.
auto-mode-alist), on the `#!' line
(using interpreter-mode-alist), or on the file's local
variables list. However, this function does not look for the
`mode:' local variable near the end of a file; the
hack-local-variables function does that. See section
`How Major Modes are Chosen' in The GNU Emacs
Manual.
fundamental-mode. If the value of default-major-mode is
nil, Emacs uses the (previously) current buffer's
major mode for the major mode of a new buffer. However, if that
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.
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.
lisp-interaction-mode.
(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 (see section Functions that Expand Filenames)
matches a regexp, set-auto-mode calls the
corresponding mode-function. This feature enables Emacs
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, Emacs searches
auto-mode-alist again for a match against the portion
of the file name that did not match before. This feature is useful
for uncompression packages: an entry of the form ("\\.gz\\'"
function t) 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 (within directory) starts with a dot.
'(("/\\.[^/]*\\'" . fundamental-mode)
;; File name has no dot.
("[^\\./]*\\'" . fundamental-mode)
;; File name ends in `.C'.
("\\.C\\'" . c++-mode))
auto-mode-alist))
(interpreter . mode);
for example, ("perl" . perl-mode) is one element
present by default. The element says to use mode mode if
the file specifies an interpreter which matches
interpreter. The value of interpreter is
actually a regular expression. This variable is applicable only when the
auto-mode-alist does not indicate which major mode to
use.
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.
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.
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. (See section Access to Documentation Strings.)
describe-mode function uses the
documentation string of the function as the documentation of the
major mode.
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.
The new command variant is defined to call the function parent, then override certain aspects of that parent mode:
variant-map.
define-derived-mode initializes this map to inherit
from parent-map, if it is not already
set.
variant-syntax-table.
define-derived-mode initializes this variable by
copying parent-syntax-table, if it is not
already set.
variant-abbrev-table.
define-derived-mode initializes this variable by
copying parent-abbrev-table, if it is not
already set.
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)
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 would be unwieldy.
A minor mode is not usually a modification of single major mode. For example, Auto Fill mode works with 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.
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.
nil to disable; anything else to
enable). If it is possible, implement the mode so that setting the
variable automatically enables or disables the mode. Then the minor
mode command does not need to do anything except set the variable.
This variable is used in conjunction with the
minor-mode-alist to display the minor mode name in the
mode line. It can also enable or disable a minor mode keymap.
Individual commands or hooks can also check the variable's value.
If you want the minor mode to be enabled separately in each buffer,
make the variable buffer-local.
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)))
minor-mode-alist for each minor
mode (see section Variables Used in the
Mode Line), if you want to indicate the minor mode in the mode
line. This element should be a list of the following form:
(mode-variable string)Here mode-variable is the variable that controls enabling of the minor mode, and string is a short string, starting with a space, to represent the mode in the mode line. These strings must be short so that there is room for several of them at once. When you add an element to
minor-mode-alist, use assq to check for
an existing element, to avoid duplication. For example:
(or (assq 'leif-mode minor-mode-alist)
(setq minor-mode-alist
(cons '(leif-mode " Leif") minor-mode-alist)))
You can also use add-to-list to add an element to
this list just once (see section How to
Alter a Variable Value).
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. See section
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.)
The key sequences bound in a minor mode should consist of C-c followed by a punctuation character other than {, }, <, >, : or ;. (Those few punctuation characters are reserved for major modes.)
The easy-mmode package provides a convenient way of implementing a minor mode; with it, you can specify all about a simple minor mode in one self-contained definition.
This macro defines a command named mode which toggles the minor mode, and has doc as its documentation string.
It also defines a variable named mode, which is set
to t or nil by enabling or disabling the
mode. The variable is initialized to init-value.
The string mode-indicator says what to display in the
mode line when the mode is enabled; if it is nil, the
mode is not displayed in the mode line.
The optional argument keymap specifies the keymap for the minor mode. It can be a variable name, whose value is the keymap, or it can be an alist specifying bindings in this form:
(key-sequence . definition)
Here is an example of using
easy-mmode-define-minor-mode:
(easy-mmode-define-minor-mode hungry-mode
"Toggle Hungry mode.
With no argument, this command toggles the mode.
Non-null prefix argument turns on the mode.
Null prefix argument turns off the mode.
When Hungry mode is enabled, the control delete key
gobbles all preceding whitespace except the last.
See the command \\[hungry-electric-delete]."
;; The initial value.
nil
;; The indicator for the mode line.
" Hungry"
;; The minor mode bindings.
'(("\C-\^?" . hungry-electric-delete)
("\C-\M-\^?"
. (lambda ()
(interactive)
(hungry-electric-delete t)))))
This defines a minor mode named "Hungry mode", a command named
hungry-mode to toggle it, a variable named
hungry-mode which indicates whether the mode is
enabled, and a variable named hungry-mode-map which
holds the keymap that is active when the mode is enabled. It
initializes the keymap with key bindings for C-DEL and
C-M-DEL.
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. We include it in this chapter because much of the information displayed in the mode line relates to the enabled major and minor modes.
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
mode-line-format and their mode lines appear the same
(except for scrolling percentages, and line and column
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 nil to t or
vice-versa. If you modify any of the variables referenced by
mode-line-format (see section Variables Used in the Mode Line), or
any other variables and data structures that affect how text is
displayed (see section Emacs
Display), you may want to force an update of the mode line so
as to display the new information or display it in the new way.
The mode line is usually displayed in inverse video; see
mode-line-inverse-video in section Inverse Video.
The mode line 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 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 (see section Frame Titles).
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 mode-line-format incorporates
the values of variables such as mode-name and
minor-mode-alist. Because of this, very few modes need
to alter mode-line-format itself. For most purposes,
it is sufficient to alter some of the variables that
mode-line-format refers to.
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.
string%-constructs. Decimal digits after the
`%' specify the field width for space filling on the
right (i.e., the data is left justified). See section %-Constructs in the Mode
Line.
symbolt 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...)(symbol then
else)nil, the second element, then, is
processed recursively as a mode line 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...)(-3 "%p").
If you do alter mode-line-format itself, the new
value should use the same variables that appear in the default
value (see section Variables Used in
the Mode Line), 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
mode-line-format that might be useful for
shell-mode, since it contains the host name and
default directory.
(setq mode-line-format
(list "-"
'mode-line-mule-info
'mode-line-modified
'mode-line-frame-identification
"%b--"
;; Note that this is evaluated while making the list.
;; It makes a mode line construct which is just a string.
(getenv "HOST")
":"
'default-directory
" "
'global-mode-string
" %[("
'mode-name
'mode-line-process
'minor-mode-alist
"%n"
")%]--"
'(which-func-mode ("" which-func-format "--"))
'(line-number-mode "L%l--")
'(column-number-mode "C%c--")
'(-3 . "%p")
"-%-"))
(The variables line-number-mode,
column-number-mode and which-func-mode
enable particular minor modes; as usual, these variable names are
also the minor mode command names.)
This section describes variables incorporated by the standard
value of 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
mode-line-format were changed to use them.
The default value of mode-line-modified is
("%1*%1+"). This means that the mode line 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 mode line.
" " if you are using a window system
which can show multiple frames, or "-%F " on an
ordinary terminal which shows only one frame at a time.
("%12b"), which displays the buffer name, padded with
spaces to at least 12 columns.
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 that is obsolete, since the
variable is included in the mode line from
mode-line-format.
minor-mode-alist should be a two-element list: (minor-mode-variable mode-line-string)
More generally, mode-line-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 itself is not buffer-local. Each
variable mentioned in the alist should be buffer-local if its minor
mode can be enabled separately in each buffer.
(":%s"), which allows the shell to display its status
along with the major mode as: `(Shell: run)'. Normally
this variable is nil.
mode-line-format for buffers that do not
override it. This is the same as (default-value
'mode-line-format). The default value of default-mode-line-format is
this list:
("-"
mode-line-mule-info
mode-line-modified
mode-line-frame-identification
mode-line-buffer-identification
" "
global-mode-string
" %[("
mode-name
mode-line-process
minor-mode-alist
"%n"
")%]--"
(which-func-mode ("" which-func-format "--"))
(line-number-mode "L%l--")
(column-number-mode "C%c--")
(-3 . "%p")
"-%-")
vc-mode, buffer-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.
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.
%bbuffer-name function. See section Buffer Names.
%fbuffer-file-name function. See section Buffer File Name.
%F%c%l%*buffer-read-only);buffer-modified-p);%+buffer-modified-p);buffer-read-only);%&%sprocess-status. See section Process Information.
%t%p%P%nnarrow-to-region in section Narrowing).
%[%]%%%-constructs are allowed.
%-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.
%mmode-name.
%Mglobal-mode-string. Currently, only
display-time modifies the value of
global-mode-string.
Imenu is a feature that lets users select a definition or section in the buffer, from a menu which lists all of them, to go directly to that location in the buffer. Imenu works by constructing a buffer index which lists the names and positions of the definitions or portions of in the buffer, so the user can pick one of them to move to. This section explains how to customize Imenu for a major mode.
The usual and simplest way is to set the variable
imenu-generic-expression:
nil, specifies regular expressions for finding
definitions for Imenu. In the simplest case, elements should look
like this: (menu-title regexp subexp)
Here, if menu-title is non-nil, it says
that the matches for this element should go in a submenu of the
buffer index; menu-title itself specifies the name for
the submenu. If menu-title is nil, the
matches for this element go directly in the top level of the buffer
index.
The second item in the list, regexp, is a regular expression (see section Regular Expressions); wherever it matches, that is a definition to mention in the buffer index. The third item, subexp, indicates which subexpression in regexp matches the definition's name.
An element can also look like this:
(menu-title regexp index function arguments...)
Each match for this element creates a special index item which, if selected by the user, calls function with arguments item-name, the buffer position, and arguments.
For Emacs Lisp mode, pattern could look like this:
((nil "^\\s-*(def\\(un\\|subst\\|macro\\|advice\\)\
\\s-+\\([-A-Za-z0-9+]+\\)" 2)
("*Vars*" "^\\s-*(def\\(var\\|const\\)\
\\s-+\\([-A-Za-z0-9+]+\\)" 2)
("*Types*"
"^\\s-*\
(def\\(type\\|struct\\|class\\|ine-condition\\)\
\\s-+\\([-A-Za-z0-9+]+\\)" 2))
Setting this variable makes it buffer-local in the current buffer.
t, the default, means matching should
ignore case. Setting this variable makes it buffer-local in the current buffer.
imenu-generic-expression, to override the syntax table
of the current buffer. Each element should have this form: (characters . syntax-description)
The CAR, characters, can be either a character or a
string. The element says to give that character or characters the
syntax specified by syntax-description, which is passed
to modify-syntax-entry (see section Syntax Table Functions).
This feature is typically used to give word syntax to characters
which normally have symbol syntax, and thus to simplify
imenu-generic-expression and speed up matching. For
example, Fortran mode uses it this way:
(setq imenu-syntax-alist '(("_$" . "w")))
The imenu-generic-expression patterns can then use
`\\sw+' instead of `\\(\\sw\\|\\s_\\)+'.
Note that this technique may be inconvenient to use when the mode
needs to limit the initial character of a name to a smaller set of
characters than are allowed in the rest of a name.
Setting this variable makes it buffer-local in the current buffer.
Another way to customize Imenu for a major mode is to set the
variables imenu-prev-index-position-function and
imenu-extract-index-name-function:
nil, its value should be a function for finding
the next definition to mention in the buffer index, moving
backwards in the file. The function should leave point at the place to be connected to
the index item; it should return nil if it doesn't
find another item.
Setting this variable makes it buffer-local in the current buffer.
nil, its value should be a function to return the
name for a definition, assuming point is in that definition as the
imenu-prev-index-position-function function would
leave it. Setting this variable makes it buffer-local in the current buffer.
The last way to customize Imenu for a major mode is to set the
variables imenu-create-index-function:
save-excursion, so where it leaves
point makes no difference. The default value is a function that uses
imenu-generic-expression to produce the index alist.
If you specify a different function, then
imenu-generic-expression is not used.
Setting this variable makes it buffer-local in the current buffer.
Simple elements in the alist look like
(index-name . index-position).
Selecting a simple element has the effect of moving to position
index-position in the buffer.
Special elements look like (index-name
position function
arguments...). Selecting a special element
performs
(funcall function index-name position arguments...)
A nested sub-alist element looks like
(index-name sub-alist).
Font Lock mode is a feature that automatically attaches
face properties to certain parts of the buffer based
on their syntactic role. How it parses the buffer depends on the
major mode; most major modes define syntactic criteria for which
faces to use, in which contexts. This section explains how to
customize Font Lock for a particular language--in other words, for
a particular major mode.
Font Lock mode finds text to highlight in two ways: through
syntactic parsing based on the syntax table, and through searching
(usually for regular expressions). Syntactic fontification happens
first; it finds comments and string constants, and highlights them
using font-lock-comment-face and
font-lock-string-face (see section Faces for Font Lock); search-based
fontification follows.
There are several variables that control how Font Lock mode
highlights text. But major modes should not set any of these
variables directly. Instead, it should set
font-lock-defaults as a buffer-local variable. The
value assigned to this variable is used, if and when Font Lock mode
is enabled, to set all the other variables.
(keywords keywords-only case-fold syntax-alist syntax-begin other-vars...)
The first element, keywords, indirectly specifies the
value of font-lock-keywords. It can be a symbol, a
variable whose value is list to use for
font-lock-keywords. It can also be a list of several
such symbols, one for each possible level of fontification. The
first symbol specifies how to do level 1 fontification, the second
symbol how to do level 2, and so on.
The second element, keywords-only, specifies the
value of the variable font-lock-keywords-only. If this
is non-nil, syntactic fontification (of strings and
comments) is not performed.
The third element, case-fold, specifies the value of
font-lock-case-fold-search. If it is
non-nil, Font Lock mode ignores case when searching as
directed by font-lock-keywords.
If the fourth element, syntax-alist, is
non-nil, it should be a list of cons cells of the form
(char-or-string . string). These
are used to set up a syntax table for fontification (see section Syntax Table Functions). The resulting
syntax table is stored in font-lock-syntax-table.
The fifth element, syntax-begin, specifies the value
of font-lock-beginning-of-syntax-function (see
below).
Any further elements other-vars are have form
(variable . value). This kind of
element means to make variable buffer-local and then set
it to value. This is used to set other variables that
affect fontification.
The most important variable for customizing Font Lock mode is
font-lock-keywords. It specifies the search criteria
for search-based fontification.
Each element of font-lock-keywords specifies how to
find certain cases of text, and how to highlight those cases. Font
Lock mode processes the elements of font-lock-keywords
one by one, and for each element, it finds and handles all matches.
Ordinarily, once part of the text has been fontified already, this
cannot be overridden by a subsequent match in the same text; but
you can specify different behavior using the override
element of a highlighter.
Each element of font-lock-keywords should have one
of these forms:
regexpfont-lock-keyword-face. For example,
;; Highlight discrete occurrences of `foo'
;; using font-lock-keyword-face.
"\\<foo\\>"
The function regexp-opt (see section Syntax of Regular Expressions) is
useful for calculating optimal regular expressions to match a
number of different keywords.
functionfont-lock-keyword-face. When
function is called, it receives one argument, the limit
of the search. It should return non-nil if it
succeeds, and set the match data to describe the match that was
found.
(matcher . match)
;; Highlight the `bar' in each occurrences of `fubar',
;; using font-lock-keyword-face.
("fu\\(bar\\)" . 1)
If you use regexp-opt to produce the regular
expression matcher, then you can use
regexp-opt-depth (see section Syntax of Regular Expressions) to
calculate the value for match.
(matcher . facename)
;; Highlight occurrences of `fubar',
;; using the face which is the value of fubar-face.
("fubar" . fubar-face)
(matcher . highlighter)(subexp facename override laxmatch)The CAR, subexp, is an integer specifying which subexpression of the match to fontify (0 means the entire matching text). The second subelement, facename, specifies the face, as described above. The last two values in highlighter, override and laxmatch, are flags. If override is
t, this element can override existing fontification
made by previous elements of font-lock-keywords. If it
is keep, then each character is fontified if it has
not been fontified already by some other element. If it is
prepend, the face facename is added to the
beginning of the face property. If it is
append, the face facename is added to the
end of the face property. If laxmatch is
non-nil, it means there should be no error if there is
no subexpression numbered subexp in matcher.
Here are some examples of elements of this kind, and what they do:
;; Highlight occurrences of either `foo' or `bar', ;; usingfoo-bar-face, even if they have already been highlighted. ;;foo-bar-faceshould be a variable whose value is a face. ("foo\\|bar" 0 foo-bar-face t) ;; Highlight the first subexpression within each occurrences ;; that the functionfubar-matchfinds, ;; using the face which is the value offubar-face. (fubar-match 1 fubar-face)
(matcher
highlighters...)(eval . form)font-lock-keywords is used in a
buffer. Its value should have one of the forms described in this
table.
Warning: Do not design an element of
font-lock-keywords to match text which spans lines;
this does not work reliably. While
font-lock-fontify-buffer handles multi-line patterns
correctly, updating when you edit the buffer does not, since it
considers text one line at a time.
This section describes additional variables that a major mode
can set by means of font-lock-defaults.
nil means
Font Lock should not fontify comments or strings syntactically; it
should only fontify based on font-lock-keywords.
nil means
that regular expression matching for the sake of
font-lock-keywords should be case-insensitive.
nil, it should be a function to move point back to
a position that is syntactically at "top level" and outside of
strings or comments. Font Lock uses this when necessary to get the
right results for syntactic fontification. This function is called with no arguments. It should leave point
at the beginning of any enclosing syntactic block. Typical values
are beginning-of-line (i.e., the start of the line is
known to be outside a syntactic block), or
beginning-of-defun for programming modes or
backward-paragraph for textual modes (i.e., the
mode-dependent function is known to move outside a syntactic
block).
If the value is nil, the beginning of the buffer is
used as a position outside of a syntactic block. This cannot be
wrong, but it can be slow.
nil, it should be a function that is called with
no arguments, to choose an enclosing range of text for
refontification for the command M-g M-g
(font-lock-fontify-block). The function should report its choice by placing the region
around it. A good choice is a range of text large enough to give
proper results, but not too large so that refontification becomes
slow. Typical values are mark-defun for programming
modes or mark-paragraph for textual modes.
Many major modes offer three different levels of fontification.
You can define multiple levels by using a list of symbols for
keywords in font-lock-defaults. Each symbol
specifies one level of fontification; it is up to the user to
choose one of these levels. The chosen level's symbol value is used
to initialize font-lock-keywords.
Here are the conventions for how to define the levels of fontification:
You can make Font Lock mode use any face, but several faces are
defined specifically for Font Lock mode. Each of these symbols is
both a face name, and a variable whose default value is the symbol
itself. Thus, the default value of
font-lock-comment-face is
font-lock-comment-face. This means you can write
font-lock-comment-face in a context such as
font-lock-keywords where a face-name-valued expression
is used.
font-lock-comment-facefont-lock-string-facefont-lock-keyword-facefor and if in C.
font-lock-builtin-facefont-lock-function-name-facefont-lock-variable-name-facefont-lock-type-facefont-lock-constant-facefont-lock-warning-face#error directives in C.
Font Lock mode can be used to update syntax-table
properties automatically. This is useful in languages for which a
single syntax table by itself is not sufficient.
(matcher subexp syntax override laxmatch)
The parts of this element have the same meanings as in the
corresponding sort of element of
font-lock-keywords,
(matcher subexp facename override laxmatch)
However, instead of specifying the value facename to
use for the face property, it specifies the value
syntax to use for the syntax-table
property. Here, syntax can be a variable whose value is
a syntax table, a syntax entry of the form
(syntax-code . matching-char),
or an expression whose value is one of those two types.
A 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 `.emacs' file, but Lisp programs can set them also. See section Standard Hooks, for a list of standard hook variables.
Most of the hooks in Emacs are normal hooks. These variables contain lists of functions to be called with no arguments. When the hook name ends in `-hook', that tells you it is normal. We try to make all hooks normal, as much as possible, so that you can use them in a uniform way.
Every major mode function is supposed to run a normal hook
called the 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 buffer-local variable assignments already
made by the mode. But hooks are used in other contexts too. For
example, the hook suspend-hook runs just before Emacs
suspends itself (see section Suspending
Emacs).
The recommended way to add a hook function to a normal hook is
by calling add-hook (see below). The hook functions
may be any of the valid kinds of functions that
funcall accepts (see section What Is a Function?). Most normal hook
variables are initially void; add-hook knows how to
deal with this.
If the hook variable's name does not end with `-hook', that indicates it is probably an abnormal hook; you should look at its documentation to see how to use the hook properly.
If the variable's name ends in `-functions' or
`-hooks', then the value is a list of functions, but
it is abnormal in that either these functions are called with
arguments or their values are used in some way. You can use
add-hook to add a function to the list, but you must
take care in writing the function. (A few of these variables are
actually normal hooks which were named before we established the
convention of using `-hook' for them.)
If the variable's name ends in `-function', then its value is just a single function, not a list of functions.
Here's an example that uses a mode hook to turn on Auto Fill mode when in Lisp Interaction mode:
(add-hook 'lisp-interaction-mode-hook 'turn-on-auto-fill)
At the appropriate time, Emacs uses the run-hooks
function to run particular hooks. This function calls the hook
functions that have been added with add-hook.
If a hook variable has a non-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.
Nowadays, storing a single function in the hook variable is
semi-obsolete; you should always use a list of functions.
For example, here's how emacs-lisp-mode runs its
mode hook:
(run-hooks 'emacs-lisp-mode-hook)
nil. Then it stops,
and returns nil if some hook function did, and
otherwise returns a non-nil value.
nil. Then it stops, and returns whatever was
returned by the last hook function that was called.
(add-hook 'text-mode-hook 'my-text-hook-function)
adds my-text-hook-function to the hook called
text-mode-hook.
You can use 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, function goes at the front of the hook list,
so it will be executed first (barring another add-hook
call). If the optional argument append is
non-nil, the new hook function goes at the end of the
hook list and will be executed last.
If local is non-nil, that says to make
the new hook function buffer-local in the current buffer. Before
you can do this, you must make the hook itself buffer-local by
calling make-local-hook (not
make-local-variable). If the hook itself is not
buffer-local, then the value of local makes no
difference--the hook function is always global.
If local is non-nil, that says to remove
function from the buffer-local hook list instead of from
the global hook list. If the hook variable itself is not
buffer-local, then the value of local makes no
difference.
hook buffer-local in the current buffer.
When a hook variable is buffer-local, it can have buffer-local and
global hook functions, and run-hooks runs all of them.
This function works by making 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
buffer-local value. Since run-hooks understands this
flag, 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 t.
Do not use make-local-variable directly for hook
variables; it is not sufficient.