2 @c This is part of the GNU Emacs Lisp Reference Manual.
3 @c Copyright (C) 1990-1995, 1998-1999, 2001-2012 Free Software Foundation, Inc.
4 @c See the file elisp.texi for copying conditions.
5 @setfilename ../../info/modes
6 @node Modes, Documentation, Keymaps, Top
7 @chapter Major and Minor Modes
10 A @dfn{mode} is a set of definitions that customize Emacs and can be
11 turned on and off while you edit. There are two varieties of modes:
12 @dfn{major modes}, which are mutually exclusive and used for editing
13 particular kinds of text, and @dfn{minor modes}, which provide features
14 that users can enable individually.
16 This chapter describes how to write both major and minor modes, how to
17 indicate them in the mode line, and how they run hooks supplied by the
18 user. For related topics such as keymaps and syntax tables, see
19 @ref{Keymaps}, and @ref{Syntax Tables}.
22 * Hooks:: How to use hooks; how to write code that provides hooks.
23 * Major Modes:: Defining major modes.
24 * Minor Modes:: Defining minor modes.
25 * Mode Line Format:: Customizing the text that appears in the mode line.
26 * Imenu:: Providing a menu of definitions made in a buffer.
27 * Font Lock Mode:: How modes can highlight text according to syntax.
28 * Auto-Indentation:: How to teach Emacs to indent for a major mode.
29 * Desktop Save Mode:: How modes can have buffer state saved between
37 A @dfn{hook} is a variable where you can store a function or functions
38 to be called on a particular occasion by an existing program. Emacs
39 provides hooks for the sake of customization. Most often, hooks are set
40 up in the init file (@pxref{Init File}), but Lisp programs can set them also.
41 @xref{Standard Hooks}, for a list of standard hook variables.
44 Most of the hooks in Emacs are @dfn{normal hooks}. These variables
45 contain lists of functions to be called with no arguments. By
46 convention, whenever the hook name ends in @samp{-hook}, that tells
47 you it is normal. We try to make all hooks normal, as much as
48 possible, so that you can use them in a uniform way.
50 Every major mode command is supposed to run a normal hook called the
51 @dfn{mode hook} as the one of the last steps of initialization. This
52 makes it easy for a user to customize the behavior of the mode, by
53 overriding the buffer-local variable assignments already made by the
54 mode. Most minor mode functions also run a mode hook at the end. But
55 hooks are used in other contexts too. For example, the hook
56 @code{suspend-hook} runs just before Emacs suspends itself
57 (@pxref{Suspending Emacs}).
59 The recommended way to add a hook function to a normal hook is by
60 calling @code{add-hook} (see below). The hook functions may be any of
61 the valid kinds of functions that @code{funcall} accepts (@pxref{What
62 Is a Function}). Most normal hook variables are initially void;
63 @code{add-hook} knows how to deal with this. You can add hooks either
64 globally or buffer-locally with @code{add-hook}.
67 If the hook variable's name does not end with @samp{-hook}, that
68 indicates it is probably an @dfn{abnormal hook}. That means the hook
69 functions are called with arguments, or their return values are used
70 in some way. The hook's documentation says how the functions are
71 called. You can use @code{add-hook} to add a function to an abnormal
72 hook, but you must write the function to follow the hook's calling
75 By convention, abnormal hook names end in @samp{-functions} or
76 @samp{-hooks}. If the variable's name ends in @samp{-function}, then
77 its value is just a single function, not a list of functions.
80 * Running Hooks:: How to run a hook.
81 * Setting Hooks:: How to put functions on a hook, or remove them.
85 @subsection Running Hooks
87 In this section, we document the @code{run-hooks} function, which is
88 used to run a normal hook. We also document the functions for running
89 various kinds of abnormal hooks.
91 @defun run-hooks &rest hookvars
92 This function takes one or more normal hook variable names as
93 arguments, and runs each hook in turn. Each argument should be a
94 symbol that is a normal hook variable. These arguments are processed
95 in the order specified.
97 If a hook variable has a non-@code{nil} value, that value should be a
98 list of functions. @code{run-hooks} calls all the functions, one by
99 one, with no arguments.
101 The hook variable's value can also be a single function---either a
102 lambda expression or a symbol with a function definition---which
103 @code{run-hooks} calls. But this usage is obsolete.
105 If the hook variable is buffer-local, the buffer-local variable will
106 be used instead of the global variable. However, if the buffer-local
107 variable contains the element @code{t}, the global hook variable will
111 @defun run-hook-with-args hook &rest args
112 This function runs an abnormal hook by calling all the hook functions in
113 @var{hook}, passing each one the arguments @var{args}.
116 @defun run-hook-with-args-until-failure hook &rest args
117 This function runs an abnormal hook by calling each hook function in
118 turn, stopping if one of them ``fails'' by returning @code{nil}. Each
119 hook function is passed the arguments @var{args}. If this function
120 stops because one of the hook functions fails, it returns @code{nil};
121 otherwise it returns a non-@code{nil} value.
124 @defun run-hook-with-args-until-success hook &rest args
125 This function runs an abnormal hook by calling each hook function,
126 stopping if one of them ``succeeds'' by returning a non-@code{nil}
127 value. Each hook function is passed the arguments @var{args}. If this
128 function stops because one of the hook functions returns a
129 non-@code{nil} value, it returns that value; otherwise it returns
133 @defmac with-wrapper-hook hook args &rest body
134 This macro runs the abnormal hook @code{hook} as a series of nested
135 ``wrapper functions'' around the @var{body} forms. The effect is
136 similar to nested @code{around} advices (@pxref{Around-Advice}).
138 Each hook function must accept an argument list consisting of a function
139 @var{fun}, followed by the additional arguments listed in @var{args}.
140 The function @var{fun} passed to the very first hook function in
141 @var{hook} does the same as @var{body}, if it is called with arguments
142 @var{args}. The @var{fun} passed to each successive hook function is
143 constructed from all the preceding hook functions (and @var{body}); if
144 this @var{fun} is called with arguments @var{args}, it does what the
145 @code{with-wrapper-hook} call would if the preceding hook functions were
146 the only ones in @var{hook}.
148 In the function definition of the hook function, @var{fun} can be called
149 any number of times (including not calling it at all). This function
150 definition is then used to construct the @var{fun} passed to the next
151 hook function in @var{hook}, if any. The last or ``outermost''
152 @var{fun} is called once to produce the effect.
156 @subsection Setting Hooks
158 Here's an example that uses a mode hook to turn on Auto Fill mode when
159 in Lisp Interaction mode:
162 (add-hook 'lisp-interaction-mode-hook 'turn-on-auto-fill)
165 @defun add-hook hook function &optional append local
166 This function is the handy way to add function @var{function} to hook
167 variable @var{hook}. You can use it for abnormal hooks as well as for
168 normal hooks. @var{function} can be any Lisp function that can accept
169 the proper number of arguments for @var{hook}. For example,
172 (add-hook 'text-mode-hook 'my-text-hook-function)
176 adds @code{my-text-hook-function} to the hook called @code{text-mode-hook}.
178 If @var{function} is already present in @var{hook} (comparing using
179 @code{equal}), then @code{add-hook} does not add it a second time.
181 If @var{function} has a non-@code{nil} property
182 @code{permanent-local-hook}, then @code{kill-all-local-variables} (or
183 changing major modes) won't delete it from the hook variable's local
186 It is best to design your hook functions so that the order in which
187 they are executed does not matter. Any dependence on the order is
188 asking for trouble. However, the order is predictable: normally,
189 @var{function} goes at the front of the hook list, so it will be
190 executed first (barring another @code{add-hook} call). If the
191 optional argument @var{append} is non-@code{nil}, the new hook
192 function goes at the end of the hook list and will be executed last.
194 @code{add-hook} can handle the cases where @var{hook} is void or its
195 value is a single function; it sets or changes the value to a list of
198 If @var{local} is non-@code{nil}, that says to add @var{function} to the
199 buffer-local hook list instead of to the global hook list. This makes
200 the hook buffer-local and adds @code{t} to the buffer-local value. The
201 latter acts as a flag to run the hook functions in the default value as
202 well as in the local value.
205 @defun remove-hook hook function &optional local
206 This function removes @var{function} from the hook variable
207 @var{hook}. It compares @var{function} with elements of @var{hook}
208 using @code{equal}, so it works for both symbols and lambda
211 If @var{local} is non-@code{nil}, that says to remove @var{function}
212 from the buffer-local hook list instead of from the global hook list.
219 @cindex major mode command
220 Major modes specialize Emacs for editing particular kinds of text.
221 Each buffer has one major mode at a time. Every major mode is
222 associated with a @dfn{major mode command}, whose name should end in
223 @samp{-mode}. This command takes care of switching to that mode in the
224 current buffer, by setting various buffer-local variables such as a
225 local keymap. @xref{Major Mode Conventions}.
227 The least specialized major mode is called @dfn{Fundamental mode},
228 which has no mode-specific definitions or variable settings.
230 @deffn Command fundamental-mode
231 This is the major mode command for Fundamental mode. Unlike other mode
232 commands, it does @emph{not} run any mode hooks (@pxref{Major Mode
233 Conventions}), since you are not supposed to customize this mode.
236 The easiest way to write a major mode is to use the macro
237 @code{define-derived-mode}, which sets up the new mode as a variant of
238 an existing major mode. @xref{Derived Modes}. We recommend using
239 @code{define-derived-mode} even if the new mode is not an obvious
240 derivative of another mode, as it automatically enforces many coding
241 conventions for you. @xref{Basic Major Modes}, for common modes to
244 The standard GNU Emacs Lisp directory tree contains the code for
245 several major modes, in files such as @file{text-mode.el},
246 @file{texinfo.el}, @file{lisp-mode.el}, and @file{rmail.el}. You can
247 study these libraries to see how modes are written.
250 The buffer-local value of this variable holds the symbol for the current
251 major mode. Its default value holds the default major mode for new
252 buffers. The standard default value is @code{fundamental-mode}.
254 If the default value is @code{nil}, then whenever Emacs creates a new
255 buffer via a command such as @kbd{C-x b} (@code{switch-to-buffer}), the
256 new buffer is put in the major mode of the previously current buffer.
257 As an exception, if the major mode of the previous buffer has a
258 @code{mode-class} symbol property with value @code{special}, the new
259 buffer is put in Fundamental mode (@pxref{Major Mode Conventions}).
263 * Major Mode Conventions:: Coding conventions for keymaps, etc.
264 * Auto Major Mode:: How Emacs chooses the major mode automatically.
265 * Mode Help:: Finding out how to use a mode.
266 * Derived Modes:: Defining a new major mode based on another major
268 * Basic Major Modes:: Modes that other modes are often derived from.
269 * Generic Modes:: Defining a simple major mode that supports
270 comment syntax and Font Lock mode.
271 * Mode Hooks:: Hooks run at the end of major mode commands.
272 * Example Major Modes:: Text mode and Lisp modes.
275 @node Major Mode Conventions
276 @subsection Major Mode Conventions
277 @cindex major mode conventions
278 @cindex conventions for writing major modes
280 The code for every major mode should follow various coding
281 conventions, including conventions for local keymap and syntax table
282 initialization, function and variable names, and hooks.
284 If you use the @code{define-derived-mode} macro, it will take care of
285 many of these conventions automatically. @xref{Derived Modes}. Note
286 also that fundamental mode is an exception to many of these conventions,
287 because its definition is to present the global state of Emacs.
289 The following list of conventions is only partial. Each major mode
290 should aim for consistency in general with other Emacs major modes, as
291 this makes Emacs as a whole more coherent. It is impossible to list
292 here all the possible points where this issue might come up; if the
293 Emacs developers point out an area where your major mode deviates from
294 the usual conventions, please make it compatible.
298 Define a major mode command whose name ends in @samp{-mode}. When
299 called with no arguments, this command should switch to the new mode in
300 the current buffer by setting up the keymap, syntax table, and
301 buffer-local variables in an existing buffer. It should not change the
305 Write a documentation string for this command that describes the special
306 commands available in this mode. @xref{Mode Help}.
308 The documentation string may include the special documentation
309 substrings, @samp{\[@var{command}]}, @samp{\@{@var{keymap}@}}, and
310 @samp{\<@var{keymap}>}, which allow the help display to adapt
311 automatically to the user's own key bindings. @xref{Keys in
315 The major mode command should start by calling
316 @code{kill-all-local-variables}. This runs the normal hook
317 @code{change-major-mode-hook}, then gets rid of the buffer-local
318 variables of the major mode previously in effect. @xref{Creating
322 The major mode command should set the variable @code{major-mode} to the
323 major mode command symbol. This is how @code{describe-mode} discovers
324 which documentation to print.
327 The major mode command should set the variable @code{mode-name} to the
328 ``pretty'' name of the mode, usually a string (but see @ref{Mode Line
329 Data}, for other possible forms). The name of the mode appears
333 @cindex functions in modes
334 Since all global names are in the same name space, all the global
335 variables, constants, and functions that are part of the mode should
336 have names that start with the major mode name (or with an abbreviation
337 of it if the name is long). @xref{Coding Conventions}.
340 In a major mode for editing some kind of structured text, such as a
341 programming language, indentation of text according to structure is
342 probably useful. So the mode should set @code{indent-line-function}
343 to a suitable function, and probably customize other variables
344 for indentation. @xref{Auto-Indentation}.
347 @cindex keymaps in modes
348 The major mode should usually have its own keymap, which is used as the
349 local keymap in all buffers in that mode. The major mode command should
350 call @code{use-local-map} to install this local map. @xref{Active
351 Keymaps}, for more information.
353 This keymap should be stored permanently in a global variable named
354 @code{@var{modename}-mode-map}. Normally the library that defines the
355 mode sets this variable.
357 @xref{Tips for Defining}, for advice about how to write the code to set
358 up the mode's keymap variable.
361 The key sequences bound in a major mode keymap should usually start with
362 @kbd{C-c}, followed by a control character, a digit, or @kbd{@{},
363 @kbd{@}}, @kbd{<}, @kbd{>}, @kbd{:} or @kbd{;}. The other punctuation
364 characters are reserved for minor modes, and ordinary letters are
367 A major mode can also rebind the keys @kbd{M-n}, @kbd{M-p} and
368 @kbd{M-s}. The bindings for @kbd{M-n} and @kbd{M-p} should normally
369 be some kind of ``moving forward and backward,'' but this does not
370 necessarily mean cursor motion.
372 It is legitimate for a major mode to rebind a standard key sequence if
373 it provides a command that does ``the same job'' in a way better
374 suited to the text this mode is used for. For example, a major mode
375 for editing a programming language might redefine @kbd{C-M-a} to
376 ``move to the beginning of a function'' in a way that works better for
379 It is also legitimate for a major mode to rebind a standard key
380 sequence whose standard meaning is rarely useful in that mode. For
381 instance, minibuffer modes rebind @kbd{M-r}, whose standard meaning is
382 rarely of any use in the minibuffer. Major modes such as Dired or
383 Rmail that do not allow self-insertion of text can reasonably redefine
384 letters and other printing characters as special commands.
387 Major modes for editing text should not define @key{RET} to do
388 anything other than insert a newline. However, it is ok for
389 specialized modes for text that users don't directly edit, such as
390 Dired and Info modes, to redefine @key{RET} to do something entirely
394 Major modes should not alter options that are primarily a matter of user
395 preference, such as whether Auto-Fill mode is enabled. Leave this to
396 each user to decide. However, a major mode should customize other
397 variables so that Auto-Fill mode will work usefully @emph{if} the user
401 @cindex syntax tables in modes
402 The mode may have its own syntax table or may share one with other
403 related modes. If it has its own syntax table, it should store this in
404 a variable named @code{@var{modename}-mode-syntax-table}. @xref{Syntax
408 If the mode handles a language that has a syntax for comments, it should
409 set the variables that define the comment syntax. @xref{Options for
410 Comments,, Options Controlling Comments, emacs, The GNU Emacs Manual}.
413 @cindex abbrev tables in modes
414 The mode may have its own abbrev table or may share one with other
415 related modes. If it has its own abbrev table, it should store this
416 in a variable named @code{@var{modename}-mode-abbrev-table}. If the
417 major mode command defines any abbrevs itself, it should pass @code{t}
418 for the @var{system-flag} argument to @code{define-abbrev}.
419 @xref{Defining Abbrevs}.
422 The mode should specify how to do highlighting for Font Lock mode, by
423 setting up a buffer-local value for the variable
424 @code{font-lock-defaults} (@pxref{Font Lock Mode}).
427 Each face that the mode defines should, if possible, inherit from an
428 existing Emacs face. @xref{Basic Faces}, and @ref{Faces for Font Lock}.
431 The mode should specify how Imenu should find the definitions or
432 sections of a buffer, by setting up a buffer-local value for the
433 variable @code{imenu-generic-expression}, for the two variables
434 @code{imenu-prev-index-position-function} and
435 @code{imenu-extract-index-name-function}, or for the variable
436 @code{imenu-create-index-function} (@pxref{Imenu}).
439 The mode can specify a local value for
440 @code{eldoc-documentation-function} to tell ElDoc mode how to handle
444 The mode can specify how to complete various keywords by adding
445 to the special hook @code{completion-at-point-functions}.
448 Use @code{defvar} or @code{defcustom} to set mode-related variables, so
449 that they are not reinitialized if they already have a value. (Such
450 reinitialization could discard customizations made by the user.)
453 @cindex buffer-local variables in modes
454 To make a buffer-local binding for an Emacs customization variable, use
455 @code{make-local-variable} in the major mode command, not
456 @code{make-variable-buffer-local}. The latter function would make the
457 variable local to every buffer in which it is subsequently set, which
458 would affect buffers that do not use this mode. It is undesirable for a
459 mode to have such global effects. @xref{Buffer-Local Variables}.
461 With rare exceptions, the only reasonable way to use
462 @code{make-variable-buffer-local} in a Lisp package is for a variable
463 which is used only within that package. Using it on a variable used by
464 other packages would interfere with them.
468 @cindex major mode hook
469 Each major mode should have a normal @dfn{mode hook} named
470 @code{@var{modename}-mode-hook}. The very last thing the major mode command
471 should do is to call @code{run-mode-hooks}. This runs the normal
472 hook @code{change-major-mode-after-body-hook}, the mode hook,
473 and then the normal hook @code{after-change-major-mode-hook}.
477 The major mode command may start by calling some other major mode
478 command (called the @dfn{parent mode}) and then alter some of its
479 settings. A mode that does this is called a @dfn{derived mode}. The
480 recommended way to define one is to use the @code{define-derived-mode}
481 macro, but this is not required. Such a mode should call the parent
482 mode command inside a @code{delay-mode-hooks} form. (Using
483 @code{define-derived-mode} does this automatically.) @xref{Derived
484 Modes}, and @ref{Mode Hooks}.
487 If something special should be done if the user switches a buffer from
488 this mode to any other major mode, this mode can set up a buffer-local
489 value for @code{change-major-mode-hook} (@pxref{Creating Buffer-Local}).
492 If this mode is appropriate only for specially-prepared text produced by
493 the mode itself (rather than by the user typing at the keyboard or by an
494 external file), then the major mode command symbol should have a
495 property named @code{mode-class} with value @code{special}, put on as
498 @kindex mode-class @r{(property)}
499 @cindex @code{special} modes
501 (put 'funny-mode 'mode-class 'special)
505 This tells Emacs that new buffers created while the current buffer is in
506 Funny mode should not be put in Funny mode, even though the default
507 value of @code{major-mode} is @code{nil}. By default, the value of
508 @code{nil} for @code{major-mode} means to use the current buffer's major
509 mode when creating new buffers (@pxref{Auto Major Mode}), but with such
510 @code{special} modes, Fundamental mode is used instead. Modes such as
511 Dired, Rmail, and Buffer List use this feature.
513 The @code{define-derived-mode} macro automatically marks the derived
514 mode as special if the parent mode is special. Special mode is a
515 convenient parent for such modes to inherit from; @xref{Basic Major
519 If you want to make the new mode the default for files with certain
520 recognizable names, add an element to @code{auto-mode-alist} to select
521 the mode for those file names (@pxref{Auto Major Mode}). If you
522 define the mode command to autoload, you should add this element in
523 the same file that calls @code{autoload}. If you use an autoload
524 cookie for the mode command, you can also use an autoload cookie for
525 the form that adds the element (@pxref{autoload cookie}). If you do
526 not autoload the mode command, it is sufficient to add the element in
527 the file that contains the mode definition.
530 In the comments that document the file, you should provide a sample
531 @code{autoload} form and an example of how to add to
532 @code{auto-mode-alist}, that users can include in their init files
537 The top-level forms in the file defining the mode should be written so
538 that they may be evaluated more than once without adverse consequences.
539 Even if you never load the file more than once, someone else will.
542 @node Auto Major Mode
543 @subsection How Emacs Chooses a Major Mode
544 @cindex major mode, automatic selection
546 Based on information in the file name or in the file itself, Emacs
547 automatically selects a major mode for the new buffer when a file is
548 visited. It also processes local variables specified in the file text.
550 @deffn Command normal-mode &optional find-file
551 This function establishes the proper major mode and buffer-local variable
552 bindings for the current buffer. First it calls @code{set-auto-mode}
553 (see below), then it runs @code{hack-local-variables} to parse, and
554 bind or evaluate as appropriate, the file's local variables
555 (@pxref{File Local Variables}).
557 If the @var{find-file} argument to @code{normal-mode} is non-@code{nil},
558 @code{normal-mode} assumes that the @code{find-file} function is calling
559 it. In this case, it may process local variables in the @samp{-*-}
560 line or at the end of the file. The variable
561 @code{enable-local-variables} controls whether to do so. @xref{File
562 Variables, , Local Variables in Files, emacs, The GNU Emacs Manual},
563 for the syntax of the local variables section of a file.
565 If you run @code{normal-mode} interactively, the argument
566 @var{find-file} is normally @code{nil}. In this case,
567 @code{normal-mode} unconditionally processes any file local variables.
569 The function calls @code{set-auto-mode} to choose a major mode. If this
570 does not specify a mode, the buffer stays in the major mode determined
571 by the default value of @code{major-mode} (see below).
573 @cindex file mode specification error
574 @code{normal-mode} uses @code{condition-case} around the call to the
575 major mode command, so errors are caught and reported as a @samp{File
576 mode specification error}, followed by the original error message.
579 @defun set-auto-mode &optional keep-mode-if-same
580 @cindex visited file mode
581 This function selects the major mode that is appropriate for the
582 current buffer. It bases its decision (in order of precedence) on the
583 @w{@samp{-*-}} line, on any @samp{mode:} local variable near the end of
584 a file, on the @w{@samp{#!}} line (using @code{interpreter-mode-alist}),
585 on the text at the beginning of the buffer (using
586 @code{magic-mode-alist}), and finally on the visited file name (using
587 @code{auto-mode-alist}). @xref{Choosing Modes, , How Major Modes are
588 Chosen, emacs, The GNU Emacs Manual}. If @code{enable-local-variables}
589 is @code{nil}, @code{set-auto-mode} does not check the @w{@samp{-*-}}
590 line, or near the end of the file, for any mode tag.
592 @vindex inhibit-local-variables-regexps
593 There are some file types where it is not appropriate to scan the file
594 contents for a mode specifier. For example, a tar archive may happen to
595 contain, near the end of the file, a member file that has a local
596 variables section specifying a mode for that particular file. This
597 should not be applied to the containing tar file. Similarly, a tiff
598 image file might just happen to contain a first line that seems to
599 match the @w{@samp{-*-}} pattern. For these reasons, both these file
600 extensions are members of the list @var{inhibit-local-variables-regexps}.
601 Add patterns to this list to prevent Emacs searching them for local
602 variables of any kind (not just mode specifiers).
604 If @var{keep-mode-if-same} is non-@code{nil}, this function does not
605 call the mode command if the buffer is already in the proper major
606 mode. For instance, @code{set-visited-file-name} sets this to
607 @code{t} to avoid killing buffer local variables that the user may
611 @defun set-buffer-major-mode buffer
612 This function sets the major mode of @var{buffer} to the default value of
613 @code{major-mode}; if that is @code{nil}, it uses the
614 current buffer's major mode (if that is suitable). As an exception,
615 if @var{buffer}'s name is @samp{*scratch*}, it sets the mode to
616 @code{initial-major-mode}.
618 The low-level primitives for creating buffers do not use this function,
619 but medium-level commands such as @code{switch-to-buffer} and
620 @code{find-file-noselect} use it whenever they create buffers.
623 @defopt initial-major-mode
624 @cindex @samp{*scratch*}
625 The value of this variable determines the major mode of the initial
626 @samp{*scratch*} buffer. The value should be a symbol that is a major
627 mode command. The default value is @code{lisp-interaction-mode}.
630 @defvar interpreter-mode-alist
631 This variable specifies major modes to use for scripts that specify a
632 command interpreter in a @samp{#!} line. Its value is an alist with
633 elements of the form @code{(@var{interpreter} . @var{mode})}; for
634 example, @code{("perl" . perl-mode)} is one element present by
635 default. The element says to use mode @var{mode} if the file
636 specifies an interpreter which matches @var{interpreter}.
639 @defvar magic-mode-alist
640 This variable's value is an alist with elements of the form
641 @code{(@var{regexp} . @var{function})}, where @var{regexp} is a
642 regular expression and @var{function} is a function or @code{nil}.
643 After visiting a file, @code{set-auto-mode} calls @var{function} if
644 the text at the beginning of the buffer matches @var{regexp} and
645 @var{function} is non-@code{nil}; if @var{function} is @code{nil},
646 @code{auto-mode-alist} gets to decide the mode.
649 @defvar magic-fallback-mode-alist
650 This works like @code{magic-mode-alist}, except that it is handled
651 only if @code{auto-mode-alist} does not specify a mode for this file.
654 @defvar auto-mode-alist
655 This variable contains an association list of file name patterns
656 (regular expressions) and corresponding major mode commands. Usually,
657 the file name patterns test for suffixes, such as @samp{.el} and
658 @samp{.c}, but this need not be the case. An ordinary element of the
659 alist looks like @code{(@var{regexp} . @var{mode-function})}.
665 (("\\`/tmp/fol/" . text-mode)
666 ("\\.texinfo\\'" . texinfo-mode)
667 ("\\.texi\\'" . texinfo-mode)
670 ("\\.el\\'" . emacs-lisp-mode)
677 When you visit a file whose expanded file name (@pxref{File Name
678 Expansion}), with version numbers and backup suffixes removed using
679 @code{file-name-sans-versions} (@pxref{File Name Components}), matches
680 a @var{regexp}, @code{set-auto-mode} calls the corresponding
681 @var{mode-function}. This feature enables Emacs to select the proper
682 major mode for most files.
684 If an element of @code{auto-mode-alist} has the form @code{(@var{regexp}
685 @var{function} t)}, then after calling @var{function}, Emacs searches
686 @code{auto-mode-alist} again for a match against the portion of the file
687 name that did not match before. This feature is useful for
688 uncompression packages: an entry of the form @code{("\\.gz\\'"
689 @var{function} t)} can uncompress the file and then put the uncompressed
690 file in the proper mode according to the name sans @samp{.gz}.
692 Here is an example of how to prepend several pattern pairs to
693 @code{auto-mode-alist}. (You might use this sort of expression in your
698 (setq auto-mode-alist
700 ;; @r{File name (within directory) starts with a dot.}
701 '(("/\\.[^/]*\\'" . fundamental-mode)
702 ;; @r{File name has no dot.}
703 ("/[^\\./]*\\'" . fundamental-mode)
704 ;; @r{File name ends in @samp{.C}.}
705 ("\\.C\\'" . c++-mode))
712 @subsection Getting Help about a Major Mode
714 @cindex help for major mode
715 @cindex documentation for major mode
717 The @code{describe-mode} function provides information about major
718 modes. It is normally bound to @kbd{C-h m}. It uses the value of the
719 variable @code{major-mode} (@pxref{Major Modes}), which is why every
720 major mode command needs to set that variable.
722 @deffn Command describe-mode
723 This function displays the documentation of the current major mode.
725 The @code{describe-mode} function calls the @code{documentation}
726 function using the value of @code{major-mode} as an argument. Thus, it
727 displays the documentation string of the major mode command.
728 (@xref{Accessing Documentation}.)
732 @subsection Defining Derived Modes
735 The recommended way to define a new major mode is to derive it from an
736 existing one using @code{define-derived-mode}. If there is no closely
737 related mode, you should inherit from either @code{text-mode},
738 @code{special-mode}, or @code{prog-mode}. @xref{Basic Major Modes}. If
739 none of these are suitable, you can inherit from @code{fundamental-mode}
740 (@pxref{Major Modes}).
742 @defmac define-derived-mode variant parent name docstring keyword-args@dots{} body@dots{}
743 This macro defines @var{variant} as a major mode command, using
744 @var{name} as the string form of the mode name. @var{variant} and
745 @var{parent} should be unquoted symbols.
747 The new command @var{variant} is defined to call the function
748 @var{parent}, then override certain aspects of that parent mode:
752 The new mode has its own sparse keymap, named
753 @code{@var{variant}-map}. @code{define-derived-mode}
754 makes the parent mode's keymap the parent of the new map, unless
755 @code{@var{variant}-map} is already set and already has a parent.
758 The new mode has its own syntax table, kept in the variable
759 @code{@var{variant}-syntax-table}, unless you override this using the
760 @code{:syntax-table} keyword (see below). @code{define-derived-mode}
761 makes the parent mode's syntax-table the parent of
762 @code{@var{variant}-syntax-table}, unless the latter is already set
763 and already has a parent different from the standard syntax table.
766 The new mode has its own abbrev table, kept in the variable
767 @code{@var{variant}-abbrev-table}, unless you override this using the
768 @code{:abbrev-table} keyword (see below).
771 The new mode has its own mode hook, @code{@var{variant}-hook}. It
772 runs this hook, after running the hooks of its ancestor modes, with
773 @code{run-mode-hooks}, as the last thing it does. @xref{Mode Hooks}.
776 In addition, you can specify how to override other aspects of
777 @var{parent} with @var{body}. The command @var{variant}
778 evaluates the forms in @var{body} after setting up all its usual
779 overrides, just before running the mode hooks.
781 If @var{parent} has a non-@code{nil} @code{mode-class} symbol
782 property, then @code{define-derived-mode} sets the @code{mode-class}
783 property of @var{variant} to the same value. This ensures, for
784 example, that if @var{parent} is a special mode, then @var{variant} is
785 also a special mode (@pxref{Major Mode Conventions}).
787 You can also specify @code{nil} for @var{parent}. This gives the new
788 mode no parent. Then @code{define-derived-mode} behaves as described
789 above, but, of course, omits all actions connected with @var{parent}.
791 The argument @var{docstring} specifies the documentation string for
792 the new mode. @code{define-derived-mode} adds some general
793 information about the mode's hook, followed by the mode's keymap, at
794 the end of this docstring. If you omit @var{docstring},
795 @code{define-derived-mode} generates a documentation string.
797 The @var{keyword-args} are pairs of keywords and values. The values
798 are evaluated. The following keywords are currently supported:
802 You can use this to explicitly specify a syntax table for the new
803 mode. If you specify a @code{nil} value, the new mode uses the same
804 syntax table as @var{parent}, or the standard syntax table if
805 @var{parent} is @code{nil}. (Note that this does @emph{not} follow
806 the convention used for non-keyword arguments that a @code{nil} value
807 is equivalent with not specifying the argument.)
810 You can use this to explicitly specify an abbrev table for the new
811 mode. If you specify a @code{nil} value, the new mode uses the same
812 abbrev table as @var{parent}, or @code{fundamental-mode-abbrev-table}
813 if @var{parent} is @code{nil}. (Again, a @code{nil} value is
814 @emph{not} equivalent to not specifying this keyword.)
817 If this is specified, the value should be the customization group for
818 this mode. (Not all major modes have one.) Only the (still
819 experimental and unadvertised) command @code{customize-mode} currently
820 uses this. @code{define-derived-mode} does @emph{not} automatically
821 define the specified customization group.
824 Here is a hypothetical example:
827 (define-derived-mode hypertext-mode
828 text-mode "Hypertext"
829 "Major mode for hypertext.
830 \\@{hypertext-mode-map@}"
831 (setq case-fold-search nil))
833 (define-key hypertext-mode-map
834 [down-mouse-3] 'do-hyper-link)
837 Do not write an @code{interactive} spec in the definition;
838 @code{define-derived-mode} does that automatically.
841 @defun derived-mode-p &rest modes
842 This function returns non-@code{nil} if the current major mode is
843 derived from any of the major modes given by the symbols @var{modes}.
846 @node Basic Major Modes
847 @subsection Basic Major Modes
849 Apart from Fundamental mode, there are three major modes that other
850 major modes commonly derive from: Text mode, Prog mode, and Special
851 mode. While Text mode is useful in its own right (e.g. for editing
852 files ending in @file{.txt}), Prog mode and Special mode exist mainly to
853 let other modes derive from them.
855 @vindex prog-mode-hook
856 As far as possible, new major modes should be derived, either directly
857 or indirectly, from one of these three modes. One reason is that this
858 allows users to customize a single mode hook
859 (e.g. @code{prog-mode-hook}) for an entire family of relevant modes
860 (e.g. all programming language modes).
862 @deffn Command text-mode
863 Text mode is a major mode for editing human languages. It defines the
864 @samp{"} and @samp{\} characters as having punctuation syntax
865 (@pxref{Syntax Class Table}), and binds @kbd{M-@key{TAB}} to
866 @code{ispell-complete-word} (@pxref{Spelling,,, emacs, The GNU Emacs
869 An example of a major mode derived from Text mode is HTML mode.
870 @xref{HTML Mode,,SGML and HTML Modes, emacs, The GNU Emacs Manual}.
873 @deffn Command prog-mode
874 Prog mode is a basic major mode for buffers containing programming
875 language source code. Most of the programming language major modes
876 built into Emacs are derived from it.
878 Prog mode binds @code{parse-sexp-ignore-comments} to @code{t}
879 (@pxref{Motion via Parsing}) and @code{bidi-paragraph-direction} to
880 @code{left-to-right} (@pxref{Bidirectional Display}).
883 @deffn Command special-mode
884 Special mode is a basic major mode for buffers containing text that is
885 produced specially by Emacs, rather than from a file. Major modes
886 derived from Special mode are given a @code{mode-class} property of
887 @code{special} (@pxref{Major Mode Conventions}).
889 Special mode sets the buffer to read-only. Its keymap defines several
890 common bindings, including @kbd{q} for @code{quit-window}, @kbd{z} for
891 @code{kill-this-buffer}, and @kbd{g} for @code{revert-buffer}
894 An example of a major mode derived from Special mode is Buffer Menu
895 mode, which is used by the @samp{*Buffer List*} buffer. @xref{List
896 Buffers,,Listing Existing Buffers, emacs, The GNU Emacs Manual}.
900 @subsection Generic Modes
903 @dfn{Generic modes} are simple major modes with basic support for
904 comment syntax and Font Lock mode. To define a generic mode, use the
905 macro @code{define-generic-mode}. See the file @file{generic-x.el}
906 for some examples of the use of @code{define-generic-mode}.
908 @defmac define-generic-mode mode comment-list keyword-list font-lock-list auto-mode-list function-list &optional docstring
909 This macro defines a generic mode command named @var{mode} (a symbol,
910 not quoted). The optional argument @var{docstring} is the
911 documentation for the mode command. If you do not supply it,
912 @code{define-generic-mode} generates one by default.
914 The argument @var{comment-list} is a list in which each element is
915 either a character, a string of one or two characters, or a cons cell.
916 A character or a string is set up in the mode's syntax table as a
917 ``comment starter.'' If the entry is a cons cell, the @sc{car} is set
918 up as a ``comment starter'' and the @sc{cdr} as a ``comment ender.''
919 (Use @code{nil} for the latter if you want comments to end at the end
920 of the line.) Note that the syntax table mechanism has limitations
921 about what comment starters and enders are actually possible.
922 @xref{Syntax Tables}.
924 The argument @var{keyword-list} is a list of keywords to highlight
925 with @code{font-lock-keyword-face}. Each keyword should be a string.
926 Meanwhile, @var{font-lock-list} is a list of additional expressions to
927 highlight. Each element of this list should have the same form as an
928 element of @code{font-lock-keywords}. @xref{Search-based
931 The argument @var{auto-mode-list} is a list of regular expressions to
932 add to the variable @code{auto-mode-alist}. They are added by the execution
933 of the @code{define-generic-mode} form, not by expanding the macro call.
935 Finally, @var{function-list} is a list of functions for the mode
936 command to call for additional setup. It calls these functions just
937 before it runs the mode hook variable @code{@var{mode}-hook}.
941 @subsection Mode Hooks
943 Every major mode command should finish by running the mode-independent
944 normal hook @code{change-major-mode-after-body-hook}, its mode hook,
945 and the normal hook @code{after-change-major-mode-hook}.
946 It does this by calling @code{run-mode-hooks}. If the major mode is a
947 derived mode, that is if it calls another major mode (the parent mode)
948 in its body, it should do this inside @code{delay-mode-hooks} so that
949 the parent won't run these hooks itself. Instead, the derived mode's
950 call to @code{run-mode-hooks} runs the parent's mode hook too.
951 @xref{Major Mode Conventions}.
953 Emacs versions before Emacs 22 did not have @code{delay-mode-hooks}.
954 Versions before 24 did not have @code{change-major-mode-after-body-hook}.
955 When user-implemented major modes do not use @code{run-mode-hooks} and
956 have not been updated to use these newer features, they won't entirely
957 follow these conventions: they may run the parent's mode hook too early,
958 or fail to run @code{after-change-major-mode-hook}. If you encounter
959 such a major mode, please correct it to follow these conventions.
961 When you defined a major mode using @code{define-derived-mode}, it
962 automatically makes sure these conventions are followed. If you
963 define a major mode ``by hand,'' not using @code{define-derived-mode},
964 use the following functions to handle these conventions automatically.
966 @defun run-mode-hooks &rest hookvars
967 Major modes should run their mode hook using this function. It is
968 similar to @code{run-hooks} (@pxref{Hooks}), but it also runs
969 @code{change-major-mode-after-body-hook} and
970 @code{after-change-major-mode-hook}.
972 When this function is called during the execution of a
973 @code{delay-mode-hooks} form, it does not run the hooks immediately.
974 Instead, it arranges for the next call to @code{run-mode-hooks} to run
978 @defmac delay-mode-hooks body@dots{}
979 When one major mode command calls another, it should do so inside of
980 @code{delay-mode-hooks}.
982 This macro executes @var{body}, but tells all @code{run-mode-hooks}
983 calls during the execution of @var{body} to delay running their hooks.
984 The hooks will actually run during the next call to
985 @code{run-mode-hooks} after the end of the @code{delay-mode-hooks}
989 @defvar change-major-mode-after-body-hook
990 This is a normal hook run by @code{run-mode-hooks}. It is run before
994 @defvar after-change-major-mode-hook
995 This is a normal hook run by @code{run-mode-hooks}. It is run at the
996 very end of every properly-written major mode command.
999 @node Example Major Modes
1000 @subsection Major Mode Examples
1002 Text mode is perhaps the simplest mode besides Fundamental mode.
1003 Here are excerpts from @file{text-mode.el} that illustrate many of
1004 the conventions listed above:
1008 ;; @r{Create the syntax table for this mode.}
1009 (defvar text-mode-syntax-table
1010 (let ((st (make-syntax-table)))
1011 (modify-syntax-entry ?\" ". " st)
1012 (modify-syntax-entry ?\\ ". " st)
1013 ;; Add `p' so M-c on `hello' leads to `Hello', not `hello'.
1014 (modify-syntax-entry ?' "w p" st)
1016 "Syntax table used while in `text-mode'.")
1019 ;; @r{Create the keymap for this mode.}
1021 (defvar text-mode-map
1022 (let ((map (make-sparse-keymap)))
1023 (define-key map "\e\t" 'ispell-complete-word)
1025 "Keymap for `text-mode'.
1026 Many other modes, such as `mail-mode', `outline-mode' and
1027 `indented-text-mode', inherit all the commands defined in this map.")
1031 Here is how the actual mode command is defined now:
1035 (define-derived-mode text-mode nil "Text"
1036 "Major mode for editing text written for humans to read.
1037 In this mode, paragraphs are delimited only by blank or white lines.
1038 You can thus get the full benefit of adaptive filling
1039 (see the variable `adaptive-fill-mode').
1041 Turning on Text mode runs the normal hook `text-mode-hook'."
1044 (set (make-local-variable 'text-mode-variant) t)
1045 (set (make-local-variable 'require-final-newline)
1046 mode-require-final-newline)
1047 (set (make-local-variable 'indent-line-function) 'indent-relative))
1052 (The last line is redundant nowadays, since @code{indent-relative} is
1053 the default value, and we'll delete it in a future version.)
1055 @cindex @file{lisp-mode.el}
1056 The three Lisp modes (Lisp mode, Emacs Lisp mode, and Lisp Interaction
1057 mode) have more features than Text mode and the code is correspondingly
1058 more complicated. Here are excerpts from @file{lisp-mode.el} that
1059 illustrate how these modes are written.
1061 Here is how the Lisp mode syntax and abbrev tables are defined:
1063 @cindex syntax table example
1066 ;; @r{Create mode-specific table variables.}
1067 (defvar lisp-mode-abbrev-table nil)
1068 (define-abbrev-table 'lisp-mode-abbrev-table ())
1070 (defvar lisp-mode-syntax-table
1071 (let ((table (copy-syntax-table emacs-lisp-mode-syntax-table)))
1072 (modify-syntax-entry ?\[ "_ " table)
1073 (modify-syntax-entry ?\] "_ " table)
1074 (modify-syntax-entry ?# "' 14" table)
1075 (modify-syntax-entry ?| "\" 23bn" table)
1077 "Syntax table used in `lisp-mode'.")
1081 The three modes for Lisp share much of their code. For instance,
1082 each calls the following function to set various variables:
1086 (defun lisp-mode-variables (&optional lisp-syntax keywords-case-insensitive)
1088 (set-syntax-table lisp-mode-syntax-table))
1089 (setq local-abbrev-table lisp-mode-abbrev-table)
1095 Amongst other things, this function sets up the @code{comment-start}
1096 variable to handle Lisp comments:
1100 (make-local-variable 'comment-start)
1101 (setq comment-start ";")
1106 Each of the different Lisp modes has a slightly different keymap. For
1107 example, Lisp mode binds @kbd{C-c C-z} to @code{run-lisp}, but the other
1108 Lisp modes do not. However, all Lisp modes have some commands in
1109 common. The following code sets up the common commands:
1113 (defvar lisp-mode-shared-map
1114 (let ((map (make-sparse-keymap)))
1115 (define-key map "\e\C-q" 'indent-sexp)
1116 (define-key map "\177" 'backward-delete-char-untabify)
1118 "Keymap for commands shared by all sorts of Lisp modes.")
1123 And here is the code to set up the keymap for Lisp mode:
1127 (defvar lisp-mode-map
1128 (let ((map (make-sparse-keymap))
1129 (menu-map (make-sparse-keymap "Lisp")))
1130 (set-keymap-parent map lisp-mode-shared-map)
1131 (define-key map "\e\C-x" 'lisp-eval-defun)
1132 (define-key map "\C-c\C-z" 'run-lisp)
1135 "Keymap for ordinary Lisp mode.
1136 All commands in `lisp-mode-shared-map' are inherited by this map.")
1141 Finally, here is the major mode command for Lisp mode:
1145 (define-derived-mode lisp-mode prog-mode "Lisp"
1146 "Major mode for editing Lisp code for Lisps other than GNU Emacs Lisp.
1148 Delete converts tabs to spaces as it moves back.
1149 Blank lines separate paragraphs. Semicolons start comments.
1152 Note that `run-lisp' may be used either to start an inferior Lisp job
1153 or to switch back to an existing one.
1157 Entry to this mode calls the value of `lisp-mode-hook'
1158 if that value is non-nil."
1159 (lisp-mode-variables nil t)
1160 (set (make-local-variable 'find-tag-default-function) 'lisp-find-tag-default)
1161 (make-local-variable 'comment-start-skip)
1162 (setq comment-start-skip
1163 "\\(\\(^\\|[^\\\\\n]\\)\\(\\\\\\\\\\)*\\)\\(;+\\|#|\\) *")
1164 (setq imenu-case-fold-search t))
1169 @section Minor Modes
1172 A @dfn{minor mode} provides features that users may enable or disable
1173 independently of the choice of major mode. Minor modes can be enabled
1174 individually or in combination. Minor modes would be better named
1175 ``generally available, optional feature modes,'' except that such a name
1178 A minor mode is not usually meant as a variation of a single major mode.
1179 Usually they are general and can apply to many major modes. For
1180 example, Auto Fill mode works with any major mode that permits text
1181 insertion. To be general, a minor mode must be effectively independent
1182 of the things major modes do.
1184 A minor mode is often much more difficult to implement than a major
1185 mode. One reason is that you should be able to activate and deactivate
1186 minor modes in any order. A minor mode should be able to have its
1187 desired effect regardless of the major mode and regardless of the other
1188 minor modes in effect.
1190 Often the biggest problem in implementing a minor mode is finding a
1191 way to insert the necessary hook into the rest of Emacs. Minor mode
1192 keymaps make this easier than it used to be.
1194 @defvar minor-mode-list
1195 The value of this variable is a list of all minor mode commands.
1199 * Minor Mode Conventions:: Tips for writing a minor mode.
1200 * Keymaps and Minor Modes:: How a minor mode can have its own keymap.
1201 * Defining Minor Modes:: A convenient facility for defining minor modes.
1204 @node Minor Mode Conventions
1205 @subsection Conventions for Writing Minor Modes
1206 @cindex minor mode conventions
1207 @cindex conventions for writing minor modes
1209 There are conventions for writing minor modes just as there are for
1210 major modes. Several of the major mode conventions apply to minor
1211 modes as well: those regarding the name of the mode initialization
1212 function, the names of global symbols, the use of a hook at the end of
1213 the initialization function, and the use of keymaps and other tables.
1215 In addition, there are several conventions that are specific to
1216 minor modes. (The easiest way to follow all the conventions is to use
1217 the macro @code{define-minor-mode}; @ref{Defining Minor Modes}.)
1221 @cindex mode variable
1222 Make a variable whose name ends in @samp{-mode} to control the minor
1223 mode. We call this the @dfn{mode variable}. The minor mode command
1224 should set this variable (@code{nil} to disable; anything else to
1227 If possible, implement the mode so that setting the variable
1228 automatically enables or disables the mode. Then the minor mode command
1229 does not need to do anything except set the variable.
1231 This variable is used in conjunction with the @code{minor-mode-alist} to
1232 display the minor mode name in the mode line. It can also enable
1233 or disable a minor mode keymap. Individual commands or hooks can also
1234 check the variable's value.
1236 If you want the minor mode to be enabled separately in each buffer,
1237 make the variable buffer-local.
1240 Define a command whose name is the same as the mode variable.
1241 Its job is to enable and disable the mode by setting the variable.
1243 The command should accept one optional argument. If the argument is
1244 @code{nil}, it should toggle the mode (turn it on if it is off, and
1245 off if it is on). It should turn the mode on if the argument is a
1246 positive integer, the symbol @code{t}, or a list whose @sc{car} is one
1247 of those. It should turn the mode off if the argument is a negative
1248 integer or zero, the symbol @code{-}, or a list whose @sc{car} is a
1249 negative integer or zero. The meaning of other arguments is not
1252 Here is an example taken from the definition of @code{transient-mark-mode}.
1253 It shows the use of @code{transient-mark-mode} as a variable that enables or
1254 disables the mode's behavior, and also shows the proper way to toggle,
1255 enable or disable the minor mode based on the raw prefix argument value.
1259 (setq transient-mark-mode
1260 (if (null arg) (not transient-mark-mode)
1261 (> (prefix-numeric-value arg) 0)))
1266 Add an element to @code{minor-mode-alist} for each minor mode
1267 (@pxref{Definition of minor-mode-alist}), if you want to indicate the
1268 minor mode in the mode line. This element should be a list of the
1272 (@var{mode-variable} @var{string})
1275 Here @var{mode-variable} is the variable that controls enabling of the
1276 minor mode, and @var{string} is a short string, starting with a space,
1277 to represent the mode in the mode line. These strings must be short so
1278 that there is room for several of them at once.
1280 When you add an element to @code{minor-mode-alist}, use @code{assq} to
1281 check for an existing element, to avoid duplication. For example:
1285 (unless (assq 'leif-mode minor-mode-alist)
1286 (setq minor-mode-alist
1287 (cons '(leif-mode " Leif") minor-mode-alist)))
1292 or like this, using @code{add-to-list} (@pxref{List Variables}):
1296 (add-to-list 'minor-mode-alist '(leif-mode " Leif"))
1301 Global minor modes distributed with Emacs should if possible support
1302 enabling and disabling via Custom (@pxref{Customization}). To do this,
1303 the first step is to define the mode variable with @code{defcustom}, and
1304 specify @code{:type 'boolean}.
1306 If just setting the variable is not sufficient to enable the mode, you
1307 should also specify a @code{:set} method which enables the mode by
1308 invoking the mode command. Note in the variable's documentation string that
1309 setting the variable other than via Custom may not take effect.
1311 Also mark the definition with an autoload cookie (@pxref{autoload cookie}),
1312 and specify a @code{:require} so that customizing the variable will load
1313 the library that defines the mode. This will copy suitable definitions
1314 into @file{loaddefs.el} so that users can use @code{customize-option} to
1315 enable the mode. For example:
1321 (defcustom msb-mode nil
1323 Setting this variable directly does not take effect;
1324 use either \\[customize] or the function `msb-mode'."
1325 :set 'custom-set-minor-mode
1326 :initialize 'custom-initialize-default
1334 @node Keymaps and Minor Modes
1335 @subsection Keymaps and Minor Modes
1337 Each minor mode can have its own keymap, which is active when the mode
1338 is enabled. To set up a keymap for a minor mode, add an element to the
1339 alist @code{minor-mode-map-alist}. @xref{Definition of minor-mode-map-alist}.
1341 @cindex @code{self-insert-command}, minor modes
1342 One use of minor mode keymaps is to modify the behavior of certain
1343 self-inserting characters so that they do something else as well as
1344 self-insert. (Another way to customize @code{self-insert-command} is
1345 through @code{post-self-insert-hook}. Apart from this, the facilities
1346 for customizing @code{self-insert-command} are limited to special cases,
1347 designed for abbrevs and Auto Fill mode. Do not try substituting your
1348 own definition of @code{self-insert-command} for the standard one. The
1349 editor command loop handles this function specially.)
1351 The key sequences bound in a minor mode should consist of @kbd{C-c}
1352 followed by one of @kbd{.,/?`'"[]\|~!#$%^&*()-_+=}. (The other
1353 punctuation characters are reserved for major modes.)
1355 @node Defining Minor Modes
1356 @subsection Defining Minor Modes
1358 The macro @code{define-minor-mode} offers a convenient way of
1359 implementing a mode in one self-contained definition.
1361 @defmac define-minor-mode mode doc [init-value [lighter [keymap]]] keyword-args@dots{} body@dots{}
1362 This macro defines a new minor mode whose name is @var{mode} (a
1363 symbol). It defines a command named @var{mode} to toggle the minor
1364 mode, with @var{doc} as its documentation string. By default, it also
1365 defines a variable named @var{mode}, which is set to @code{t} or
1366 @code{nil} by enabling or disabling the mode. The variable is
1367 initialized to @var{init-value}. Except in unusual circumstances (see
1368 below), this value must be @code{nil}.
1370 The string @var{lighter} says what to display in the mode line
1371 when the mode is enabled; if it is @code{nil}, the mode is not displayed
1374 The optional argument @var{keymap} specifies the keymap for the minor
1375 mode. If non-@code{nil}, it should be a variable name (whose value is
1376 a keymap), a keymap, or an alist of the form
1379 (@var{key-sequence} . @var{definition})
1383 where each @var{key-sequence} and @var{definition} are arguments
1384 suitable for passing to @code{define-key} (@pxref{Changing Key
1385 Bindings}). If @var{keymap} is a keymap or an alist, this also
1386 defines the variable @code{@var{mode}-map}.
1388 The above three arguments @var{init-value}, @var{lighter}, and
1389 @var{keymap} can be (partially) omitted when @var{keyword-args} are
1390 used. The @var{keyword-args} consist of keywords followed by
1391 corresponding values. A few keywords have special meanings:
1394 @item :group @var{group}
1395 Custom group name to use in all generated @code{defcustom} forms.
1396 Defaults to @var{mode} without the possible trailing @samp{-mode}.
1397 @strong{Warning:} don't use this default group name unless you have
1398 written a @code{defgroup} to define that group properly. @xref{Group
1401 @item :global @var{global}
1402 If non-@code{nil}, this specifies that the minor mode should be global
1403 rather than buffer-local. It defaults to @code{nil}.
1405 One of the effects of making a minor mode global is that the
1406 @var{mode} variable becomes a customization variable. Toggling it
1407 through the Custom interface turns the mode on and off, and its value
1408 can be saved for future Emacs sessions (@pxref{Saving
1409 Customizations,,, emacs, The GNU Emacs Manual}. For the saved
1410 variable to work, you should ensure that the @code{define-minor-mode}
1411 form is evaluated each time Emacs starts; for packages that are not
1412 part of Emacs, the easiest way to do this is to specify a
1413 @code{:require} keyword.
1415 @item :init-value @var{init-value}
1416 This is equivalent to specifying @var{init-value} positionally.
1418 @item :lighter @var{lighter}
1419 This is equivalent to specifying @var{lighter} positionally.
1421 @item :keymap @var{keymap}
1422 This is equivalent to specifying @var{keymap} positionally.
1424 @item :variable @var{place}
1425 This replaces the default variable @var{mode}, used to store the state
1426 of the mode. If you specify this, the @var{mode} variable is not
1427 defined, and any @var{init-value} argument is unused. @var{place}
1428 can be a different named variable (which you must define yourself), or
1429 anything that can be used with the @code{setf} function
1430 (@pxref{Generalized Variables,,, cl, Common Lisp Extensions}).
1431 @var{place} can also be a cons @code{(@var{get} . @var{set})},
1432 where @var{get} is an expression that returns the current state,
1433 and @var{set} is a function of one argument (a state) that sets it.
1436 Any other keyword arguments are passed directly to the
1437 @code{defcustom} generated for the variable @var{mode}.
1439 The command named @var{mode} first performs the standard actions such
1440 as setting the variable named @var{mode} and then executes the
1441 @var{body} forms, if any. It finishes by running the mode hook
1442 variable @code{@var{mode}-hook}.
1445 The initial value must be @code{nil} except in cases where (1) the
1446 mode is preloaded in Emacs, or (2) it is painless for loading to
1447 enable the mode even though the user did not request it. For
1448 instance, if the mode has no effect unless something else is enabled,
1449 and will always be loaded by that time, enabling it by default is
1450 harmless. But these are unusual circumstances. Normally, the
1451 initial value must be @code{nil}.
1453 @findex easy-mmode-define-minor-mode
1454 The name @code{easy-mmode-define-minor-mode} is an alias
1457 Here is an example of using @code{define-minor-mode}:
1460 (define-minor-mode hungry-mode
1461 "Toggle Hungry mode.
1462 With no argument, this command toggles the mode.
1463 Non-null prefix argument turns on the mode.
1464 Null prefix argument turns off the mode.
1466 When Hungry mode is enabled, the control delete key
1467 gobbles all preceding whitespace except the last.
1468 See the command \\[hungry-electric-delete]."
1469 ;; The initial value.
1471 ;; The indicator for the mode line.
1473 ;; The minor mode bindings.
1474 '(([C-backspace] . hungry-electric-delete))
1479 This defines a minor mode named ``Hungry mode,'' a command named
1480 @code{hungry-mode} to toggle it, a variable named @code{hungry-mode}
1481 which indicates whether the mode is enabled, and a variable named
1482 @code{hungry-mode-map} which holds the keymap that is active when the
1483 mode is enabled. It initializes the keymap with a key binding for
1484 @kbd{C-@key{DEL}}. It puts the variable @code{hungry-mode} into
1485 custom group @code{hunger}. There are no @var{body} forms---many
1486 minor modes don't need any.
1488 Here's an equivalent way to write it:
1491 (define-minor-mode hungry-mode
1492 "Toggle Hungry mode.
1493 With no argument, this command toggles the mode.
1494 Non-null prefix argument turns on the mode.
1495 Null prefix argument turns off the mode.
1497 When Hungry mode is enabled, the control delete key
1498 gobbles all preceding whitespace except the last.
1499 See the command \\[hungry-electric-delete]."
1500 ;; The initial value.
1502 ;; The indicator for the mode line.
1504 ;; The minor mode bindings.
1506 '(([C-backspace] . hungry-electric-delete)
1510 (hungry-electric-delete t))))
1514 @defmac define-globalized-minor-mode global-mode mode turn-on keyword-args@dots{}
1515 This defines a global toggle named @var{global-mode} whose meaning is
1516 to enable or disable the buffer-local minor mode @var{mode} in all
1517 buffers. To turn on the minor mode in a buffer, it uses the function
1518 @var{turn-on}; to turn off the minor mode, it calls @code{mode} with
1519 @minus{}1 as argument.
1521 Globally enabling the mode also affects buffers subsequently created
1522 by visiting files, and buffers that use a major mode other than
1523 Fundamental mode; but it does not detect the creation of a new buffer
1524 in Fundamental mode.
1526 This defines the customization option @var{global-mode} (@pxref{Customization}),
1527 which can be toggled in the Custom interface to turn the minor mode on
1528 and off. As with @code{define-minor-mode}, you should ensure that the
1529 @code{define-globalized-minor-mode} form is evaluated each time Emacs
1530 starts, for example by providing a @code{:require} keyword.
1532 Use @code{:group @var{group}} in @var{keyword-args} to specify the
1533 custom group for the mode variable of the global minor mode.
1535 Generally speaking, when you define a globalized minor mode, you should
1536 also define a non-globalized version, so that people can use (or
1537 disable) it in individual buffers. This also allows them to disable a
1538 globally enabled minor mode in a specific major mode, by using that
1543 @node Mode Line Format
1544 @section Mode-Line Format
1547 Each Emacs window (aside from minibuffer windows) typically has a mode
1548 line at the bottom, which displays status information about the buffer
1549 displayed in the window. The mode line contains information about the
1550 buffer, such as its name, associated file, depth of recursive editing,
1551 and major and minor modes. A window can also have a @dfn{header
1552 line}, which is much like the mode line but appears at the top of the
1555 This section describes how to control the contents of the mode line
1556 and header line. We include it in this chapter because much of the
1557 information displayed in the mode line relates to the enabled major and
1561 * Base: Mode Line Basics. Basic ideas of mode line control.
1562 * Data: Mode Line Data. The data structure that controls the mode line.
1563 * Top: Mode Line Top. The top level variable, mode-line-format.
1564 * Mode Line Variables:: Variables used in that data structure.
1565 * %-Constructs:: Putting information into a mode line.
1566 * Properties in Mode:: Using text properties in the mode line.
1567 * Header Lines:: Like a mode line, but at the top.
1568 * Emulating Mode Line:: Formatting text as the mode line would.
1571 @node Mode Line Basics
1572 @subsection Mode Line Basics
1574 @code{mode-line-format} is a buffer-local variable that holds a
1575 @dfn{mode line construct}, a kind of template, which controls what is
1576 displayed on the mode line of the current buffer. The value of
1577 @code{header-line-format} specifies the buffer's header line in the
1578 same way. All windows for the same buffer use the same
1579 @code{mode-line-format} and @code{header-line-format}.
1581 For efficiency, Emacs does not continuously recompute the mode
1582 line and header line of a window. It does so when circumstances
1583 appear to call for it---for instance, if you change the window
1584 configuration, switch buffers, narrow or widen the buffer, scroll, or
1585 change the buffer's modification status. If you modify any of the
1586 variables referenced by @code{mode-line-format} (@pxref{Mode Line
1587 Variables}), or any other variables and data structures that affect
1588 how text is displayed (@pxref{Display}), you may want to force an
1589 update of the mode line so as to display the new information or
1590 display it in the new way.
1592 @defun force-mode-line-update &optional all
1593 Force redisplay of the current buffer's mode line and header line.
1594 The next redisplay will update the mode line and header line based on
1595 the latest values of all relevant variables. With optional
1596 non-@code{nil} @var{all}, force redisplay of all mode lines and header
1599 This function also forces recomputation of the menu bar menus
1600 and the frame title.
1603 The selected window's mode line is usually displayed in a different
1604 color using the face @code{mode-line}. Other windows' mode lines
1605 appear in the face @code{mode-line-inactive} instead. @xref{Faces}.
1607 @node Mode Line Data
1608 @subsection The Data Structure of the Mode Line
1609 @cindex mode-line construct
1611 The mode-line contents are controlled by a data structure called a
1612 @dfn{mode-line construct}, made up of lists, strings, symbols, and
1613 numbers kept in buffer-local variables. Each data type has a specific
1614 meaning for the mode-line appearance, as described below. The same
1615 data structure is used for constructing frame titles (@pxref{Frame
1616 Titles}) and header lines (@pxref{Header Lines}).
1618 A mode-line construct may be as simple as a fixed string of text,
1619 but it usually specifies how to combine fixed strings with variables'
1620 values to construct the text. Many of these variables are themselves
1621 defined to have mode-line constructs as their values.
1623 Here are the meanings of various data types as mode-line constructs:
1626 @cindex percent symbol in mode line
1628 A string as a mode-line construct appears verbatim except for
1629 @dfn{@code{%}-constructs} in it. These stand for substitution of
1630 other data; see @ref{%-Constructs}.
1632 If parts of the string have @code{face} properties, they control
1633 display of the text just as they would text in the buffer. Any
1634 characters which have no @code{face} properties are displayed, by
1635 default, in the face @code{mode-line} or @code{mode-line-inactive}
1636 (@pxref{Standard Faces,,, emacs, The GNU Emacs Manual}). The
1637 @code{help-echo} and @code{local-map} properties in @var{string} have
1638 special meanings. @xref{Properties in Mode}.
1641 A symbol as a mode-line construct stands for its value. The value of
1642 @var{symbol} is used as a mode-line construct, in place of @var{symbol}.
1643 However, the symbols @code{t} and @code{nil} are ignored, as is any
1644 symbol whose value is void.
1646 There is one exception: if the value of @var{symbol} is a string, it is
1647 displayed verbatim: the @code{%}-constructs are not recognized.
1649 Unless @var{symbol} is marked as ``risky'' (i.e., it has a
1650 non-@code{nil} @code{risky-local-variable} property), all text
1651 properties specified in @var{symbol}'s value are ignored. This
1652 includes the text properties of strings in @var{symbol}'s value, as
1653 well as all @code{:eval} and @code{:propertize} forms in it. (The
1654 reason for this is security: non-risky variables could be set
1655 automatically from file variables without prompting the user.)
1657 @item (@var{string} @var{rest}@dots{})
1658 @itemx (@var{list} @var{rest}@dots{})
1659 A list whose first element is a string or list means to process all the
1660 elements recursively and concatenate the results. This is the most
1661 common form of mode-line construct.
1663 @item (:eval @var{form})
1664 A list whose first element is the symbol @code{:eval} says to evaluate
1665 @var{form}, and use the result as a string to display. Make sure this
1666 evaluation cannot load any files, as doing so could cause infinite
1669 @item (:propertize @var{elt} @var{props}@dots{})
1670 A list whose first element is the symbol @code{:propertize} says to
1671 process the mode-line construct @var{elt} recursively, then add the text
1672 properties specified by @var{props} to the result. The argument
1673 @var{props} should consist of zero or more pairs @var{text-property}
1674 @var{value}. (This feature is new as of Emacs 22.1.)
1676 @item (@var{symbol} @var{then} @var{else})
1677 A list whose first element is a symbol that is not a keyword specifies
1678 a conditional. Its meaning depends on the value of @var{symbol}. If
1679 @var{symbol} has a non-@code{nil} value, the second element,
1680 @var{then}, is processed recursively as a mode-line element.
1681 Otherwise, the third element, @var{else}, is processed recursively.
1682 You may omit @var{else}; then the mode-line element displays nothing
1683 if the value of @var{symbol} is @code{nil} or void.
1685 @item (@var{width} @var{rest}@dots{})
1686 A list whose first element is an integer specifies truncation or
1687 padding of the results of @var{rest}. The remaining elements
1688 @var{rest} are processed recursively as mode-line constructs and
1689 concatenated together. When @var{width} is positive, the result is
1690 space filled on the right if its width is less than @var{width}. When
1691 @var{width} is negative, the result is truncated on the right to
1692 @minus{}@var{width} columns if its width exceeds @minus{}@var{width}.
1694 For example, the usual way to show what percentage of a buffer is above
1695 the top of the window is to use a list like this: @code{(-3 "%p")}.
1699 @subsection The Top Level of Mode Line Control
1701 The variable in overall control of the mode line is
1702 @code{mode-line-format}.
1704 @defopt mode-line-format
1705 The value of this variable is a mode-line construct that controls the
1706 contents of the mode-line. It is always buffer-local in all buffers.
1708 If you set this variable to @code{nil} in a buffer, that buffer does
1709 not have a mode line. (A window that is just one line tall never
1710 displays a mode line.)
1713 The default value of @code{mode-line-format} is designed to use the
1714 values of other variables such as @code{mode-line-position} and
1715 @code{mode-line-modes} (which in turn incorporates the values of the
1716 variables @code{mode-name} and @code{minor-mode-alist}). Very few
1717 modes need to alter @code{mode-line-format} itself. For most
1718 purposes, it is sufficient to alter some of the variables that
1719 @code{mode-line-format} either directly or indirectly refers to.
1721 If you do alter @code{mode-line-format} itself, the new value should
1722 use the same variables that appear in the default value (@pxref{Mode
1723 Line Variables}), rather than duplicating their contents or displaying
1724 the information in another fashion. This way, customizations made by
1725 the user or by Lisp programs (such as @code{display-time} and major
1726 modes) via changes to those variables remain effective.
1728 Here is an example of a @code{mode-line-format} that might be
1729 useful for @code{shell-mode}, since it contains the host name and default
1734 (setq mode-line-format
1736 'mode-line-mule-info
1738 'mode-line-frame-identification
1742 ;; @r{Note that this is evaluated while making the list.}
1743 ;; @r{It makes a mode-line construct which is just a string.}
1751 '(:eval (mode-line-mode-name))
1757 '(which-func-mode ("" which-func-format "--"))
1758 '(line-number-mode "L%l--")
1759 '(column-number-mode "C%c--")
1766 (The variables @code{line-number-mode}, @code{column-number-mode}
1767 and @code{which-func-mode} enable particular minor modes; as usual,
1768 these variable names are also the minor mode command names.)
1770 @node Mode Line Variables
1771 @subsection Variables Used in the Mode Line
1773 This section describes variables incorporated by the standard value
1774 of @code{mode-line-format} into the text of the mode line. There is
1775 nothing inherently special about these variables; any other variables
1776 could have the same effects on the mode line if
1777 @code{mode-line-format}'s value were changed to use them. However,
1778 various parts of Emacs set these variables on the understanding that
1779 they will control parts of the mode line; therefore, practically
1780 speaking, it is essential for the mode line to use them.
1782 @defvar mode-line-mule-info
1783 This variable holds the value of the mode-line construct that displays
1784 information about the language environment, buffer coding system, and
1785 current input method. @xref{Non-ASCII Characters}.
1788 @defvar mode-line-modified
1789 This variable holds the value of the mode-line construct that displays
1790 whether the current buffer is modified. Its default value displays
1791 @samp{**} if the buffer is modified, @samp{--} if the buffer is not
1792 modified, @samp{%%} if the buffer is read only, and @samp{%*} if the
1793 buffer is read only and modified.
1795 Changing this variable does not force an update of the mode line.
1798 @defvar mode-line-frame-identification
1799 This variable identifies the current frame. Its default value
1800 displays @code{" "} if you are using a window system which can show
1801 multiple frames, or @code{"-%F "} on an ordinary terminal which shows
1802 only one frame at a time.
1805 @defvar mode-line-buffer-identification
1806 This variable identifies the buffer being displayed in the window.
1807 Its default value displays the buffer name, padded with spaces to at
1811 @defopt mode-line-position
1812 This variable indicates the position in the buffer. Its default value
1813 displays the buffer percentage and, optionally, the buffer size, the
1814 line number and the column number.
1818 The variable @code{vc-mode}, buffer-local in each buffer, records
1819 whether the buffer's visited file is maintained with version control,
1820 and, if so, which kind. Its value is a string that appears in the mode
1821 line, or @code{nil} for no version control.
1824 @defopt mode-line-modes
1825 This variable displays the buffer's major and minor modes. Its
1826 default value also displays the recursive editing level, information
1827 on the process status, and whether narrowing is in effect.
1830 @defopt mode-line-remote
1831 This variable is used to show whether @code{default-directory} for the
1832 current buffer is remote.
1835 @defopt mode-line-client
1836 This variable is used to identify @code{emacsclient} frames.
1839 The following three variables are used in @code{mode-line-modes}:
1842 This buffer-local variable holds the ``pretty'' name of the current
1843 buffer's major mode. Each major mode should set this variable so that
1844 the mode name will appear in the mode line. The value does not have
1845 to be a string, but can use any of the data types valid in a mode-line
1846 construct (@pxref{Mode Line Data}). To compute the string that will
1847 identify the mode name in the mode line, use @code{format-mode-line}
1848 (@pxref{Emulating Mode Line}).
1851 @defvar mode-line-process
1852 This buffer-local variable contains the mode-line information on process
1853 status in modes used for communicating with subprocesses. It is
1854 displayed immediately following the major mode name, with no intervening
1855 space. For example, its value in the @samp{*shell*} buffer is
1856 @code{(":%s")}, which allows the shell to display its status along
1857 with the major mode as: @samp{(Shell:run)}. Normally this variable
1861 @defvar minor-mode-alist
1862 @anchor{Definition of minor-mode-alist}
1863 This variable holds an association list whose elements specify how the
1864 mode line should indicate that a minor mode is active. Each element of
1865 the @code{minor-mode-alist} should be a two-element list:
1868 (@var{minor-mode-variable} @var{mode-line-string})
1871 More generally, @var{mode-line-string} can be any mode-line spec. It
1872 appears in the mode line when the value of @var{minor-mode-variable}
1873 is non-@code{nil}, and not otherwise. These strings should begin with
1874 spaces so that they don't run together. Conventionally, the
1875 @var{minor-mode-variable} for a specific mode is set to a
1876 non-@code{nil} value when that minor mode is activated.
1878 @code{minor-mode-alist} itself is not buffer-local. Each variable
1879 mentioned in the alist should be buffer-local if its minor mode can be
1880 enabled separately in each buffer.
1883 @defvar global-mode-string
1884 This variable holds a mode-line spec that, by default, appears in the
1885 mode line just after the @code{which-func-mode} minor mode if set,
1886 else after @code{mode-line-modes}. The command @code{display-time}
1887 sets @code{global-mode-string} to refer to the variable
1888 @code{display-time-string}, which holds a string containing the time
1889 and load information.
1891 The @samp{%M} construct substitutes the value of
1892 @code{global-mode-string}, but that is obsolete, since the variable is
1893 included in the mode line from @code{mode-line-format}.
1896 Here is a simplified version of the default value of
1897 @code{mode-line-format}. The real default value also
1898 specifies addition of text properties.
1905 mode-line-frame-identification
1906 mode-line-buffer-identification
1914 (which-func-mode ("" which-func-format "--"))
1915 (global-mode-string ("--" global-mode-string))
1921 @subsection @code{%}-Constructs in the Mode Line
1923 Strings used as mode-line constructs can use certain
1924 @code{%}-constructs to substitute various kinds of data. Here is a
1925 list of the defined @code{%}-constructs, and what they mean. In any
1926 construct except @samp{%%}, you can add a decimal integer after the
1927 @samp{%} to specify a minimum field width. If the width is less, the
1928 field is padded with spaces to the right.
1932 The current buffer name, obtained with the @code{buffer-name} function.
1933 @xref{Buffer Names}.
1936 The current column number of point.
1939 When Emacs is nearly out of memory for Lisp objects, a brief message
1940 saying so. Otherwise, this is empty.
1943 The visited file name, obtained with the @code{buffer-file-name}
1944 function. @xref{Buffer File Name}.
1947 The title (only on a window system) or the name of the selected frame.
1948 @xref{Basic Parameters}.
1951 The size of the accessible part of the current buffer; basically
1952 @code{(- (point-max) (point-min))}.
1955 Like @samp{%i}, but the size is printed in a more readable way by using
1956 @samp{k} for 10^3, @samp{M} for 10^6, @samp{G} for 10^9, etc., to
1960 The current line number of point, counting within the accessible portion
1964 @samp{Narrow} when narrowing is in effect; nothing otherwise (see
1965 @code{narrow-to-region} in @ref{Narrowing}).
1968 The percentage of the buffer text above the @strong{top} of window, or
1969 @samp{Top}, @samp{Bottom} or @samp{All}. Note that the default
1970 mode-line specification truncates this to three characters.
1973 The percentage of the buffer text that is above the @strong{bottom} of
1974 the window (which includes the text visible in the window, as well as
1975 the text above the top), plus @samp{Top} if the top of the buffer is
1976 visible on screen; or @samp{Bottom} or @samp{All}.
1979 The status of the subprocess belonging to the current buffer, obtained with
1980 @code{process-status}. @xref{Process Information}.
1983 Whether the visited file is a text file or a binary file. This is a
1984 meaningful distinction only on certain operating systems (@pxref{MS-DOS
1988 The mnemonics of keyboard, terminal, and buffer coding systems.
1991 Like @samp{%z}, but including the end-of-line format.
1994 @samp{%} if the buffer is read only (see @code{buffer-read-only}); @*
1995 @samp{*} if the buffer is modified (see @code{buffer-modified-p}); @*
1996 @samp{-} otherwise. @xref{Buffer Modification}.
1999 @samp{*} if the buffer is modified (see @code{buffer-modified-p}); @*
2000 @samp{%} if the buffer is read only (see @code{buffer-read-only}); @*
2001 @samp{-} otherwise. This differs from @samp{%*} only for a modified
2002 read-only buffer. @xref{Buffer Modification}.
2005 @samp{*} if the buffer is modified, and @samp{-} otherwise.
2008 An indication of the depth of recursive editing levels (not counting
2009 minibuffer levels): one @samp{[} for each editing level.
2010 @xref{Recursive Editing}.
2013 One @samp{]} for each recursive editing level (not counting minibuffer
2017 Dashes sufficient to fill the remainder of the mode line.
2020 The character @samp{%}---this is how to include a literal @samp{%} in a
2021 string in which @code{%}-constructs are allowed.
2024 The following two @code{%}-constructs are still supported, but they are
2025 obsolete, since you can get the same results with the variables
2026 @code{mode-name} and @code{global-mode-string}.
2030 The value of @code{mode-name}.
2033 The value of @code{global-mode-string}.
2036 @node Properties in Mode
2037 @subsection Properties in the Mode Line
2038 @cindex text properties in the mode line
2040 Certain text properties are meaningful in the
2041 mode line. The @code{face} property affects the appearance of text; the
2042 @code{help-echo} property associates help strings with the text, and
2043 @code{local-map} can make the text mouse-sensitive.
2045 There are four ways to specify text properties for text in the mode
2050 Put a string with a text property directly into the mode-line data
2054 Put a text property on a mode-line %-construct such as @samp{%12b}; then
2055 the expansion of the %-construct will have that same text property.
2058 Use a @code{(:propertize @var{elt} @var{props}@dots{})} construct to
2059 give @var{elt} a text property specified by @var{props}.
2062 Use a list containing @code{:eval @var{form}} in the mode-line data
2063 structure, and make @var{form} evaluate to a string that has a text
2067 You can use the @code{local-map} property to specify a keymap. This
2068 keymap only takes real effect for mouse clicks; binding character keys
2069 and function keys to it has no effect, since it is impossible to move
2070 point into the mode line.
2072 When the mode line refers to a variable which does not have a
2073 non-@code{nil} @code{risky-local-variable} property, any text
2074 properties given or specified within that variable's values are
2075 ignored. This is because such properties could otherwise specify
2076 functions to be called, and those functions could come from file
2080 @subsection Window Header Lines
2081 @cindex header line (of a window)
2082 @cindex window header line
2084 A window can have a @dfn{header line} at the
2085 top, just as it can have a mode line at the bottom. The header line
2086 feature works just like the mode-line feature, except that it's
2087 controlled by different variables.
2089 @defvar header-line-format
2090 This variable, local in every buffer, specifies how to display the
2091 header line, for windows displaying the buffer. The format of the value
2092 is the same as for @code{mode-line-format} (@pxref{Mode Line Data}).
2093 It is normally @code{nil}, so that ordinary buffers have no header line.
2096 A window that is just one line tall never displays a header line. A
2097 window that is two lines tall cannot display both a mode line and a
2098 header line at once; if it has a mode line, then it does not display a
2101 @node Emulating Mode Line
2102 @subsection Emulating Mode-Line Formatting
2104 You can use the function @code{format-mode-line} to compute
2105 the text that would appear in a mode line or header line
2106 based on a certain mode-line specification.
2108 @defun format-mode-line format &optional face window buffer
2109 This function formats a line of text according to @var{format} as if it
2110 were generating the mode line for @var{window}, but it also returns the
2111 text as a string. The argument @var{window} defaults to the selected
2112 window. If @var{buffer} is non-@code{nil}, all the information used is
2113 taken from @var{buffer}; by default, it comes from @var{window}'s
2116 The value string normally has text properties that correspond to the
2117 faces, keymaps, etc., that the mode line would have. Any character for
2118 which no @code{face} property is specified by @var{format} gets a
2119 default value determined by @var{face}. If @var{face} is @code{t}, that
2120 stands for either @code{mode-line} if @var{window} is selected,
2121 otherwise @code{mode-line-inactive}. If @var{face} is @code{nil} or
2122 omitted, that stands for the default face. If @var{face} is an integer,
2123 the value returned by this function will have no text properties.
2125 You can also specify other valid faces as the value of @var{face}.
2126 If specified, that face provides the @code{face} property for characters
2127 whose face is not specified by @var{format}.
2129 Note that using @code{mode-line}, @code{mode-line-inactive}, or
2130 @code{header-line} as @var{face} will actually redisplay the mode line
2131 or the header line, respectively, using the current definitions of the
2132 corresponding face, in addition to returning the formatted string.
2133 (Other faces do not cause redisplay.)
2135 For example, @code{(format-mode-line header-line-format)} returns the
2136 text that would appear in the selected window's header line (@code{""}
2137 if it has no header line). @code{(format-mode-line header-line-format
2138 'header-line)} returns the same text, with each character
2139 carrying the face that it will have in the header line itself, and also
2140 redraws the header line.
2147 @dfn{Imenu} is a feature that lets users select a definition or
2148 section in the buffer, from a menu which lists all of them, to go
2149 directly to that location in the buffer. Imenu works by constructing
2150 a buffer index which lists the names and buffer positions of the
2151 definitions, or other named portions of the buffer; then the user can
2152 choose one of them and move point to it. Major modes can add a menu
2153 bar item to use Imenu using @code{imenu-add-to-menubar}.
2155 @defun imenu-add-to-menubar name
2156 This function defines a local menu bar item named @var{name}
2160 The user-level commands for using Imenu are described in the Emacs
2161 Manual (@pxref{Imenu,, Imenu, emacs, the Emacs Manual}). This section
2162 explains how to customize Imenu's method of finding definitions or
2163 buffer portions for a particular major mode.
2165 The usual and simplest way is to set the variable
2166 @code{imenu-generic-expression}:
2168 @defvar imenu-generic-expression
2169 This variable, if non-@code{nil}, is a list that specifies regular
2170 expressions for finding definitions for Imenu. Simple elements of
2171 @code{imenu-generic-expression} look like this:
2174 (@var{menu-title} @var{regexp} @var{index})
2177 Here, if @var{menu-title} is non-@code{nil}, it says that the matches
2178 for this element should go in a submenu of the buffer index;
2179 @var{menu-title} itself specifies the name for the submenu. If
2180 @var{menu-title} is @code{nil}, the matches for this element go directly
2181 in the top level of the buffer index.
2183 The second item in the list, @var{regexp}, is a regular expression
2184 (@pxref{Regular Expressions}); anything in the buffer that it matches
2185 is considered a definition, something to mention in the buffer index.
2186 The third item, @var{index}, is a non-negative integer that indicates
2187 which subexpression in @var{regexp} matches the definition's name.
2189 An element can also look like this:
2192 (@var{menu-title} @var{regexp} @var{index} @var{function} @var{arguments}@dots{})
2195 Each match for this element creates an index item, and when the index
2196 item is selected by the user, it calls @var{function} with arguments
2197 consisting of the item name, the buffer position, and @var{arguments}.
2199 For Emacs Lisp mode, @code{imenu-generic-expression} could look like
2202 @c should probably use imenu-syntax-alist and \\sw rather than [-A-Za-z0-9+]
2205 ((nil "^\\s-*(def\\(un\\|subst\\|macro\\|advice\\)\
2206 \\s-+\\([-A-Za-z0-9+]+\\)" 2)
2209 ("*Vars*" "^\\s-*(def\\(var\\|const\\)\
2210 \\s-+\\([-A-Za-z0-9+]+\\)" 2)
2215 (def\\(type\\|struct\\|class\\|ine-condition\\)\
2216 \\s-+\\([-A-Za-z0-9+]+\\)" 2))
2220 Setting this variable makes it buffer-local in the current buffer.
2223 @defvar imenu-case-fold-search
2224 This variable controls whether matching against the regular
2225 expressions in the value of @code{imenu-generic-expression} is
2226 case-sensitive: @code{t}, the default, means matching should ignore
2229 Setting this variable makes it buffer-local in the current buffer.
2232 @defvar imenu-syntax-alist
2233 This variable is an alist of syntax table modifiers to use while
2234 processing @code{imenu-generic-expression}, to override the syntax table
2235 of the current buffer. Each element should have this form:
2238 (@var{characters} . @var{syntax-description})
2241 The @sc{car}, @var{characters}, can be either a character or a string.
2242 The element says to give that character or characters the syntax
2243 specified by @var{syntax-description}, which is passed to
2244 @code{modify-syntax-entry} (@pxref{Syntax Table Functions}).
2246 This feature is typically used to give word syntax to characters which
2247 normally have symbol syntax, and thus to simplify
2248 @code{imenu-generic-expression} and speed up matching.
2249 For example, Fortran mode uses it this way:
2252 (setq imenu-syntax-alist '(("_$" . "w")))
2255 The @code{imenu-generic-expression} regular expressions can then use
2256 @samp{\\sw+} instead of @samp{\\(\\sw\\|\\s_\\)+}. Note that this
2257 technique may be inconvenient when the mode needs to limit the initial
2258 character of a name to a smaller set of characters than are allowed in
2261 Setting this variable makes it buffer-local in the current buffer.
2264 Another way to customize Imenu for a major mode is to set the
2265 variables @code{imenu-prev-index-position-function} and
2266 @code{imenu-extract-index-name-function}:
2268 @defvar imenu-prev-index-position-function
2269 If this variable is non-@code{nil}, its value should be a function that
2270 finds the next ``definition'' to put in the buffer index, scanning
2271 backward in the buffer from point. It should return @code{nil} if it
2272 doesn't find another ``definition'' before point. Otherwise it should
2273 leave point at the place it finds a ``definition'' and return any
2274 non-@code{nil} value.
2276 Setting this variable makes it buffer-local in the current buffer.
2279 @defvar imenu-extract-index-name-function
2280 If this variable is non-@code{nil}, its value should be a function to
2281 return the name for a definition, assuming point is in that definition
2282 as the @code{imenu-prev-index-position-function} function would leave
2285 Setting this variable makes it buffer-local in the current buffer.
2288 The last way to customize Imenu for a major mode is to set the
2289 variable @code{imenu-create-index-function}:
2291 @defvar imenu-create-index-function
2292 This variable specifies the function to use for creating a buffer
2293 index. The function should take no arguments, and return an index
2294 alist for the current buffer. It is called within
2295 @code{save-excursion}, so where it leaves point makes no difference.
2297 The index alist can have three types of elements. Simple elements
2301 (@var{index-name} . @var{index-position})
2304 Selecting a simple element has the effect of moving to position
2305 @var{index-position} in the buffer. Special elements look like this:
2308 (@var{index-name} @var{index-position} @var{function} @var{arguments}@dots{})
2311 Selecting a special element performs:
2314 (funcall @var{function}
2315 @var{index-name} @var{index-position} @var{arguments}@dots{})
2318 A nested sub-alist element looks like this:
2321 (@var{menu-title} @var{sub-alist})
2324 It creates the submenu @var{menu-title} specified by @var{sub-alist}.
2326 The default value of @code{imenu-create-index-function} is
2327 @code{imenu-default-create-index-function}. This function calls the
2328 value of @code{imenu-prev-index-position-function} and the value of
2329 @code{imenu-extract-index-name-function} to produce the index alist.
2330 However, if either of these two variables is @code{nil}, the default
2331 function uses @code{imenu-generic-expression} instead.
2333 Setting this variable makes it buffer-local in the current buffer.
2336 @node Font Lock Mode
2337 @section Font Lock Mode
2338 @cindex Font Lock mode
2340 @dfn{Font Lock mode} is a feature that automatically attaches
2341 @code{face} properties to certain parts of the buffer based on their
2342 syntactic role. How it parses the buffer depends on the major mode;
2343 most major modes define syntactic criteria for which faces to use in
2344 which contexts. This section explains how to customize Font Lock for a
2345 particular major mode.
2347 Font Lock mode finds text to highlight in two ways: through
2348 syntactic parsing based on the syntax table, and through searching
2349 (usually for regular expressions). Syntactic fontification happens
2350 first; it finds comments and string constants and highlights them.
2351 Search-based fontification happens second.
2354 * Font Lock Basics:: Overview of customizing Font Lock.
2355 * Search-based Fontification:: Fontification based on regexps.
2356 * Customizing Keywords:: Customizing search-based fontification.
2357 * Other Font Lock Variables:: Additional customization facilities.
2358 * Levels of Font Lock:: Each mode can define alternative levels
2359 so that the user can select more or less.
2360 * Precalculated Fontification:: How Lisp programs that produce the buffer
2361 contents can also specify how to fontify it.
2362 * Faces for Font Lock:: Special faces specifically for Font Lock.
2363 * Syntactic Font Lock:: Fontification based on syntax tables.
2364 * Setting Syntax Properties:: Defining character syntax based on context
2365 using the Font Lock mechanism.
2366 * Multiline Font Lock:: How to coerce Font Lock into properly
2367 highlighting multiline constructs.
2370 @node Font Lock Basics
2371 @subsection Font Lock Basics
2373 There are several variables that control how Font Lock mode highlights
2374 text. But major modes should not set any of these variables directly.
2375 Instead, they should set @code{font-lock-defaults} as a buffer-local
2376 variable. The value assigned to this variable is used, if and when Font
2377 Lock mode is enabled, to set all the other variables.
2379 @defvar font-lock-defaults
2380 This variable is set by major modes, as a buffer-local variable, to
2381 specify how to fontify text in that mode. It automatically becomes
2382 buffer-local when you set it. If its value is @code{nil}, Font-Lock
2383 mode does no highlighting, and you can use the @samp{Faces} menu
2384 (under @samp{Edit} and then @samp{Text Properties} in the menu bar) to
2385 assign faces explicitly to text in the buffer.
2387 If non-@code{nil}, the value should look like this:
2390 (@var{keywords} [@var{keywords-only} [@var{case-fold}
2391 [@var{syntax-alist} [@var{syntax-begin} @var{other-vars}@dots{}]]]])
2394 The first element, @var{keywords}, indirectly specifies the value of
2395 @code{font-lock-keywords} which directs search-based fontification.
2396 It can be a symbol, a variable or a function whose value is the list
2397 to use for @code{font-lock-keywords}. It can also be a list of
2398 several such symbols, one for each possible level of fontification.
2399 The first symbol specifies the @samp{mode default} level of
2400 fontification, the next symbol level 1 fontification, the next level 2,
2401 and so on. The @samp{mode default} level is normally the same as level
2402 1. It is used when @code{font-lock-maximum-decoration} has a @code{nil}
2403 value. @xref{Levels of Font Lock}.
2405 The second element, @var{keywords-only}, specifies the value of the
2406 variable @code{font-lock-keywords-only}. If this is omitted or
2407 @code{nil}, syntactic fontification (of strings and comments) is also
2408 performed. If this is non-@code{nil}, such fontification is not
2409 performed. @xref{Syntactic Font Lock}.
2411 The third element, @var{case-fold}, specifies the value of
2412 @code{font-lock-keywords-case-fold-search}. If it is non-@code{nil},
2413 Font Lock mode ignores case when searching as directed by
2414 @code{font-lock-keywords}.
2416 If the fourth element, @var{syntax-alist}, is non-@code{nil}, it
2417 should be a list of cons cells of the form @code{(@var{char-or-string}
2418 . @var{string})}. These are used to set up a syntax table for
2419 syntactic fontification (@pxref{Syntax Table Functions}). The
2420 resulting syntax table is stored in @code{font-lock-syntax-table}.
2422 The fifth element, @var{syntax-begin}, specifies the value of
2423 @code{font-lock-beginning-of-syntax-function}. We recommend setting
2424 this variable to @code{nil} and using @code{syntax-begin-function}
2427 All the remaining elements (if any) are collectively called
2428 @var{other-vars}. Each of these elements should have the form
2429 @code{(@var{variable} . @var{value})}---which means, make
2430 @var{variable} buffer-local and then set it to @var{value}. You can
2431 use these @var{other-vars} to set other variables that affect
2432 fontification, aside from those you can control with the first five
2433 elements. @xref{Other Font Lock Variables}.
2436 If your mode fontifies text explicitly by adding
2437 @code{font-lock-face} properties, it can specify @code{(nil t)} for
2438 @code{font-lock-defaults} to turn off all automatic fontification.
2439 However, this is not required; it is possible to fontify some things
2440 using @code{font-lock-face} properties and set up automatic
2441 fontification for other parts of the text.
2443 @node Search-based Fontification
2444 @subsection Search-based Fontification
2446 The most important variable for customizing Font Lock mode is
2447 @code{font-lock-keywords}. It specifies the search criteria for
2448 search-based fontification. You should specify the value of this
2449 variable with @var{keywords} in @code{font-lock-defaults}.
2451 @defvar font-lock-keywords
2452 This variable's value is a list of the keywords to highlight. Be
2453 careful when composing regular expressions for this list; a poorly
2454 written pattern can dramatically slow things down!
2457 Each element of @code{font-lock-keywords} specifies how to find
2458 certain cases of text, and how to highlight those cases. Font Lock mode
2459 processes the elements of @code{font-lock-keywords} one by one, and for
2460 each element, it finds and handles all matches. Ordinarily, once
2461 part of the text has been fontified already, this cannot be overridden
2462 by a subsequent match in the same text; but you can specify different
2463 behavior using the @var{override} element of a @var{subexp-highlighter}.
2465 Each element of @code{font-lock-keywords} should have one of these
2470 Highlight all matches for @var{regexp} using
2471 @code{font-lock-keyword-face}. For example,
2474 ;; @r{Highlight occurrences of the word @samp{foo}}
2475 ;; @r{using @code{font-lock-keyword-face}.}
2479 The function @code{regexp-opt} (@pxref{Regexp Functions}) is useful
2480 for calculating optimal regular expressions to match a number of
2483 @item @var{function}
2484 Find text by calling @var{function}, and highlight the matches
2485 it finds using @code{font-lock-keyword-face}.
2487 When @var{function} is called, it receives one argument, the limit of
2488 the search; it should begin searching at point, and not search beyond the
2489 limit. It should return non-@code{nil} if it succeeds, and set the
2490 match data to describe the match that was found. Returning @code{nil}
2491 indicates failure of the search.
2493 Fontification will call @var{function} repeatedly with the same limit,
2494 and with point where the previous invocation left it, until
2495 @var{function} fails. On failure, @var{function} need not reset point
2496 in any particular way.
2498 @item (@var{matcher} . @var{subexp})
2499 In this kind of element, @var{matcher} is either a regular
2500 expression or a function, as described above. The @sc{cdr},
2501 @var{subexp}, specifies which subexpression of @var{matcher} should be
2502 highlighted (instead of the entire text that @var{matcher} matched).
2505 ;; @r{Highlight the @samp{bar} in each occurrence of @samp{fubar},}
2506 ;; @r{using @code{font-lock-keyword-face}.}
2510 If you use @code{regexp-opt} to produce the regular expression
2511 @var{matcher}, you can use @code{regexp-opt-depth} (@pxref{Regexp
2512 Functions}) to calculate the value for @var{subexp}.
2514 @item (@var{matcher} . @var{facespec})
2515 In this kind of element, @var{facespec} is an expression whose value
2516 specifies the face to use for highlighting. In the simplest case,
2517 @var{facespec} is a Lisp variable (a symbol) whose value is a face
2521 ;; @r{Highlight occurrences of @samp{fubar},}
2522 ;; @r{using the face which is the value of @code{fubar-face}.}
2523 ("fubar" . fubar-face)
2526 However, @var{facespec} can also evaluate to a list of this form:
2529 (face @var{face} @var{prop1} @var{val1} @var{prop2} @var{val2}@dots{})
2533 to specify the face @var{face} and various additional text properties
2534 to put on the text that matches. If you do this, be sure to add the
2535 other text property names that you set in this way to the value of
2536 @code{font-lock-extra-managed-props} so that the properties will also
2537 be cleared out when they are no longer appropriate. Alternatively,
2538 you can set the variable @code{font-lock-unfontify-region-function} to
2539 a function that clears these properties. @xref{Other Font Lock
2542 @item (@var{matcher} . @var{subexp-highlighter})
2543 In this kind of element, @var{subexp-highlighter} is a list
2544 which specifies how to highlight matches found by @var{matcher}.
2548 (@var{subexp} @var{facespec} [@var{override} [@var{laxmatch}]])
2551 The @sc{car}, @var{subexp}, is an integer specifying which subexpression
2552 of the match to fontify (0 means the entire matching text). The second
2553 subelement, @var{facespec}, is an expression whose value specifies the
2554 face, as described above.
2556 The last two values in @var{subexp-highlighter}, @var{override} and
2557 @var{laxmatch}, are optional flags. If @var{override} is @code{t},
2558 this element can override existing fontification made by previous
2559 elements of @code{font-lock-keywords}. If it is @code{keep}, then
2560 each character is fontified if it has not been fontified already by
2561 some other element. If it is @code{prepend}, the face specified by
2562 @var{facespec} is added to the beginning of the @code{font-lock-face}
2563 property. If it is @code{append}, the face is added to the end of the
2564 @code{font-lock-face} property.
2566 If @var{laxmatch} is non-@code{nil}, it means there should be no error
2567 if there is no subexpression numbered @var{subexp} in @var{matcher}.
2568 Obviously, fontification of the subexpression numbered @var{subexp} will
2569 not occur. However, fontification of other subexpressions (and other
2570 regexps) will continue. If @var{laxmatch} is @code{nil}, and the
2571 specified subexpression is missing, then an error is signaled which
2572 terminates search-based fontification.
2574 Here are some examples of elements of this kind, and what they do:
2577 ;; @r{Highlight occurrences of either @samp{foo} or @samp{bar}, using}
2578 ;; @r{@code{foo-bar-face}, even if they have already been highlighted.}
2579 ;; @r{@code{foo-bar-face} should be a variable whose value is a face.}
2580 ("foo\\|bar" 0 foo-bar-face t)
2582 ;; @r{Highlight the first subexpression within each occurrence}
2583 ;; @r{that the function @code{fubar-match} finds,}
2584 ;; @r{using the face which is the value of @code{fubar-face}.}
2585 (fubar-match 1 fubar-face)
2588 @item (@var{matcher} . @var{anchored-highlighter})
2589 In this kind of element, @var{anchored-highlighter} specifies how to
2590 highlight text that follows a match found by @var{matcher}. So a
2591 match found by @var{matcher} acts as the anchor for further searches
2592 specified by @var{anchored-highlighter}. @var{anchored-highlighter}
2593 is a list of the following form:
2596 (@var{anchored-matcher} @var{pre-form} @var{post-form}
2597 @var{subexp-highlighters}@dots{})
2600 Here, @var{anchored-matcher}, like @var{matcher}, is either a regular
2601 expression or a function. After a match of @var{matcher} is found,
2602 point is at the end of the match. Now, Font Lock evaluates the form
2603 @var{pre-form}. Then it searches for matches of
2604 @var{anchored-matcher} and uses @var{subexp-highlighters} to highlight
2605 these. A @var{subexp-highlighter} is as described above. Finally,
2606 Font Lock evaluates @var{post-form}.
2608 The forms @var{pre-form} and @var{post-form} can be used to initialize
2609 before, and cleanup after, @var{anchored-matcher} is used. Typically,
2610 @var{pre-form} is used to move point to some position relative to the
2611 match of @var{matcher}, before starting with @var{anchored-matcher}.
2612 @var{post-form} might be used to move back, before resuming with
2615 After Font Lock evaluates @var{pre-form}, it does not search for
2616 @var{anchored-matcher} beyond the end of the line. However, if
2617 @var{pre-form} returns a buffer position that is greater than the
2618 position of point after @var{pre-form} is evaluated, then the position
2619 returned by @var{pre-form} is used as the limit of the search instead.
2620 It is generally a bad idea to return a position greater than the end
2621 of the line; in other words, the @var{anchored-matcher} search should
2627 ;; @r{Highlight occurrences of the word @samp{item} following}
2628 ;; @r{an occurrence of the word @samp{anchor} (on the same line)}
2629 ;; @r{in the value of @code{item-face}.}
2630 ("\\<anchor\\>" "\\<item\\>" nil nil (0 item-face))
2633 Here, @var{pre-form} and @var{post-form} are @code{nil}. Therefore
2634 searching for @samp{item} starts at the end of the match of
2635 @samp{anchor}, and searching for subsequent instances of @samp{anchor}
2636 resumes from where searching for @samp{item} concluded.
2638 @item (@var{matcher} @var{highlighters}@dots{})
2639 This sort of element specifies several @var{highlighter} lists for a
2640 single @var{matcher}. A @var{highlighter} list can be of the type
2641 @var{subexp-highlighter} or @var{anchored-highlighter} as described
2647 ;; @r{Highlight occurrences of the word @samp{anchor} in the value}
2648 ;; @r{of @code{anchor-face}, and subsequent occurrences of the word}
2649 ;; @r{@samp{item} (on the same line) in the value of @code{item-face}.}
2650 ("\\<anchor\\>" (0 anchor-face)
2651 ("\\<item\\>" nil nil (0 item-face)))
2654 @item (eval . @var{form})
2655 Here @var{form} is an expression to be evaluated the first time
2656 this value of @code{font-lock-keywords} is used in a buffer.
2657 Its value should have one of the forms described in this table.
2660 @strong{Warning:} Do not design an element of @code{font-lock-keywords}
2661 to match text which spans lines; this does not work reliably.
2662 For details, see @xref{Multiline Font Lock}.
2664 You can use @var{case-fold} in @code{font-lock-defaults} to specify
2665 the value of @code{font-lock-keywords-case-fold-search} which says
2666 whether search-based fontification should be case-insensitive.
2668 @defvar font-lock-keywords-case-fold-search
2669 Non-@code{nil} means that regular expression matching for the sake of
2670 @code{font-lock-keywords} should be case-insensitive.
2673 @node Customizing Keywords
2674 @subsection Customizing Search-Based Fontification
2676 You can use @code{font-lock-add-keywords} to add additional
2677 search-based fontification rules to a major mode, and
2678 @code{font-lock-remove-keywords} to remove rules.
2680 @defun font-lock-add-keywords mode keywords &optional how
2681 This function adds highlighting @var{keywords}, for the current buffer
2682 or for major mode @var{mode}. The argument @var{keywords} should be a
2683 list with the same format as the variable @code{font-lock-keywords}.
2685 If @var{mode} is a symbol which is a major mode command name, such as
2686 @code{c-mode}, the effect is that enabling Font Lock mode in
2687 @var{mode} will add @var{keywords} to @code{font-lock-keywords}.
2688 Calling with a non-@code{nil} value of @var{mode} is correct only in
2689 your @file{~/.emacs} file.
2691 If @var{mode} is @code{nil}, this function adds @var{keywords} to
2692 @code{font-lock-keywords} in the current buffer. This way of calling
2693 @code{font-lock-add-keywords} is usually used in mode hook functions.
2695 By default, @var{keywords} are added at the beginning of
2696 @code{font-lock-keywords}. If the optional argument @var{how} is
2697 @code{set}, they are used to replace the value of
2698 @code{font-lock-keywords}. If @var{how} is any other non-@code{nil}
2699 value, they are added at the end of @code{font-lock-keywords}.
2701 Some modes provide specialized support you can use in additional
2702 highlighting patterns. See the variables
2703 @code{c-font-lock-extra-types}, @code{c++-font-lock-extra-types},
2704 and @code{java-font-lock-extra-types}, for example.
2706 @strong{Warning:} major mode commands must not call
2707 @code{font-lock-add-keywords} under any circumstances, either directly
2708 or indirectly, except through their mode hooks. (Doing so would lead to
2709 incorrect behavior for some minor modes.) They should set up their
2710 rules for search-based fontification by setting
2711 @code{font-lock-keywords}.
2714 @defun font-lock-remove-keywords mode keywords
2715 This function removes @var{keywords} from @code{font-lock-keywords}
2716 for the current buffer or for major mode @var{mode}. As in
2717 @code{font-lock-add-keywords}, @var{mode} should be a major mode
2718 command name or @code{nil}. All the caveats and requirements for
2719 @code{font-lock-add-keywords} apply here too.
2722 For example, this code
2725 (font-lock-add-keywords 'c-mode
2726 '(("\\<\\(FIXME\\):" 1 font-lock-warning-face prepend)
2727 ("\\<\\(and\\|or\\|not\\)\\>" . font-lock-keyword-face)))
2731 adds two fontification patterns for C mode: one to fontify the word
2732 @samp{FIXME}, even in comments, and another to fontify the words
2733 @samp{and}, @samp{or} and @samp{not} as keywords.
2736 That example affects only C mode proper. To add the same patterns to
2737 C mode @emph{and} all modes derived from it, do this instead:
2740 (add-hook 'c-mode-hook
2742 (font-lock-add-keywords nil
2743 '(("\\<\\(FIXME\\):" 1 font-lock-warning-face prepend)
2744 ("\\<\\(and\\|or\\|not\\)\\>" .
2745 font-lock-keyword-face)))))
2748 @node Other Font Lock Variables
2749 @subsection Other Font Lock Variables
2751 This section describes additional variables that a major mode can
2752 set by means of @var{other-vars} in @code{font-lock-defaults}
2753 (@pxref{Font Lock Basics}).
2755 @defvar font-lock-mark-block-function
2756 If this variable is non-@code{nil}, it should be a function that is
2757 called with no arguments, to choose an enclosing range of text for
2758 refontification for the command @kbd{M-o M-o}
2759 (@code{font-lock-fontify-block}).
2761 The function should report its choice by placing the region around it.
2762 A good choice is a range of text large enough to give proper results,
2763 but not too large so that refontification becomes slow. Typical values
2764 are @code{mark-defun} for programming modes or @code{mark-paragraph} for
2768 @defvar font-lock-extra-managed-props
2769 This variable specifies additional properties (other than
2770 @code{font-lock-face}) that are being managed by Font Lock mode. It
2771 is used by @code{font-lock-default-unfontify-region}, which normally
2772 only manages the @code{font-lock-face} property. If you want Font
2773 Lock to manage other properties as well, you must specify them in a
2774 @var{facespec} in @code{font-lock-keywords} as well as add them to
2775 this list. @xref{Search-based Fontification}.
2778 @defvar font-lock-fontify-buffer-function
2779 Function to use for fontifying the buffer. The default value is
2780 @code{font-lock-default-fontify-buffer}.
2783 @defvar font-lock-unfontify-buffer-function
2784 Function to use for unfontifying the buffer. This is used when
2785 turning off Font Lock mode. The default value is
2786 @code{font-lock-default-unfontify-buffer}.
2789 @defvar font-lock-fontify-region-function
2790 Function to use for fontifying a region. It should take two
2791 arguments, the beginning and end of the region, and an optional third
2792 argument @var{verbose}. If @var{verbose} is non-@code{nil}, the
2793 function should print status messages. The default value is
2794 @code{font-lock-default-fontify-region}.
2797 @defvar font-lock-unfontify-region-function
2798 Function to use for unfontifying a region. It should take two
2799 arguments, the beginning and end of the region. The default value is
2800 @code{font-lock-default-unfontify-region}.
2803 @defun jit-lock-register function &optional contextual
2804 This function tells Font Lock mode to run the Lisp function
2805 @var{function} any time it has to fontify or refontify part of the
2806 current buffer. It calls @var{function} before calling the default
2807 fontification functions, and gives it two arguments, @var{start} and
2808 @var{end}, which specify the region to be fontified or refontified.
2810 The optional argument @var{contextual}, if non-@code{nil}, forces Font
2811 Lock mode to always refontify a syntactically relevant part of the
2812 buffer, and not just the modified lines. This argument can usually be
2816 @defun jit-lock-unregister function
2817 If @var{function} was previously registered as a fontification
2818 function using @code{jit-lock-register}, this function unregisters it.
2821 @node Levels of Font Lock
2822 @subsection Levels of Font Lock
2824 Many major modes offer three different levels of fontification. You
2825 can define multiple levels by using a list of symbols for @var{keywords}
2826 in @code{font-lock-defaults}. Each symbol specifies one level of
2827 fontification; it is up to the user to choose one of these levels,
2828 normally by setting @code{font-lock-maximum-decoration} (@pxref{Font
2829 Lock,,, emacs, the GNU Emacs Manual}). The chosen level's symbol
2830 value is used to initialize @code{font-lock-keywords}.
2832 Here are the conventions for how to define the levels of
2837 Level 1: highlight function declarations, file directives (such as include or
2838 import directives), strings and comments. The idea is speed, so only
2839 the most important and top-level components are fontified.
2842 Level 2: in addition to level 1, highlight all language keywords,
2843 including type names that act like keywords, as well as named constant
2844 values. The idea is that all keywords (either syntactic or semantic)
2845 should be fontified appropriately.
2848 Level 3: in addition to level 2, highlight the symbols being defined in
2849 function and variable declarations, and all builtin function names,
2850 wherever they appear.
2853 @node Precalculated Fontification
2854 @subsection Precalculated Fontification
2856 Some major modes such as @code{list-buffers} and @code{occur}
2857 construct the buffer text programmatically. The easiest way for them
2858 to support Font Lock mode is to specify the faces of text when they
2859 insert the text in the buffer.
2861 The way to do this is to specify the faces in the text with the
2862 special text property @code{font-lock-face} (@pxref{Special
2863 Properties}). When Font Lock mode is enabled, this property controls
2864 the display, just like the @code{face} property. When Font Lock mode
2865 is disabled, @code{font-lock-face} has no effect on the display.
2867 It is ok for a mode to use @code{font-lock-face} for some text and
2868 also use the normal Font Lock machinery. But if the mode does not use
2869 the normal Font Lock machinery, it should not set the variable
2870 @code{font-lock-defaults}.
2872 @node Faces for Font Lock
2873 @subsection Faces for Font Lock
2874 @cindex faces for font lock
2875 @cindex font lock faces
2877 Font Lock mode can highlight using any face, but Emacs defines several
2878 faces specifically for syntactic highlighting. These @dfn{Font Lock
2879 faces} are listed below. They can also be used by major modes for
2880 syntactic highlighting outside of Font Lock mode (@pxref{Major Mode
2883 Each of these symbols is both a face name, and a variable whose
2884 default value is the symbol itself. Thus, the default value of
2885 @code{font-lock-comment-face} is @code{font-lock-comment-face}.
2887 The faces are listed with descriptions of their typical usage, and in
2888 order of greater to lesser ``prominence''. If a mode's syntactic
2889 categories do not fit well with the usage descriptions, the faces can be
2890 assigned using the ordering as a guide.
2893 @item font-lock-warning-face
2894 @vindex font-lock-warning-face
2895 for a construct that is peculiar, or that greatly changes the meaning of
2896 other text, like @samp{;;;###autoload} in Emacs Lisp and @samp{#error}
2899 @item font-lock-function-name-face
2900 @vindex font-lock-function-name-face
2901 for the name of a function being defined or declared.
2903 @item font-lock-variable-name-face
2904 @vindex font-lock-variable-name-face
2905 for the name of a variable being defined or declared.
2907 @item font-lock-keyword-face
2908 @vindex font-lock-keyword-face
2909 for a keyword with special syntactic significance, like @samp{for} and
2912 @item font-lock-comment-face
2913 @vindex font-lock-comment-face
2916 @item font-lock-comment-delimiter-face
2917 @vindex font-lock-comment-delimiter-face
2918 for comments delimiters, like @samp{/*} and @samp{*/} in C. On most
2919 terminals, this inherits from @code{font-lock-comment-face}.
2921 @item font-lock-type-face
2922 @vindex font-lock-type-face
2923 for the names of user-defined data types.
2925 @item font-lock-constant-face
2926 @vindex font-lock-constant-face
2927 for the names of constants, like @samp{NULL} in C.
2929 @item font-lock-builtin-face
2930 @vindex font-lock-builtin-face
2931 for the names of built-in functions.
2933 @item font-lock-preprocessor-face
2934 @vindex font-lock-preprocessor-face
2935 for preprocessor commands. This inherits, by default, from
2936 @code{font-lock-builtin-face}.
2938 @item font-lock-string-face
2939 @vindex font-lock-string-face
2940 for string constants.
2942 @item font-lock-doc-face
2943 @vindex font-lock-doc-face
2944 for documentation strings in the code. This inherits, by default, from
2945 @code{font-lock-string-face}.
2947 @item font-lock-negation-char-face
2948 @vindex font-lock-negation-char-face
2949 for easily-overlooked negation characters.
2952 @node Syntactic Font Lock
2953 @subsection Syntactic Font Lock
2954 @cindex syntactic font lock
2956 Syntactic fontification uses the syntax table to find comments and
2957 string constants (@pxref{Syntax Tables}). It highlights them using
2958 @code{font-lock-comment-face} and @code{font-lock-string-face}
2959 (@pxref{Faces for Font Lock}), or whatever
2960 @code{font-lock-syntactic-face-function} chooses. There are several
2961 variables that affect syntactic fontification; you should set them by
2962 means of @code{font-lock-defaults} (@pxref{Font Lock Basics}).
2964 @defvar font-lock-keywords-only
2965 Non-@code{nil} means Font Lock should not do syntactic fontification;
2966 it should only fontify based on @code{font-lock-keywords}. The normal
2967 way for a mode to set this variable to @code{t} is with
2968 @var{keywords-only} in @code{font-lock-defaults}.
2971 @defvar font-lock-syntax-table
2972 This variable holds the syntax table to use for fontification of
2973 comments and strings. Specify it using @var{syntax-alist} in
2974 @code{font-lock-defaults}. If this is @code{nil}, fontification uses
2975 the buffer's syntax table.
2978 @defvar font-lock-beginning-of-syntax-function
2979 If this variable is non-@code{nil}, it should be a function to move
2980 point back to a position that is syntactically at ``top level'' and
2981 outside of strings or comments. Font Lock uses this when necessary
2982 to get the right results for syntactic fontification.
2984 This function is called with no arguments. It should leave point at
2985 the beginning of any enclosing syntactic block. Typical values are
2986 @code{beginning-of-line} (used when the start of the line is known to
2987 be outside a syntactic block), or @code{beginning-of-defun} for
2988 programming modes, or @code{backward-paragraph} for textual modes.
2990 If the value is @code{nil}, Font Lock uses
2991 @code{syntax-begin-function} to move back outside of any comment,
2992 string, or sexp. This variable is semi-obsolete; we recommend setting
2993 @code{syntax-begin-function} instead.
2995 Specify this variable using @var{syntax-begin} in
2996 @code{font-lock-defaults}.
2999 @defvar font-lock-syntactic-face-function
3000 A function to determine which face to use for a given syntactic
3001 element (a string or a comment). The function is called with one
3002 argument, the parse state at point returned by
3003 @code{parse-partial-sexp}, and should return a face. The default
3004 value returns @code{font-lock-comment-face} for comments and
3005 @code{font-lock-string-face} for strings.
3007 This can be used to highlighting different kinds of strings or
3008 comments differently. It is also sometimes abused together with
3009 @code{font-lock-syntactic-keywords} to highlight constructs that span
3010 multiple lines, but this is too esoteric to document here.
3012 Specify this variable using @var{other-vars} in
3013 @code{font-lock-defaults}.
3016 @node Setting Syntax Properties
3017 @subsection Setting Syntax Properties
3019 Font Lock mode can be used to update @code{syntax-table} properties
3020 automatically (@pxref{Syntax Properties}). This is useful in
3021 languages for which a single syntax table by itself is not sufficient.
3023 @defvar font-lock-syntactic-keywords
3024 This variable enables and controls updating @code{syntax-table}
3025 properties by Font Lock. Its value should be a list of elements of
3029 (@var{matcher} @var{subexp} @var{syntax} @var{override} @var{laxmatch})
3032 The parts of this element have the same meanings as in the corresponding
3033 sort of element of @code{font-lock-keywords},
3036 (@var{matcher} @var{subexp} @var{facespec} @var{override} @var{laxmatch})
3039 However, instead of specifying the value @var{facespec} to use for the
3040 @code{face} property, it specifies the value @var{syntax} to use for
3041 the @code{syntax-table} property. Here, @var{syntax} can be a string
3042 (as taken by @code{modify-syntax-entry}), a syntax table, a cons cell
3043 (as returned by @code{string-to-syntax}), or an expression whose value
3044 is one of those two types. @var{override} cannot be @code{prepend} or
3047 For example, an element of the form:
3050 ("\\$\\(#\\)" 1 ".")
3053 highlights syntactically a hash character when following a dollar
3054 character, with a SYNTAX of @code{"."} (meaning punctuation syntax).
3055 Assuming that the buffer syntax table specifies hash characters to
3056 have comment start syntax, the element will only highlight hash
3057 characters that do not follow dollar characters as comments
3060 An element of the form:
3068 highlights syntactically both single quotes which surround a single
3069 character, with a SYNTAX of @code{"\""} (meaning string quote syntax).
3070 Assuming that the buffer syntax table does not specify single quotes
3071 to have quote syntax, the element will only highlight single quotes of
3072 the form @samp{'@var{c}'} as strings syntactically. Other forms, such
3073 as @samp{foo'bar} or @samp{'fubar'}, will not be highlighted as
3076 Major modes normally set this variable with @var{other-vars} in
3077 @code{font-lock-defaults}.
3080 @node Multiline Font Lock
3081 @subsection Multiline Font Lock Constructs
3082 @cindex multiline font lock
3084 Normally, elements of @code{font-lock-keywords} should not match
3085 across multiple lines; that doesn't work reliably, because Font Lock
3086 usually scans just part of the buffer, and it can miss a multi-line
3087 construct that crosses the line boundary where the scan starts. (The
3088 scan normally starts at the beginning of a line.)
3090 Making elements that match multiline constructs work properly has
3091 two aspects: correct @emph{identification} and correct
3092 @emph{rehighlighting}. The first means that Font Lock finds all
3093 multiline constructs. The second means that Font Lock will correctly
3094 rehighlight all the relevant text when a multiline construct is
3095 changed---for example, if some of the text that was previously part of
3096 a multiline construct ceases to be part of it. The two aspects are
3097 closely related, and often getting one of them to work will appear to
3098 make the other also work. However, for reliable results you must
3099 attend explicitly to both aspects.
3101 There are three ways to ensure correct identification of multiline
3106 Add a function to @code{font-lock-extend-region-functions} that does
3107 the @emph{identification} and extends the scan so that the scanned
3108 text never starts or ends in the middle of a multiline construct.
3110 Use the @code{font-lock-fontify-region-function} hook similarly to
3111 extend the scan so that the scanned text never starts or ends in the
3112 middle of a multiline construct.
3114 Somehow identify the multiline construct right when it gets inserted
3115 into the buffer (or at any point after that but before font-lock
3116 tries to highlight it), and mark it with a @code{font-lock-multiline}
3117 which will instruct font-lock not to start or end the scan in the
3118 middle of the construct.
3121 There are three ways to do rehighlighting of multiline constructs:
3125 Place a @code{font-lock-multiline} property on the construct. This
3126 will rehighlight the whole construct if any part of it is changed. In
3127 some cases you can do this automatically by setting the
3128 @code{font-lock-multiline} variable, which see.
3130 Make sure @code{jit-lock-contextually} is set and rely on it doing its
3131 job. This will only rehighlight the part of the construct that
3132 follows the actual change, and will do it after a short delay.
3133 This only works if the highlighting of the various parts of your
3134 multiline construct never depends on text in subsequent lines.
3135 Since @code{jit-lock-contextually} is activated by default, this can
3136 be an attractive solution.
3138 Place a @code{jit-lock-defer-multiline} property on the construct.
3139 This works only if @code{jit-lock-contextually} is used, and with the
3140 same delay before rehighlighting, but like @code{font-lock-multiline},
3141 it also handles the case where highlighting depends on
3146 * Font Lock Multiline:: Marking multiline chunks with a text property.
3147 * Region to Refontify:: Controlling which region gets refontified
3148 after a buffer change.
3151 @node Font Lock Multiline
3152 @subsubsection Font Lock Multiline
3154 One way to ensure reliable rehighlighting of multiline Font Lock
3155 constructs is to put on them the text property @code{font-lock-multiline}.
3156 It should be present and non-@code{nil} for text that is part of a
3157 multiline construct.
3159 When Font Lock is about to highlight a range of text, it first
3160 extends the boundaries of the range as necessary so that they do not
3161 fall within text marked with the @code{font-lock-multiline} property.
3162 Then it removes any @code{font-lock-multiline} properties from the
3163 range, and highlights it. The highlighting specification (mostly
3164 @code{font-lock-keywords}) must reinstall this property each time,
3165 whenever it is appropriate.
3167 @strong{Warning:} don't use the @code{font-lock-multiline} property
3168 on large ranges of text, because that will make rehighlighting slow.
3170 @defvar font-lock-multiline
3171 If the @code{font-lock-multiline} variable is set to @code{t}, Font
3172 Lock will try to add the @code{font-lock-multiline} property
3173 automatically on multiline constructs. This is not a universal
3174 solution, however, since it slows down Font Lock somewhat. It can
3175 miss some multiline constructs, or make the property larger or smaller
3178 For elements whose @var{matcher} is a function, the function should
3179 ensure that submatch 0 covers the whole relevant multiline construct,
3180 even if only a small subpart will be highlighted. It is often just as
3181 easy to add the @code{font-lock-multiline} property by hand.
3184 The @code{font-lock-multiline} property is meant to ensure proper
3185 refontification; it does not automatically identify new multiline
3186 constructs. Identifying the requires that Font-Lock operate on large
3187 enough chunks at a time. This will happen by accident on many cases,
3188 which may give the impression that multiline constructs magically work.
3189 If you set the @code{font-lock-multiline} variable non-@code{nil},
3190 this impression will be even stronger, since the highlighting of those
3191 constructs which are found will be properly updated from then on.
3192 But that does not work reliably.
3194 To find multiline constructs reliably, you must either manually
3195 place the @code{font-lock-multiline} property on the text before
3196 Font-Lock looks at it, or use
3197 @code{font-lock-fontify-region-function}.
3199 @node Region to Refontify
3200 @subsubsection Region to Fontify after a Buffer Change
3202 When a buffer is changed, the region that Font Lock refontifies is
3203 by default the smallest sequence of whole lines that spans the change.
3204 While this works well most of the time, sometimes it doesn't---for
3205 example, when a change alters the syntactic meaning of text on an
3208 You can enlarge (or even reduce) the region to refontify by setting
3209 the following variable:
3211 @defvar font-lock-extend-after-change-region-function
3212 This buffer-local variable is either @code{nil} or a function for
3213 Font-Lock to call to determine the region to scan and fontify.
3215 The function is given three parameters, the standard @var{beg},
3216 @var{end}, and @var{old-len} from @code{after-change-functions}
3217 (@pxref{Change Hooks}). It should return either a cons of the
3218 beginning and end buffer positions (in that order) of the region to
3219 fontify, or @code{nil} (which means choose the region in the standard
3220 way). This function needs to preserve point, the match-data, and the
3221 current restriction. The region it returns may start or end in the
3224 Since this function is called after every buffer change, it should be
3228 @node Auto-Indentation
3229 @section Auto-indentation of code
3231 For programming languages, an important feature of a major mode is to
3232 provide automatic indentation. This is controlled in Emacs by
3233 @code{indent-line-function} (@pxref{Mode-Specific Indent}).
3234 Writing a good indentation function can be difficult and to a large
3235 extent it is still a black art.
3237 Many major mode authors will start by writing a simple indentation
3238 function that works for simple cases, for example by comparing with the
3239 indentation of the previous text line. For most programming languages
3240 that are not really line-based, this tends to scale very poorly:
3241 improving such a function to let it handle more diverse situations tends
3242 to become more and more difficult, resulting in the end with a large,
3243 complex, unmaintainable indentation function which nobody dares to touch.
3245 A good indentation function will usually need to actually parse the
3246 text, according to the syntax of the language. Luckily, it is not
3247 necessary to parse the text in as much detail as would be needed
3248 for a compiler, but on the other hand, the parser embedded in the
3249 indentation code will want to be somewhat friendly to syntactically
3252 Good maintainable indentation functions usually fall into 2 categories:
3253 either parsing forward from some ``safe'' starting point until the
3254 position of interest, or parsing backward from the position of interest.
3255 Neither of the two is a clearly better choice than the other: parsing
3256 backward is often more difficult than parsing forward because
3257 programming languages are designed to be parsed forward, but for the
3258 purpose of indentation it has the advantage of not needing to
3259 guess a ``safe'' starting point, and it generally enjoys the property
3260 that only a minimum of text will be analyzed to decide the indentation
3261 of a line, so indentation will tend to be unaffected by syntax errors in
3262 some earlier unrelated piece of code. Parsing forward on the other hand
3263 is usually easier and has the advantage of making it possible to
3264 reindent efficiently a whole region at a time, with a single parse.
3266 Rather than write your own indentation function from scratch, it is
3267 often preferable to try and reuse some existing ones or to rely
3268 on a generic indentation engine. There are sadly few such
3269 engines. The CC-mode indentation code (used with C, C++, Java, Awk
3270 and a few other such modes) has been made more generic over the years,
3271 so if your language seems somewhat similar to one of those languages,
3272 you might try to use that engine. @c FIXME: documentation?
3273 Another one is SMIE which takes an approach in the spirit
3274 of Lisp sexps and adapts it to non-Lisp languages.
3277 * SMIE:: A simple minded indentation engine
3281 @subsection Simple Minded Indentation Engine
3283 SMIE is a package that provides a generic navigation and indentation
3284 engine. Based on a very simple parser using an ``operator precedence
3285 grammar'', it lets major modes extend the sexp-based navigation of Lisp
3286 to non-Lisp languages as well as provide a simple to use but reliable
3289 Operator precedence grammar is a very primitive technology for parsing
3290 compared to some of the more common techniques used in compilers.
3291 It has the following characteristics: its parsing power is very limited,
3292 and it is largely unable to detect syntax errors, but it has the
3293 advantage of being algorithmically efficient and able to parse forward
3294 just as well as backward. In practice that means that SMIE can use it
3295 for indentation based on backward parsing, that it can provide both
3296 @code{forward-sexp} and @code{backward-sexp} functionality, and that it
3297 will naturally work on syntactically incorrect code without any extra
3298 effort. The downside is that it also means that most programming
3299 languages cannot be parsed correctly using SMIE, at least not without
3300 resorting to some special tricks (@pxref{SMIE Tricks}).
3303 * SMIE setup:: SMIE setup and features
3304 * Operator Precedence Grammars:: A very simple parsing technique
3305 * SMIE Grammar:: Defining the grammar of a language
3306 * SMIE Lexer:: Defining tokens
3307 * SMIE Tricks:: Working around the parser's limitations
3308 * SMIE Indentation:: Specifying indentation rules
3309 * SMIE Indentation Helpers:: Helper functions for indentation rules
3310 * SMIE Indentation Example:: Sample indentation rules
3314 @subsubsection SMIE Setup and Features
3316 SMIE is meant to be a one-stop shop for structural navigation and
3317 various other features which rely on the syntactic structure of code, in
3318 particular automatic indentation. The main entry point is
3319 @code{smie-setup} which is a function typically called while setting
3322 @defun smie-setup grammar rules-function &rest keywords
3323 Setup SMIE navigation and indentation.
3324 @var{grammar} is a grammar table generated by @code{smie-prec2->grammar}.
3325 @var{rules-function} is a set of indentation rules for use on
3326 @code{smie-rules-function}.
3327 @var{keywords} are additional arguments, which can include the following
3331 @code{:forward-token} @var{fun}: Specify the forward lexer to use.
3333 @code{:backward-token} @var{fun}: Specify the backward lexer to use.
3337 Calling this function is sufficient to make commands such as
3338 @code{forward-sexp}, @code{backward-sexp}, and @code{transpose-sexps} be
3339 able to properly handle structural elements other than just the paired
3340 parentheses already handled by syntax tables. For example, if the
3341 provided grammar is precise enough, @code{transpose-sexps} can correctly
3342 transpose the two arguments of a @code{+} operator, taking into account
3343 the precedence rules of the language.
3345 Calling `smie-setup' is also sufficient to make TAB indentation work in
3346 the expected way, extends @code{blink-matching-paren} to apply to
3347 elements like @code{begin...end}, and provides some commands that you
3348 can bind in the major mode keymap.
3350 @deffn Command smie-close-block
3351 This command closes the most recently opened (and not yet closed) block.
3354 @deffn Command smie-down-list &optional arg
3355 This command is like @code{down-list} but it also pays attention to
3356 nesting of tokens other than parentheses, such as @code{begin...end}.
3359 @node Operator Precedence Grammars
3360 @subsubsection Operator Precedence Grammars
3362 SMIE's precedence grammars simply give to each token a pair of
3363 precedences: the left-precedence and the right-precedence. We say
3364 @code{T1 < T2} if the right-precedence of token @code{T1} is less than
3365 the left-precedence of token @code{T2}. A good way to read this
3366 @code{<} is as a kind of parenthesis: if we find @code{... T1 something
3367 T2 ...} then that should be parsed as @code{... T1 (something T2 ...}
3368 rather than as @code{... T1 something) T2 ...}. The latter
3369 interpretation would be the case if we had @code{T1 > T2}. If we have
3370 @code{T1 = T2}, it means that token T2 follows token T1 in the same
3371 syntactic construction, so typically we have @code{"begin" = "end"}.
3372 Such pairs of precedences are sufficient to express left-associativity
3373 or right-associativity of infix operators, nesting of tokens like
3374 parentheses and many other cases.
3376 @c Let's leave this undocumented to leave it more open for change!
3377 @c @defvar smie-grammar
3378 @c The value of this variable is an alist specifying the left and right
3379 @c precedence of each token. It is meant to be initialized by using one of
3380 @c the functions below.
3383 @defun smie-prec2->grammar table
3384 This function takes a @emph{prec2} grammar @var{table} and returns an
3385 alist suitable for use in @code{smie-setup}. The @emph{prec2}
3386 @var{table} is itself meant to be built by one of the functions below.
3389 @defun smie-merge-prec2s &rest tables
3390 This function takes several @emph{prec2} @var{tables} and merges them
3391 into a new @emph{prec2} table.
3394 @defun smie-precs->prec2 precs
3395 This function builds a @emph{prec2} table from a table of precedences
3396 @var{precs}. @var{precs} should be a list, sorted by precedence (for
3397 example @code{"+"} will come before @code{"*"}), of elements of the form
3398 @code{(@var{assoc} @var{op} ...)}, where each @var{op} is a token that
3399 acts as an operator; @var{assoc} is their associativity, which can be
3400 either @code{left}, @code{right}, @code{assoc}, or @code{nonassoc}.
3401 All operators in a given element share the same precedence level
3405 @defun smie-bnf->prec2 bnf &rest resolvers
3406 This function lets you specify the grammar using a BNF notation.
3407 It accepts a @var{bnf} description of the grammar along with a set of
3408 conflict resolution rules @var{resolvers}, and
3409 returns a @emph{prec2} table.
3411 @var{bnf} is a list of nonterminal definitions of the form
3412 @code{(@var{nonterm} @var{rhs1} @var{rhs2} ...)} where each @var{rhs}
3413 is a (non-empty) list of terminals (aka tokens) or non-terminals.
3415 Not all grammars are accepted:
3418 An @var{rhs} cannot be an empty list (an empty list is never needed,
3419 since SMIE allows all non-terminals to match the empty string anyway).
3421 An @var{rhs} cannot have 2 consecutive non-terminals: each pair of
3422 non-terminals needs to be separated by a terminal (aka token).
3423 This is a fundamental limitation of operator precedence grammars.
3426 Additionally, conflicts can occur:
3429 The returned @emph{prec2} table holds constraints between pairs of tokens, and
3430 for any given pair only one constraint can be present: T1 < T2,
3431 T1 = T2, or T1 > T2.
3433 A token can be an @code{opener} (something similar to an open-paren),
3434 a @code{closer} (like a close-paren), or @code{neither} of the two
3435 (e.g. an infix operator, or an inner token like @code{"else"}).
3438 Precedence conflicts can be resolved via @var{resolvers}, which
3439 is a list of @emph{precs} tables (see @code{smie-precs->prec2}): for
3440 each precedence conflict, if those @code{precs} tables
3441 specify a particular constraint, then the conflict is resolved by using
3442 this constraint instead, else a conflict is reported and one of the
3443 conflicting constraints is picked arbitrarily and the others are
3448 @subsubsection Defining the Grammar of a Language
3450 The usual way to define the SMIE grammar of a language is by
3451 defining a new global variable that holds the precedence table by
3452 giving a set of BNF rules.
3453 For example, the grammar definition for a small Pascal-like language
3458 (defvar sample-smie-grammar
3459 (smie-prec2->grammar
3464 (inst ("begin" insts "end")
3465 ("if" exp "then" inst "else" inst)
3468 (insts (insts ";" insts) (inst))
3472 (exps (exps "," exps) (exp)))
3477 '((assoc "+") (assoc "*")))))
3482 A few things to note:
3486 The above grammar does not explicitly mention the syntax of function
3487 calls: SMIE will automatically allow any sequence of sexps, such as
3488 identifiers, balanced parentheses, or @code{begin ... end} blocks
3489 to appear anywhere anyway.
3491 The grammar category @code{id} has no right hand side: this does not
3492 mean that it can match only the empty string, since as mentioned any
3493 sequence of sexps can appear anywhere anyway.
3495 Because non terminals cannot appear consecutively in the BNF grammar, it
3496 is difficult to correctly handle tokens that act as terminators, so the
3497 above grammar treats @code{";"} as a statement @emph{separator} instead,
3498 which SMIE can handle very well.
3500 Separators used in sequences (such as @code{","} and @code{";"} above)
3501 are best defined with BNF rules such as @code{(foo (foo "separator" foo) ...)}
3502 which generate precedence conflicts which are then resolved by giving
3503 them an explicit @code{(assoc "separator")}.
3505 The @code{("(" exps ")")} rule was not needed to pair up parens, since
3506 SMIE will pair up any characters that are marked as having paren syntax
3507 in the syntax table. What this rule does instead (together with the
3508 definition of @code{exps}) is to make it clear that @code{","} should
3509 not appear outside of parentheses.
3511 Rather than have a single @emph{precs} table to resolve conflicts, it is
3512 preferable to have several tables, so as to let the BNF part of the
3513 grammar specify relative precedences where possible.
3515 Unless there is a very good reason to prefer @code{left} or
3516 @code{right}, it is usually preferable to mark operators as associative,
3517 using @code{assoc}. For that reason @code{"+"} and @code{"*"} are
3518 defined above as @code{assoc}, although the language defines them
3519 formally as left associative.
3523 @subsubsection Defining Tokens
3525 SMIE comes with a predefined lexical analyzer which uses syntax tables
3526 in the following way: any sequence of characters that have word or
3527 symbol syntax is considered a token, and so is any sequence of
3528 characters that have punctuation syntax. This default lexer is
3529 often a good starting point but is rarely actually correct for any given
3530 language. For example, it will consider @code{"2,+3"} to be composed
3531 of 3 tokens: @code{"2"}, @code{",+"}, and @code{"3"}.
3533 To describe the lexing rules of your language to SMIE, you need
3534 2 functions, one to fetch the next token, and another to fetch the
3535 previous token. Those functions will usually first skip whitespace and
3536 comments and then look at the next chunk of text to see if it
3537 is a special token. If so it should skip the token and
3538 return a description of this token. Usually this is simply the string
3539 extracted from the buffer, but it can be anything you want.
3543 (defvar sample-keywords-regexp
3544 (regexp-opt '("+" "*" "," ";" ">" ">=" "<" "<=" ":=" "=")))
3547 (defun sample-smie-forward-token ()
3548 (forward-comment (point-max))
3550 ((looking-at sample-keywords-regexp)
3551 (goto-char (match-end 0))
3552 (match-string-no-properties 0))
3553 (t (buffer-substring-no-properties
3555 (progn (skip-syntax-forward "w_")
3559 (defun sample-smie-backward-token ()
3560 (forward-comment (- (point)))
3562 ((looking-back sample-keywords-regexp (- (point) 2) t)
3563 (goto-char (match-beginning 0))
3564 (match-string-no-properties 0))
3565 (t (buffer-substring-no-properties
3567 (progn (skip-syntax-backward "w_")
3572 Notice how those lexers return the empty string when in front of
3573 parentheses. This is because SMIE automatically takes care of the
3574 parentheses defined in the syntax table. More specifically if the lexer
3575 returns nil or an empty string, SMIE tries to handle the corresponding
3576 text as a sexp according to syntax tables.
3579 @subsubsection Living With a Weak Parser
3581 The parsing technique used by SMIE does not allow tokens to behave
3582 differently in different contexts. For most programming languages, this
3583 manifests itself by precedence conflicts when converting the
3586 Sometimes, those conflicts can be worked around by expressing the
3587 grammar slightly differently. For example, for Modula-2 it might seem
3588 natural to have a BNF grammar that looks like this:
3592 (inst ("IF" exp "THEN" insts "ELSE" insts "END")
3593 ("CASE" exp "OF" cases "END")
3595 (cases (cases "|" cases)
3596 (caselabel ":" insts)
3601 But this will create conflicts for @code{"ELSE"}: on the one hand, the
3602 IF rule implies (among many other things) that @code{"ELSE" = "END"};
3603 but on the other hand, since @code{"ELSE"} appears within @code{cases},
3604 which appears left of @code{"END"}, we also have @code{"ELSE" > "END"}.
3605 We can solve the conflict either by using:
3608 (inst ("IF" exp "THEN" insts "ELSE" insts "END")
3609 ("CASE" exp "OF" cases "END")
3610 ("CASE" exp "OF" cases "ELSE" insts "END")
3612 (cases (cases "|" cases) (caselabel ":" insts))
3618 (inst ("IF" exp "THEN" else "END")
3619 ("CASE" exp "OF" cases "END")
3621 (else (insts "ELSE" insts))
3622 (cases (cases "|" cases) (caselabel ":" insts) (else))
3626 Reworking the grammar to try and solve conflicts has its downsides, tho,
3627 because SMIE assumes that the grammar reflects the logical structure of
3628 the code, so it is preferable to keep the BNF closer to the intended
3629 abstract syntax tree.
3631 Other times, after careful consideration you may conclude that those
3632 conflicts are not serious and simply resolve them via the
3633 @var{resolvers} argument of @code{smie-bnf->prec2}. Usually this is
3634 because the grammar is simply ambiguous: the conflict does not affect
3635 the set of programs described by the grammar, but only the way those
3636 programs are parsed. This is typically the case for separators and
3637 associative infix operators, where you want to add a resolver like
3638 @code{'((assoc "|"))}. Another case where this can happen is for the
3639 classic @emph{dangling else} problem, where you will use @code{'((assoc
3640 "else" "then"))}. It can also happen for cases where the conflict is
3641 real and cannot really be resolved, but it is unlikely to pose a problem
3644 Finally, in many cases some conflicts will remain despite all efforts to
3645 restructure the grammar. Do not despair: while the parser cannot be
3646 made more clever, you can make the lexer as smart as you want. So, the
3647 solution is then to look at the tokens involved in the conflict and to
3648 split one of those tokens into 2 (or more) different tokens. E.g. if
3649 the grammar needs to distinguish between two incompatible uses of the
3650 token @code{"begin"}, make the lexer return different tokens (say
3651 @code{"begin-fun"} and @code{"begin-plain"}) depending on which kind of
3652 @code{"begin"} it finds. This pushes the work of distinguishing the
3653 different cases to the lexer, which will thus have to look at the
3654 surrounding text to find ad-hoc clues.
3656 @node SMIE Indentation
3657 @subsubsection Specifying Indentation Rules
3659 Based on the provided grammar, SMIE will be able to provide automatic
3660 indentation without any extra effort. But in practice, this default
3661 indentation style will probably not be good enough. You will want to
3662 tweak it in many different cases.
3664 SMIE indentation is based on the idea that indentation rules should be
3665 as local as possible. To this end, it relies on the idea of
3666 @emph{virtual} indentation, which is the indentation that a particular
3667 program point would have if it were at the beginning of a line.
3668 Of course, if that program point is indeed at the beginning of a line,
3669 its virtual indentation is its current indentation. But if not, then
3670 SMIE uses the indentation algorithm to compute the virtual indentation
3671 of that point. Now in practice, the virtual indentation of a program
3672 point does not have to be identical to the indentation it would have if
3673 we inserted a newline before it. To see how this works, the SMIE rule
3674 for indentation after a @code{@{} in C does not care whether the
3675 @code{@{} is standing on a line of its own or is at the end of the
3676 preceding line. Instead, these different cases are handled in the
3677 indentation rule that decides how to indent before a @code{@{}.
3679 Another important concept is the notion of @emph{parent}: The
3680 @emph{parent} of a token, is the head token of the nearest enclosing
3681 syntactic construct. For example, the parent of an @code{else} is the
3682 @code{if} to which it belongs, and the parent of an @code{if}, in turn,
3683 is the lead token of the surrounding construct. The command
3684 @code{backward-sexp} jumps from a token to its parent, but there are
3685 some caveats: for @emph{openers} (tokens which start a construct, like
3686 @code{if}), you need to start with point before the token, while for
3687 others you need to start with point after the token.
3688 @code{backward-sexp} stops with point before the parent token if that is
3689 the @emph{opener} of the token of interest, and otherwise it stops with
3690 point after the parent token.
3692 SMIE indentation rules are specified using a function that takes two
3693 arguments @var{method} and @var{arg} where the meaning of @var{arg} and the
3694 expected return value depend on @var{method}.
3696 @var{method} can be:
3699 @code{:after}, in which case @var{arg} is a token and the function
3700 should return the @var{offset} to use for indentation after @var{arg}.
3702 @code{:before}, in which case @var{arg} is a token and the function
3703 should return the @var{offset} to use to indent @var{arg} itself.
3705 @code{:elem}, in which case the function should return either the offset
3706 to use to indent function arguments (if @var{arg} is the symbol
3707 @code{arg}) or the basic indentation step (if @var{arg} is the symbol
3710 @code{:list-intro}, in which case @var{arg} is a token and the function
3711 should return non-@code{nil} if the token is followed by a list of
3712 expressions (not separated by any token) rather than an expression.
3715 When @var{arg} is a token, the function is called with point just before
3716 that token. A return value of nil always means to fallback on the
3717 default behavior, so the function should return nil for arguments it
3720 @var{offset} can be:
3723 @code{nil}: use the default indentation rule.
3725 @code{(column . @var{column})}: indent to column @var{column}.
3727 @var{number}: offset by @var{number}, relative to a base token which is
3728 the current token for @code{:after} and its parent for @code{:before}.
3731 @node SMIE Indentation Helpers
3732 @subsubsection Helper Functions for Indentation Rules
3734 SMIE provides various functions designed specifically for use in the
3735 indentation rules function (several of those functions break if used in
3736 another context). These functions all start with the prefix
3739 @defun smie-rule-bolp
3740 Return non-@code{nil} if the current token is the first on the line.
3743 @defun smie-rule-hanging-p
3744 Return non-@code{nil} if the current token is @emph{hanging}.
3745 A token is @emph{hanging} if it is the last token on the line
3746 and if it is preceded by other tokens: a lone token on a line is not
3750 @defun smie-rule-next-p &rest tokens
3751 Return non-@code{nil} if the next token is among @var{tokens}.
3754 @defun smie-rule-prev-p &rest tokens
3755 Return non-@code{nil} if the previous token is among @var{tokens}.
3758 @defun smie-rule-parent-p &rest parents
3759 Return non-@code{nil} if the current token's parent is among @var{parents}.
3762 @defun smie-rule-sibling-p
3763 Return non-nil if the current token's parent is actually a sibling.
3764 This is the case for example when the parent of a @code{","} is just the
3765 previous @code{","}.
3768 @defun smie-rule-parent &optional offset
3769 Return the proper offset to align the current token with the parent.
3770 If non-@code{nil}, @var{offset} should be an integer giving an
3771 additional offset to apply.
3774 @defun smie-rule-separator method
3775 Indent current token as a @emph{separator}.
3777 By @emph{separator}, we mean here a token whose sole purpose is to
3778 separate various elements within some enclosing syntactic construct, and
3779 which does not have any semantic significance in itself (i.e. it would
3780 typically not exist as a node in an abstract syntax tree).
3782 Such a token is expected to have an associative syntax and be closely
3783 tied to its syntactic parent. Typical examples are @code{","} in lists
3784 of arguments (enclosed inside parentheses), or @code{";"} in sequences
3785 of instructions (enclosed in a @code{@{...@}} or @code{begin...end}
3788 @var{method} should be the method name that was passed to
3789 `smie-rules-function'.
3792 @node SMIE Indentation Example
3793 @subsubsection Sample Indentation Rules
3795 Here is an example of an indentation function:
3798 (defun sample-smie-rules (kind token)
3799 (pcase (cons kind token)
3800 (`(:elem . basic) sample-indent-basic)
3801 (`(,_ . ",") (smie-rule-separator kind))
3802 (`(:after . ":=") sample-indent-basic)
3803 (`(:before . ,(or `"begin" `"(" `"@{")))
3804 (if (smie-rule-hanging-p) (smie-rule-parent)))
3806 (and (not (smie-rule-bolp)) (smie-rule-prev-p "else")
3807 (smie-rule-parent)))))
3811 A few things to note:
3815 The first case indicates the basic indentation increment to use.
3816 If @code{sample-indent-basic} is nil, then SMIE uses the global
3817 setting @code{smie-indent-basic}. The major mode could have set
3818 @code{smie-indent-basic} buffer-locally instead, but that
3822 The rule for the token @code{","} make SMIE try to be more clever when
3823 the comma separator is placed at the beginning of lines. It tries to
3824 outdent the separator so as to align the code after the comma; for
3828 x = longfunctionname (
3835 The rule for indentation after @code{":="} exists because otherwise
3836 SMIE would treat @code{":="} as an infix operator and would align the
3837 right argument with the left one.
3840 The rule for indentation before @code{"begin"} is an example of the use
3841 of virtual indentation: This rule is used only when @code{"begin"} is
3842 hanging, which can happen only when @code{"begin"} is not at the
3843 beginning of a line. So this is not used when indenting
3844 @code{"begin"} itself but only when indenting something relative to this
3845 @code{"begin"}. Concretely, this rule changes the indentation from:
3860 The rule for indentation before @code{"if"} is similar to the one for
3861 @code{"begin"}, but where the purpose is to treat @code{"else if"}
3862 as a single unit, so as to align a sequence of tests rather than indent
3863 each test further to the right. This function does this only in the
3864 case where the @code{"if"} is not placed on a separate line, hence the
3865 @code{smie-rule-bolp} test.
3867 If we know that the @code{"else"} is always aligned with its @code{"if"}
3868 and is always at the beginning of a line, we can use a more efficient
3872 (and (not (smie-rule-bolp))
3873 (smie-rule-prev-p "else")
3875 (sample-smie-backward-token)
3876 (cons 'column (current-column)))))
3879 The advantage of this formulation is that it reuses the indentation of
3880 the previous @code{"else"}, rather than going all the way back to the
3881 first @code{"if"} of the sequence.
3884 @node Desktop Save Mode
3885 @section Desktop Save Mode
3886 @cindex desktop save mode
3888 @dfn{Desktop Save Mode} is a feature to save the state of Emacs from
3889 one session to another. The user-level commands for using Desktop
3890 Save Mode are described in the GNU Emacs Manual (@pxref{Saving Emacs
3891 Sessions,,, emacs, the GNU Emacs Manual}). Modes whose buffers visit
3892 a file, don't have to do anything to use this feature.
3894 For buffers not visiting a file to have their state saved, the major
3895 mode must bind the buffer local variable @code{desktop-save-buffer} to
3896 a non-@code{nil} value.
3898 @defvar desktop-save-buffer
3899 If this buffer-local variable is non-@code{nil}, the buffer will have
3900 its state saved in the desktop file at desktop save. If the value is
3901 a function, it is called at desktop save with argument
3902 @var{desktop-dirname}, and its value is saved in the desktop file along
3903 with the state of the buffer for which it was called. When file names
3904 are returned as part of the auxiliary information, they should be
3905 formatted using the call
3908 (desktop-file-name @var{file-name} @var{desktop-dirname})
3913 For buffers not visiting a file to be restored, the major mode must
3914 define a function to do the job, and that function must be listed in
3915 the alist @code{desktop-buffer-mode-handlers}.
3917 @defvar desktop-buffer-mode-handlers
3921 (@var{major-mode} . @var{restore-buffer-function})
3924 The function @var{restore-buffer-function} will be called with
3928 (@var{buffer-file-name} @var{buffer-name} @var{desktop-buffer-misc})
3931 and it should return the restored buffer.
3932 Here @var{desktop-buffer-misc} is the value returned by the function
3933 optionally bound to @code{desktop-save-buffer}.