elisp: Major Mode Conventions
22.2.1 Major Mode Conventions
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The code for every major mode should follow various coding conventions,
including conventions for local keymap and syntax table initialization,
function and variable names, and hooks.
If you use the ‘define-derived-mode’ macro, it will take care of many
of these conventions automatically. 
Derived Modes. Note also
that Fundamental mode is an exception to many of these conventions,
because it represents the default state of Emacs.
The following list of conventions is only partial. Each major mode
should aim for consistency in general with other Emacs major modes, as
this makes Emacs as a whole more coherent. It is impossible to list
here all the possible points where this issue might come up; if the
Emacs developers point out an area where your major mode deviates from
the usual conventions, please make it compatible.
• Define a major mode command whose name ends in ‘-mode’. When
called with no arguments, this command should switch to the new
mode in the current buffer by setting up the keymap, syntax table,
and buffer-local variables in an existing buffer. It should not
change the buffer’s contents.
• Write a documentation string for this command that describes the
special commands available in this mode. Mode Help.
The documentation string may include the special documentation
substrings, ‘\[COMMAND]’, ‘\{KEYMAP}’, and ‘\<KEYMAP>’, which allow
the help display to adapt automatically to the user’s own key
bindings. Keys in Documentation.
• The major mode command should start by calling
‘kill-all-local-variables’. This runs the normal hook
‘change-major-mode-hook’, then gets rid of the buffer-local
variables of the major mode previously in effect. Creating
Buffer-Local.
• The major mode command should set the variable ‘major-mode’ to the
major mode command symbol. This is how ‘describe-mode’ discovers
which documentation to print.
• The major mode command should set the variable ‘mode-name’ to the
“pretty” name of the mode, usually a string (but see Mode
Line Data, for other possible forms). The name of the mode
appears in the mode line.
• Since all global names are in the same name space, all the global
variables, constants, and functions that are part of the mode
should have names that start with the major mode name (or with an
abbreviation of it if the name is long). Coding
Conventions.
• In a major mode for editing some kind of structured text, such as a
programming language, indentation of text according to structure is
probably useful. So the mode should set ‘indent-line-function’ to
a suitable function, and probably customize other variables for
indentation. Auto-Indentation.
• The major mode should usually have its own keymap, which is used as
the local keymap in all buffers in that mode. The major mode
command should call ‘use-local-map’ to install this local map.
Active Keymaps, for more information.
This keymap should be stored permanently in a global variable named
‘MODENAME-mode-map’. Normally the library that defines the mode
sets this variable.
Tips for Defining, for advice about how to write the code
to set up the mode’s keymap variable.
• The key sequences bound in a major mode keymap should usually start
with ‘C-c’, followed by a control character, a digit, or ‘{’, ‘}’,
‘<’, ‘>’, ‘:’ or ‘;’. The other punctuation characters are
reserved for minor modes, and ordinary letters are reserved for
users.
A major mode can also rebind the keys ‘M-n’, ‘M-p’ and ‘M-s’. The
bindings for ‘M-n’ and ‘M-p’ should normally be some kind of moving
forward and backward, but this does not necessarily mean cursor
motion.
It is legitimate for a major mode to rebind a standard key sequence
if it provides a command that does the same job in a way better
suited to the text this mode is used for. For example, a major
mode for editing a programming language might redefine ‘C-M-a’ to
move to the beginning of a function in a way that works better for
that language.
It is also legitimate for a major mode to rebind a standard key
sequence whose standard meaning is rarely useful in that mode. For
instance, minibuffer modes rebind ‘M-r’, whose standard meaning is
rarely of any use in the minibuffer. Major modes such as Dired or
Rmail that do not allow self-insertion of text can reasonably
redefine letters and other printing characters as special commands.
• Major modes for editing text should not define <RET> to do anything
other than insert a newline. However, it is ok for specialized
modes for text that users don’t directly edit, such as Dired and
Info modes, to redefine <RET> to do something entirely different.
• Major modes should not alter options that are primarily a matter of
user preference, such as whether Auto-Fill mode is enabled. Leave
this to each user to decide. However, a major mode should
customize other variables so that Auto-Fill mode will work usefully
_if_ the user decides to use it.
• The mode may have its own syntax table or may share one with other
related modes. If it has its own syntax table, it should store
this in a variable named ‘MODENAME-mode-syntax-table’.
Syntax Tables.
• If the mode handles a language that has a syntax for comments, it
should set the variables that define the comment syntax.
Options Controlling Comments (emacs)Options for Comments.
• The mode may have its own abbrev table or may share one with other
related modes. If it has its own abbrev table, it should store
this in a variable named ‘MODENAME-mode-abbrev-table’. If the
major mode command defines any abbrevs itself, it should pass ‘t’
for the SYSTEM-FLAG argument to ‘define-abbrev’. Defining
Abbrevs.
• The mode should specify how to do highlighting for Font Lock mode,
by setting up a buffer-local value for the variable
‘font-lock-defaults’ (Font Lock Mode).
• Each face that the mode defines should, if possible, inherit from
DONTPRINTYET an existing Emacs face. Basic Faces, and *noteFaces for
DONTPRINTYET an existing Emacs face. Basic Faces, and Faces for
Font Lock.
• The mode should specify how Imenu should find the definitions or
sections of a buffer, by setting up a buffer-local value for the
variable ‘imenu-generic-expression’, for the two variables
‘imenu-prev-index-position-function’ and
‘imenu-extract-index-name-function’, or for the variable
‘imenu-create-index-function’ (Imenu).
• The mode can specify a local value for
‘eldoc-documentation-function’ to tell ElDoc mode how to handle
this mode.
• The mode can specify how to complete various keywords by adding one
or more buffer-local entries to the special hook
‘completion-at-point-functions’. Completion in Buffers.
• To make a buffer-local binding for an Emacs customization variable,
use ‘make-local-variable’ in the major mode command, not
‘make-variable-buffer-local’. The latter function would make the
variable local to every buffer in which it is subsequently set,
which would affect buffers that do not use this mode. It is
undesirable for a mode to have such global effects.
Buffer-Local Variables.
With rare exceptions, the only reasonable way to use
‘make-variable-buffer-local’ in a Lisp package is for a variable
which is used only within that package. Using it on a variable
used by other packages would interfere with them.
• Each major mode should have a normal “mode hook” named
‘MODENAME-mode-hook’. The very last thing the major mode command
should do is to call ‘run-mode-hooks’. This runs the normal hook
‘change-major-mode-after-body-hook’, the mode hook, and then the
normal hook ‘after-change-major-mode-hook’. Mode Hooks.
• The major mode command may start by calling some other major mode
command (called the “parent mode”) and then alter some of its
settings. A mode that does this is called a “derived mode”. The
recommended way to define one is to use the ‘define-derived-mode’
macro, but this is not required. Such a mode should call the
parent mode command inside a ‘delay-mode-hooks’ form. (Using
‘define-derived-mode’ does this automatically.) Derived
Modes, and Mode Hooks.
• If something special should be done if the user switches a buffer
from this mode to any other major mode, this mode can set up a
buffer-local value for ‘change-major-mode-hook’ (Creating
Buffer-Local).
• If this mode is appropriate only for specially-prepared text
produced by the mode itself (rather than by the user typing at the
keyboard or by an external file), then the major mode command
symbol should have a property named ‘mode-class’ with value
‘special’, put on as follows:
(put 'funny-mode 'mode-class 'special)
This tells Emacs that new buffers created while the current buffer
is in Funny mode should not be put in Funny mode, even though the
default value of ‘major-mode’ is ‘nil’. By default, the value of
‘nil’ for ‘major-mode’ means to use the current buffer’s major mode
when creating new buffers (Auto Major Mode), but with such
‘special’ modes, Fundamental mode is used instead. Modes such as
Dired, Rmail, and Buffer List use this feature.
The function ‘view-buffer’ does not enable View mode in buffers
whose mode-class is special, because such modes usually provide
their own View-like bindings.
The ‘define-derived-mode’ macro automatically marks the derived
mode as special if the parent mode is special. Special mode is a
convenient parent for such modes to inherit from; Basic Major
Modes.
• If you want to make the new mode the default for files with certain
recognizable names, add an element to ‘auto-mode-alist’ to select
the mode for those file names (Auto Major Mode). If you
define the mode command to autoload, you should add this element in
the same file that calls ‘autoload’. If you use an autoload cookie
for the mode command, you can also use an autoload cookie for the
form that adds the element (autoload cookie). If you do
not autoload the mode command, it is sufficient to add the element
in the file that contains the mode definition.
• The top-level forms in the file defining the mode should be written
so that they may be evaluated more than once without adverse
consequences. For instance, use ‘defvar’ or ‘defcustom’ to set
mode-related variables, so that they are not reinitialized if they
already have a value (Defining Variables).
Info Catalog
elisp: Major Modes
elisp: Auto Major Mode