23.1.1 Major Mode Conventions

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

This list of conventions is only partial, because each major mode should aim for consistency in general with other Emacs major modes. 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 command whose name ends in ‘-mode’, with no arguments, that switches to the new mode in the current buffer. This command should set up the keymap, syntax table, and buffer-local variables in an existing buffer, without changing the buffer's contents.
• Write a documentation string for this command that describes the special commands available in this mode. C-h m (describe-mode) in your mode will display this string.

  The documentation string may include the special documentation substrings, ‘\[command]’, ‘\{keymap}’, and ‘\<keymap>’, which enable the documentation to adapt automatically to the user's own key bindings. See Keys in Documentation.

• The major mode command should start by calling kill-all-local-variables. This is what gets rid of the buffer-local variables of the major mode previously in effect.
• The major mode command should set the variable major-mode to the major mode command symbol. This is how describe-mode discovers which documentation to print.
• The major mode command should set the variable mode-name to the “pretty” name of the mode, as a string. This string 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). See 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.
• 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. See Active Keymaps, for more information.

  This keymap should be stored permanently in a global variable named modename-mode-map. Normally the library that defines the mode sets this variable.

  See 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.

  It is reasonable for a major mode to rebind a key sequence with a standard meaning, if it implements a command that does “the same job” in a way that fits the major mode better. 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.

  Major modes such as Dired or Rmail that do not allow self-insertion of text can reasonably redefine letters and other printing characters as editing commands. Dired and Rmail both do this.

• Major modes must not define <RET> to do anything other than insert a newline. The command to insert a newline and then indent is C-j. Please keep this distinction uniform for all major modes.
• Major modes should not alter options that are primary 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. See Syntax Tables.
• If the mode handles a language that has a syntax for comments, it should set the variables that define the comment syntax. See Options Controlling 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. See Abbrev Tables.
• 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 (see Font Lock Mode).
• 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 or imenu-create-index-function (see Imenu).
• Use defvar or defcustom to set mode-related variables, so that they are not reinitialized if they already have a value. (Such reinitialization could discard customizations made by the user.)
• 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. See 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 mode hook named modename-mode-hook. The major mode command should run that hook, with run-hooks, as the very last thing it does. See Hooks.
• The major mode command may also run the hooks of some more basic modes. For example, indented-text-mode runs text-mode-hook as well as indented-text-mode-hook. It may run these other hooks immediately before the mode's own hook (that is, after everything else), or it may run them earlier.
• If something special should be done if the user switches a buffer from this mode to any other major mode, this mode can set up a buffer-local value for change-major-mode-hook (see Creating Buffer-Local).
• If this mode is appropriate only for specially-prepared text, then the major mode command symbol should have a property named mode-class with value special, put on as follows:

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

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

• If you want to make the new mode the default for files with certain recognizable names, add an element to auto-mode-alist to select the mode for those file names. If you define the mode command to autoload, you should add this element in the same file that calls autoload. Otherwise, it is sufficient to add the element in the file that contains the mode definition. See Auto Major Mode.
• In the documentation, you should provide a sample autoload form and an example of how to add to auto-mode-alist, that users can include in their init files (see Init File).
• The top-level forms in the file defining the mode should be written so that they may be evaluated more than once without adverse consequences. Even if you never load the file more than once, someone else will.