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(elisp)Major Modes

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Major Modes
===========

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

   The least specialized major mode is called "Fundamental mode".  This
mode has no mode-specific definitions or variable settings, so each
Emacs command behaves in its default manner, and each option is in its
default state.  All other major modes redefine various keys and options.
For example, Lisp Interaction mode provides special key bindings for
`C-j' (`eval-print-last-sexp'), <TAB> (`lisp-indent-line'), and other
keys.

   When you need to write several editing commands to help you perform a
specialized editing task, creating a new major mode is usually a good
idea.  In practice, writing a major mode is easy (in contrast to
writing a minor mode, which is often difficult).

   If the new mode is similar to an old one, it is often unwise to
modify the old one to serve two purposes, since it may become harder to
use and maintain.  Instead, copy and rename an existing major mode
definition and alter the copy--or define a "derived mode" (Note:
Derived Modes).  For example, Rmail Edit mode, which is in
`emacs/lisp/mail/rmailedit.el', is a major mode that is very similar to
Text mode except that it provides two additional commands.  Its
definition is distinct from that of Text mode, but uses that of Text
mode.

   Even if the new mode is not an obvious derivative of any other mode,
it can be convenient to define it as a derivative of
`fundamental-mode', so that `define-derived-mode' can automatically
enforce the most important coding conventions for you.

   Rmail Edit mode offers an example of changing the major mode
temporarily for a buffer, so it can be edited in a different way (with
ordinary Emacs commands rather than Rmail commands).  In such cases, the
temporary major mode usually provides a command to switch back to the
buffer's usual mode (Rmail mode, in this case).  You might be tempted to
present the temporary redefinitions inside a recursive edit and restore
the usual ones when the user exits; but this is a bad idea because it
constrains the user's options when it is done in more than one buffer:
recursive edits must be exited most-recently-entered first.  Using an
alternative major mode avoids this limitation.  Note: Recursive
Editing.

   The standard GNU Emacs Lisp library directory tree contains the code
for several major modes, in files such as `text-mode.el', `texinfo.el',
`lisp-mode.el', `c-mode.el', and `rmail.el'.  They are found in various
subdirectories of the `lisp' directory.  You can study these libraries
to see how modes are written.  Text mode is perhaps the simplest major
mode aside from Fundamental mode.  Rmail mode is a complicated and
specialized mode.

Major Mode Conventions

Coding conventions for keymaps, etc.

Example Major Modes

Text mode and Lisp modes.

Auto Major Mode

How Emacs chooses the major mode automatically.

Mode Help

Finding out how to use a mode.

Derived Modes

Defining a new major mode based on another major

mode.