You can specify variables using
defcustom so that you and
others can then use Emacs's
customize feature to set their
values. (You cannot use
customize to write function
definitions; but you can write
defuns in your .emacs
file. Indeed, you can write any Lisp expression in your .emacs
customize feature depends on the
form. Although you can use
setq for variables
that users set, the
defcustom special form is designed for the
You can use your knowledge of
defvar for writing the
first three arguments for
defcustom. The first argument to
defcustom is the name of the variable. The second argument is
the variable's initial value, if any; and this value is set only if
the value has not already been set. The third argument is the
The fourth and subsequent arguments to
defcustom specify types
and options; these are not featured in
arguments are optional.)
Each of these arguments consists of a keyword followed by a value. Each keyword starts with the colon character ‘:’.
For example, the customizable user option variable
text-mode-hook looks like this:
(defcustom text-mode-hook nil "Normal hook run when entering Text mode and many related modes." :type 'hook :options '(turn-on-auto-fill flyspell-mode) :group 'wp)
The name of the variable is
text-mode-hook; it has no default
value; and its documentation string tells you what it does.
:type keyword tells Emacs the kind of data to which
text-mode-hook should be set and how to display the value in a
:options keyword specifies a suggested list of values for
the variable. Usually,
:options applies to a hook.
The list is only a suggestion; it is not exclusive; a person who sets
the variable may set it to other values; the list shown following the
:options keyword is intended to offer convenient choices to a
:group keyword tells the Emacs Customization
command in which group the variable is located. This tells where to
defcustom function recognizes more than a dozen keywords.
For more information, see Writing Customization Definitions.
text-mode-hook as an example.
There are two ways to customize this variable. You can use the customization command or write the appropriate expressions yourself.
Using the customization command, you can type:
and find that the group for editing files of data is called `data'.
Enter that group. Text Mode Hook is the first member. You can click
on its various options, such as
turn-on-auto-fill, to set the
values. After you click on the button to
Save for Future Sessions
Emacs will write an expression into your .emacs file. It will look like this:
(custom-set-variables ;; custom-set-variables was added by Custom. ;; If you edit it by hand, you could mess it up, so be careful. ;; Your init file should contain only one such instance. ;; If there is more than one, they won't work right. '(text-mode-hook (quote (turn-on-auto-fill text-mode-hook-identify))))
text-mode-hook-identify function tells
toggle-text-mode-auto-fill which buffers are in Text mode.
It comes on automatically.)
custom-set-variables function works somewhat differently
setq. While I have never learned the differences, I
custom-set-variables expressions in my .emacs
file by hand: I make the changes in what appears to me to be a
reasonable manner and have not had any problems. Others prefer to use
the Customization command and let Emacs do the work for them.
custom-set-... function is
This function sets the various font faces. Over time, I have set a
considerable number of faces. Some of the time, I re-set them using
customize; other times, I simply edit the
custom-set-faces expression in my .emacs file itself.
The second way to customize your
text-mode-hook is to set it
yourself in your .emacs file using code that has nothing to do
When you do this, and later use
customize, you will see a
message that says
CHANGED outside Customize; operating on it here may be unreliable.
This message is only a warning. If you click on the button to
Save for Future Sessions
Emacs will write a
custom-set-... expression near the end
of your .emacs file that will be evaluated after your
hand-written expression. It will, therefore, overrule your
hand-written expression. No harm will be done. When you do this,
however, be careful to remember which expression is active; if you
forget, you may confuse yourself.
So long as you remember where the values are set, you will have no trouble. In any event, the values are always set in your initialization file, which is usually called .emacs.
I myself use
customize for hardly anything. Mostly, I write
Incidentally, to be more complete concerning defines:
defines an inline function. The syntax is just like that of
defconst defines a symbol as a constant. The
intent is that neither programs nor users should ever change a value
defconst. (You can change it; the value set is a
variable; but please do not.)