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Specifying Variables using `defcustom'
======================================

   You can specify variables using `defcustom' so that you and others
can then can use Emacs' `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' file.)

   The `customize' feature depends on the `defcustom' special form.
Although you can use `defvar' or `setq' for variables that users set,
the `defcustom' special form is designed for the job.

   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 documentation.

   The fourth and subsequent arguments to `defcustom' specify types and
options; these are not featured in `defvar'.  (These arguments are
optional.)

   Each of these arguments consists of a keyword followed by a value.
Each keyword starts with the 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 'data)

The name of the variable is `text-mode-hook'; it has no default value;
and its documentation string tells you what it does.

   The `:type' keyword tells Emacs what kind of data `text-mode-hook'
should be set to and how to display the value in a Customization buffer.

   The `:options' keyword specifies a suggested list of values for the
variable.  Currently, you can use `:options' only for 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 user.

   Finally, the `:group' keyword tells the Emacs Customization command
in which group the variable is located.  This tells where to find it.

   For more information, see *Note Writing Customization Definitions:
(elisp)Customization.

   Consider `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:

     M-x customize

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 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 --
       ;;                           don't edit or cut/paste it!
       ;; Your init file should contain only one such instance.
      '(text-mode-hook (quote (turn-on-auto-fill text-mode-hook-identify))))

(The `text-mode-hook-identify' function tells
`toggle-text-mode-auto-fill' which buffers are in Text mode.)

   In spite of the warning, you certainly may edit, cut, and paste the
expression!  I do all time.  The purpose of the warning is to scare
those who do not know what they are doing, so they do not inadvertently
generate an error.

   The `custom-set-variables' works somewhat differently than a `setq'.
While I have never learned the differences, I do modify the
`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.

   Another `custom-set-...' function is `custom-set-faces'.  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 with
the `custom-set-...' functions.

   When you do this, and later use `customize', you will see a message
that says

     this option has been changed outside the customize buffer.

   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
expressions myself.