defvar
Unlike the delete-region function, the copy-region-as-kill
function is written in Emacs Lisp. It copies a region in a buffer
and saves it in a variable called the kill-ring. This section
describes how this variable is created and initialized.
(Again we note that the term kill-ring is a misnomer. The text
that is clipped out of the buffer can be brought back; it is not a ring
of corpses, but a ring of resurrectable text.)
In Emacs Lisp, a variable such as the kill-ring is created and
given an initial value by using the defvar special form. The
name comes from "define variable".
The defvar special form is similar to setq in that it sets
the value of a variable. It is unlike setq in two ways: first,
it only sets the value of the variable if the variable does not already
have a value. If the variable already has a value, defvar does
not override the existing value. Second, defvar has a
documentation string.
You can see the current value of a variable, any variable, by using
the describe-variable function, which is usually invoked by
typing C-h v. If you type C-h v and then kill-ring
(followed by RET) when prompted, you will see what is in your
current kill ring--this may be quite a lot! Conversely, if you have
been doing nothing this Emacs session except read this document, you
may have nothing in it. At the end of the `*Help*' buffer, you
will see the documentation for kill-ring:
Documentation: List of killed text sequences.
The kill ring is defined by a defvar in the following way:
(defvar kill-ring nil "List of killed text sequences.")
In this variable definition, the variable is given an initial value of
nil, which makes sense, since if you have saved nothing, you want
nothing back if you give a yank command. The documentation
string is written just like the documentation string of a defun.
As with the documentation string of the defun, the first line of
the documentation should be a complete sentence, since some commands,
like apropos, print only the first line of documentation.
Succeeding lines should not be indented; otherwise they look odd when
you use C-h v (describe-variable).
Most variables are internal to Emacs, but some are intended as options
that you can readily set with the edit-options command. (These
settings last only for the duration of an editing session; to set a
value permanently, write a `.emacs' file. See section Your `.emacs' File.)
A readily settable variable is distinguished from others in Emacs by an asterisk, `*', in the first column of its documentation string.
For example:
(defvar line-number-mode nil "*Non-nil means display line number in mode line.")
This means that you can use the edit-options command to change
the value of line-number-mode.
Of course, you can also change the value of line-number-mode by
evaluating it within a setq expression, like this:
(setq line-number-mode t)
See section Using setq.
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