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(emacs-lisp-intro.info)defvar

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Initializing a Variable with `defvar'
=====================================

   Unlike the `delete-and-extract-region' function, the
`copy-region-as-kill' function is written in Emacs Lisp.  Two functions
within it, `kill-append' and `kill-new', copy a region in a buffer and
save it in a variable called the `kill-ring'.  This section describes
how the `kill-ring' variable is created and initialized using the
`defvar' special form.

   (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.

   (Another special form, `defcustom', is designed for variables that
people customize.  It has more features than `defvar'.  (Note: Setting
Variables with `defcustom'.)

See variable current value

defvar and asterisk

An old-time convention.