In Emacs Lisp, certain symbols normally evaluate to themselves. These
include nil and t, as well as any symbol whose name starts
with ‘:’ (these are called keywords). These symbols cannot
be rebound, nor can their values be changed. Any attempt to set or bind
nil or t signals a setting-constant error. The
same is true for a keyword (a symbol whose name starts with ‘:’),
if it is interned in the standard obarray, except that setting such a
symbol to itself is not an error.
nil ≡ 'nil
⇒ nil
(setq nil 500) error→ Attempt to set constant symbol: nil
function returns t if object is a symbol whose name
starts with ‘:’, interned in the standard obarray, and returns
nil otherwise.
These constants are fundamentally different from the constants
defined using the defconst special form (see Defining Global Variables). A defconst form serves to inform human readers
that you do not intend to change the value of a variable, but Emacs
does not raise an error if you actually change it.
A small number of additional symbols are made read-only for various
practical reasons. These include enable-multibyte-characters,
most-positive-fixnum, most-negative-fixnum, and a few
others. Any attempt to set or bind these also signals a
setting-constant error.