In Emacs Lisp, the symbol
nil has three separate meanings: it
is a symbol with the name ‘nil’; it is the logical truth value
false; and it is the empty list—the list of zero elements.
When used as a variable,
nil always has the value
As far as the Lisp reader is concerned, ‘()’ and ‘nil’ are
identical: they stand for the same object, the symbol
different ways of writing the symbol are intended entirely for human
readers. After the Lisp reader has read either ‘()’ or ‘nil’,
there is no way to determine which representation was actually written
by the programmer.
In this manual, we write
() when we wish to emphasize that it
means the empty list, and we write
nil when we wish to emphasize
that it means the truth value false. That is a good convention to use
in Lisp programs also.
(cons 'foo ()) ; Emphasize the empty list (setq foo-flag nil) ; Emphasize the truth value false
In contexts where a truth value is expected, any non-
is considered to be true. However,
t is the preferred way
to represent the truth value true. When you need to choose a
value that represents true, and there is no other basis for
t. The symbol
t always has the value
In Emacs Lisp,
t are special symbols that always
evaluate to themselves. This is so that you do not need to quote them
to use them as constants in a program. An attempt to change their
values results in a
setting-constant error. See Constant Variables.