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nil
and t
In 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 nil
.
As far as the Lisp reader is concerned, ‘()’ and ‘nil’ are
identical: they stand for the same object, the symbol nil
. The
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 use ()
when we wish to emphasize that it
means the empty list, and we use 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 (not nil) ; Emphasize the truth value false
In contexts where a truth value is expected, any non-nil
value
is considered to be true. However, t
is the preferred way
to represent the truth value true. When you need to choose a
value which represents true, and there is no other basis for
choosing, use t
. The symbol t
always has value t
.
In SXEmacs Lisp, nil
and 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 Accessing Variables.
Next: Evaluation Notation, Previous: Some Terms, Up: Conventions [Contents][Index]