(lispref.info)nil and t


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`nil' and `t'
-------------

   In Lisp, the symbol `nil' is overloaded with three 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 Emacs 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.  Note: Accessing
Variables.


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