@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
-@c Copyright (C) 1990, 1991, 1992, 1993, 1994, 2002
-@c Free Software Foundation, Inc.
+@c Copyright (C) 1990, 1991, 1992, 1993, 1994, 2002, 2003, 2004,
+@c 2005, 2006 Free Software Foundation, Inc.
@c See the file elisp.texi for copying conditions.
@setfilename ../info/intro
addressed as ``you''. ``The user'' is the person who uses Lisp
programs, including those you write.
-@cindex fonts
+@cindex fonts in this manual
Examples of Lisp code are formatted like this: @code{(list 1 2 3)}.
Names that represent metasyntactic variables, or arguments to a function
being described, are formatted like this: @var{first-number}.
there is no way to determine which representation was actually written
by the programmer.
- In this manual, we use @code{()} when we wish to emphasize that it
-means the empty list, and we use @code{nil} when we wish to emphasize
+ In this manual, we write @code{()} when we wish to emphasize that it
+means the empty list, and we write @code{nil} when we wish to emphasize
that it means the truth value @var{false}. That is a good convention to use
in Lisp programs also.
@example
(cons 'foo ()) ; @r{Emphasize the empty list}
-(not nil) ; @r{Emphasize the truth value @var{false}}
+(setq foo-flag nil) ; @r{Emphasize the truth value @var{false}}
@end example
-@cindex @code{t} and truth
+@cindex @code{t}, uses of
@cindex true
In contexts where a truth value is expected, any non-@code{nil} value
is considered to be @var{true}. However, @code{t} is the preferred way
In Emacs Lisp, @code{nil} and @code{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 @code{setting-constant} error. The same is true of
-any symbol whose name starts with a colon (@samp{:}). @xref{Constant
+values results in a @code{setting-constant} error. @xref{Constant
Variables}.
+@defun booleanp object
+Return non-nil iff @var{object} is one of the two canonical boolean
+values: @code{t} or @code{nil}.
+@end defun
+
@node Evaluation Notation
@subsection Evaluation Notation
@cindex evaluation notation
@cindex documentation notation
+@cindex notation
A Lisp expression that you can evaluate is called a @dfn{form}.
Evaluating a form always produces a result, which is a Lisp object. In
arguments default to @code{nil}). Do not write @code{&optional} when
you call the function.
- The keyword @code{&rest} (which must be followed by a single argument
-name) indicates that any number of arguments can follow. The single
-following argument name will have a value, as a variable, which is a
-list of all these remaining arguments. Do not write @code{&rest} when
-you call the function.
+ The keyword @code{&rest} (which must be followed by a single
+argument name) indicates that any number of arguments can follow. The
+single argument name following @code{&rest} will receive, as its
+value, a list of all the remaining arguments passed to the function.
+Do not write @code{&rest} when you call the function.
Here is a description of an imaginary function @code{foo}:
@cindex variable descriptions
@cindex option descriptions
- A @dfn{variable} is a name that can hold a value. Although any
-variable can be set by the user, certain variables that exist
-specifically so that users can change them are called @dfn{user
+ A @dfn{variable} is a name that can hold a value. Although nearly
+all variables can be set by the user, certain variables exist
+specifically so that users can change them; these are called @dfn{user
options}. Ordinary variables and user options are described using a
format like that for functions except that there are no arguments.