197 |
(not nil) ; @r{Emphasize the truth value @var{false}} |
(not nil) ; @r{Emphasize the truth value @var{false}} |
198 |
@end example |
@end example |
199 |
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200 |
@cindex @code{t} and truth |
@cindex @code{t}, uses of |
201 |
@cindex true |
@cindex true |
202 |
In contexts where a truth value is expected, any non-@code{nil} value |
In contexts where a truth value is expected, any non-@code{nil} value |
203 |
is considered to be @var{true}. However, @code{t} is the preferred way |
is considered to be @var{true}. However, @code{t} is the preferred way |
209 |
In Emacs Lisp, @code{nil} and @code{t} are special symbols that always |
In Emacs Lisp, @code{nil} and @code{t} are special symbols that always |
210 |
evaluate to themselves. This is so that you do not need to quote them |
evaluate to themselves. This is so that you do not need to quote them |
211 |
to use them as constants in a program. An attempt to change their |
to use them as constants in a program. An attempt to change their |
212 |
values results in a @code{setting-constant} error. The same is true of |
values results in a @code{setting-constant} error. @xref{Constant |
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any symbol whose name starts with a colon (@samp{:}). @xref{Constant |
|
213 |
Variables}. |
Variables}. |
214 |
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|
215 |
@node Evaluation Notation |
@node Evaluation Notation |
216 |
@subsection Evaluation Notation |
@subsection Evaluation Notation |
217 |
@cindex evaluation notation |
@cindex evaluation notation |
218 |
@cindex documentation notation |
@cindex documentation notation |
219 |
|
@cindex notation |
220 |
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|
221 |
A Lisp expression that you can evaluate is called a @dfn{form}. |
A Lisp expression that you can evaluate is called a @dfn{form}. |
222 |
Evaluating a form always produces a result, which is a Lisp object. In |
Evaluating a form always produces a result, which is a Lisp object. In |
355 |
arguments default to @code{nil}). Do not write @code{&optional} when |
arguments default to @code{nil}). Do not write @code{&optional} when |
356 |
you call the function. |
you call the function. |
357 |
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|
358 |
The keyword @code{&rest} (which must be followed by a single argument |
The keyword @code{&rest} (which must be followed by a single |
359 |
name) indicates that any number of arguments can follow. The single |
argument name) indicates that any number of arguments can follow. The |
360 |
following argument name will have a value, as a variable, which is a |
single argument name following @code{&rest} will receive, as its |
361 |
list of all these remaining arguments. Do not write @code{&rest} when |
value, a list of all the remaining arguments passed to the function. |
362 |
you call the function. |
Do not write @code{&rest} when you call the function. |
363 |
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|
364 |
Here is a description of an imaginary function @code{foo}: |
Here is a description of an imaginary function @code{foo}: |
365 |
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|
450 |
@cindex variable descriptions |
@cindex variable descriptions |
451 |
@cindex option descriptions |
@cindex option descriptions |
452 |
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|
453 |
A @dfn{variable} is a name that can hold a value. Although any |
A @dfn{variable} is a name that can hold a value. Although nearly |
454 |
variable can be set by the user, certain variables that exist |
all variables can be set by the user, certain variables exist |
455 |
specifically so that users can change them are called @dfn{user |
specifically so that users can change them; these are called @dfn{user |
456 |
options}. Ordinary variables and user options are described using a |
options}. Ordinary variables and user options are described using a |
457 |
format like that for functions except that there are no arguments. |
format like that for functions except that there are no arguments. |
458 |
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