In Lisp, a function is a list that starts with @code{lambda}, a
byte-code function compiled from such a list, or alternatively a
-primitive subr-object; names are ``extra.'' Although usually functions
-are defined with @code{defun} and given names at the same time, it is
-occasionally more concise to use an explicit lambda expression---an
+primitive subr-object; names are ``extra.'' Although functions are
+usually defined with @code{defun} and given names at the same time, it
+is occasionally more concise to use an explicit lambda expression---an
anonymous function. Such a list is valid wherever a function name is.
Any method of creating such a list makes a valid function. Even this:
@end example
@noindent
-(It does @emph{not} work to write @code{(silly 1)}, because this function
-is not the @emph{function definition} of @code{silly}. We have not given
-@code{silly} any function definition, just a value as a variable.)
+It does @emph{not} work to write @code{(silly 1)}, because this
+function is not the @emph{function definition} of @code{silly}. We
+have not given @code{silly} any function definition, just a value as a
+variable.
Most of the time, anonymous functions are constants that appear in
-your program. For example, you might want to pass one as an argument to
-the function @code{mapcar}, which applies any given function to each
-element of a list.
+your program. For instance, you might want to pass one as an argument
+to the function @code{mapcar}, which applies any given function to
+each element of a list (@pxref{Mapping Functions}).
+@xref{describe-symbols example}, for a realistic example of this.
- Here we define a function @code{change-property} which
-uses a function as its third argument:
+ In the following example, we define a @code{change-property}
+function that takes a function as its third argument, followed by a
+@code{double-property} function that makes use of
+@code{change-property} by passing it an anonymous function:
@example
@group
(let ((value (get symbol prop)))
(put symbol prop (funcall function value))))
@end group
-@end example
-
-@noindent
-Here we define a function that uses @code{change-property},
-passing it a function to double a number:
-@example
-@group
-(defun double-property (symbol prop)
- (change-property symbol prop '(lambda (x) (* 2 x))))
-@end group
-@end example
-
-@noindent
-In such cases, we usually use the special form @code{function} instead
-of simple quotation to quote the anonymous function, like this:
-
-@example
@group
(defun double-property (symbol prop)
- (change-property symbol prop
- (function (lambda (x) (* 2 x)))))
+ (change-property symbol prop (lambda (x) (* 2 x))))
@end group
@end example
-Using @code{function} instead of @code{quote} makes a difference if you
-compile the function @code{double-property}. For example, if you
-compile the second definition of @code{double-property}, the anonymous
-function is compiled as well. By contrast, if you compile the first
-definition which uses ordinary @code{quote}, the argument passed to
-@code{change-property} is the precise list shown:
-
-@example
-(lambda (x) (* x 2))
-@end example
-
@noindent
-The Lisp compiler cannot assume this list is a function, even though it
-looks like one, since it does not know what @code{change-property} will
-do with the list. Perhaps it will check whether the @sc{car} of the third
-element is the symbol @code{*}! Using @code{function} tells the
-compiler it is safe to go ahead and compile the constant function.
+In the @code{double-property} function, we did not quote the
+@code{lambda} form. This is permissible, because a @code{lambda} form
+is @dfn{self-quoting}: evaluating the form yields the form itself.
- Nowadays it is possible to omit @code{function} entirely, like this:
+Whether or not you quote a @code{lambda} form makes a difference if
+you compile the code (@pxref{Byte Compilation}). If the @code{lambda}
+form is unquoted, as in the above example, the anonymous function is
+also compiled. Suppose, however, that we quoted the @code{lambda}
+form:
@example
@group
(defun double-property (symbol prop)
- (change-property symbol prop (lambda (x) (* 2 x))))
+ (change-property symbol prop '(lambda (x) (* 2 x))))
@end group
@end example
@noindent
-This is because @code{lambda} itself implies @code{function}.
-
- We sometimes write @code{function} instead of @code{quote} when
-quoting the name of a function, but this usage is just a sort of
-comment:
+If you compile this, the argument passed to @code{change-property} is
+the precise list shown:
@example
-(function @var{symbol}) @equiv{} (quote @var{symbol}) @equiv{} '@var{symbol}
-@end example
-
-@cindex @samp{#'} syntax
- The read syntax @code{#'} is a short-hand for using @code{function}.
-For example,
-
-@example
-#'(lambda (x) (* x x))
+(lambda (x) (* x 2))
@end example
@noindent
-is equivalent to
+The Lisp compiler cannot assume this list is a function, even though
+it looks like one, since it does not know what @code{change-property}
+will do with the list. Perhaps it will check whether the @sc{car} of
+the third element is the symbol @code{*}!
-@example
-(function (lambda (x) (* x x)))
-@end example
+@findex function
+The @code{function} special form explicitly tells the byte-compiler
+that its argument is a function:
@defspec function function-object
@cindex function quoting
Contrast this with @code{quote}, in @ref{Quoting}.
@end defspec
- @xref{describe-symbols example}, for a realistic example using
-@code{function} and an anonymous function.
+@cindex @samp{#'} syntax
+The read syntax @code{#'} is a short-hand for using @code{function}.
+Generally, it is not necessary to use either @code{#'} or
+@code{function}; just use an unquoted @code{lambda} form instead.
+(Actually, @code{lambda} is a macro defined using @code{function}.)
+The following forms are all equivalent:
+
+@example
+#'(lambda (x) (* x x))
+(function (lambda (x) (* x x)))
+(lambda (x) (* x x))
+@end example
+
+ We sometimes write @code{function} instead of @code{quote} when
+quoting the name of a function, but this usage is just a sort of
+comment:
+
+@example
+(function @var{symbol}) @equiv{} (quote @var{symbol}) @equiv{} '@var{symbol}
+@end example
@node Function Cells
@section Accessing Function Cell Contents
projects / emacs.git / commitdiff