@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
-@c Copyright (C) 1990, 1991, 1992, 1993, 1994 Free Software Foundation, Inc.
+@c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999, 2002, 2003,
+@c 2004, 2005, 2006 Free Software Foundation, Inc.
@c See the file elisp.texi for copying conditions.
@setfilename ../info/functions
@node Functions, Macros, Variables, Top
* Defining Functions:: Lisp expressions for defining functions.
* Calling Functions:: How to use an existing function.
* Mapping Functions:: Applying a function to each element of a list, etc.
-* Anonymous Functions:: Lambda expressions are functions with no names.
+* Anonymous Functions:: Lambda expressions are functions with no names.
* Function Cells:: Accessing or setting the function definition
of a symbol.
+* Obsolete Functions:: Declaring functions obsolete.
* Inline Functions:: Defining functions that the compiler will open code.
+* Function Safety:: Determining whether a function is safe to call.
* Related Topics:: Cross-references to specific Lisp primitives
that have a special bearing on how functions work.
@end menu
@cindex built-in function
A @dfn{primitive} is a function callable from Lisp that is written in C,
such as @code{car} or @code{append}. These functions are also called
-@dfn{built-in} functions or @dfn{subrs}. (Special forms are also
+@dfn{built-in functions}, or @dfn{subrs}. (Special forms are also
considered primitives.)
-Usually the reason that a function is a primitives is because it is
-fundamental, because it provides a low-level interface to operating
-system services, or because it needs to run fast. Primitives can be
-modified or added only by changing the C sources and recompiling the
-editor. See @ref{Writing Emacs Primitives}.
+Usually the reason we implement a function as a primitive is either
+because it is fundamental, because it provides a low-level interface
+to operating system services, or because it needs to run fast.
+Primitives can be modified or added only by changing the C sources and
+recompiling the editor. See @ref{Writing Emacs Primitives}.
@item lambda expression
A @dfn{lambda expression} is a function written in Lisp.
These are described in the following section.
-@ifinfo
+@ifnottex
@xref{Lambda Expressions}.
-@end ifinfo
+@end ifnottex
@item special form
A @dfn{special form} is a primitive that is like a function but does not
A @dfn{macro} is a construct defined in Lisp by the programmer. It
differs from a function in that it translates a Lisp expression that you
write into an equivalent expression to be evaluated instead of the
-original expression. @xref{Macros}, for how to define and use macros.
+original expression. Macros enable Lisp programmers to do the sorts of
+things that special forms can do. @xref{Macros}, for how to define and
+use macros.
@item command
@cindex command
they are not functions. A symbol is a command if its function
definition is a command; such symbols can be invoked with @kbd{M-x}.
The symbol is a function as well if the definition is a function.
-@xref{Command Overview}.
+@xref{Interactive Call}.
@item keystroke command
@cindex keystroke command
byte compiler. @xref{Byte-Code Type}.
@end table
+@defun functionp object
+This function returns @code{t} if @var{object} is any kind of
+function, or a special form, or, recursively, a symbol whose function
+definition is a function or special form. (This does not include
+macros.)
+@end defun
+
+Unlike @code{functionp}, the next three functions do @emph{not}
+treat a symbol as its function definition.
+
@defun subrp object
This function returns @code{t} if @var{object} is a built-in function
(i.e., a Lisp primitive).
@end example
@end defun
+@defun subr-arity subr
+@tindex subr-arity
+This function provides information about the argument list of a
+primitive, @var{subr}. The returned value is a pair
+@code{(@var{min} . @var{max})}. @var{min} is the minimum number of
+args. @var{max} is the maximum number or the symbol @code{many}, for a
+function with @code{&rest} arguments, or the symbol @code{unevalled} if
+@var{subr} is a special form.
+@end defun
+
@node Lambda Expressions
@section Lambda Expressions
@cindex lambda expression
@end example
@noindent
-(Such a list is called a @dfn{lambda expression} for historical reasons,
-even though it is not really an expression at all---it is not a form
-that can be evaluated meaningfully.)
+Such a list is called a @dfn{lambda expression}. In Emacs Lisp, it
+actually is valid as an expression---it evaluates to itself. In some
+other Lisp dialects, a lambda expression is not a valid expression at
+all. In either case, its main use is not to be evaluated as an
+expression, but to be called as a function.
@menu
* Lambda Components:: The parts of a lambda expression.
@node Lambda Components
@subsection Components of a Lambda Expression
-@ifinfo
+@ifnottex
A function written in Lisp (a ``lambda expression'') is a list that
looks like this:
[@var{interactive-declaration}]
@var{body-forms}@dots{})
@end example
-@end ifinfo
+@end ifnottex
@cindex lambda list
The first element of a lambda expression is always the symbol
other lists, intended for other uses, will not accidentally be valid as
functions.
- The second element is a list of symbols--the argument variable names.
+ The second element is a list of symbols---the argument variable names.
This is called the @dfn{lambda list}. When a Lisp function is called,
the argument values are matched up against the variables in the lambda
list, which are given local bindings with the values provided.
@xref{Local Variables}.
- The documentation string is an actual string that serves to describe
-the function for the Emacs help facilities. @xref{Function Documentation}.
+ The documentation string is a Lisp string object placed within the
+function definition to describe the function for the Emacs help
+facilities. @xref{Function Documentation}.
The interactive declaration is a list of the form @code{(interactive
@var{code-string})}. This declares how to provide arguments if the
@noindent
This evaluates the arguments @code{1}, @code{(* 2 3)}, and @code{(- 5
-4)} from left to right. Then it applies the lambda expression applied
-to the argument values 1, 6 and 1 to produce the value 8.
+4)} from left to right. Then it applies the lambda expression to the
+argument values 1, 6 and 1 to produce the value 8.
It is not often useful to write a lambda expression as the @sc{car} of
a form in this way. You can get the same result, of making local
variables.
@node Argument List
-@subsection Advanced Features of Argument Lists
+@subsection Other Features of Argument Lists
@kindex wrong-number-of-arguments
@cindex argument binding
@cindex binding arguments
accepts three arguments---a string, the start index and the end
index---but the third argument defaults to the @var{length} of the
string if you omit it. It is also convenient for certain functions to
-accept an indefinite number of arguments, as the functions @code{and}
+accept an indefinite number of arguments, as the functions @code{list}
and @code{+} do.
@cindex optional arguments
be any number of extra actual arguments.
If actual arguments for the optional and rest variables are omitted,
-then they always default to @code{nil}. However, the body of the function
-is free to consider @code{nil} an abbreviation for some other meaningful
-value. This is what @code{substring} does; @code{nil} as the third argument
-means to use the length of the string supplied. There is no way for the
+then they always default to @code{nil}. There is no way for the
function to distinguish between an explicit argument of @code{nil} and
-an omitted argument.
+an omitted argument. However, the body of the function is free to
+consider @code{nil} an abbreviation for some other meaningful value.
+This is what @code{substring} does; @code{nil} as the third argument to
+@code{substring} means to use the length of the string supplied.
@cindex CL note---default optional arg
@quotation
@b{Common Lisp note:} Common Lisp allows the function to specify what
default value to use when an optional argument is omitted; Emacs Lisp
-always uses @code{nil}.
+always uses @code{nil}. Emacs Lisp does not support ``supplied-p''
+variables that tell you whether an argument was explicitly passed.
@end quotation
For example, an argument list that looks like this:
There is no way to have required arguments following optional
ones---it would not make sense. To see why this must be so, suppose
that @code{c} in the example were optional and @code{d} were required.
-Suppose three actual arguments are given; which variable would the third
-argument be for? Similarly, it makes no sense to have any more
-arguments (either required or optional) after a @code{&rest} argument.
+Suppose three actual arguments are given; which variable would the
+third argument be for? Would it be used for the @var{c}, or for
+@var{d}? One can argue for both possibilities. Similarly, it makes
+no sense to have any more arguments (either required or optional)
+after a @code{&rest} argument.
Here are some examples of argument lists and proper calls:
facilities. @xref{Documentation}, for how the @var{documentation-string} is
accessed.
- It is a good idea to provide documentation strings for all commands,
-and for all other functions in your program that users of your program
-should know about; internal functions might as well have only comments,
-since comments don't take up any room when your program is loaded.
+ It is a good idea to provide documentation strings for all the
+functions in your program, even those that are called only from within
+your program. Documentation strings are like comments, except that they
+are easier to access.
The first line of the documentation string should stand on its own,
because @code{apropos} displays just this first line. It should consist
of one or two complete sentences that summarize the function's purpose.
- The start of the documentation string is usually indented, but since
-these spaces come before the starting double-quote, they are not part of
-the string. Some people make a practice of indenting any additional
-lines of the string so that the text lines up. @emph{This is a
-mistake.} The indentation of the following lines is inside the string;
-what looks nice in the source code will look ugly when displayed by the
-help commands.
+ The start of the documentation string is usually indented in the
+source file, but since these spaces come before the starting
+double-quote, they are not part of the string. Some people make a
+practice of indenting any additional lines of the string so that the
+text lines up in the program source. @emph{That is a mistake.} The
+indentation of the following lines is inside the string; what looks
+nice in the source code will look ugly when displayed by the help
+commands.
You may wonder how the documentation string could be optional, since
there are required components of the function that follow it (the body).
documentation string; if the only body form is a string then it serves both
as the return value and as the documentation.
+ The last line of the documentation string can specify calling
+conventions different from the actual function arguments. Write
+text like this:
+
+@example
+\(fn @var{arglist})
+@end example
+
+@noindent
+following a blank line, at the beginning of the line, with no newline
+following it inside the documentation string. (The @samp{\} is used
+to avoid confusing the Emacs motion commands.) The calling convention
+specified in this way appears in help messages in place of the one
+derived from the actual arguments of the function.
+
+ This feature is particularly useful for macro definitions, since the
+arguments written in a macro definition often do not correspond to the
+way users think of the parts of the macro call.
+
@node Function Names
@section Naming a Function
@cindex function definition
In most computer languages, every function has a name; the idea of a
function without a name is nonsensical. In Lisp, a function in the
strictest sense has no name. It is simply a list whose first element is
-@code{lambda}, or a primitive subr-object.
+@code{lambda}, a byte-code function object, or a primitive subr-object.
However, a symbol can serve as the name of a function. This happens
when you put the function in the symbol's @dfn{function cell}
We often identify functions with the symbols used to name them. For
example, we often speak of ``the function @code{car}'', not
distinguishing between the symbol @code{car} and the primitive
-subr-object that is its function definition. For most purposes, there
-is no need to distinguish.
+subr-object that is its function definition. For most purposes, the
+distinction is not important.
Even so, keep in mind that a function need not have a unique name. While
a given function object @emph{usually} appears in the function cell of only
it in several symbols using @code{fset}; then each of the symbols is
equally well a name for the same function.
- A symbol used as a function name may also be used as a variable;
-these two uses of a symbol are independent and do not conflict.
+ A symbol used as a function name may also be used as a variable; these
+two uses of a symbol are independent and do not conflict. (Some Lisp
+dialects, such as Scheme, do not distinguish between a symbol's value
+and its function definition; a symbol's value as a variable is also its
+function definition.) If you have not given a symbol a function
+definition, you cannot use it as a function; whether the symbol has a
+value as a variable makes no difference to this.
@node Defining Functions
-@section Defining Named Functions
+@section Defining Functions
@cindex defining a function
We usually give a name to a function when it is first created. This
@var{name}. It returns the value @var{name}, but usually we ignore this
value.
-As described previously (@pxref{Lambda Expressions}),
-@var{argument-list} is a list of argument names and may include the
-keywords @code{&optional} and @code{&rest}. Also, the first two forms
-in @var{body-forms} may be a documentation string and an interactive
+As described previously, @var{argument-list} is a list of argument
+names and may include the keywords @code{&optional} and @code{&rest}
+(@pxref{Lambda Expressions}). Also, the first two of the
+@var{body-forms} may be a documentation string and an interactive
declaration.
There is no conflict if the same symbol @var{name} is also used as a
deliberate redefinition from unintentional redefinition.
@end defspec
-@defun defalias name definition
+@defun defalias name definition &optional docstring
+@anchor{Definition of defalias}
This special form defines the symbol @var{name} as a function, with
-definition @var{definition}. It's best to use this rather than
-@code{fset} when defining a function in a file, because @code{defalias}
-records which file defined the function (@pxref{Unloading}).
+definition @var{definition} (which can be any valid Lisp function).
+It returns @var{definition}.
+
+If @var{docstring} is non-@code{nil}, it becomes the function
+documentation of @var{name}. Otherwise, any documentation provided by
+@var{definition} is used.
+
+The proper place to use @code{defalias} is where a specific function
+name is being defined---especially where that name appears explicitly in
+the source file being loaded. This is because @code{defalias} records
+which file defined the function, just like @code{defun}
+(@pxref{Unloading}).
+
+By contrast, in programs that manipulate function definitions for other
+purposes, it is better to use @code{fset}, which does not keep such
+records. @xref{Function Cells}.
@end defun
+ You cannot create a new primitive function with @code{defun} or
+@code{defalias}, but you can use them to change the function definition of
+any symbol, even one such as @code{car} or @code{x-popup-menu} whose
+normal definition is a primitive. However, this is risky: for
+instance, it is next to impossible to redefine @code{car} without
+breaking Lisp completely. Redefining an obscure function such as
+@code{x-popup-menu} is less dangerous, but it still may not work as
+you expect. If there are calls to the primitive from C code, they
+call the primitive's C definition directly, so changing the symbol's
+definition will have no effect on them.
+
+ See also @code{defsubst}, which defines a function like @code{defun}
+and tells the Lisp compiler to open-code it. @xref{Inline Functions}.
+
@node Calling Functions
@section Calling Functions
@cindex function invocation
anything until you @dfn{call} them, i.e., tell them to run. Calling a
function is also known as @dfn{invocation}.
- The most common way of invoking a function is by evaluating a list. For
-example, evaluating the list @code{(concat "a" "b")} calls the function
-@code{concat}. @xref{Evaluation}, for a description of evaluation.
+ The most common way of invoking a function is by evaluating a list.
+For example, evaluating the list @code{(concat "a" "b")} calls the
+function @code{concat} with arguments @code{"a"} and @code{"b"}.
+@xref{Evaluation}, for a description of evaluation.
- When you write a list as an expression in your program, the function
-name is part of the program. This means that you choose which function
-to call, and how many arguments to give it, when you write the program.
-Usually that's just what you want. Occasionally you need to decide at
-run time which function to call. To do that, use the functions
-@code{funcall} and @code{apply}.
+ When you write a list as an expression in your program, you specify
+which function to call, and how many arguments to give it, in the text
+of the program. Usually that's just what you want. Occasionally you
+need to compute at run time which function to call. To do that, use
+the function @code{funcall}. When you also need to determine at run
+time how many arguments to pass, use @code{apply}.
@defun funcall function &rest arguments
@code{funcall} calls @var{function} with @var{arguments}, and returns
Since @code{funcall} is a function, all of its arguments, including
@var{function}, are evaluated before @code{funcall} is called. This
means that you can use any expression to obtain the function to be
-called. It also means that @code{funcall} does not see the expressions
-you write for the @var{arguments}, only their values. These values are
-@emph{not} evaluated a second time in the act of calling @var{function};
-@code{funcall} enters the normal procedure for calling a function at the
-place where the arguments have already been evaluated.
+called. It also means that @code{funcall} does not see the
+expressions you write for the @var{arguments}, only their values.
+These values are @emph{not} evaluated a second time in the act of
+calling @var{function}; the operation of @code{funcall} is like the
+normal procedure for calling a function, once its arguments have
+already been evaluated.
The argument @var{function} must be either a Lisp function or a
primitive function. Special forms and macros are not allowed, because
@end group
@end example
-Compare these example with the examples of @code{apply}.
+Compare these examples with the examples of @code{apply}.
@end defun
@defun apply function &rest arguments
@code{apply} calls @var{function} with @var{arguments}, just like
@code{funcall} but with one difference: the last of @var{arguments} is a
-list of arguments to give to @var{function}, rather than a single
-argument. We also say that this list is @dfn{appended} to the other
-arguments.
+list of objects, which are passed to @var{function} as separate
+arguments, rather than a single list. We say that @code{apply}
+@dfn{spreads} this list so that each individual element becomes an
+argument.
@code{apply} returns the result of calling @var{function}. As with
@code{funcall}, @var{function} must either be a Lisp function or a
@end group
@end example
-For an interesting example of using @code{apply}, see the description of
-@code{mapcar}, in @ref{Mapping Functions}.
+For an interesting example of using @code{apply}, see @ref{Definition
+of mapcar}.
@end defun
@cindex functionals
lists) and call them using @code{funcall} or @code{apply}. Functions
that accept function arguments are often called @dfn{functionals}.
- Sometimes, when you call such a function, it is useful to supply a
-no-op function as the argument. Here are two different kinds of no-op
+ Sometimes, when you call a functional, it is useful to supply a no-op
+function as the argument. Here are two different kinds of no-op
function:
@defun identity arg
@section Mapping Functions
@cindex mapping functions
- A @dfn{mapping function} applies a given function to each element of a
-list or other collection. Emacs Lisp has three such functions;
-@code{mapcar} and @code{mapconcat}, which scan a list, are described
-here. For the third mapping function, @code{mapatoms}, see
-@ref{Creating Symbols}.
+ A @dfn{mapping function} applies a given function (@emph{not} a
+special form or macro) to each element of a list or other collection.
+Emacs Lisp has several such functions; @code{mapcar} and
+@code{mapconcat}, which scan a list, are described here.
+@xref{Definition of mapatoms}, for the function @code{mapatoms} which
+maps over the symbols in an obarray. @xref{Definition of maphash},
+for the function @code{maphash} which maps over key/value associations
+in a hash table.
+
+ These mapping functions do not allow char-tables because a char-table
+is a sparse array whose nominal range of indices is very large. To map
+over a char-table in a way that deals properly with its sparse nature,
+use the function @code{map-char-table} (@pxref{Char-Tables}).
@defun mapcar function sequence
-@code{mapcar} applies @var{function} to each element of @var{sequence} in
-turn. The results are made into a @code{nil}-terminated list.
+@anchor{Definition of mapcar}
+@code{mapcar} applies @var{function} to each element of @var{sequence}
+in turn, and returns a list of the results.
-The argument @var{sequence} may be a list, a vector or a string. The
+The argument @var{sequence} can be any kind of sequence except a
+char-table; that is, a list, a vector, a bool-vector, or a string. The
result is always a list. The length of the result is the same as the
length of @var{sequence}.
@end group
@group
-(defun mapcar* (f &rest args)
+(defun mapcar* (function &rest args)
"Apply FUNCTION to successive cars of all ARGS.
Return the list of results."
;; @r{If no list is exhausted,}
- (if (not (memq 'nil args))
- ;; @r{apply function to @sc{CAR}s.}
- (cons (apply f (mapcar 'car args))
- (apply 'mapcar* f
+ (if (not (memq nil args))
+ ;; @r{apply function to @sc{car}s.}
+ (cons (apply function (mapcar 'car args))
+ (apply 'mapcar* function
;; @r{Recurse for rest of elements.}
(mapcar 'cdr args)))))
@end group
@end smallexample
@end defun
+@defun mapc function sequence
+@tindex mapc
+@code{mapc} is like @code{mapcar} except that @var{function} is used for
+side-effects only---the values it returns are ignored, not collected
+into a list. @code{mapc} always returns @var{sequence}.
+@end defun
+
@defun mapconcat function sequence separator
@code{mapconcat} applies @var{function} to each element of
@var{sequence}: the results, which must be strings, are concatenated.
other suitable punctuation.
The argument @var{function} must be a function that can take one
-argument and return a string.
-
+argument and return a string. The argument @var{sequence} can be any
+kind of sequence except a char-table; that is, a list, a vector, a
+bool-vector, or a string.
+
@smallexample
@group
(mapconcat 'symbol-name
@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. Here we pass an anonymous function that multiplies
-a number by two:
+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.
+
+ Here we define a function @code{change-property} which
+uses a function as its third argument:
@example
@group
-(defun double-each (list)
- (mapcar '(lambda (x) (* 2 x)) list))
-@result{} double-each
+(defun change-property (symbol prop function)
+ (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
-(double-each '(2 11))
-@result{} (4 22)
+(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.
-
-@defspec function function-object
-@cindex function quoting
-This special form returns @var{function-object} without evaluating it.
-In this, it is equivalent to @code{quote}. However, it serves as a
-note to the Emacs Lisp compiler that @var{function-object} is intended
-to be used only as a function, and therefore can safely be compiled.
-Contrast this with @code{quote}, in @ref{Quoting}.
-@end defspec
-
- Using @code{function} instead of @code{quote} makes a difference
-inside a function or macro that you are going to compile. For example:
+of simple quotation to quote the anonymous function, like this:
@example
@group
-(defun double-each (list)
- (mapcar (function (lambda (x) (* 2 x))) list))
-@result{} double-each
-@end group
-@group
-(double-each '(2 11))
-@result{} (4 22)
+(defun double-property (symbol prop)
+ (change-property symbol prop
+ (function (lambda (x) (* 2 x)))))
@end group
@end example
-@noindent
-If this definition of @code{double-each} is compiled, the anonymous
-function is compiled as well. By contrast, in the previous definition
-where ordinary @code{quote} is used, the argument passed to
-@code{mapcar} is the precise list shown:
+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))
@noindent
The Lisp compiler cannot assume this list is a function, even though it
-looks like one, since it does not know what @code{mapcar} does with the
-list. Perhaps @code{mapcar} will check that the @sc{car} of the third
-element is the symbol @code{*}! The advantage of @code{function} is
-that it tells the compiler to go ahead and compile the constant
-function.
+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.
+
+ Nowadays it is possible to omit @code{function} entirely, like this:
+
+@example
+@group
+(defun double-property (symbol prop)
+ (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.
+comment:
@example
(function @var{symbol}) @equiv{} (quote @var{symbol}) @equiv{} '@var{symbol}
@end example
- See @code{documentation} in @ref{Accessing Documentation}, 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}.
+For example,
+
+@example
+#'(lambda (x) (* x x))
+@end example
+
+@noindent
+is equivalent to
+
+@example
+(function (lambda (x) (* x x)))
+@end example
+
+@defspec function function-object
+@cindex function quoting
+This special form returns @var{function-object} without evaluating it.
+In this, it is equivalent to @code{quote}. However, it serves as a
+note to the Emacs Lisp compiler that @var{function-object} is intended
+to be used only as a function, and therefore can safely be compiled.
+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.
@node Function Cells
@section Accessing Function Cell Contents
function cell of the symbol. The functions described here access, test,
and set the function cell of symbols.
+ See also the function @code{indirect-function}. @xref{Definition of
+indirect-function}.
+
@defun symbol-function symbol
@kindex void-function
This returns the object in the function cell of @var{symbol}. If the
@code{void}. The symbols @code{nil} and @code{void} are Lisp objects,
and can be stored into a function cell just as any other object can be
(and they can be valid functions if you define them in turn with
-@code{defun}); but @code{nil} or @code{void} is @emph{an object}. A
-void function cell contains no object whatsoever.
+@code{defun}). A void function cell contains no object whatsoever.
You can test the voidness of a symbol's function definition with
@code{fboundp}. After you have given a symbol a function definition, you
@defun fmakunbound symbol
This function makes @var{symbol}'s function cell void, so that a
-subsequent attempt to access this cell will cause a @code{void-function}
-error. (See also @code{makunbound}, in @ref{Local Variables}.)
+subsequent attempt to access this cell will cause a
+@code{void-function} error. It returns @var{symbol}. (See also
+@code{makunbound}, in @ref{Void Variables}.)
@example
@group
(defun foo (x) x)
- @result{} x
+ @result{} foo
+@end group
+@group
+(foo 1)
+ @result{}1
@end group
@group
(fmakunbound 'foo)
- @result{} x
+ @result{} foo
@end group
@group
(foo 1)
@end example
@end defun
-@defun fset symbol object
-This function stores @var{object} in the function cell of @var{symbol}.
-The result is @var{object}. Normally @var{object} should be a function
-or the name of a function, but this is not checked.
+@defun fset symbol definition
+This function stores @var{definition} in the function cell of
+@var{symbol}. The result is @var{definition}. Normally
+@var{definition} should be a function or the name of a function, but
+this is not checked. The argument @var{symbol} is an ordinary evaluated
+argument.
There are three normal uses of this function:
@itemize @bullet
@item
-Copying one symbol's function definition to another. (In other words,
-making an alternate name for a function.)
+Copying one symbol's function definition to another---in other words,
+making an alternate name for a function. (If you think of this as the
+definition of the new name, you should use @code{defalias} instead of
+@code{fset}; see @ref{Definition of defalias}.)
@item
Giving a symbol a function definition that is not a list and therefore
-cannot be made with @code{defun}. @xref{Classifying Lists}, for an
-example of this usage.
+cannot be made with @code{defun}. For example, you can use @code{fset}
+to give a symbol @code{s1} a function definition which is another symbol
+@code{s2}; then @code{s1} serves as an alias for whatever definition
+@code{s2} presently has. (Once again use @code{defalias} instead of
+@code{fset} if you think of this as the definition of @code{s1}.)
@item
In constructs for defining or altering functions. If @code{defun}
@code{fset}.
@end itemize
-Here are examples of the first two uses:
+Here are examples of these uses:
@example
@group
-;; @r{Give @code{first} the same definition @code{car} has.}
-(fset 'first (symbol-function 'car))
- @result{} #<subr car>
-@end group
-@group
-(first '(1 2 3))
- @result{} 1
+;; @r{Save @code{foo}'s definition in @code{old-foo}.}
+(fset 'old-foo (symbol-function 'foo))
@end group
@group
;; @r{Make the symbol @code{car} the function definition of @code{xfirst}.}
+;; @r{(Most likely, @code{defalias} would be better than @code{fset} here.)}
(fset 'xfirst 'car)
@result{} car
@end group
(fset 'kill-two-lines "\^u2\^k")
@result{} "\^u2\^k"
@end group
-@end example
-@end defun
-
- When writing a function that extends a previously defined function,
-the following idiom is often used:
-@example
-(fset 'old-foo (symbol-function 'foo))
-(defun foo ()
- "Just like old-foo, except more so."
@group
- (old-foo)
- (more-so))
+;; @r{Here is a function that alters other functions.}
+(defun copy-function-definition (new old)
+ "Define NEW with the same function definition as OLD."
+ (fset new (symbol-function old)))
@end group
@end example
+@end defun
-@noindent
-This does not work properly if @code{foo} has been defined to autoload.
-In such a case, when @code{foo} calls @code{old-foo}, Lisp attempts
-to define @code{old-foo} by loading a file. Since this presumably
-defines @code{foo} rather than @code{old-foo}, it does not produce the
-proper results. The only way to avoid this problem is to make sure the
-file is loaded before moving aside the old definition of @code{foo}.
+ @code{fset} is sometimes used to save the old definition of a
+function before redefining it. That permits the new definition to
+invoke the old definition. But it is unmodular and unclean for a Lisp
+file to redefine a function defined elsewhere. If you want to modify
+a function defined by another package, it is cleaner to use
+@code{defadvice} (@pxref{Advising Functions}).
+
+@node Obsolete Functions
+@section Declaring Functions Obsolete
+
+You can use @code{make-obsolete} to declare a function obsolete. This
+indicates that the function may be removed at some stage in the future.
+
+@defun make-obsolete obsolete-name current-name &optional when
+This function makes the byte compiler warn that the function
+@var{obsolete-name} is obsolete. If @var{current-name} is a symbol, the
+warning message says to use @var{current-name} instead of
+@var{obsolete-name}. @var{current-name} does not need to be an alias for
+@var{obsolete-name}; it can be a different function with similar
+functionality. If @var{current-name} is a string, it is the warning
+message.
+
+If provided, @var{when} should be a string indicating when the function
+was first made obsolete---for example, a date or a release number.
+@end defun
-See also the function @code{indirect-function} in @ref{Function
-Indirection}.
+You can define a function as an alias and declare it obsolete at the
+same time using the macro @code{define-obsolete-function-alias}.
+
+@defmac define-obsolete-function-alias obsolete-name current-name &optional when docstring
+This macro marks the function @var{obsolete-name} obsolete and also
+defines it as an alias for the function @var{current-name}. It is
+equivalent to the following:
+
+@example
+(defalias @var{obsolete-name} @var{current-name} @var{docstring})
+(make-obsolete @var{obsolete-name} @var{current-name} @var{when})
+@end example
+@end defmac
@node Inline Functions
@section Inline Functions
generally should not make large functions inline.
It's possible to define a macro to expand into the same code that an
-inline function would execute. But the macro would have a limitation:
-you can use it only explicitly---a macro cannot be called with
+inline function would execute. (@xref{Macros}.) But the macro would be
+limited to direct use in expressions---a macro cannot be called with
@code{apply}, @code{mapcar} and so on. Also, it takes some work to
-convert an ordinary function into a macro. (@xref{Macros}.) To convert
-it into an inline function is very easy; simply replace @code{defun}
-with @code{defsubst}. Since each argument of an inline function is
-evaluated exactly once, you needn't worry about how many times the
-body uses the arguments, as you do for macros. (@xref{Argument
-Evaluation}.)
-
-Inline functions can be used and open coded later on in the same file,
+convert an ordinary function into a macro. To convert it into an inline
+function is very easy; simply replace @code{defun} with @code{defsubst}.
+Since each argument of an inline function is evaluated exactly once, you
+needn't worry about how many times the body uses the arguments, as you
+do for macros. (@xref{Argument Evaluation}.)
+
+Inline functions can be used and open-coded later on in the same file,
following the definition, just like macros.
-Emacs versions prior to 19 did not have inline functions.
+@node Function Safety
+@section Determining whether a Function is Safe to Call
+@cindex function safety
+@cindex safety of functions
+
+Some major modes such as SES call functions that are stored in user
+files. (@inforef{Top, ,ses}, for more information on SES.) User
+files sometimes have poor pedigrees---you can get a spreadsheet from
+someone you've just met, or you can get one through email from someone
+you've never met. So it is risky to call a function whose source code
+is stored in a user file until you have determined that it is safe.
+
+@defun unsafep form &optional unsafep-vars
+Returns @code{nil} if @var{form} is a @dfn{safe} Lisp expression, or
+returns a list that describes why it might be unsafe. The argument
+@var{unsafep-vars} is a list of symbols known to have temporary
+bindings at this point; it is mainly used for internal recursive
+calls. The current buffer is an implicit argument, which provides a
+list of buffer-local bindings.
+@end defun
+
+Being quick and simple, @code{unsafep} does a very light analysis and
+rejects many Lisp expressions that are actually safe. There are no
+known cases where @code{unsafep} returns @code{nil} for an unsafe
+expression. However, a ``safe'' Lisp expression can return a string
+with a @code{display} property, containing an associated Lisp
+expression to be executed after the string is inserted into a buffer.
+This associated expression can be a virus. In order to be safe, you
+must delete properties from all strings calculated by user code before
+inserting them into buffers.
+
+@ignore
+What is a safe Lisp expression? Basically, it's an expression that
+calls only built-in functions with no side effects (or only innocuous
+ones). Innocuous side effects include displaying messages and
+altering non-risky buffer-local variables (but not global variables).
+
+@table @dfn
+@item Safe expression
+@itemize
+@item
+An atom or quoted thing.
+@item
+A call to a safe function (see below), if all its arguments are
+safe expressions.
+@item
+One of the special forms @code{and}, @code{catch}, @code{cond},
+@code{if}, @code{or}, @code{prog1}, @code{prog2}, @code{progn},
+@code{while}, and @code{unwind-protect}], if all its arguments are
+safe.
+@item
+A form that creates temporary bindings (@code{condition-case},
+@code{dolist}, @code{dotimes}, @code{lambda}, @code{let}, or
+@code{let*}), if all args are safe and the symbols to be bound are not
+explicitly risky (see @pxref{File Local Variables}).
+@item
+An assignment using @code{add-to-list}, @code{setq}, @code{push}, or
+@code{pop}, if all args are safe and the symbols to be assigned are
+not explicitly risky and they already have temporary or buffer-local
+bindings.
+@item
+One of [apply, mapc, mapcar, mapconcat] if the first argument is a
+safe explicit lambda and the other args are safe expressions.
+@end itemize
+
+@item Safe function
+@itemize
+@item
+A lambda containing safe expressions.
+@item
+A symbol on the list @code{safe-functions}, so the user says it's safe.
+@item
+A symbol with a non-@code{nil} @code{side-effect-free} property.
+@item
+A symbol with a non-@code{nil} @code{safe-function} property. Value t
+indicates a function that is safe but has innocuous side effects.
+Other values will someday indicate functions with classes of side
+effects that are not always safe.
+@end itemize
+
+The @code{side-effect-free} and @code{safe-function} properties are
+provided for built-in functions and for low-level functions and macros
+defined in @file{subr.el}. You can assign these properties for the
+functions you write.
+@end table
+@end ignore
@node Related Topics
@section Other Topics Related to Functions
@item funcall
See @ref{Calling Functions}.
+@item function
+See @ref{Anonymous Functions}.
+
@item ignore
See @ref{Calling Functions}.
@item mapcar
See @ref{Mapping Functions}.
+@item map-char-table
+See @ref{Char-Tables}.
+
@item mapconcat
See @ref{Mapping Functions}.
@item undefined
-See @ref{Key Lookup}.
+See @ref{Functions for Key Lookup}.
@end table
+@ignore
+ arch-tag: 39100cdf-8a55-4898-acba-595db619e8e2
+@end ignore