@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, 2001,
+@c 2002, 2003, 2004, 2005, 2006, 2007 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
+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.
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
+@cindex argument lists, features
Our simple sample function, @code{(lambda (a b c) (+ a b c))},
specifies three argument variables, so it must be called with three
@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 in the program source.
-@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}
practice).
We often identify functions with the symbols used to name them. For
-example, we often speak of ``the function @code{car}'', not
+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 Functions
@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
+@cindex function aliases
+@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} (which can be any valid Lisp function).
-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}), while @code{fset} does not.
+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
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 @code{apply} @dfn{spreads} this list so that
-each individual element becomes an argument.
+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
+@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}.
+length of @var{sequence}. For example:
@smallexample
@group
-@exdent @r{For example:}
-
(mapcar 'car '((a b) (c d) (e f)))
@result{} (a c e)
(mapcar '1+ [1 2 3])
@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
+@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
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
+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
anonymous function. Such a list is valid wherever a function name is.
@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} in @ref{Function
-Indirection}.
+ See also the function @code{indirect-function}. @xref{Definition of
+indirect-function}.
@defun symbol-function symbol
@kindex void-function
@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)
@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}. 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.
+@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
+
+@group
+;; @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
-See also the related function @code{defalias}, in @ref{Defining
-Functions}.
+ @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
- When writing a function that extends a previously defined function,
-the following idiom is often used:
+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
-(fset 'old-foo (symbol-function 'foo))
-(defun foo ()
- "Just like old-foo, except more so."
-@group
- (old-foo)
- (more-so))
-@end group
+(defalias @var{obsolete-name} @var{current-name} @var{docstring})
+(make-obsolete @var{obsolete-name} @var{current-name} @var{when})
@end example
-
-@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}.
+@end defmac
@node Inline Functions
@section Inline Functions
the function's definition is open-coded into the caller.
Making a function inline makes explicit calls run faster. But it also
-has disadvantages. For one thing, it reduces flexibility; if you change
-the definition of the function, calls already inlined still use the old
-definition until you recompile them. Since the flexibility of
-redefining functions is an important feature of Emacs, you should not
-make a function inline unless its speed is really crucial.
+has disadvantages. For one thing, it reduces flexibility; if you
+change the definition of the function, calls already inlined still use
+the old definition until you recompile them.
Another disadvantage is that making a large function inline can increase
the size of compiled code both in files and in memory. Since the speed
advantage of inline functions is greatest for small functions, you
generally should not make large functions inline.
+Also, inline functions do not behave well with respect to debugging,
+tracing, and advising (@pxref{Advising Functions}). Since ease of
+debugging and the flexibility of redefining functions are important
+features of Emacs, you should not make a function inline, even if it's
+small, unless its speed is really crucial, and you've timed the code
+to verify that using @code{defun} actually has performance problems.
+
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}.)
+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