@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
-@c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999
-@c 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 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}.
+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.
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
@end table
@defun functionp object
-This function returns @code{t} if @var{object} is any kind of function,
-or a special form or macro.
+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).
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 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{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
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
@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 of the
+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.
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} (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
By contrast, in programs that manipulate function definitions for other
purposes, it is better to use @code{fset}, which does not keep such
-records.
+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}.
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 it calls is written in your 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 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}.
+ 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
-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
@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 several such functions;
-@code{mapcar} and @code{mapconcat}, which scan a list, are described
-here. @xref{Creating Symbols}, for the function @code{mapatoms} which
-maps over the symbols in an obarray. @xref{Hash Access}, for the
-function @code{maphash} which maps over key/value associations in a
-hash table.
+ 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
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.
"Apply FUNCTION to successive cars of all ARGS.
Return the list of results."
;; @r{If no list is exhausted,}
- (if (not (memq 'nil args))
+ (if (not (memq nil args))
;; @r{apply function to @sc{car}s.}
- (cons (apply function (mapcar 'car args))
- (apply 'mapcar* function
+ (cons (apply function (mapcar 'car args))
+ (apply 'mapcar* function
;; @r{Recurse for rest of elements.}
(mapcar 'cdr args)))))
@end group
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
@smallexample
@group
-(setq silly (append (lambda (x)) (list (list '+ (* 3 4) 'x))))
+(setq silly (append '(lambda (x)) (list (list '+ (* 3 4) 'x))))
@result{} (lambda (x) (+ 12 x))
@end group
@end smallexample
the function @code{mapcar}, which applies any given function to each
element of a list.
- Here we define a function @code{change-property} which
+ Here we define a function @code{change-property} which
uses a function as its third argument:
@example
@cindex @samp{#'} syntax
The read syntax @code{#'} is a short-hand for using @code{function}.
-For example,
+For example,
@example
#'(lambda (x) (* x x))
Contrast this with @code{quote}, in @ref{Quoting}.
@end defspec
- See @code{documentation} in @ref{Accessing Documentation}, for a
-realistic example using @code{function} and an anonymous function.
+ @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{Void 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
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{Defining Functions}.)
+@code{fset}; see @ref{Definition of defalias}.)
@item
Giving a symbol a function definition that is not a list and therefore
@end example
@end defun
- When writing a function that extends a previously defined function,
-the following idiom is sometimes used:
+ @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
-@example
-(fset 'old-foo (symbol-function 'foo))
-(defun foo ()
- "Just like old-foo, except more so."
-@group
- (old-foo)
- (more-so))
-@end group
-@end example
+You can define a function as an alias and declare it obsolete at the
+same time using the macro @code{define-obsolete-function-alias}.
-@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}.
+@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:
- But it is unmodular and unclean, in any case, for a Lisp file to
-redefine a function defined elsewhere. It is cleaner to use the advice
-facility (@pxref{Advising Functions}).
+@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
Inline functions can be used and open-coded later on in the same file,
following the definition, just like macros.
-@c 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
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