2 @c This is part of the GNU Emacs Lisp Reference Manual.
3 @c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999, 2001,
4 @c 2002, 2003, 2004, 2005, 2006, 2007, 2008 Free Software Foundation, Inc.
5 @c See the file elisp.texi for copying conditions.
6 @setfilename ../../info/functions
7 @node Functions, Macros, Variables, Top
10 A Lisp program is composed mainly of Lisp functions. This chapter
11 explains what functions are, how they accept arguments, and how to
15 * What Is a Function:: Lisp functions vs. primitives; terminology.
16 * Lambda Expressions:: How functions are expressed as Lisp objects.
17 * Function Names:: A symbol can serve as the name of a function.
18 * Defining Functions:: Lisp expressions for defining functions.
19 * Calling Functions:: How to use an existing function.
20 * Mapping Functions:: Applying a function to each element of a list, etc.
21 * Anonymous Functions:: Lambda expressions are functions with no names.
22 * Function Cells:: Accessing or setting the function definition
24 * Obsolete Functions:: Declaring functions obsolete.
25 * Inline Functions:: Defining functions that the compiler will open code.
26 * Declaring Functions:: Telling the compiler that a function is defined.
27 * Function Safety:: Determining whether a function is safe to call.
28 * Related Topics:: Cross-references to specific Lisp primitives
29 that have a special bearing on how functions work.
32 @node What Is a Function
33 @section What Is a Function?
35 In a general sense, a function is a rule for carrying on a computation
36 given several values called @dfn{arguments}. The result of the
37 computation is called the value of the function. The computation can
38 also have side effects: lasting changes in the values of variables or
39 the contents of data structures.
41 Here are important terms for functions in Emacs Lisp and for other
42 function-like objects.
47 In Emacs Lisp, a @dfn{function} is anything that can be applied to
48 arguments in a Lisp program. In some cases, we use it more
49 specifically to mean a function written in Lisp. Special forms and
50 macros are not functions.
55 @cindex built-in function
56 A @dfn{primitive} is a function callable from Lisp that is written in C,
57 such as @code{car} or @code{append}. These functions are also called
58 @dfn{built-in functions}, or @dfn{subrs}. (Special forms are also
59 considered primitives.)
61 Usually the reason we implement a function as a primitive is either
62 because it is fundamental, because it provides a low-level interface
63 to operating system services, or because it needs to run fast.
64 Primitives can be modified or added only by changing the C sources and
65 recompiling the editor. See @ref{Writing Emacs Primitives}.
67 @item lambda expression
68 A @dfn{lambda expression} is a function written in Lisp.
69 These are described in the following section.
71 @xref{Lambda Expressions}.
75 A @dfn{special form} is a primitive that is like a function but does not
76 evaluate all of its arguments in the usual way. It may evaluate only
77 some of the arguments, or may evaluate them in an unusual order, or
78 several times. Many special forms are described in @ref{Control
83 A @dfn{macro} is a construct defined in Lisp by the programmer. It
84 differs from a function in that it translates a Lisp expression that you
85 write into an equivalent expression to be evaluated instead of the
86 original expression. Macros enable Lisp programmers to do the sorts of
87 things that special forms can do. @xref{Macros}, for how to define and
92 A @dfn{command} is an object that @code{command-execute} can invoke; it
93 is a possible definition for a key sequence. Some functions are
94 commands; a function written in Lisp is a command if it contains an
95 interactive declaration (@pxref{Defining Commands}). Such a function
96 can be called from Lisp expressions like other functions; in this case,
97 the fact that the function is a command makes no difference.
99 Keyboard macros (strings and vectors) are commands also, even though
100 they are not functions. A symbol is a command if its function
101 definition is a command; such symbols can be invoked with @kbd{M-x}.
102 The symbol is a function as well if the definition is a function.
103 @xref{Interactive Call}.
105 @item keystroke command
106 @cindex keystroke command
107 A @dfn{keystroke command} is a command that is bound to a key sequence
108 (typically one to three keystrokes). The distinction is made here
109 merely to avoid confusion with the meaning of ``command'' in non-Emacs
110 editors; for Lisp programs, the distinction is normally unimportant.
112 @item byte-code function
113 A @dfn{byte-code function} is a function that has been compiled by the
114 byte compiler. @xref{Byte-Code Type}.
117 @defun functionp object
118 This function returns @code{t} if @var{object} is any kind of
119 function, i.e. can be passed to @code{funcall}.
122 Unlike @code{functionp}, the next three functions do @emph{not}
123 treat a symbol as its function definition.
126 This function returns @code{t} if @var{object} is a built-in function
127 (i.e., a Lisp primitive).
131 (subrp 'message) ; @r{@code{message} is a symbol,}
132 @result{} nil ; @r{not a subr object.}
135 (subrp (symbol-function 'message))
141 @defun byte-code-function-p object
142 This function returns @code{t} if @var{object} is a byte-code
143 function. For example:
147 (byte-code-function-p (symbol-function 'next-line))
153 @defun subr-arity subr
154 This function provides information about the argument list of a
155 primitive, @var{subr}. The returned value is a pair
156 @code{(@var{min} . @var{max})}. @var{min} is the minimum number of
157 args. @var{max} is the maximum number or the symbol @code{many}, for a
158 function with @code{&rest} arguments, or the symbol @code{unevalled} if
159 @var{subr} is a special form.
162 @node Lambda Expressions
163 @section Lambda Expressions
164 @cindex lambda expression
166 A function written in Lisp is a list that looks like this:
169 (lambda (@var{arg-variables}@dots{})
170 @r{[}@var{documentation-string}@r{]}
171 @r{[}@var{interactive-declaration}@r{]}
172 @var{body-forms}@dots{})
176 Such a list is called a @dfn{lambda expression}. In Emacs Lisp, it
177 actually is valid as an expression---it evaluates to itself. In some
178 other Lisp dialects, a lambda expression is not a valid expression at
179 all. In either case, its main use is not to be evaluated as an
180 expression, but to be called as a function.
183 * Lambda Components:: The parts of a lambda expression.
184 * Simple Lambda:: A simple example.
185 * Argument List:: Details and special features of argument lists.
186 * Function Documentation:: How to put documentation in a function.
189 @node Lambda Components
190 @subsection Components of a Lambda Expression
194 A function written in Lisp (a ``lambda expression'') is a list that
198 (lambda (@var{arg-variables}@dots{})
199 [@var{documentation-string}]
200 [@var{interactive-declaration}]
201 @var{body-forms}@dots{})
206 The first element of a lambda expression is always the symbol
207 @code{lambda}. This indicates that the list represents a function. The
208 reason functions are defined to start with @code{lambda} is so that
209 other lists, intended for other uses, will not accidentally be valid as
212 The second element is a list of symbols---the argument variable names.
213 This is called the @dfn{lambda list}. When a Lisp function is called,
214 the argument values are matched up against the variables in the lambda
215 list, which are given local bindings with the values provided.
216 @xref{Local Variables}.
218 The documentation string is a Lisp string object placed within the
219 function definition to describe the function for the Emacs help
220 facilities. @xref{Function Documentation}.
222 The interactive declaration is a list of the form @code{(interactive
223 @var{code-string})}. This declares how to provide arguments if the
224 function is used interactively. Functions with this declaration are called
225 @dfn{commands}; they can be called using @kbd{M-x} or bound to a key.
226 Functions not intended to be called in this way should not have interactive
227 declarations. @xref{Defining Commands}, for how to write an interactive
230 @cindex body of function
231 The rest of the elements are the @dfn{body} of the function: the Lisp
232 code to do the work of the function (or, as a Lisp programmer would say,
233 ``a list of Lisp forms to evaluate''). The value returned by the
234 function is the value returned by the last element of the body.
237 @subsection A Simple Lambda-Expression Example
239 Consider for example the following function:
242 (lambda (a b c) (+ a b c))
246 We can call this function by writing it as the @sc{car} of an
247 expression, like this:
251 ((lambda (a b c) (+ a b c))
257 This call evaluates the body of the lambda expression with the variable
258 @code{a} bound to 1, @code{b} bound to 2, and @code{c} bound to 3.
259 Evaluation of the body adds these three numbers, producing the result 6;
260 therefore, this call to the function returns the value 6.
262 Note that the arguments can be the results of other function calls, as in
267 ((lambda (a b c) (+ a b c))
273 This evaluates the arguments @code{1}, @code{(* 2 3)}, and @code{(- 5
274 4)} from left to right. Then it applies the lambda expression to the
275 argument values 1, 6 and 1 to produce the value 8.
277 It is not often useful to write a lambda expression as the @sc{car} of
278 a form in this way. You can get the same result, of making local
279 variables and giving them values, using the special form @code{let}
280 (@pxref{Local Variables}). And @code{let} is clearer and easier to use.
281 In practice, lambda expressions are either stored as the function
282 definitions of symbols, to produce named functions, or passed as
283 arguments to other functions (@pxref{Anonymous Functions}).
285 However, calls to explicit lambda expressions were very useful in the
286 old days of Lisp, before the special form @code{let} was invented. At
287 that time, they were the only way to bind and initialize local
291 @subsection Other Features of Argument Lists
292 @kindex wrong-number-of-arguments
293 @cindex argument binding
294 @cindex binding arguments
295 @cindex argument lists, features
297 Our simple sample function, @code{(lambda (a b c) (+ a b c))},
298 specifies three argument variables, so it must be called with three
299 arguments: if you try to call it with only two arguments or four
300 arguments, you get a @code{wrong-number-of-arguments} error.
302 It is often convenient to write a function that allows certain
303 arguments to be omitted. For example, the function @code{substring}
304 accepts three arguments---a string, the start index and the end
305 index---but the third argument defaults to the @var{length} of the
306 string if you omit it. It is also convenient for certain functions to
307 accept an indefinite number of arguments, as the functions @code{list}
310 @cindex optional arguments
311 @cindex rest arguments
314 To specify optional arguments that may be omitted when a function
315 is called, simply include the keyword @code{&optional} before the optional
316 arguments. To specify a list of zero or more extra arguments, include the
317 keyword @code{&rest} before one final argument.
319 Thus, the complete syntax for an argument list is as follows:
323 (@var{required-vars}@dots{}
324 @r{[}&optional @var{optional-vars}@dots{}@r{]}
325 @r{[}&rest @var{rest-var}@r{]})
330 The square brackets indicate that the @code{&optional} and @code{&rest}
331 clauses, and the variables that follow them, are optional.
333 A call to the function requires one actual argument for each of the
334 @var{required-vars}. There may be actual arguments for zero or more of
335 the @var{optional-vars}, and there cannot be any actual arguments beyond
336 that unless the lambda list uses @code{&rest}. In that case, there may
337 be any number of extra actual arguments.
339 If actual arguments for the optional and rest variables are omitted,
340 then they always default to @code{nil}. There is no way for the
341 function to distinguish between an explicit argument of @code{nil} and
342 an omitted argument. However, the body of the function is free to
343 consider @code{nil} an abbreviation for some other meaningful value.
344 This is what @code{substring} does; @code{nil} as the third argument to
345 @code{substring} means to use the length of the string supplied.
347 @cindex CL note---default optional arg
349 @b{Common Lisp note:} Common Lisp allows the function to specify what
350 default value to use when an optional argument is omitted; Emacs Lisp
351 always uses @code{nil}. Emacs Lisp does not support ``supplied-p''
352 variables that tell you whether an argument was explicitly passed.
355 For example, an argument list that looks like this:
358 (a b &optional c d &rest e)
362 binds @code{a} and @code{b} to the first two actual arguments, which are
363 required. If one or two more arguments are provided, @code{c} and
364 @code{d} are bound to them respectively; any arguments after the first
365 four are collected into a list and @code{e} is bound to that list. If
366 there are only two arguments, @code{c} is @code{nil}; if two or three
367 arguments, @code{d} is @code{nil}; if four arguments or fewer, @code{e}
370 There is no way to have required arguments following optional
371 ones---it would not make sense. To see why this must be so, suppose
372 that @code{c} in the example were optional and @code{d} were required.
373 Suppose three actual arguments are given; which variable would the
374 third argument be for? Would it be used for the @var{c}, or for
375 @var{d}? One can argue for both possibilities. Similarly, it makes
376 no sense to have any more arguments (either required or optional)
377 after a @code{&rest} argument.
379 Here are some examples of argument lists and proper calls:
382 ((lambda (n) (1+ n)) ; @r{One required:}
383 1) ; @r{requires exactly one argument.}
385 ((lambda (n &optional n1) ; @r{One required and one optional:}
386 (if n1 (+ n n1) (1+ n))) ; @r{1 or 2 arguments.}
389 ((lambda (n &rest ns) ; @r{One required and one rest:}
390 (+ n (apply '+ ns))) ; @r{1 or more arguments.}
395 @node Function Documentation
396 @subsection Documentation Strings of Functions
397 @cindex documentation of function
399 A lambda expression may optionally have a @dfn{documentation string} just
400 after the lambda list. This string does not affect execution of the
401 function; it is a kind of comment, but a systematized comment which
402 actually appears inside the Lisp world and can be used by the Emacs help
403 facilities. @xref{Documentation}, for how the @var{documentation-string} is
406 It is a good idea to provide documentation strings for all the
407 functions in your program, even those that are called only from within
408 your program. Documentation strings are like comments, except that they
409 are easier to access.
411 The first line of the documentation string should stand on its own,
412 because @code{apropos} displays just this first line. It should consist
413 of one or two complete sentences that summarize the function's purpose.
415 The start of the documentation string is usually indented in the
416 source file, but since these spaces come before the starting
417 double-quote, they are not part of the string. Some people make a
418 practice of indenting any additional lines of the string so that the
419 text lines up in the program source. @emph{That is a mistake.} The
420 indentation of the following lines is inside the string; what looks
421 nice in the source code will look ugly when displayed by the help
424 You may wonder how the documentation string could be optional, since
425 there are required components of the function that follow it (the body).
426 Since evaluation of a string returns that string, without any side effects,
427 it has no effect if it is not the last form in the body. Thus, in
428 practice, there is no confusion between the first form of the body and the
429 documentation string; if the only body form is a string then it serves both
430 as the return value and as the documentation.
432 The last line of the documentation string can specify calling
433 conventions different from the actual function arguments. Write
441 following a blank line, at the beginning of the line, with no newline
442 following it inside the documentation string. (The @samp{\} is used
443 to avoid confusing the Emacs motion commands.) The calling convention
444 specified in this way appears in help messages in place of the one
445 derived from the actual arguments of the function.
447 This feature is particularly useful for macro definitions, since the
448 arguments written in a macro definition often do not correspond to the
449 way users think of the parts of the macro call.
452 @section Naming a Function
453 @cindex function definition
454 @cindex named function
455 @cindex function name
457 In most computer languages, every function has a name; the idea of a
458 function without a name is nonsensical. In Lisp, a function in the
459 strictest sense has no name. It is simply a list whose first element is
460 @code{lambda}, a byte-code function object, or a primitive subr-object.
462 However, a symbol can serve as the name of a function. This happens
463 when you put the function in the symbol's @dfn{function cell}
464 (@pxref{Symbol Components}). Then the symbol itself becomes a valid,
465 callable function, equivalent to the list or subr-object that its
466 function cell refers to. The contents of the function cell are also
467 called the symbol's @dfn{function definition}. The procedure of using a
468 symbol's function definition in place of the symbol is called
469 @dfn{symbol function indirection}; see @ref{Function Indirection}.
471 In practice, nearly all functions are given names in this way and
472 referred to through their names. For example, the symbol @code{car} works
473 as a function and does what it does because the primitive subr-object
474 @code{#<subr car>} is stored in its function cell.
476 We give functions names because it is convenient to refer to them by
477 their names in Lisp expressions. For primitive subr-objects such as
478 @code{#<subr car>}, names are the only way you can refer to them: there
479 is no read syntax for such objects. For functions written in Lisp, the
480 name is more convenient to use in a call than an explicit lambda
481 expression. Also, a function with a name can refer to itself---it can
482 be recursive. Writing the function's name in its own definition is much
483 more convenient than making the function definition point to itself
484 (something that is not impossible but that has various disadvantages in
487 We often identify functions with the symbols used to name them. For
488 example, we often speak of ``the function @code{car},'' not
489 distinguishing between the symbol @code{car} and the primitive
490 subr-object that is its function definition. For most purposes, the
491 distinction is not important.
493 Even so, keep in mind that a function need not have a unique name. While
494 a given function object @emph{usually} appears in the function cell of only
495 one symbol, this is just a matter of convenience. It is easy to store
496 it in several symbols using @code{fset}; then each of the symbols is
497 equally well a name for the same function.
499 A symbol used as a function name may also be used as a variable; these
500 two uses of a symbol are independent and do not conflict. (Some Lisp
501 dialects, such as Scheme, do not distinguish between a symbol's value
502 and its function definition; a symbol's value as a variable is also its
503 function definition.) If you have not given a symbol a function
504 definition, you cannot use it as a function; whether the symbol has a
505 value as a variable makes no difference to this.
507 @node Defining Functions
508 @section Defining Functions
509 @cindex defining a function
511 We usually give a name to a function when it is first created. This
512 is called @dfn{defining a function}, and it is done with the
513 @code{defun} special form.
515 @defspec defun name argument-list body-forms
516 @code{defun} is the usual way to define new Lisp functions. It
517 defines the symbol @var{name} as a function that looks like this:
520 (lambda @var{argument-list} . @var{body-forms})
523 @code{defun} stores this lambda expression in the function cell of
524 @var{name}. It returns the value @var{name}, but usually we ignore this
527 As described previously, @var{argument-list} is a list of argument
528 names and may include the keywords @code{&optional} and @code{&rest}
529 (@pxref{Lambda Expressions}). Also, the first two of the
530 @var{body-forms} may be a documentation string and an interactive
533 There is no conflict if the same symbol @var{name} is also used as a
534 variable, since the symbol's value cell is independent of the function
535 cell. @xref{Symbol Components}.
537 Here are some examples:
550 (defun bar (a &optional b &rest c)
556 @result{} (1 2 (3 4 5))
560 @result{} (1 nil nil)
564 @error{} Wrong number of arguments.
568 (defun capitalize-backwards ()
569 "Upcase the last letter of a word."
575 @result{} capitalize-backwards
579 Be careful not to redefine existing functions unintentionally.
580 @code{defun} redefines even primitive functions such as @code{car}
581 without any hesitation or notification. Redefining a function already
582 defined is often done deliberately, and there is no way to distinguish
583 deliberate redefinition from unintentional redefinition.
586 @cindex function aliases
587 @defun defalias name definition &optional docstring
588 @anchor{Definition of defalias}
589 This special form defines the symbol @var{name} as a function, with
590 definition @var{definition} (which can be any valid Lisp function).
591 It returns @var{definition}.
593 If @var{docstring} is non-@code{nil}, it becomes the function
594 documentation of @var{name}. Otherwise, any documentation provided by
595 @var{definition} is used.
597 The proper place to use @code{defalias} is where a specific function
598 name is being defined---especially where that name appears explicitly in
599 the source file being loaded. This is because @code{defalias} records
600 which file defined the function, just like @code{defun}
603 By contrast, in programs that manipulate function definitions for other
604 purposes, it is better to use @code{fset}, which does not keep such
605 records. @xref{Function Cells}.
608 You cannot create a new primitive function with @code{defun} or
609 @code{defalias}, but you can use them to change the function definition of
610 any symbol, even one such as @code{car} or @code{x-popup-menu} whose
611 normal definition is a primitive. However, this is risky: for
612 instance, it is next to impossible to redefine @code{car} without
613 breaking Lisp completely. Redefining an obscure function such as
614 @code{x-popup-menu} is less dangerous, but it still may not work as
615 you expect. If there are calls to the primitive from C code, they
616 call the primitive's C definition directly, so changing the symbol's
617 definition will have no effect on them.
619 See also @code{defsubst}, which defines a function like @code{defun}
620 and tells the Lisp compiler to open-code it. @xref{Inline Functions}.
622 @node Calling Functions
623 @section Calling Functions
624 @cindex function invocation
625 @cindex calling a function
627 Defining functions is only half the battle. Functions don't do
628 anything until you @dfn{call} them, i.e., tell them to run. Calling a
629 function is also known as @dfn{invocation}.
631 The most common way of invoking a function is by evaluating a list.
632 For example, evaluating the list @code{(concat "a" "b")} calls the
633 function @code{concat} with arguments @code{"a"} and @code{"b"}.
634 @xref{Evaluation}, for a description of evaluation.
636 When you write a list as an expression in your program, you specify
637 which function to call, and how many arguments to give it, in the text
638 of the program. Usually that's just what you want. Occasionally you
639 need to compute at run time which function to call. To do that, use
640 the function @code{funcall}. When you also need to determine at run
641 time how many arguments to pass, use @code{apply}.
643 @defun funcall function &rest arguments
644 @code{funcall} calls @var{function} with @var{arguments}, and returns
645 whatever @var{function} returns.
647 Since @code{funcall} is a function, all of its arguments, including
648 @var{function}, are evaluated before @code{funcall} is called. This
649 means that you can use any expression to obtain the function to be
650 called. It also means that @code{funcall} does not see the
651 expressions you write for the @var{arguments}, only their values.
652 These values are @emph{not} evaluated a second time in the act of
653 calling @var{function}; the operation of @code{funcall} is like the
654 normal procedure for calling a function, once its arguments have
655 already been evaluated.
657 The argument @var{function} must be either a Lisp function or a
658 primitive function. Special forms and macros are not allowed, because
659 they make sense only when given the ``unevaluated'' argument
660 expressions. @code{funcall} cannot provide these because, as we saw
661 above, it never knows them in the first place.
673 (funcall f 'x 'y '(z))
678 @error{} Invalid function: #<subr and>
682 Compare these examples with the examples of @code{apply}.
685 @defun apply function &rest arguments
686 @code{apply} calls @var{function} with @var{arguments}, just like
687 @code{funcall} but with one difference: the last of @var{arguments} is a
688 list of objects, which are passed to @var{function} as separate
689 arguments, rather than a single list. We say that @code{apply}
690 @dfn{spreads} this list so that each individual element becomes an
693 @code{apply} returns the result of calling @var{function}. As with
694 @code{funcall}, @var{function} must either be a Lisp function or a
695 primitive function; special forms and macros do not make sense in
705 @error{} Wrong type argument: listp, z
708 (apply '+ 1 2 '(3 4))
712 (apply '+ '(1 2 3 4))
717 (apply 'append '((a b c) nil (x y z) nil))
718 @result{} (a b c x y z)
722 For an interesting example of using @code{apply}, see @ref{Definition
727 It is common for Lisp functions to accept functions as arguments or
728 find them in data structures (especially in hook variables and property
729 lists) and call them using @code{funcall} or @code{apply}. Functions
730 that accept function arguments are often called @dfn{functionals}.
732 Sometimes, when you call a functional, it is useful to supply a no-op
733 function as the argument. Here are two different kinds of no-op
737 This function returns @var{arg} and has no side effects.
740 @defun ignore &rest args
741 This function ignores any arguments and returns @code{nil}.
744 @node Mapping Functions
745 @section Mapping Functions
746 @cindex mapping functions
748 A @dfn{mapping function} applies a given function (@emph{not} a
749 special form or macro) to each element of a list or other collection.
750 Emacs Lisp has several such functions; @code{mapcar} and
751 @code{mapconcat}, which scan a list, are described here.
752 @xref{Definition of mapatoms}, for the function @code{mapatoms} which
753 maps over the symbols in an obarray. @xref{Definition of maphash},
754 for the function @code{maphash} which maps over key/value associations
757 These mapping functions do not allow char-tables because a char-table
758 is a sparse array whose nominal range of indices is very large. To map
759 over a char-table in a way that deals properly with its sparse nature,
760 use the function @code{map-char-table} (@pxref{Char-Tables}).
762 @defun mapcar function sequence
763 @anchor{Definition of mapcar}
764 @code{mapcar} applies @var{function} to each element of @var{sequence}
765 in turn, and returns a list of the results.
767 The argument @var{sequence} can be any kind of sequence except a
768 char-table; that is, a list, a vector, a bool-vector, or a string. The
769 result is always a list. The length of the result is the same as the
770 length of @var{sequence}. For example:
774 (mapcar 'car '((a b) (c d) (e f)))
778 (mapcar 'char-to-string "abc")
779 @result{} ("a" "b" "c")
783 ;; @r{Call each function in @code{my-hooks}.}
784 (mapcar 'funcall my-hooks)
788 (defun mapcar* (function &rest args)
789 "Apply FUNCTION to successive cars of all ARGS.
790 Return the list of results."
791 ;; @r{If no list is exhausted,}
792 (if (not (memq nil args))
793 ;; @r{apply function to @sc{car}s.}
794 (cons (apply function (mapcar 'car args))
795 (apply 'mapcar* function
796 ;; @r{Recurse for rest of elements.}
797 (mapcar 'cdr args)))))
801 (mapcar* 'cons '(a b c) '(1 2 3 4))
802 @result{} ((a . 1) (b . 2) (c . 3))
807 @defun mapc function sequence
808 @code{mapc} is like @code{mapcar} except that @var{function} is used for
809 side-effects only---the values it returns are ignored, not collected
810 into a list. @code{mapc} always returns @var{sequence}.
813 @defun mapconcat function sequence separator
814 @code{mapconcat} applies @var{function} to each element of
815 @var{sequence}: the results, which must be strings, are concatenated.
816 Between each pair of result strings, @code{mapconcat} inserts the string
817 @var{separator}. Usually @var{separator} contains a space or comma or
818 other suitable punctuation.
820 The argument @var{function} must be a function that can take one
821 argument and return a string. The argument @var{sequence} can be any
822 kind of sequence except a char-table; that is, a list, a vector, a
823 bool-vector, or a string.
827 (mapconcat 'symbol-name
828 '(The cat in the hat)
830 @result{} "The cat in the hat"
834 (mapconcat (function (lambda (x) (format "%c" (1+ x))))
842 @node Anonymous Functions
843 @section Anonymous Functions
844 @cindex anonymous function
846 In Lisp, a function is a list that starts with @code{lambda}, a
847 byte-code function compiled from such a list, or alternatively a
848 primitive subr-object; names are ``extra.'' Although usually functions
849 are defined with @code{defun} and given names at the same time, it is
850 occasionally more concise to use an explicit lambda expression---an
851 anonymous function. Such a list is valid wherever a function name is.
853 Any method of creating such a list makes a valid function. Even this:
857 (setq silly (append '(lambda (x)) (list (list '+ (* 3 4) 'x))))
858 @result{} (lambda (x) (+ 12 x))
863 This computes a list that looks like @code{(lambda (x) (+ 12 x))} and
864 makes it the value (@emph{not} the function definition!) of
867 Here is how we might call this function:
877 (It does @emph{not} work to write @code{(silly 1)}, because this function
878 is not the @emph{function definition} of @code{silly}. We have not given
879 @code{silly} any function definition, just a value as a variable.)
881 Most of the time, anonymous functions are constants that appear in
882 your program. For example, you might want to pass one as an argument to
883 the function @code{mapcar}, which applies any given function to each
886 Here we define a function @code{change-property} which
887 uses a function as its third argument:
891 (defun change-property (symbol prop function)
892 (let ((value (get symbol prop)))
893 (put symbol prop (funcall function value))))
898 Here we define a function that uses @code{change-property},
899 passing it a function to double a number:
903 (defun double-property (symbol prop)
904 (change-property symbol prop '(lambda (x) (* 2 x))))
909 In such cases, we usually use the special form @code{function} instead
910 of simple quotation to quote the anonymous function, like this:
914 (defun double-property (symbol prop)
915 (change-property symbol prop
916 (function (lambda (x) (* 2 x)))))
920 Using @code{function} instead of @code{quote} makes a difference if you
921 compile the function @code{double-property}. For example, if you
922 compile the second definition of @code{double-property}, the anonymous
923 function is compiled as well. By contrast, if you compile the first
924 definition which uses ordinary @code{quote}, the argument passed to
925 @code{change-property} is the precise list shown:
932 The Lisp compiler cannot assume this list is a function, even though it
933 looks like one, since it does not know what @code{change-property} will
934 do with the list. Perhaps it will check whether the @sc{car} of the third
935 element is the symbol @code{*}! Using @code{function} tells the
936 compiler it is safe to go ahead and compile the constant function.
938 Nowadays it is possible to omit @code{function} entirely, like this:
942 (defun double-property (symbol prop)
943 (change-property symbol prop (lambda (x) (* 2 x))))
948 This is because @code{lambda} itself implies @code{function}.
950 We sometimes write @code{function} instead of @code{quote} when
951 quoting the name of a function, but this usage is just a sort of
955 (function @var{symbol}) @equiv{} (quote @var{symbol}) @equiv{} '@var{symbol}
958 @cindex @samp{#'} syntax
959 The read syntax @code{#'} is a short-hand for using @code{function}.
963 #'(lambda (x) (* x x))
970 (function (lambda (x) (* x x)))
973 @defspec function function-object
974 @cindex function quoting
975 This special form returns @var{function-object} without evaluating it.
976 In this, it is equivalent to @code{quote}. However, it serves as a
977 note to the Emacs Lisp compiler that @var{function-object} is intended
978 to be used only as a function, and therefore can safely be compiled.
979 Contrast this with @code{quote}, in @ref{Quoting}.
982 @xref{describe-symbols example}, for a realistic example using
983 @code{function} and an anonymous function.
986 @section Accessing Function Cell Contents
988 The @dfn{function definition} of a symbol is the object stored in the
989 function cell of the symbol. The functions described here access, test,
990 and set the function cell of symbols.
992 See also the function @code{indirect-function}. @xref{Definition of
995 @defun symbol-function symbol
996 @kindex void-function
997 This returns the object in the function cell of @var{symbol}. If the
998 symbol's function cell is void, a @code{void-function} error is
1001 This function does not check that the returned object is a legitimate
1006 (defun bar (n) (+ n 2))
1010 (symbol-function 'bar)
1011 @result{} (lambda (n) (+ n 2))
1018 (symbol-function 'baz)
1024 @cindex void function cell
1025 If you have never given a symbol any function definition, we say that
1026 that symbol's function cell is @dfn{void}. In other words, the function
1027 cell does not have any Lisp object in it. If you try to call such a symbol
1028 as a function, it signals a @code{void-function} error.
1030 Note that void is not the same as @code{nil} or the symbol
1031 @code{void}. The symbols @code{nil} and @code{void} are Lisp objects,
1032 and can be stored into a function cell just as any other object can be
1033 (and they can be valid functions if you define them in turn with
1034 @code{defun}). A void function cell contains no object whatsoever.
1036 You can test the voidness of a symbol's function definition with
1037 @code{fboundp}. After you have given a symbol a function definition, you
1038 can make it void once more using @code{fmakunbound}.
1040 @defun fboundp symbol
1041 This function returns @code{t} if the symbol has an object in its
1042 function cell, @code{nil} otherwise. It does not check that the object
1043 is a legitimate function.
1046 @defun fmakunbound symbol
1047 This function makes @var{symbol}'s function cell void, so that a
1048 subsequent attempt to access this cell will cause a
1049 @code{void-function} error. It returns @var{symbol}. (See also
1050 @code{makunbound}, in @ref{Void Variables}.)
1067 @error{} Symbol's function definition is void: foo
1072 @defun fset symbol definition
1073 This function stores @var{definition} in the function cell of
1074 @var{symbol}. The result is @var{definition}. Normally
1075 @var{definition} should be a function or the name of a function, but
1076 this is not checked. The argument @var{symbol} is an ordinary evaluated
1079 There are three normal uses of this function:
1083 Copying one symbol's function definition to another---in other words,
1084 making an alternate name for a function. (If you think of this as the
1085 definition of the new name, you should use @code{defalias} instead of
1086 @code{fset}; see @ref{Definition of defalias}.)
1089 Giving a symbol a function definition that is not a list and therefore
1090 cannot be made with @code{defun}. For example, you can use @code{fset}
1091 to give a symbol @code{s1} a function definition which is another symbol
1092 @code{s2}; then @code{s1} serves as an alias for whatever definition
1093 @code{s2} presently has. (Once again use @code{defalias} instead of
1094 @code{fset} if you think of this as the definition of @code{s1}.)
1097 In constructs for defining or altering functions. If @code{defun}
1098 were not a primitive, it could be written in Lisp (as a macro) using
1102 Here are examples of these uses:
1106 ;; @r{Save @code{foo}'s definition in @code{old-foo}.}
1107 (fset 'old-foo (symbol-function 'foo))
1111 ;; @r{Make the symbol @code{car} the function definition of @code{xfirst}.}
1112 ;; @r{(Most likely, @code{defalias} would be better than @code{fset} here.)}
1121 (symbol-function 'xfirst)
1125 (symbol-function (symbol-function 'xfirst))
1126 @result{} #<subr car>
1130 ;; @r{Define a named keyboard macro.}
1131 (fset 'kill-two-lines "\^u2\^k")
1136 ;; @r{Here is a function that alters other functions.}
1137 (defun copy-function-definition (new old)
1138 "Define NEW with the same function definition as OLD."
1139 (fset new (symbol-function old)))
1144 @code{fset} is sometimes used to save the old definition of a
1145 function before redefining it. That permits the new definition to
1146 invoke the old definition. But it is unmodular and unclean for a Lisp
1147 file to redefine a function defined elsewhere. If you want to modify
1148 a function defined by another package, it is cleaner to use
1149 @code{defadvice} (@pxref{Advising Functions}).
1151 @node Obsolete Functions
1152 @section Declaring Functions Obsolete
1154 You can use @code{make-obsolete} to declare a function obsolete. This
1155 indicates that the function may be removed at some stage in the future.
1157 @defun make-obsolete obsolete-name current-name &optional when
1158 This function makes the byte compiler warn that the function
1159 @var{obsolete-name} is obsolete. If @var{current-name} is a symbol, the
1160 warning message says to use @var{current-name} instead of
1161 @var{obsolete-name}. @var{current-name} does not need to be an alias for
1162 @var{obsolete-name}; it can be a different function with similar
1163 functionality. If @var{current-name} is a string, it is the warning
1166 If provided, @var{when} should be a string indicating when the function
1167 was first made obsolete---for example, a date or a release number.
1170 You can define a function as an alias and declare it obsolete at the
1171 same time using the macro @code{define-obsolete-function-alias}.
1173 @defmac define-obsolete-function-alias obsolete-name current-name &optional when docstring
1174 This macro marks the function @var{obsolete-name} obsolete and also
1175 defines it as an alias for the function @var{current-name}. It is
1176 equivalent to the following:
1179 (defalias @var{obsolete-name} @var{current-name} @var{docstring})
1180 (make-obsolete @var{obsolete-name} @var{current-name} @var{when})
1184 @node Inline Functions
1185 @section Inline Functions
1186 @cindex inline functions
1189 You can define an @dfn{inline function} by using @code{defsubst} instead
1190 of @code{defun}. An inline function works just like an ordinary
1191 function except for one thing: when you compile a call to the function,
1192 the function's definition is open-coded into the caller.
1194 Making a function inline makes explicit calls run faster. But it also
1195 has disadvantages. For one thing, it reduces flexibility; if you
1196 change the definition of the function, calls already inlined still use
1197 the old definition until you recompile them.
1199 Another disadvantage is that making a large function inline can increase
1200 the size of compiled code both in files and in memory. Since the speed
1201 advantage of inline functions is greatest for small functions, you
1202 generally should not make large functions inline.
1204 Also, inline functions do not behave well with respect to debugging,
1205 tracing, and advising (@pxref{Advising Functions}). Since ease of
1206 debugging and the flexibility of redefining functions are important
1207 features of Emacs, you should not make a function inline, even if it's
1208 small, unless its speed is really crucial, and you've timed the code
1209 to verify that using @code{defun} actually has performance problems.
1211 It's possible to define a macro to expand into the same code that an
1212 inline function would execute. (@xref{Macros}.) But the macro would be
1213 limited to direct use in expressions---a macro cannot be called with
1214 @code{apply}, @code{mapcar} and so on. Also, it takes some work to
1215 convert an ordinary function into a macro. To convert it into an inline
1216 function is very easy; simply replace @code{defun} with @code{defsubst}.
1217 Since each argument of an inline function is evaluated exactly once, you
1218 needn't worry about how many times the body uses the arguments, as you
1219 do for macros. (@xref{Argument Evaluation}.)
1221 Inline functions can be used and open-coded later on in the same file,
1222 following the definition, just like macros.
1224 @node Declaring Functions
1225 @section Telling the Compiler that a Function is Defined
1226 @cindex function declaration
1227 @cindex declaring functions
1228 @findex declare-function
1230 Byte-compiling a file often produces warnings about functions that the
1231 compiler doesn't know about (@pxref{Compiler Errors}). Sometimes this
1232 indicates a real problem, but usually the functions in question are
1233 defined in other files which would be loaded if that code is run. For
1234 example, byte-compiling @file{fortran.el} used to warn:
1238 fortran.el:2152:1:Warning: the function `gud-find-c-expr' is not known to be defined.
1241 In fact, @code{gud-find-c-expr} is only used in the function that
1242 Fortran mode uses for the local value of
1243 @code{gud-find-expr-function}, which is a callback from GUD; if it is
1244 called, the GUD functions will be loaded. When you know that such a
1245 warning does not indicate a real problem, it is good to suppress the
1246 warning. That makes new warnings which might mean real problems more
1247 visible. You do that with @code{declare-function}.
1249 All you need to do is add a @code{declare-function} statement before the
1250 first use of the function in question:
1253 (declare-function gud-find-c-expr "gud.el" nil)
1256 This says that @code{gud-find-c-expr} is defined in @file{gud.el} (the
1257 @samp{.el} can be omitted). The compiler takes for granted that that file
1258 really defines the function, and does not check.
1260 The optional third argument specifies the argument list of
1261 @code{gud-find-c-expr}. In this case, it takes no arguments
1262 (@code{nil} is different from not specifying a value). In other
1263 cases, this might be something like @code{(file &optional overwrite)}.
1264 You don't have to specify the argument list, but if you do the
1265 byte compiler can check that the calls match the declaration.
1267 @defmac declare-function function file &optional arglist fileonly
1268 Tell the byte compiler to assume that @var{function} is defined, with
1269 arguments @var{arglist}, and that the definition should come from
1270 the file @var{file}. @var{fileonly} non-nil means only check that
1271 @var{file} exists, not that it actually defines @var{function}.
1274 To verify that these functions really are declared where
1275 @code{declare-function} says they are, use @code{check-declare-file}
1276 to check all @code{declare-function} calls in one source file, or use
1277 @code{check-declare-directory} check all the files in and under a
1280 These commands find the file that ought to contain a function's
1281 definition using @code{locate-library}; if that finds no file, they
1282 expand the definition file name relative to the directory of the file
1283 that contains the @code{declare-function} call.
1285 You can also say that a function is defined by C code by specifying
1286 a file name ending in @samp{.c}. @code{check-declare-file} looks for
1287 these files in the C source code directory. This is useful only when
1288 you call a function that is defined only on certain systems. Most
1289 of the primitive functions of Emacs are always defined so they will
1290 never give you a warning.
1292 Sometimes a file will optionally use functions from an external package.
1293 If you prefix the filename in the @code{declare-function} statement with
1294 @samp{ext:}, then it will be checked if it is found, otherwise skipped
1297 There are some function definitions that @samp{check-declare} does not
1298 understand (e.g. @code{defstruct} and some other macros). In such cases,
1299 you can pass a non-@code{nil} @var{fileonly} argument to
1300 @code{declare-function}, meaning to only check that the file exists, not
1301 that it actually defines the function. Note that to do this without
1302 having to specify an argument list, you should set the @var{arglist}
1303 argument to @code{t} (because @code{nil} means an empty argument list, as
1304 opposed to an unspecified one).
1306 @node Function Safety
1307 @section Determining whether a Function is Safe to Call
1308 @cindex function safety
1309 @cindex safety of functions
1311 Some major modes such as SES call functions that are stored in user
1312 files. (@inforef{Top, ,ses}, for more information on SES.) User
1313 files sometimes have poor pedigrees---you can get a spreadsheet from
1314 someone you've just met, or you can get one through email from someone
1315 you've never met. So it is risky to call a function whose source code
1316 is stored in a user file until you have determined that it is safe.
1318 @defun unsafep form &optional unsafep-vars
1319 Returns @code{nil} if @var{form} is a @dfn{safe} Lisp expression, or
1320 returns a list that describes why it might be unsafe. The argument
1321 @var{unsafep-vars} is a list of symbols known to have temporary
1322 bindings at this point; it is mainly used for internal recursive
1323 calls. The current buffer is an implicit argument, which provides a
1324 list of buffer-local bindings.
1327 Being quick and simple, @code{unsafep} does a very light analysis and
1328 rejects many Lisp expressions that are actually safe. There are no
1329 known cases where @code{unsafep} returns @code{nil} for an unsafe
1330 expression. However, a ``safe'' Lisp expression can return a string
1331 with a @code{display} property, containing an associated Lisp
1332 expression to be executed after the string is inserted into a buffer.
1333 This associated expression can be a virus. In order to be safe, you
1334 must delete properties from all strings calculated by user code before
1335 inserting them into buffers.
1338 What is a safe Lisp expression? Basically, it's an expression that
1339 calls only built-in functions with no side effects (or only innocuous
1340 ones). Innocuous side effects include displaying messages and
1341 altering non-risky buffer-local variables (but not global variables).
1344 @item Safe expression
1347 An atom or quoted thing.
1349 A call to a safe function (see below), if all its arguments are
1352 One of the special forms @code{and}, @code{catch}, @code{cond},
1353 @code{if}, @code{or}, @code{prog1}, @code{prog2}, @code{progn},
1354 @code{while}, and @code{unwind-protect}], if all its arguments are
1357 A form that creates temporary bindings (@code{condition-case},
1358 @code{dolist}, @code{dotimes}, @code{lambda}, @code{let}, or
1359 @code{let*}), if all args are safe and the symbols to be bound are not
1360 explicitly risky (see @pxref{File Local Variables}).
1362 An assignment using @code{add-to-list}, @code{setq}, @code{push}, or
1363 @code{pop}, if all args are safe and the symbols to be assigned are
1364 not explicitly risky and they already have temporary or buffer-local
1367 One of [apply, mapc, mapcar, mapconcat] if the first argument is a
1368 safe explicit lambda and the other args are safe expressions.
1374 A lambda containing safe expressions.
1376 A symbol on the list @code{safe-functions}, so the user says it's safe.
1378 A symbol with a non-@code{nil} @code{side-effect-free} property.
1380 A symbol with a non-@code{nil} @code{safe-function} property. Value t
1381 indicates a function that is safe but has innocuous side effects.
1382 Other values will someday indicate functions with classes of side
1383 effects that are not always safe.
1386 The @code{side-effect-free} and @code{safe-function} properties are
1387 provided for built-in functions and for low-level functions and macros
1388 defined in @file{subr.el}. You can assign these properties for the
1389 functions you write.
1393 @node Related Topics
1394 @section Other Topics Related to Functions
1396 Here is a table of several functions that do things related to
1397 function calling and function definitions. They are documented
1398 elsewhere, but we provide cross references here.
1402 See @ref{Calling Functions}.
1407 @item call-interactively
1408 See @ref{Interactive Call}.
1410 @item called-interactively-p
1411 See @ref{Distinguish Interactive}.
1414 See @ref{Interactive Call}.
1417 See @ref{Accessing Documentation}.
1423 See @ref{Calling Functions}.
1426 See @ref{Anonymous Functions}.
1429 See @ref{Calling Functions}.
1431 @item indirect-function
1432 See @ref{Function Indirection}.
1435 See @ref{Using Interactive}.
1438 See @ref{Distinguish Interactive}.
1441 See @ref{Creating Symbols}.
1444 See @ref{Mapping Functions}.
1446 @item map-char-table
1447 See @ref{Char-Tables}.
1450 See @ref{Mapping Functions}.
1453 See @ref{Functions for Key Lookup}.
1457 arch-tag: 39100cdf-8a55-4898-acba-595db619e8e2