@c -*-texinfo-*- @c This is part of the XEmacs Lisp Reference Manual. @c Copyright (C) 1990, 1991, 1992, 1993, 1994 Free Software Foundation, Inc. @c See the file lispref.texi for copying conditions. @setfilename ../../info/functions.info @node Functions, Macros, Variables, Top @chapter Functions A Lisp program is composed mainly of Lisp functions. This chapter explains what functions are, how they accept arguments, and how to define them. @menu * What Is a Function:: Lisp functions vs. primitives; terminology. * Lambda Expressions:: How functions are expressed as Lisp objects. * Function Names:: A symbol can serve as the name of a function. * 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. * Function Cells:: Accessing or setting the function definition of a symbol. * Inline Functions:: Defining functions that the compiler will open code. * Related Topics:: Cross-references to specific Lisp primitives that have a special bearing on how functions work. @end menu @node What Is a Function @section What Is a Function? In a general sense, a function is a rule for carrying on a computation given several values called @dfn{arguments}. The result of the computation is called the value of the function. The computation can also have side effects: lasting changes in the values of variables or the contents of data structures. Here are important terms for functions in XEmacs Lisp and for other function-like objects. @table @dfn @item function @cindex function In XEmacs Lisp, a @dfn{function} is anything that can be applied to arguments in a Lisp program. In some cases, we use it more specifically to mean a function written in Lisp. Special forms and macros are not functions. @item primitive @cindex primitive @cindex subr @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 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 Lisp Primitives,,, internals, XEmacs Internals Manual}. @item lambda expression A @dfn{lambda expression} is a function written in Lisp. These are described in the following section. @ifinfo @xref{Lambda Expressions}. @end ifinfo @item special form A @dfn{special form} is a primitive that is like a function but does not evaluate all of its arguments in the usual way. It may evaluate only some of the arguments, or may evaluate them in an unusual order, or several times. Many special forms are described in @ref{Control Structures}. @item macro @cindex macro 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. 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 A @dfn{command} is an object that @code{command-execute} can invoke; it is a possible definition for a key sequence. Some functions are commands; a function written in Lisp is a command if it contains an interactive declaration (@pxref{Defining Commands}). Such a function can be called from Lisp expressions like other functions; in this case, the fact that the function is a command makes no difference. Keyboard macros (strings and vectors) are commands also, even though 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}. @item keystroke command @cindex keystroke command A @dfn{keystroke command} is a command that is bound to a key sequence (typically one to three keystrokes). The distinction is made here merely to avoid confusion with the meaning of ``command'' in non-Emacs editors; for Lisp programs, the distinction is normally unimportant. @item compiled function A @dfn{compiled function} is a function that has been compiled by the byte compiler. @xref{Compiled-Function Type}. @end table @defun subrp object This function returns @code{t} if @var{object} is a built-in function (i.e., a Lisp primitive). @example @group (subrp 'message) ; @r{@code{message} is a symbol,} @result{} nil ; @r{not a subr object.} @end group @group (subrp (symbol-function 'message)) @result{} t @end group @end example @end defun @defun compiled-function-p object This function returns @code{t} if @var{object} is a compiled function. For example: @example @group (compiled-function-p (symbol-function 'next-line)) @result{} t @end group @end example @end defun @node Lambda Expressions @section Lambda Expressions @cindex lambda expression A function written in Lisp is a list that looks like this: @example (lambda (@var{arg-variables}@dots{}) @r{[}@var{documentation-string}@r{]} @r{[}@var{interactive-declaration}@r{]} @var{body-forms}@dots{}) @end example @noindent Such a list is called a @dfn{lambda expression}. In XEmacs 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. * Simple Lambda:: A simple example. * Argument List:: Details and special features of argument lists. * Function Documentation:: How to put documentation in a function. @end menu @node Lambda Components @subsection Components of a Lambda Expression @ifinfo A function written in Lisp (a ``lambda expression'') is a list that looks like this: @example (lambda (@var{arg-variables}@dots{}) [@var{documentation-string}] [@var{interactive-declaration}] @var{body-forms}@dots{}) @end example @end ifinfo @cindex lambda list The first element of a lambda expression is always the symbol @code{lambda}. This indicates that the list represents a function. The reason functions are defined to start with @code{lambda} is so that 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. This is called the @dfn{lambda list}. When a Lisp function is called, the argument values are matched up against the variables in the lambda list, which are given local bindings with the values provided. @xref{Local Variables}. The documentation string is a Lisp string object placed within the function definition to describe the function for the XEmacs help facilities. @xref{Function Documentation}. The interactive declaration is a list of the form @code{(interactive @var{code-string})}. This declares how to provide arguments if the function is used interactively. Functions with this declaration are called @dfn{commands}; they can be called using @kbd{M-x} or bound to a key. Functions not intended to be called in this way should not have interactive declarations. @xref{Defining Commands}, for how to write an interactive declaration. @cindex body of function The rest of the elements are the @dfn{body} of the function: the Lisp code to do the work of the function (or, as a Lisp programmer would say, ``a list of Lisp forms to evaluate''). The value returned by the function is the value returned by the last element of the body. @node Simple Lambda @subsection A Simple Lambda-Expression Example Consider for example the following function: @example (lambda (a b c) (+ a b c)) @end example @noindent We can call this function by writing it as the @sc{car} of an expression, like this: @example @group ((lambda (a b c) (+ a b c)) 1 2 3) @end group @end example @noindent This call evaluates the body of the lambda expression with the variable @code{a} bound to 1, @code{b} bound to 2, and @code{c} bound to 3. Evaluation of the body adds these three numbers, producing the result 6; therefore, this call to the function returns the value 6. Note that the arguments can be the results of other function calls, as in this example: @example @group ((lambda (a b c) (+ a b c)) 1 (* 2 3) (- 5 4)) @end group @end example @noindent This evaluates the arguments @code{1}, @code{(* 2 3)}, and @code{(- 5 4)} from left to right. Then it applies the lambda expression to the argument values 1, 6 and 1 to produce the value 8. It is not often useful to write a lambda expression as the @sc{car} of a form in this way. You can get the same result, of making local variables and giving them values, using the special form @code{let} (@pxref{Local Variables}). And @code{let} is clearer and easier to use. In practice, lambda expressions are either stored as the function definitions of symbols, to produce named functions, or passed as arguments to other functions (@pxref{Anonymous Functions}). However, calls to explicit lambda expressions were very useful in the old days of Lisp, before the special form @code{let} was invented. At that time, they were the only way to bind and initialize local variables. @node Argument List @subsection Advanced Features of Argument Lists @kindex wrong-number-of-arguments @cindex argument binding @cindex binding arguments Our simple sample function, @code{(lambda (a b c) (+ a b c))}, specifies three argument variables, so it must be called with three arguments: if you try to call it with only two arguments or four arguments, you get a @code{wrong-number-of-arguments} error. It is often convenient to write a function that allows certain arguments to be omitted. For example, the function @code{substring} accepts three arguments---a string, the start index and the end index---but the third argument defaults to the @var{length} of the string if you omit it. It is also convenient for certain functions to accept an indefinite number of arguments, as the functions @code{list} and @code{+} do. @cindex optional arguments @cindex rest arguments @kindex &optional @kindex &rest To specify optional arguments that may be omitted when a function is called, simply include the keyword @code{&optional} before the optional arguments. To specify a list of zero or more extra arguments, include the keyword @code{&rest} before one final argument. Thus, the complete syntax for an argument list is as follows: @example @group (@var{required-vars}@dots{} @r{[}&optional @var{optional-vars}@dots{}@r{]} @r{[}&rest @var{rest-var}@r{]}) @end group @end example @noindent The square brackets indicate that the @code{&optional} and @code{&rest} clauses, and the variables that follow them, are optional. A call to the function requires one actual argument for each of the @var{required-vars}. There may be actual arguments for zero or more of the @var{optional-vars}, and there cannot be any actual arguments beyond that unless the lambda list uses @code{&rest}. In that case, there may be any number of extra actual arguments. If actual arguments for the optional and rest variables are omitted, then they always default to @code{nil}. There is no way for the function to distinguish between an explicit argument of @code{nil} and an omitted argument. However, the body of the function is free to consider @code{nil} an abbreviation for some other meaningful value. This is what @code{substring} does; @code{nil} as the third argument to @code{substring} means to use the length of the string supplied. @cindex CL note---default optional arg @quotation @b{Common Lisp note:} Common Lisp allows the function to specify what default value to use when an optional argument is omitted; XEmacs Lisp always uses @code{nil}. @end quotation For example, an argument list that looks like this: @example (a b &optional c d &rest e) @end example @noindent binds @code{a} and @code{b} to the first two actual arguments, which are required. If one or two more arguments are provided, @code{c} and @code{d} are bound to them respectively; any arguments after the first four are collected into a list and @code{e} is bound to that list. If there are only two arguments, @code{c} is @code{nil}; if two or three arguments, @code{d} is @code{nil}; if four arguments or fewer, @code{e} is @code{nil}. 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. Here are some examples of argument lists and proper calls: @smallexample ((lambda (n) (1+ n)) ; @r{One required:} 1) ; @r{requires exactly one argument.} @result{} 2 ((lambda (n &optional n1) ; @r{One required and one optional:} (if n1 (+ n n1) (1+ n))) ; @r{1 or 2 arguments.} 1 2) @result{} 3 ((lambda (n &rest ns) ; @r{One required and one rest:} (+ n (apply '+ ns))) ; @r{1 or more arguments.} 1 2 3 4 5) @result{} 15 @end smallexample @node Function Documentation @subsection Documentation Strings of Functions @cindex documentation of function A lambda expression may optionally have a @dfn{documentation string} just after the lambda list. This string does not affect execution of the function; it is a kind of comment, but a systematized comment which actually appears inside the Lisp world and can be used by the XEmacs help facilities. @xref{Documentation}, for how the @var{documentation-string} is accessed. It is a good idea to provide documentation strings for all the functions in your program, even those that are only called 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 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. You may wonder how the documentation string could be optional, since there are required components of the function that follow it (the body). Since evaluation of a string returns that string, without any side effects, it has no effect if it is not the last form in the body. Thus, in practice, there is no confusion between the first form of the body and the documentation string; if the only body form is a string then it serves both as the return value and as the documentation. @node Function Names @section Naming a Function @cindex function definition @cindex named function @cindex function name 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. 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} (@pxref{Symbol Components}). Then the symbol itself becomes a valid, callable function, equivalent to the list or subr-object that its function cell refers to. The contents of the function cell are also called the symbol's @dfn{function definition}. The procedure of using a symbol's function definition in place of the symbol is called @dfn{symbol function indirection}; see @ref{Function Indirection}. In practice, nearly all functions are given names in this way and referred to through their names. For example, the symbol @code{car} works as a function and does what it does because the primitive subr-object @code{#} is stored in its function cell. We give functions names because it is convenient to refer to them by their names in Lisp expressions. For primitive subr-objects such as @code{#}, names are the only way you can refer to them: there is no read syntax for such objects. For functions written in Lisp, the name is more convenient to use in a call than an explicit lambda expression. Also, a function with a name can refer to itself---it can be recursive. Writing the function's name in its own definition is much more convenient than making the function definition point to itself (something that is not impossible but that has various disadvantages in practice). 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. 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 one symbol, this is just a matter of convenience. It is easy to store 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. @node Defining Functions @section Defining Functions @cindex defining a function We usually give a name to a function when it is first created. This is called @dfn{defining a function}, and it is done with the @code{defun} special form. @defspec defun name argument-list body-forms @code{defun} is the usual way to define new Lisp functions. It defines the symbol @var{name} as a function that looks like this: @example (lambda @var{argument-list} . @var{body-forms}) @end example @code{defun} stores this lambda expression in the function cell of @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 declaration. There is no conflict if the same symbol @var{name} is also used as a variable, since the symbol's value cell is independent of the function cell. @xref{Symbol Components}. Here are some examples: @example @group (defun foo () 5) @result{} foo @end group @group (foo) @result{} 5 @end group @group (defun bar (a &optional b &rest c) (list a b c)) @result{} bar @end group @group (bar 1 2 3 4 5) @result{} (1 2 (3 4 5)) @end group @group (bar 1) @result{} (1 nil nil) @end group @group (bar) @error{} Wrong number of arguments. @end group @group (defun capitalize-backwards () "Upcase the last letter of a word." (interactive) (backward-word 1) (forward-word 1) (backward-char 1) (capitalize-word 1)) @result{} capitalize-backwards @end group @end example Be careful not to redefine existing functions unintentionally. @code{defun} redefines even primitive functions such as @code{car} without any hesitation or notification. Redefining a function already defined is often done deliberately, and there is no way to distinguish deliberate redefinition from unintentional redefinition. @end defspec @defun define-function name definition @defunx defalias name definition These equivalent special forms define the symbol @var{name} as a function, with definition @var{definition} (which can be any valid Lisp function). The proper place to use @code{define-function} or @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{define-function} and @code{defalias} record 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. @end defun 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 @cindex calling a function Defining functions is only half the battle. Functions don't do anything until you @dfn{call} them, i.e., tell them to run. Calling a function is also known as @dfn{invocation}. The most common way of invoking a function is by evaluating a list. For example, evaluating the list @code{(concat "a" "b")} calls the function @code{concat} 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}. @defun funcall function &rest arguments @code{funcall} calls @var{function} with @var{arguments}, and returns whatever @var{function} 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. The argument @var{function} must be either a Lisp function or a primitive function. Special forms and macros are not allowed, because they make sense only when given the ``unevaluated'' argument expressions. @code{funcall} cannot provide these because, as we saw above, it never knows them in the first place. @example @group (setq f 'list) @result{} list @end group @group (funcall f 'x 'y 'z) @result{} (x y z) @end group @group (funcall f 'x 'y '(z)) @result{} (x y (z)) @end group @group (funcall 'and t nil) @error{} Invalid function: # @end group @end example Compare these example 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. @code{apply} returns the result of calling @var{function}. As with @code{funcall}, @var{function} must either be a Lisp function or a primitive function; special forms and macros do not make sense in @code{apply}. @example @group (setq f 'list) @result{} list @end group @group (apply f 'x 'y 'z) @error{} Wrong type argument: listp, z @end group @group (apply '+ 1 2 '(3 4)) @result{} 10 @end group @group (apply '+ '(1 2 3 4)) @result{} 10 @end group @group (apply 'append '((a b c) nil (x y z) nil)) @result{} (a b c x y z) @end group @end example For an interesting example of using @code{apply}, see the description of @code{mapcar}, in @ref{Mapping Functions}. @end defun @cindex functionals It is common for Lisp functions to accept functions as arguments or find them in data structures (especially in hook variables and property lists) and call them using @code{funcall} or @code{apply}. Functions that accept function arguments are often called @dfn{functionals}. 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 This function returns @var{arg} and has no side effects. @end defun @defun ignore &rest args This function ignores any arguments and returns @code{nil}. @end defun @node Mapping Functions @section Mapping Functions @cindex mapping functions A @dfn{mapping function} applies a given function to each element of a list or other collection. XEmacs 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}. @defun mapcar function sequence @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 result is always a list. The length of the result is the same as the length of @var{sequence}. @smallexample @group @exdent @r{For example:} (mapcar 'car '((a b) (c d) (e f))) @result{} (a c e) (mapcar '1+ [1 2 3]) @result{} (2 3 4) (mapcar 'char-to-string "abc") @result{} ("a" "b" "c") @end group @group ;; @r{Call each function in @code{my-hooks}.} (mapcar 'funcall my-hooks) @end group @group (defun mapcar* (f &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 ;; @r{Recurse for rest of elements.} (mapcar 'cdr args))))) @end group @group (mapcar* 'cons '(a b c) '(1 2 3 4)) @result{} ((a . 1) (b . 2) (c . 3)) @end group @end smallexample @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. Between each pair of result strings, @code{mapconcat} inserts the string @var{separator}. Usually @var{separator} contains a space or comma or other suitable punctuation. The argument @var{function} must be a function that can take one argument and return a string. @smallexample @group (mapconcat 'symbol-name '(The cat in the hat) " ") @result{} "The cat in the hat" @end group @group (mapconcat (function (lambda (x) (format "%c" (1+ x)))) "HAL-8000" "") @result{} "IBM.9111" @end group @end smallexample @end defun @node Anonymous Functions @section Anonymous Functions @cindex anonymous function In Lisp, a function is a list that starts with @code{lambda}, a byte-code function compiled from such a list, or alternatively a primitive subr-object; names are ``extra''. Although usually functions are defined with @code{defun} and given names at the same time, it is occasionally more concise to use an explicit lambda expression---an anonymous function. Such a list is valid wherever a function name is. Any method of creating such a list makes a valid function. Even this: @smallexample @group (setq silly (append '(lambda (x)) (list (list '+ (* 3 4) 'x)))) @result{} (lambda (x) (+ 12 x)) @end group @end smallexample @noindent This computes a list that looks like @code{(lambda (x) (+ 12 x))} and makes it the value (@emph{not} the function definition!) of @code{silly}. Here is how we might call this function: @example @group (funcall silly 1) @result{} 13 @end group @end example @noindent (It does @emph{not} work to write @code{(silly 1)}, because this function is not the @emph{function definition} of @code{silly}. We have not given @code{silly} any function definition, just a value as a variable.) 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: @example @group (defun double-each (list) (mapcar '(lambda (x) (* 2 x)) list)) @result{} double-each @end group @group (double-each '(2 11)) @result{} (4 22) @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 XEmacs 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: @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) @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: @example (lambda (x) (* x 2)) @end example @noindent The Lisp compiler cannot assume this list is a function, even though it looks like one, since it does not know what @code{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. We sometimes write @code{function} instead of @code{quote} when quoting the name of a function, but this usage is just a sort of comment. @example (function @var{symbol}) @equiv{} (quote @var{symbol}) @equiv{} '@var{symbol} @end example See @code{documentation} in @ref{Accessing Documentation}, for a realistic example using @code{function} and an anonymous function. @node Function Cells @section Accessing Function Cell Contents The @dfn{function definition} of a symbol is the object stored in the 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}. @defun symbol-function symbol @kindex void-function This returns the object in the function cell of @var{symbol}. If the symbol's function cell is void, a @code{void-function} error is signaled. This function does not check that the returned object is a legitimate function. @example @group (defun bar (n) (+ n 2)) @result{} bar @end group @group (symbol-function 'bar) @result{} (lambda (n) (+ n 2)) @end group @group (fset 'baz 'bar) @result{} bar @end group @group (symbol-function 'baz) @result{} bar @end group @end example @end defun @cindex void function cell If you have never given a symbol any function definition, we say that that symbol's function cell is @dfn{void}. In other words, the function cell does not have any Lisp object in it. If you try to call such a symbol as a function, it signals a @code{void-function} error. Note that void is not the same as @code{nil} or the symbol @code{void}. The symbols @code{nil} and @code{void} are Lisp objects, and can be stored into a function cell just as any other object can be (and they can be valid functions if you define them in turn with @code{defun}). A void function cell contains no object whatsoever. You can test the voidness of a symbol's function definition with @code{fboundp}. After you have given a symbol a function definition, you can make it void once more using @code{fmakunbound}. @defun fboundp symbol This function returns @code{t} if the symbol has an object in its function cell, @code{nil} otherwise. It does not check that the object is a legitimate function. @end defun @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}.) @example @group (defun foo (x) x) @result{} x @end group @group (foo 1) @result{}1 @end group @group (fmakunbound 'foo) @result{} x @end group @group (foo 1) @error{} Symbol's function definition is void: foo @end group @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. 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.) @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. @item In constructs for defining or altering functions. If @code{defun} were not a primitive, it could be written in Lisp (as a macro) using @code{fset}. @end itemize Here are examples of the first two uses: @example @group ;; @r{Give @code{first} the same definition @code{car} has.} (fset 'first (symbol-function 'car)) @result{} # @end group @group (first '(1 2 3)) @result{} 1 @end group @group ;; @r{Make the symbol @code{car} the function definition of @code{xfirst}.} (fset 'xfirst 'car) @result{} car @end group @group (xfirst '(1 2 3)) @result{} 1 @end group @group (symbol-function 'xfirst) @result{} car @end group @group (symbol-function (symbol-function 'xfirst)) @result{} # @end group @group ;; @r{Define a named keyboard macro.} (fset 'kill-two-lines "\^u2\^k") @result{} "\^u2\^k" @end group @end example See also the related functions @code{define-function} and @code{defalias}, in @ref{Defining Functions}. @end defun When writing a function that extends a previously defined function, the following idiom is sometimes used: @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 @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}. But it is unmodular and unclean, in any case, for a Lisp file to redefine a function defined elsewhere. @node Inline Functions @section Inline Functions @cindex inline functions @findex defsubst You can define an @dfn{inline function} by using @code{defsubst} instead of @code{defun}. An inline function works just like an ordinary function except for one thing: when you compile a call to the function, 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 XEmacs, you should not make a function inline unless its speed is really crucial. 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. 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 @code{apply}, @code{mapcar} and so on. Also, it takes some work to convert an ordinary function into a macro. (@xref{Macros}.) To convert it into an inline function is very easy; simply replace @code{defun} with @code{defsubst}. Since each argument of an inline function is evaluated exactly once, you needn't worry about how many times the body uses the arguments, as you do for macros. (@xref{Argument Evaluation}.) Inline functions can be used and open-coded later on in the same file, following the definition, just like macros. @c Emacs versions prior to 19 did not have inline functions. @node Related Topics @section Other Topics Related to Functions Here is a table of several functions that do things related to function calling and function definitions. They are documented elsewhere, but we provide cross references here. @table @code @item apply See @ref{Calling Functions}. @item autoload See @ref{Autoload}. @item call-interactively See @ref{Interactive Call}. @item commandp See @ref{Interactive Call}. @item documentation See @ref{Accessing Documentation}. @item eval See @ref{Eval}. @item funcall See @ref{Calling Functions}. @item ignore See @ref{Calling Functions}. @item indirect-function See @ref{Function Indirection}. @item interactive See @ref{Using Interactive}. @item interactive-p See @ref{Interactive Call}. @item mapatoms See @ref{Creating Symbols}. @item mapcar See @ref{Mapping Functions}. @item mapconcat See @ref{Mapping Functions}. @item undefined See @ref{Key Lookup}. @end table