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+@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{#<subr car>} 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{#<subr car>}, 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: #<subr and>
+@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{} #<subr car>
+@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{} #<subr car>
+@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

Mercurial > hg > xemacs-beta

comparison man/lispref/functions.texi @ 0:376386a54a3c r19-14