A Lisp program is composed mainly of Lisp functions. This chapter explains what functions are, how they accept arguments, and how to define them.
In a general sense, a function is a rule for carrying on a computation given several values called 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 Emacs Lisp and for other function-like objects.
car or append. These functions are also called
built-in functions or subrs. (Special forms are also
considered primitives.)
Usually the reason we implement a function as a primitive is either
because it is fundamental, because it provides a low-level interface to
operating system services, or because it needs to run fast. Primitives
can be modified or added only by changing the C sources and recompiling
the editor. See section Writing Emacs Primitives.
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 (see section 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 M-x.
The symbol is a function as well if the definition is a function.
See section Command Loop Overview.
t if object is any kind of function,
or a special form or macro.
t if object is a built-in function
(i.e., a Lisp primitive).
(subrp 'message) ; message is a symbol,
=> nil ; not a subr object.
(subrp (symbol-function 'message))
=> t
t if object is a byte-code
function. For example:
(byte-code-function-p (symbol-function 'next-line))
=> t
A function written in Lisp is a list that looks like this:
(lambda (arg-variables...) [documentation-string] [interactive-declaration] body-forms...)
Such a list is called a lambda expression. In Emacs Lisp, it actually is valid as an expression--it evaluates to itself. In some other Lisp dialects, a lambda expression is not a valid expression at all. In either case, its main use is not to be evaluated as an expression, but to be called as a function.
The first element of a lambda expression is always the symbol
lambda. This indicates that the list represents a function. The
reason functions are defined to start with 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 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. See section Local Variables.
The documentation string is a Lisp string object placed within the function definition to describe the function for the Emacs help facilities. See section Documentation Strings of Functions.
The interactive declaration is a list of the form (interactive
code-string). This declares how to provide arguments if the
function is used interactively. Functions with this declaration are called
commands; they can be called using M-x or bound to a key.
Functions not intended to be called in this way should not have interactive
declarations. See section Defining Commands, for how to write an interactive
declaration.
The rest of the elements are the 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.
Consider for example the following function:
(lambda (a b c) (+ a b c))
We can call this function by writing it as the CAR of an expression, like this:
((lambda (a b c) (+ a b c)) 1 2 3)
This call evaluates the body of the lambda expression with the variable
a bound to 1, b bound to 2, and 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:
((lambda (a b c) (+ a b c)) 1 (* 2 3) (- 5 4))
This evaluates the arguments 1, (* 2 3), and (- 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 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 let
(see section Local Variables). And 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 (see section Anonymous Functions).
However, calls to explicit lambda expressions were very useful in the
old days of Lisp, before the special form let was invented. At
that time, they were the only way to bind and initialize local
variables.
Our simple sample function, (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 wrong-number-of-arguments error.
It is often convenient to write a function that allows certain
arguments to be omitted. For example, the function substring
accepts three arguments--a string, the start index and the end
index--but the third argument defaults to the 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 list
and + do.
To specify optional arguments that may be omitted when a function
is called, simply include the keyword &optional before the optional
arguments. To specify a list of zero or more extra arguments, include the
keyword &rest before one final argument.
Thus, the complete syntax for an argument list is as follows:
(required-vars... [&optional optional-vars...] [&rest rest-var])
The square brackets indicate that the &optional and &rest
clauses, and the variables that follow them, are optional.
A call to the function requires one actual argument for each of the
required-vars. There may be actual arguments for zero or more of
the optional-vars, and there cannot be any actual arguments beyond
that unless the lambda list uses &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 nil. There is no way for the
function to distinguish between an explicit argument of nil and
an omitted argument. However, the body of the function is free to
consider nil an abbreviation for some other meaningful value.
This is what substring does; nil as the third argument to
substring means to use the length of the string supplied.
Common Lisp note: Common Lisp allows the function to specify what default value to use when an optional argument is omitted; Emacs Lisp always uses
nil. Emacs Lisp does not support "supplied-p" variables that tell you whether an argument was explicitly passed.
For example, an argument list that looks like this:
(a b &optional c d &rest e)
binds a and b to the first two actual arguments, which are
required. If one or two more arguments are provided, c and
d are bound to them respectively; any arguments after the first
four are collected into a list and e is bound to that list. If
there are only two arguments, c is nil; if two or three
arguments, d is nil; if four arguments or fewer, e
is 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 c in the example were optional and 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 &rest argument.
Here are some examples of argument lists and proper calls:
((lambda (n) (1+ n)) ; One required:
1) ; requires exactly one argument.
=> 2
((lambda (n &optional n1) ; One required and one optional:
(if n1 (+ n n1) (1+ n))) ; 1 or 2 arguments.
1 2)
=> 3
((lambda (n &rest ns) ; One required and one rest:
(+ n (apply '+ ns))) ; 1 or more arguments.
1 2 3 4 5)
=> 15
A lambda expression may optionally have a 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 Emacs help facilities. See section Documentation, for how the documentation-string is accessed.
It is a good idea to provide documentation strings for all the functions in your program, even those that are called only from within your program. Documentation strings are like comments, except that they are easier to access.
The first line of the documentation string should stand on its own,
because 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. 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.
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
lambda, a byte-code function object, or a primitive subr-object.
However, a symbol can serve as the name of a function. This happens when you put the function in the symbol's function cell (see section 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 function definition. The procedure of using a symbol's function definition in place of the symbol is called symbol function indirection; see section Symbol Function Indirection.
In practice, nearly all functions are given names in this way and
referred to through their names. For example, the symbol car works
as a function and does what it does because the primitive subr-object
#<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
#<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 car", not
distinguishing between the symbol 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 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 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. (Some Lisp dialects, such as Scheme, do not distinguish between a symbol's value and its function definition; a symbol's value as a variable is also its function definition.) If you have not given a symbol a function definition, you cannot use it as a function; whether the symbol has a value as a variable makes no difference to this.
We usually give a name to a function when it is first created. This
is called defining a function, and it is done with the
defun special form.
defun is the usual way to define new Lisp functions. It
defines the symbol name as a function that looks like this:
(lambda argument-list . body-forms)
defun stores this lambda expression in the function cell of
name. It returns the value name, but usually we ignore this
value.
As described previously (see section Lambda Expressions),
argument-list is a list of argument names and may include the
keywords &optional and &rest. Also, the first two of the
body-forms may be a documentation string and an interactive
declaration.
There is no conflict if the same symbol name is also used as a variable, since the symbol's value cell is independent of the function cell. See section Symbol Components.
Here are some examples:
(defun foo () 5)
=> foo
(foo)
=> 5
(defun bar (a &optional b &rest c)
(list a b c))
=> bar
(bar 1 2 3 4 5)
=> (1 2 (3 4 5))
(bar 1)
=> (1 nil nil)
(bar)
error--> Wrong number of arguments.
(defun capitalize-backwards ()
"Upcase the last letter of a word."
(interactive)
(backward-word 1)
(forward-word 1)
(backward-char 1)
(capitalize-word 1))
=> capitalize-backwards
Be careful not to redefine existing functions unintentionally.
defun redefines even primitive functions such as 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.
The proper place to use 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 defalias records
which file defined the function, just like defun
(see section Unloading).
By contrast, in programs that manipulate function definitions for other
purposes, it is better to use fset, which does not keep such
records.
See also defsubst, which defines a function like defun
and tells the Lisp compiler to open-code it. See section Inline Functions.
Defining functions is only half the battle. Functions don't do anything until you call them, i.e., tell them to run. Calling a function is also known as invocation.
The most common way of invoking a function is by evaluating a list.
For example, evaluating the list (concat "a" "b") calls the
function concat with arguments "a" and "b".
See section Evaluation, for a description of evaluation.
When you write a list as an expression in your program, the function
name it calls is written in your program. This means that you choose
which function to call, and how many arguments to give it, when you
write the program. Usually that's just what you want. Occasionally you
need to compute at run time which function to call. To do that, use the
function funcall. When you also need to determine at run time
how many arguments to pass, use apply.
funcall calls function with arguments, and returns
whatever function returns.
Since funcall is a function, all of its arguments, including
function, are evaluated before funcall is called. This
means that you can use any expression to obtain the function to be
called. It also means that funcall does not see the expressions
you write for the arguments, only their values. These values are
not evaluated a second time in the act of calling function;
funcall enters the normal procedure for calling a function at the
place where the arguments have already been evaluated.
The argument 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. funcall cannot provide these because, as we saw
above, it never knows them in the first place.
(setq f 'list)
=> list
(funcall f 'x 'y 'z)
=> (x y z)
(funcall f 'x 'y '(z))
=> (x y (z))
(funcall 'and t nil)
error--> Invalid function: #<subr and>
Compare these example with the examples of apply.
apply calls function with arguments, just like
funcall but with one difference: the last of arguments is a
list of objects, which are passed to function as separate
arguments, rather than a single list. We say that apply
spreads this list so that each individual element becomes an
argument.
apply returns the result of calling function. As with
funcall, function must either be a Lisp function or a
primitive function; special forms and macros do not make sense in
apply.
(setq f 'list)
=> list
(apply f 'x 'y 'z)
error--> Wrong type argument: listp, z
(apply '+ 1 2 '(3 4))
=> 10
(apply '+ '(1 2 3 4))
=> 10
(apply 'append '((a b c) nil (x y z) nil))
=> (a b c x y z)
For an interesting example of using apply, see the description of
mapcar, in section Mapping Functions.
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 funcall or apply. Functions
that accept function arguments are often called 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:
nil.
A mapping function applies a given function to each element of a
list or other collection. Emacs Lisp has several such functions;
mapcar and mapconcat, which scan a list, are described
here. See section Creating and Interning Symbols, for the function mapatoms which
maps over the symbols in an obarray.
These mapping functions do not allow char-tables because a char-table
is a sparse array whose nominal range of indices is very large. To map
over a char-table in a way that deals properly with its sparse nature,
use the function map-char-table (see section Char-Tables).
mapcar applies function to each element of sequence
in turn, and returns a list of the results.
The argument sequence can be any kind of sequence except a char-table; that is, a list, a vector, a bool-vector, or a string. The result is always a list. The length of the result is the same as the length of sequence.
For example:
(mapcar 'car '((a b) (c d) (e f)))
=> (a c e)
(mapcar '1+ [1 2 3])
=> (2 3 4)
(mapcar 'char-to-string "abc")
=> ("a" "b" "c")
;; Call each function in my-hooks.
(mapcar 'funcall my-hooks)
(defun mapcar* (function &rest args)
"Apply FUNCTION to successive cars of all ARGS.
Return the list of results."
;; If no list is exhausted,
(if (not (memq 'nil args))
;; apply function to CARs.
(cons (apply function (mapcar 'car args))
(apply 'mapcar* function
;; Recurse for rest of elements.
(mapcar 'cdr args)))))
(mapcar* 'cons '(a b c) '(1 2 3 4))
=> ((a . 1) (b . 2) (c . 3))
mapconcat applies function to each element of
sequence: the results, which must be strings, are concatenated.
Between each pair of result strings, mapconcat inserts the string
separator. Usually separator contains a space or comma or
other suitable punctuation.
The argument function must be a function that can take one argument and return a string. The argument sequence can be any kind of sequence except a char-table; that is, a list, a vector, a bool-vector, or a string.
(mapconcat 'symbol-name
'(The cat in the hat)
" ")
=> "The cat in the hat"
(mapconcat (function (lambda (x) (format "%c" (1+ x))))
"HAL-8000"
"")
=> "IBM.9111"
In Lisp, a function is a list that starts with 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 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:
(setq silly (append '(lambda (x)) (list (list '+ (* 3 4) 'x)))) => (lambda (x) (+ 12 x))
This computes a list that looks like (lambda (x) (+ 12 x)) and
makes it the value (not the function definition!) of
silly.
Here is how we might call this function:
(funcall silly 1) => 13
(It does not work to write (silly 1), because this function
is not the function definition of silly. We have not given
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 mapcar, which applies any given function to each
element of a list.
Here we define a function change-property which
uses a function as its third argument:
(defun change-property (symbol prop function)
(let ((value (get symbol prop)))
(put symbol prop (funcall function value))))
Here we define a function that uses change-property,
passing it a function to double a number:
(defun double-property (symbol prop) (change-property symbol prop '(lambda (x) (* 2 x))))
In such cases, we usually use the special form function instead
of simple quotation to quote the anonymous function, like this:
(defun double-property (symbol prop)
(change-property symbol prop
(function (lambda (x) (* 2 x)))))
Using function instead of quote makes a difference if you
compile the function double-property. For example, if you
compile the second definition of double-property, the anonymous
function is compiled as well. By contrast, if you compile the first
definition which uses ordinary quote, the argument passed to
change-property is the precise list shown:
(lambda (x) (* x 2))
The Lisp compiler cannot assume this list is a function, even though it
looks like one, since it does not know what change-property will
do with the list. Perhaps it will check whether the CAR of the third
element is the symbol *! Using function tells the
compiler it is safe to go ahead and compile the constant function.
We sometimes write function instead of quote when
quoting the name of a function, but this usage is just a sort of
comment:
(function symbol) == (quote symbol) == 'symbol
The read syntax #' is a short-hand for using function.
For example,
#'(lambda (x) (* x x))
is equivalent to
(function (lambda (x) (* x x)))
quote. However, it serves as a
note to the Emacs Lisp compiler that function-object is intended
to be used only as a function, and therefore can safely be compiled.
Contrast this with quote, in section Quoting.
See documentation in section Access to Documentation Strings, for a
realistic example using function and an anonymous function.
The 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 indirect-function in section Symbol Function Indirection.
void-function error is
signaled.
This function does not check that the returned object is a legitimate function.
(defun bar (n) (+ n 2))
=> bar
(symbol-function 'bar)
=> (lambda (n) (+ n 2))
(fset 'baz 'bar)
=> bar
(symbol-function 'baz)
=> bar
If you have never given a symbol any function definition, we say that
that symbol's function cell is 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 void-function error.
Note that void is not the same as nil or the symbol
void. The symbols nil and 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
defun). A void function cell contains no object whatsoever.
You can test the voidness of a symbol's function definition with
fboundp. After you have given a symbol a function definition, you
can make it void once more using fmakunbound.
t if the symbol has an object in its
function cell, nil otherwise. It does not check that the object
is a legitimate function.
void-function
error. (See also makunbound, in section When a Variable is "Void".)
(defun foo (x) x)
=> foo
(foo 1)
=>1
(fmakunbound 'foo)
=> foo
(foo 1)
error--> Symbol's function definition is void: foo
There are three normal uses of this function:
defalias instead of
fset; see section Defining Functions.)
defun. For example, you can use fset
to give a symbol s1 a function definition which is another symbol
s2; then s1 serves as an alias for whatever definition
s2 presently has. (Once again use defalias instead of
fset if you think of this as the definition of s1.)
defun
were not a primitive, it could be written in Lisp (as a macro) using
fset.
Here are examples of these uses:
;; Savefoo's definition inold-foo. (fset 'old-foo (symbol-function 'foo)) ;; Make the symbolcarthe function definition ofxfirst. ;; (Most likely,defaliaswould be better thanfsethere.) (fset 'xfirst 'car) => car (xfirst '(1 2 3)) => 1 (symbol-function 'xfirst) => car (symbol-function (symbol-function 'xfirst)) => #<subr car> ;; Define a named keyboard macro. (fset 'kill-two-lines "\^u2\^k") => "\^u2\^k" ;; Here is a function that alters other functions. (defun copy-function-definition (new old) "Define NEW with the same function definition as OLD." (fset new (symbol-function old)))
When writing a function that extends a previously defined function, the following idiom is sometimes used:
(fset 'old-foo (symbol-function 'foo)) (defun foo () "Just like old-foo, except more so." (old-foo) (more-so))
This does not work properly if foo has been defined to autoload.
In such a case, when foo calls old-foo, Lisp attempts
to define old-foo by loading a file. Since this presumably
defines foo rather than 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 foo.
But it is unmodular and unclean, in any case, for a Lisp file to redefine a function defined elsewhere. It is cleaner to use the advice facility (see section Advising Emacs Lisp Functions).
You can define an inline function by using defsubst instead
of 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 Emacs, 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. (See section Macros.) But the macro would be
limited to direct use in expressions--a macro cannot be called with
apply, mapcar and so on. Also, it takes some work to
convert an ordinary function into a macro. To convert it into an inline
function is very easy; simply replace defun with 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. (See section Evaluating Macro Arguments Repeatedly.)
Inline functions can be used and open-coded later on in the same file, following the definition, just like macros.
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.
apply
autoload
call-interactively
commandp
documentation
eval
funcall
function
ignore
indirect-function
interactive
interactive.
interactive-p
mapatoms
mapcar
map-char-table
mapconcat
undefined
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