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12. 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.

12.1 What Is a Function?  Lisp functions vs. primitives; terminology.
12.2 Lambda Expressions  How functions are expressed as Lisp objects.
12.3 Naming a Function  A symbol can serve as the name of a function.
12.4 Defining Functions  Lisp expressions for defining functions.
12.5 Calling Functions  How to use an existing function.
12.6 Mapping Functions  Applying a function to each element of a list, etc.
12.7 Anonymous Functions  Lambda expressions are functions with no names.
12.8 Accessing Function Cell Contents  Accessing or setting the function definition of a symbol.
12.9 Inline Functions  Defining functions that the compiler will open code.
12.10 Other Topics Related to Functions  Cross-references to specific Lisp primitives that have a special bearing on how functions work.

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12.1 What Is a Function?

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.

In Emacs Lisp, a 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.

A primitive is a function callable from Lisp that is written in C, such as 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 E.5 Writing Emacs Primitives.

lambda expression
A lambda expression is a function written in Lisp. These are described in the following section. See section 12.2 Lambda Expressions.

special form
A 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 10. Control Structures.

A 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. See section 13. Macros, for how to define and use macros.

A command is an object that 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 21.2 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 21.1 Command Loop Overview.

keystroke command
A 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.

byte-code function
A byte-code function is a function that has been compiled by the byte compiler. See section 2.3.16 Byte-Code Function Type.

Function: functionp object
This function returns t if object is any kind of function, or a special form or macro.

Function: subrp object
This function returns 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

Function: byte-code-function-p object
This function returns t if object is a byte-code function. For example:

(byte-code-function-p (symbol-function 'next-line))
     => t

Function: subr-arity subr
This function provides information about the argument list of a primitive, subr. The returned value is a pair (min . max). min is the minimum number of args. max is the maximum number or the symbol many, for a function with &rest arguments, or the symbol unevalled if subr is a special form.

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12.2 Lambda Expressions

A function written in Lisp is a list that looks like this:

(lambda (arg-variables...)

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.

12.2.1 Components of a Lambda Expression  The parts of a lambda expression.
12.2.2 A Simple Lambda-Expression Example  A simple example.
12.2.3 Other Features of Argument Lists  Details and special features of argument lists.
12.2.4 Documentation Strings of Functions  How to put documentation in a function.

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12.2.1 Components of a Lambda Expression

A function written in Lisp (a "lambda expression") is a list that looks like this:

(lambda (arg-variables...)

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 11.3 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 12.2.4 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 21.2 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.

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12.2.2 A Simple Lambda-Expression Example

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 11.3 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 12.7 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.

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12.2.3 Other Features of Argument Lists

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:

 [&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? Would it be used for the c, or for d? One can argue for both possibilities. 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

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12.2.4 Documentation Strings of Functions

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 24. 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.

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12.3 Naming a Function

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 8.1 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 9.2.4 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.

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12.4 Defining Functions

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.

Special Form: defun name argument-list body-forms
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 12.2 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 8.1 Symbol Components.

Here are some examples:

(defun foo () 5)
     => 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)
error--> Wrong number of arguments.

(defun capitalize-backwards ()
  "Upcase the last letter of a word."
  (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.

Function: defalias name definition
This special form defines the symbol name as a function, with definition definition (which can be any valid Lisp function).

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 15.7 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 12.9 Inline Functions.

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12.5 Calling 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 9. 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.

Function: funcall function &rest arguments
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 examples with the examples of apply.

Function: apply function &rest arguments
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 12.6 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:

Function: identity arg
This function returns arg and has no side effects.

Function: ignore &rest args
This function ignores any arguments and returns nil.

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12.6 Mapping Functions

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 8.3 Creating and Interning Symbols, for the function mapatoms which maps over the symbols in an obarray. See section 7.2 Hash Table Access, for the function maphash which maps over key/value associations in a hash table.

These mapping functions do not allow char-tables because a char-table is a sparse array whose nominal range of indices is very large. To map over a char-table in a way that deals properly with its sparse nature, use the function map-char-table (see section 6.6 Char-Tables).

Function: mapcar function sequence
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))

Function: mapc function sequence
mapc is like mapcar except that function is used for side-effects only--the values it returns are ignored, not collected into a list. mapc always returns sequence.

Function: mapconcat function sequence separator
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))))
     => "IBM.9111"

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12.7 Anonymous Functions

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.

Nowadays it is possible to omit function entirely, like this:

(defun double-property (symbol prop)
  (change-property symbol prop (lambda (x) (* 2 x))))

This is because lambda itself implies 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)))

Special Form: function function-object
This special form returns function-object without evaluating it. In this, it is equivalent to 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 9.3 Quoting.

See documentation in 24.2 Access to Documentation Strings, for a realistic example using function and an anonymous function.

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12.8 Accessing Function Cell Contents

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 9.2.4 Symbol Function Indirection.

Function: symbol-function symbol
This returns the object in the function cell of symbol. If the symbol's function cell is void, a 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.

Function: fboundp symbol
This function returns t if the symbol has an object in its function cell, nil otherwise. It does not check that the object is a legitimate function.

Function: fmakunbound symbol
This function makes symbol's function cell void, so that a subsequent attempt to access this cell will cause a void-function error. (See also makunbound, in 11.4 When a Variable is "Void".)

(defun foo (x) x)
     => foo
(foo 1)
(fmakunbound 'foo)
     => foo
(foo 1)
error--> Symbol's function definition is void: foo

Function: fset symbol definition
This function stores definition in the function cell of symbol. The result is definition. Normally definition should be a function or the name of a function, but this is not checked. The argument symbol is an ordinary evaluated argument.

There are three normal uses of this function:

Here are examples of these uses:

;; Save foo's definition in old-foo.
(fset 'old-foo (symbol-function 'foo))

;; Make the symbol car the function definition of xfirst.
;; (Most likely, defalias would be better than fset here.)
(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."

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 17. Advising Emacs Lisp Functions).

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12.9 Inline 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 13. 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 13.6.2 Evaluating Macro Arguments Repeatedly.)

Inline functions can be used and open-coded later on in the same file, following the definition, just like macros.

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12.10 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.

See 12.5 Calling Functions.

See 15.4 Autoload.

See 21.3 Interactive Call.

See 21.3 Interactive Call.

See 24.2 Access to Documentation Strings.

See 9.4 Eval.

See 12.5 Calling Functions.

See 12.7 Anonymous Functions.

See 12.5 Calling Functions.

See 9.2.4 Symbol Function Indirection.

See 21.2.1 Using interactive.

See 21.3 Interactive Call.

See 8.3 Creating and Interning Symbols.

See 12.6 Mapping Functions.

See 6.6 Char-Tables.

See 12.6 Mapping Functions.

See 22.7 Key Lookup.

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