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1 | @c -*-texinfo-*- |
2 | @c This is part of the GNU Emacs Lisp Reference Manual. | |
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3 | @c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999 |
4 | @c Free Software Foundation, Inc. | |
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5 | @c See the file elisp.texi for copying conditions. |
6 | @setfilename ../info/functions | |
7 | @node Functions, Macros, Variables, Top | |
8 | @chapter Functions | |
9 | ||
10 | A Lisp program is composed mainly of Lisp functions. This chapter | |
11 | explains what functions are, how they accept arguments, and how to | |
12 | define them. | |
13 | ||
14 | @menu | |
15 | * What Is a Function:: Lisp functions vs. primitives; terminology. | |
16 | * Lambda Expressions:: How functions are expressed as Lisp objects. | |
17 | * Function Names:: A symbol can serve as the name of a function. | |
18 | * Defining Functions:: Lisp expressions for defining functions. | |
19 | * Calling Functions:: How to use an existing function. | |
20 | * Mapping Functions:: Applying a function to each element of a list, etc. | |
21 | * Anonymous Functions:: Lambda expressions are functions with no names. | |
22 | * Function Cells:: Accessing or setting the function definition | |
23 | of a symbol. | |
24 | * Inline Functions:: Defining functions that the compiler will open code. | |
25 | * Related Topics:: Cross-references to specific Lisp primitives | |
26 | that have a special bearing on how functions work. | |
27 | @end menu | |
28 | ||
29 | @node What Is a Function | |
30 | @section What Is a Function? | |
31 | ||
32 | In a general sense, a function is a rule for carrying on a computation | |
33 | given several values called @dfn{arguments}. The result of the | |
34 | computation is called the value of the function. The computation can | |
35 | also have side effects: lasting changes in the values of variables or | |
36 | the contents of data structures. | |
37 | ||
38 | Here are important terms for functions in Emacs Lisp and for other | |
39 | function-like objects. | |
40 | ||
41 | @table @dfn | |
42 | @item function | |
43 | @cindex function | |
44 | In Emacs Lisp, a @dfn{function} is anything that can be applied to | |
45 | arguments in a Lisp program. In some cases, we use it more | |
46 | specifically to mean a function written in Lisp. Special forms and | |
47 | macros are not functions. | |
48 | ||
49 | @item primitive | |
50 | @cindex primitive | |
51 | @cindex subr | |
52 | @cindex built-in function | |
53 | A @dfn{primitive} is a function callable from Lisp that is written in C, | |
54 | such as @code{car} or @code{append}. These functions are also called | |
55 | @dfn{built-in} functions or @dfn{subrs}. (Special forms are also | |
56 | considered primitives.) | |
57 | ||
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58 | Usually the reason we implement a function as a primitive is either |
59 | because it is fundamental, because it provides a low-level interface to | |
60 | operating system services, or because it needs to run fast. Primitives | |
61 | can be modified or added only by changing the C sources and recompiling | |
62 | the editor. See @ref{Writing Emacs Primitives}. | |
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63 | |
64 | @item lambda expression | |
65 | A @dfn{lambda expression} is a function written in Lisp. | |
66 | These are described in the following section. | |
37680279 | 67 | @ifnottex |
9c52bf47 | 68 | @xref{Lambda Expressions}. |
37680279 | 69 | @end ifnottex |
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70 | |
71 | @item special form | |
72 | A @dfn{special form} is a primitive that is like a function but does not | |
73 | evaluate all of its arguments in the usual way. It may evaluate only | |
74 | some of the arguments, or may evaluate them in an unusual order, or | |
75 | several times. Many special forms are described in @ref{Control | |
76 | Structures}. | |
77 | ||
78 | @item macro | |
79 | @cindex macro | |
80 | A @dfn{macro} is a construct defined in Lisp by the programmer. It | |
81 | differs from a function in that it translates a Lisp expression that you | |
82 | write into an equivalent expression to be evaluated instead of the | |
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83 | original expression. Macros enable Lisp programmers to do the sorts of |
84 | things that special forms can do. @xref{Macros}, for how to define and | |
85 | use macros. | |
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86 | |
87 | @item command | |
88 | @cindex command | |
89 | A @dfn{command} is an object that @code{command-execute} can invoke; it | |
90 | is a possible definition for a key sequence. Some functions are | |
91 | commands; a function written in Lisp is a command if it contains an | |
92 | interactive declaration (@pxref{Defining Commands}). Such a function | |
93 | can be called from Lisp expressions like other functions; in this case, | |
94 | the fact that the function is a command makes no difference. | |
95 | ||
96 | Keyboard macros (strings and vectors) are commands also, even though | |
97 | they are not functions. A symbol is a command if its function | |
98 | definition is a command; such symbols can be invoked with @kbd{M-x}. | |
99 | The symbol is a function as well if the definition is a function. | |
100 | @xref{Command Overview}. | |
101 | ||
102 | @item keystroke command | |
103 | @cindex keystroke command | |
104 | A @dfn{keystroke command} is a command that is bound to a key sequence | |
105 | (typically one to three keystrokes). The distinction is made here | |
106 | merely to avoid confusion with the meaning of ``command'' in non-Emacs | |
107 | editors; for Lisp programs, the distinction is normally unimportant. | |
108 | ||
109 | @item byte-code function | |
110 | A @dfn{byte-code function} is a function that has been compiled by the | |
111 | byte compiler. @xref{Byte-Code Type}. | |
112 | @end table | |
113 | ||
f9f59935 | 114 | @defun functionp object |
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115 | This function returns @code{t} if @var{object} is any kind of function, |
116 | or a special form or macro. | |
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117 | @end defun |
118 | ||
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119 | @defun subrp object |
120 | This function returns @code{t} if @var{object} is a built-in function | |
121 | (i.e., a Lisp primitive). | |
122 | ||
123 | @example | |
124 | @group | |
125 | (subrp 'message) ; @r{@code{message} is a symbol,} | |
126 | @result{} nil ; @r{not a subr object.} | |
127 | @end group | |
128 | @group | |
129 | (subrp (symbol-function 'message)) | |
130 | @result{} t | |
131 | @end group | |
132 | @end example | |
133 | @end defun | |
134 | ||
135 | @defun byte-code-function-p object | |
136 | This function returns @code{t} if @var{object} is a byte-code | |
137 | function. For example: | |
138 | ||
139 | @example | |
140 | @group | |
141 | (byte-code-function-p (symbol-function 'next-line)) | |
142 | @result{} t | |
143 | @end group | |
144 | @end example | |
145 | @end defun | |
146 | ||
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147 | @defun subr-arity subr |
148 | @tindex subr-arity | |
149 | This function provides information about the argument list of a | |
150 | primitive, @var{subr}. The returned value is a pair | |
151 | @code{(@var{min} . @var{max})}. @var{min} is the minimum number of | |
152 | args. @var{max} is the maximum number or the symbol @code{many}, for a | |
153 | function with @code{&rest} arguments, or the symbol @code{unevalled} if | |
154 | @var{subr} is a special form. | |
155 | @end defun | |
156 | ||
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157 | @node Lambda Expressions |
158 | @section Lambda Expressions | |
159 | @cindex lambda expression | |
160 | ||
161 | A function written in Lisp is a list that looks like this: | |
162 | ||
163 | @example | |
164 | (lambda (@var{arg-variables}@dots{}) | |
165 | @r{[}@var{documentation-string}@r{]} | |
166 | @r{[}@var{interactive-declaration}@r{]} | |
167 | @var{body-forms}@dots{}) | |
168 | @end example | |
169 | ||
170 | @noindent | |
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171 | Such a list is called a @dfn{lambda expression}. In Emacs Lisp, it |
172 | actually is valid as an expression---it evaluates to itself. In some | |
173 | other Lisp dialects, a lambda expression is not a valid expression at | |
174 | all. In either case, its main use is not to be evaluated as an | |
175 | expression, but to be called as a function. | |
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176 | |
177 | @menu | |
178 | * Lambda Components:: The parts of a lambda expression. | |
179 | * Simple Lambda:: A simple example. | |
180 | * Argument List:: Details and special features of argument lists. | |
181 | * Function Documentation:: How to put documentation in a function. | |
182 | @end menu | |
183 | ||
184 | @node Lambda Components | |
185 | @subsection Components of a Lambda Expression | |
186 | ||
37680279 | 187 | @ifnottex |
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188 | |
189 | A function written in Lisp (a ``lambda expression'') is a list that | |
190 | looks like this: | |
191 | ||
192 | @example | |
193 | (lambda (@var{arg-variables}@dots{}) | |
194 | [@var{documentation-string}] | |
195 | [@var{interactive-declaration}] | |
196 | @var{body-forms}@dots{}) | |
197 | @end example | |
37680279 | 198 | @end ifnottex |
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199 | |
200 | @cindex lambda list | |
201 | The first element of a lambda expression is always the symbol | |
202 | @code{lambda}. This indicates that the list represents a function. The | |
203 | reason functions are defined to start with @code{lambda} is so that | |
204 | other lists, intended for other uses, will not accidentally be valid as | |
205 | functions. | |
206 | ||
f9f59935 | 207 | The second element is a list of symbols---the argument variable names. |
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208 | This is called the @dfn{lambda list}. When a Lisp function is called, |
209 | the argument values are matched up against the variables in the lambda | |
210 | list, which are given local bindings with the values provided. | |
211 | @xref{Local Variables}. | |
212 | ||
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213 | The documentation string is a Lisp string object placed within the |
214 | function definition to describe the function for the Emacs help | |
215 | facilities. @xref{Function Documentation}. | |
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216 | |
217 | The interactive declaration is a list of the form @code{(interactive | |
218 | @var{code-string})}. This declares how to provide arguments if the | |
219 | function is used interactively. Functions with this declaration are called | |
220 | @dfn{commands}; they can be called using @kbd{M-x} or bound to a key. | |
221 | Functions not intended to be called in this way should not have interactive | |
222 | declarations. @xref{Defining Commands}, for how to write an interactive | |
223 | declaration. | |
224 | ||
225 | @cindex body of function | |
226 | The rest of the elements are the @dfn{body} of the function: the Lisp | |
227 | code to do the work of the function (or, as a Lisp programmer would say, | |
228 | ``a list of Lisp forms to evaluate''). The value returned by the | |
229 | function is the value returned by the last element of the body. | |
230 | ||
231 | @node Simple Lambda | |
232 | @subsection A Simple Lambda-Expression Example | |
233 | ||
234 | Consider for example the following function: | |
235 | ||
236 | @example | |
237 | (lambda (a b c) (+ a b c)) | |
238 | @end example | |
239 | ||
240 | @noindent | |
241 | We can call this function by writing it as the @sc{car} of an | |
242 | expression, like this: | |
243 | ||
244 | @example | |
245 | @group | |
246 | ((lambda (a b c) (+ a b c)) | |
247 | 1 2 3) | |
248 | @end group | |
249 | @end example | |
250 | ||
251 | @noindent | |
252 | This call evaluates the body of the lambda expression with the variable | |
253 | @code{a} bound to 1, @code{b} bound to 2, and @code{c} bound to 3. | |
254 | Evaluation of the body adds these three numbers, producing the result 6; | |
255 | therefore, this call to the function returns the value 6. | |
256 | ||
257 | Note that the arguments can be the results of other function calls, as in | |
258 | this example: | |
259 | ||
260 | @example | |
261 | @group | |
262 | ((lambda (a b c) (+ a b c)) | |
263 | 1 (* 2 3) (- 5 4)) | |
264 | @end group | |
265 | @end example | |
266 | ||
267 | @noindent | |
268 | This evaluates the arguments @code{1}, @code{(* 2 3)}, and @code{(- 5 | |
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269 | 4)} from left to right. Then it applies the lambda expression to the |
270 | argument values 1, 6 and 1 to produce the value 8. | |
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271 | |
272 | It is not often useful to write a lambda expression as the @sc{car} of | |
273 | a form in this way. You can get the same result, of making local | |
274 | variables and giving them values, using the special form @code{let} | |
275 | (@pxref{Local Variables}). And @code{let} is clearer and easier to use. | |
276 | In practice, lambda expressions are either stored as the function | |
277 | definitions of symbols, to produce named functions, or passed as | |
278 | arguments to other functions (@pxref{Anonymous Functions}). | |
279 | ||
280 | However, calls to explicit lambda expressions were very useful in the | |
281 | old days of Lisp, before the special form @code{let} was invented. At | |
282 | that time, they were the only way to bind and initialize local | |
283 | variables. | |
284 | ||
285 | @node Argument List | |
f9f59935 | 286 | @subsection Other Features of Argument Lists |
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287 | @kindex wrong-number-of-arguments |
288 | @cindex argument binding | |
289 | @cindex binding arguments | |
290 | ||
291 | Our simple sample function, @code{(lambda (a b c) (+ a b c))}, | |
292 | specifies three argument variables, so it must be called with three | |
293 | arguments: if you try to call it with only two arguments or four | |
294 | arguments, you get a @code{wrong-number-of-arguments} error. | |
295 | ||
296 | It is often convenient to write a function that allows certain | |
297 | arguments to be omitted. For example, the function @code{substring} | |
298 | accepts three arguments---a string, the start index and the end | |
299 | index---but the third argument defaults to the @var{length} of the | |
300 | string if you omit it. It is also convenient for certain functions to | |
f25df2ab | 301 | accept an indefinite number of arguments, as the functions @code{list} |
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302 | and @code{+} do. |
303 | ||
304 | @cindex optional arguments | |
305 | @cindex rest arguments | |
306 | @kindex &optional | |
307 | @kindex &rest | |
308 | To specify optional arguments that may be omitted when a function | |
309 | is called, simply include the keyword @code{&optional} before the optional | |
310 | arguments. To specify a list of zero or more extra arguments, include the | |
311 | keyword @code{&rest} before one final argument. | |
312 | ||
313 | Thus, the complete syntax for an argument list is as follows: | |
314 | ||
315 | @example | |
316 | @group | |
317 | (@var{required-vars}@dots{} | |
318 | @r{[}&optional @var{optional-vars}@dots{}@r{]} | |
319 | @r{[}&rest @var{rest-var}@r{]}) | |
320 | @end group | |
321 | @end example | |
322 | ||
323 | @noindent | |
324 | The square brackets indicate that the @code{&optional} and @code{&rest} | |
325 | clauses, and the variables that follow them, are optional. | |
326 | ||
327 | A call to the function requires one actual argument for each of the | |
328 | @var{required-vars}. There may be actual arguments for zero or more of | |
329 | the @var{optional-vars}, and there cannot be any actual arguments beyond | |
330 | that unless the lambda list uses @code{&rest}. In that case, there may | |
331 | be any number of extra actual arguments. | |
332 | ||
333 | If actual arguments for the optional and rest variables are omitted, | |
f25df2ab | 334 | then they always default to @code{nil}. There is no way for the |
9c52bf47 | 335 | function to distinguish between an explicit argument of @code{nil} and |
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336 | an omitted argument. However, the body of the function is free to |
337 | consider @code{nil} an abbreviation for some other meaningful value. | |
338 | This is what @code{substring} does; @code{nil} as the third argument to | |
339 | @code{substring} means to use the length of the string supplied. | |
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340 | |
341 | @cindex CL note---default optional arg | |
342 | @quotation | |
343 | @b{Common Lisp note:} Common Lisp allows the function to specify what | |
344 | default value to use when an optional argument is omitted; Emacs Lisp | |
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345 | always uses @code{nil}. Emacs Lisp does not support ``supplied-p'' |
346 | variables that tell you whether an argument was explicitly passed. | |
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347 | @end quotation |
348 | ||
349 | For example, an argument list that looks like this: | |
350 | ||
351 | @example | |
352 | (a b &optional c d &rest e) | |
353 | @end example | |
354 | ||
355 | @noindent | |
356 | binds @code{a} and @code{b} to the first two actual arguments, which are | |
357 | required. If one or two more arguments are provided, @code{c} and | |
358 | @code{d} are bound to them respectively; any arguments after the first | |
359 | four are collected into a list and @code{e} is bound to that list. If | |
360 | there are only two arguments, @code{c} is @code{nil}; if two or three | |
361 | arguments, @code{d} is @code{nil}; if four arguments or fewer, @code{e} | |
362 | is @code{nil}. | |
363 | ||
364 | There is no way to have required arguments following optional | |
365 | ones---it would not make sense. To see why this must be so, suppose | |
366 | that @code{c} in the example were optional and @code{d} were required. | |
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367 | Suppose three actual arguments are given; which variable would the |
368 | third argument be for? Would it be used for the @var{c}, or for | |
369 | @var{d}? One can argue for both possibilities. Similarly, it makes | |
370 | no sense to have any more arguments (either required or optional) | |
371 | after a @code{&rest} argument. | |
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372 | |
373 | Here are some examples of argument lists and proper calls: | |
374 | ||
375 | @smallexample | |
376 | ((lambda (n) (1+ n)) ; @r{One required:} | |
377 | 1) ; @r{requires exactly one argument.} | |
378 | @result{} 2 | |
379 | ((lambda (n &optional n1) ; @r{One required and one optional:} | |
380 | (if n1 (+ n n1) (1+ n))) ; @r{1 or 2 arguments.} | |
381 | 1 2) | |
382 | @result{} 3 | |
383 | ((lambda (n &rest ns) ; @r{One required and one rest:} | |
384 | (+ n (apply '+ ns))) ; @r{1 or more arguments.} | |
385 | 1 2 3 4 5) | |
386 | @result{} 15 | |
387 | @end smallexample | |
388 | ||
389 | @node Function Documentation | |
390 | @subsection Documentation Strings of Functions | |
391 | @cindex documentation of function | |
392 | ||
393 | A lambda expression may optionally have a @dfn{documentation string} just | |
394 | after the lambda list. This string does not affect execution of the | |
395 | function; it is a kind of comment, but a systematized comment which | |
396 | actually appears inside the Lisp world and can be used by the Emacs help | |
397 | facilities. @xref{Documentation}, for how the @var{documentation-string} is | |
398 | accessed. | |
399 | ||
bfe721d1 | 400 | It is a good idea to provide documentation strings for all the |
969fe9b5 | 401 | functions in your program, even those that are called only from within |
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402 | your program. Documentation strings are like comments, except that they |
403 | are easier to access. | |
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404 | |
405 | The first line of the documentation string should stand on its own, | |
406 | because @code{apropos} displays just this first line. It should consist | |
407 | of one or two complete sentences that summarize the function's purpose. | |
408 | ||
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409 | The start of the documentation string is usually indented in the source file, |
410 | but since these spaces come before the starting double-quote, they are not part of | |
9c52bf47 | 411 | the string. Some people make a practice of indenting any additional |
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412 | lines of the string so that the text lines up in the program source. |
413 | @emph{This is a mistake.} The indentation of the following lines is | |
414 | inside the string; what looks nice in the source code will look ugly | |
415 | when displayed by the help commands. | |
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416 | |
417 | You may wonder how the documentation string could be optional, since | |
418 | there are required components of the function that follow it (the body). | |
419 | Since evaluation of a string returns that string, without any side effects, | |
420 | it has no effect if it is not the last form in the body. Thus, in | |
421 | practice, there is no confusion between the first form of the body and the | |
422 | documentation string; if the only body form is a string then it serves both | |
423 | as the return value and as the documentation. | |
424 | ||
425 | @node Function Names | |
426 | @section Naming a Function | |
427 | @cindex function definition | |
428 | @cindex named function | |
429 | @cindex function name | |
430 | ||
431 | In most computer languages, every function has a name; the idea of a | |
432 | function without a name is nonsensical. In Lisp, a function in the | |
433 | strictest sense has no name. It is simply a list whose first element is | |
969fe9b5 | 434 | @code{lambda}, a byte-code function object, or a primitive subr-object. |
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435 | |
436 | However, a symbol can serve as the name of a function. This happens | |
437 | when you put the function in the symbol's @dfn{function cell} | |
438 | (@pxref{Symbol Components}). Then the symbol itself becomes a valid, | |
439 | callable function, equivalent to the list or subr-object that its | |
440 | function cell refers to. The contents of the function cell are also | |
441 | called the symbol's @dfn{function definition}. The procedure of using a | |
442 | symbol's function definition in place of the symbol is called | |
443 | @dfn{symbol function indirection}; see @ref{Function Indirection}. | |
444 | ||
445 | In practice, nearly all functions are given names in this way and | |
446 | referred to through their names. For example, the symbol @code{car} works | |
447 | as a function and does what it does because the primitive subr-object | |
448 | @code{#<subr car>} is stored in its function cell. | |
449 | ||
450 | We give functions names because it is convenient to refer to them by | |
451 | their names in Lisp expressions. For primitive subr-objects such as | |
452 | @code{#<subr car>}, names are the only way you can refer to them: there | |
453 | is no read syntax for such objects. For functions written in Lisp, the | |
454 | name is more convenient to use in a call than an explicit lambda | |
455 | expression. Also, a function with a name can refer to itself---it can | |
456 | be recursive. Writing the function's name in its own definition is much | |
457 | more convenient than making the function definition point to itself | |
458 | (something that is not impossible but that has various disadvantages in | |
459 | practice). | |
460 | ||
461 | We often identify functions with the symbols used to name them. For | |
462 | example, we often speak of ``the function @code{car}'', not | |
463 | distinguishing between the symbol @code{car} and the primitive | |
464 | subr-object that is its function definition. For most purposes, there | |
465 | is no need to distinguish. | |
466 | ||
467 | Even so, keep in mind that a function need not have a unique name. While | |
468 | a given function object @emph{usually} appears in the function cell of only | |
469 | one symbol, this is just a matter of convenience. It is easy to store | |
470 | it in several symbols using @code{fset}; then each of the symbols is | |
471 | equally well a name for the same function. | |
472 | ||
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473 | A symbol used as a function name may also be used as a variable; these |
474 | two uses of a symbol are independent and do not conflict. (Some Lisp | |
475 | dialects, such as Scheme, do not distinguish between a symbol's value | |
476 | and its function definition; a symbol's value as a variable is also its | |
477 | function definition.) If you have not given a symbol a function | |
478 | definition, you cannot use it as a function; whether the symbol has a | |
479 | value as a variable makes no difference to this. | |
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480 | |
481 | @node Defining Functions | |
05fd2b65 | 482 | @section Defining Functions |
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483 | @cindex defining a function |
484 | ||
485 | We usually give a name to a function when it is first created. This | |
486 | is called @dfn{defining a function}, and it is done with the | |
487 | @code{defun} special form. | |
488 | ||
489 | @defspec defun name argument-list body-forms | |
490 | @code{defun} is the usual way to define new Lisp functions. It | |
491 | defines the symbol @var{name} as a function that looks like this: | |
492 | ||
493 | @example | |
494 | (lambda @var{argument-list} . @var{body-forms}) | |
495 | @end example | |
496 | ||
497 | @code{defun} stores this lambda expression in the function cell of | |
498 | @var{name}. It returns the value @var{name}, but usually we ignore this | |
499 | value. | |
500 | ||
501 | As described previously (@pxref{Lambda Expressions}), | |
502 | @var{argument-list} is a list of argument names and may include the | |
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503 | keywords @code{&optional} and @code{&rest}. Also, the first two of the |
504 | @var{body-forms} may be a documentation string and an interactive | |
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505 | declaration. |
506 | ||
507 | There is no conflict if the same symbol @var{name} is also used as a | |
508 | variable, since the symbol's value cell is independent of the function | |
509 | cell. @xref{Symbol Components}. | |
510 | ||
511 | Here are some examples: | |
512 | ||
513 | @example | |
514 | @group | |
515 | (defun foo () 5) | |
516 | @result{} foo | |
517 | @end group | |
518 | @group | |
519 | (foo) | |
520 | @result{} 5 | |
521 | @end group | |
522 | ||
523 | @group | |
524 | (defun bar (a &optional b &rest c) | |
525 | (list a b c)) | |
526 | @result{} bar | |
527 | @end group | |
528 | @group | |
529 | (bar 1 2 3 4 5) | |
530 | @result{} (1 2 (3 4 5)) | |
531 | @end group | |
532 | @group | |
533 | (bar 1) | |
534 | @result{} (1 nil nil) | |
535 | @end group | |
536 | @group | |
537 | (bar) | |
538 | @error{} Wrong number of arguments. | |
539 | @end group | |
540 | ||
541 | @group | |
542 | (defun capitalize-backwards () | |
543 | "Upcase the last letter of a word." | |
544 | (interactive) | |
545 | (backward-word 1) | |
546 | (forward-word 1) | |
547 | (backward-char 1) | |
548 | (capitalize-word 1)) | |
549 | @result{} capitalize-backwards | |
550 | @end group | |
551 | @end example | |
552 | ||
553 | Be careful not to redefine existing functions unintentionally. | |
554 | @code{defun} redefines even primitive functions such as @code{car} | |
555 | without any hesitation or notification. Redefining a function already | |
556 | defined is often done deliberately, and there is no way to distinguish | |
557 | deliberate redefinition from unintentional redefinition. | |
558 | @end defspec | |
559 | ||
560 | @defun defalias name definition | |
561 | This special form defines the symbol @var{name} as a function, with | |
f25df2ab | 562 | definition @var{definition} (which can be any valid Lisp function). |
bfe721d1 KH |
563 | |
564 | The proper place to use @code{defalias} is where a specific function | |
565 | name is being defined---especially where that name appears explicitly in | |
566 | the source file being loaded. This is because @code{defalias} records | |
567 | which file defined the function, just like @code{defun} | |
568 | (@pxref{Unloading}). | |
569 | ||
570 | By contrast, in programs that manipulate function definitions for other | |
571 | purposes, it is better to use @code{fset}, which does not keep such | |
572 | records. | |
9c52bf47 KH |
573 | @end defun |
574 | ||
bfe721d1 KH |
575 | See also @code{defsubst}, which defines a function like @code{defun} |
576 | and tells the Lisp compiler to open-code it. @xref{Inline Functions}. | |
577 | ||
9c52bf47 KH |
578 | @node Calling Functions |
579 | @section Calling Functions | |
580 | @cindex function invocation | |
581 | @cindex calling a function | |
582 | ||
583 | Defining functions is only half the battle. Functions don't do | |
584 | anything until you @dfn{call} them, i.e., tell them to run. Calling a | |
585 | function is also known as @dfn{invocation}. | |
586 | ||
f25df2ab RS |
587 | The most common way of invoking a function is by evaluating a list. |
588 | For example, evaluating the list @code{(concat "a" "b")} calls the | |
589 | function @code{concat} with arguments @code{"a"} and @code{"b"}. | |
590 | @xref{Evaluation}, for a description of evaluation. | |
9c52bf47 KH |
591 | |
592 | When you write a list as an expression in your program, the function | |
f9f59935 RS |
593 | name it calls is written in your program. This means that you choose |
594 | which function to call, and how many arguments to give it, when you | |
595 | write the program. Usually that's just what you want. Occasionally you | |
596 | need to compute at run time which function to call. To do that, use the | |
969fe9b5 | 597 | function @code{funcall}. When you also need to determine at run time |
a9f0a989 | 598 | how many arguments to pass, use @code{apply}. |
9c52bf47 KH |
599 | |
600 | @defun funcall function &rest arguments | |
601 | @code{funcall} calls @var{function} with @var{arguments}, and returns | |
602 | whatever @var{function} returns. | |
603 | ||
604 | Since @code{funcall} is a function, all of its arguments, including | |
605 | @var{function}, are evaluated before @code{funcall} is called. This | |
606 | means that you can use any expression to obtain the function to be | |
607 | called. It also means that @code{funcall} does not see the expressions | |
608 | you write for the @var{arguments}, only their values. These values are | |
609 | @emph{not} evaluated a second time in the act of calling @var{function}; | |
610 | @code{funcall} enters the normal procedure for calling a function at the | |
611 | place where the arguments have already been evaluated. | |
612 | ||
613 | The argument @var{function} must be either a Lisp function or a | |
614 | primitive function. Special forms and macros are not allowed, because | |
615 | they make sense only when given the ``unevaluated'' argument | |
616 | expressions. @code{funcall} cannot provide these because, as we saw | |
617 | above, it never knows them in the first place. | |
618 | ||
619 | @example | |
620 | @group | |
621 | (setq f 'list) | |
622 | @result{} list | |
623 | @end group | |
624 | @group | |
625 | (funcall f 'x 'y 'z) | |
626 | @result{} (x y z) | |
627 | @end group | |
628 | @group | |
629 | (funcall f 'x 'y '(z)) | |
630 | @result{} (x y (z)) | |
631 | @end group | |
632 | @group | |
633 | (funcall 'and t nil) | |
634 | @error{} Invalid function: #<subr and> | |
635 | @end group | |
636 | @end example | |
637 | ||
7f785b50 | 638 | Compare these examples with the examples of @code{apply}. |
9c52bf47 KH |
639 | @end defun |
640 | ||
641 | @defun apply function &rest arguments | |
642 | @code{apply} calls @var{function} with @var{arguments}, just like | |
643 | @code{funcall} but with one difference: the last of @var{arguments} is a | |
f9f59935 RS |
644 | list of objects, which are passed to @var{function} as separate |
645 | arguments, rather than a single list. We say that @code{apply} | |
646 | @dfn{spreads} this list so that each individual element becomes an | |
647 | argument. | |
9c52bf47 KH |
648 | |
649 | @code{apply} returns the result of calling @var{function}. As with | |
650 | @code{funcall}, @var{function} must either be a Lisp function or a | |
651 | primitive function; special forms and macros do not make sense in | |
652 | @code{apply}. | |
653 | ||
654 | @example | |
655 | @group | |
656 | (setq f 'list) | |
657 | @result{} list | |
658 | @end group | |
659 | @group | |
660 | (apply f 'x 'y 'z) | |
661 | @error{} Wrong type argument: listp, z | |
662 | @end group | |
663 | @group | |
664 | (apply '+ 1 2 '(3 4)) | |
665 | @result{} 10 | |
666 | @end group | |
667 | @group | |
668 | (apply '+ '(1 2 3 4)) | |
669 | @result{} 10 | |
670 | @end group | |
671 | ||
672 | @group | |
673 | (apply 'append '((a b c) nil (x y z) nil)) | |
674 | @result{} (a b c x y z) | |
675 | @end group | |
676 | @end example | |
677 | ||
678 | For an interesting example of using @code{apply}, see the description of | |
679 | @code{mapcar}, in @ref{Mapping Functions}. | |
680 | @end defun | |
681 | ||
682 | @cindex functionals | |
683 | It is common for Lisp functions to accept functions as arguments or | |
684 | find them in data structures (especially in hook variables and property | |
685 | lists) and call them using @code{funcall} or @code{apply}. Functions | |
686 | that accept function arguments are often called @dfn{functionals}. | |
687 | ||
bfe721d1 KH |
688 | Sometimes, when you call a functional, it is useful to supply a no-op |
689 | function as the argument. Here are two different kinds of no-op | |
9c52bf47 KH |
690 | function: |
691 | ||
692 | @defun identity arg | |
693 | This function returns @var{arg} and has no side effects. | |
694 | @end defun | |
695 | ||
696 | @defun ignore &rest args | |
697 | This function ignores any arguments and returns @code{nil}. | |
698 | @end defun | |
699 | ||
700 | @node Mapping Functions | |
701 | @section Mapping Functions | |
702 | @cindex mapping functions | |
703 | ||
704 | A @dfn{mapping function} applies a given function to each element of a | |
f9f59935 | 705 | list or other collection. Emacs Lisp has several such functions; |
9c52bf47 | 706 | @code{mapcar} and @code{mapconcat}, which scan a list, are described |
f9f59935 | 707 | here. @xref{Creating Symbols}, for the function @code{mapatoms} which |
7f785b50 GM |
708 | maps over the symbols in an obarray. @xref{Hash Access}, for the |
709 | function @code{maphash} which maps over key/value associations in a | |
710 | hash table. | |
969fe9b5 RS |
711 | |
712 | These mapping functions do not allow char-tables because a char-table | |
713 | is a sparse array whose nominal range of indices is very large. To map | |
714 | over a char-table in a way that deals properly with its sparse nature, | |
715 | use the function @code{map-char-table} (@pxref{Char-Tables}). | |
9c52bf47 KH |
716 | |
717 | @defun mapcar function sequence | |
f25df2ab RS |
718 | @code{mapcar} applies @var{function} to each element of @var{sequence} |
719 | in turn, and returns a list of the results. | |
9c52bf47 | 720 | |
969fe9b5 RS |
721 | The argument @var{sequence} can be any kind of sequence except a |
722 | char-table; that is, a list, a vector, a bool-vector, or a string. The | |
9c52bf47 KH |
723 | result is always a list. The length of the result is the same as the |
724 | length of @var{sequence}. | |
725 | ||
726 | @smallexample | |
727 | @group | |
728 | @exdent @r{For example:} | |
729 | ||
730 | (mapcar 'car '((a b) (c d) (e f))) | |
731 | @result{} (a c e) | |
732 | (mapcar '1+ [1 2 3]) | |
733 | @result{} (2 3 4) | |
734 | (mapcar 'char-to-string "abc") | |
735 | @result{} ("a" "b" "c") | |
736 | @end group | |
737 | ||
738 | @group | |
739 | ;; @r{Call each function in @code{my-hooks}.} | |
740 | (mapcar 'funcall my-hooks) | |
741 | @end group | |
742 | ||
743 | @group | |
969fe9b5 | 744 | (defun mapcar* (function &rest args) |
9c52bf47 KH |
745 | "Apply FUNCTION to successive cars of all ARGS. |
746 | Return the list of results." | |
747 | ;; @r{If no list is exhausted,} | |
748 | (if (not (memq 'nil args)) | |
969fe9b5 RS |
749 | ;; @r{apply function to @sc{car}s.} |
750 | (cons (apply function (mapcar 'car args)) | |
751 | (apply 'mapcar* function | |
9c52bf47 KH |
752 | ;; @r{Recurse for rest of elements.} |
753 | (mapcar 'cdr args))))) | |
754 | @end group | |
755 | ||
756 | @group | |
757 | (mapcar* 'cons '(a b c) '(1 2 3 4)) | |
758 | @result{} ((a . 1) (b . 2) (c . 3)) | |
759 | @end group | |
760 | @end smallexample | |
761 | @end defun | |
762 | ||
3c30cb6e DL |
763 | @defun mapc function sequence |
764 | @tindex mapc | |
765 | @code{mapc} is like @code{mapcar} except that @var{function} is used for | |
766 | side-effects only---the values it returns are ignored, not collected | |
767 | into a list. @code{mapc} always returns @var{sequence}. | |
768 | @end defun | |
769 | ||
9c52bf47 KH |
770 | @defun mapconcat function sequence separator |
771 | @code{mapconcat} applies @var{function} to each element of | |
772 | @var{sequence}: the results, which must be strings, are concatenated. | |
773 | Between each pair of result strings, @code{mapconcat} inserts the string | |
774 | @var{separator}. Usually @var{separator} contains a space or comma or | |
775 | other suitable punctuation. | |
776 | ||
777 | The argument @var{function} must be a function that can take one | |
969fe9b5 RS |
778 | argument and return a string. The argument @var{sequence} can be any |
779 | kind of sequence except a char-table; that is, a list, a vector, a | |
780 | bool-vector, or a string. | |
9c52bf47 KH |
781 | |
782 | @smallexample | |
783 | @group | |
784 | (mapconcat 'symbol-name | |
785 | '(The cat in the hat) | |
786 | " ") | |
787 | @result{} "The cat in the hat" | |
788 | @end group | |
789 | ||
790 | @group | |
791 | (mapconcat (function (lambda (x) (format "%c" (1+ x)))) | |
792 | "HAL-8000" | |
793 | "") | |
794 | @result{} "IBM.9111" | |
795 | @end group | |
796 | @end smallexample | |
797 | @end defun | |
798 | ||
799 | @node Anonymous Functions | |
800 | @section Anonymous Functions | |
801 | @cindex anonymous function | |
802 | ||
803 | In Lisp, a function is a list that starts with @code{lambda}, a | |
804 | byte-code function compiled from such a list, or alternatively a | |
805 | primitive subr-object; names are ``extra''. Although usually functions | |
806 | are defined with @code{defun} and given names at the same time, it is | |
807 | occasionally more concise to use an explicit lambda expression---an | |
808 | anonymous function. Such a list is valid wherever a function name is. | |
809 | ||
810 | Any method of creating such a list makes a valid function. Even this: | |
811 | ||
812 | @smallexample | |
813 | @group | |
ba3dafc8 | 814 | (setq silly (append '(lambda (x)) (list (list '+ (* 3 4) 'x)))) |
9c52bf47 KH |
815 | @result{} (lambda (x) (+ 12 x)) |
816 | @end group | |
817 | @end smallexample | |
818 | ||
819 | @noindent | |
820 | This computes a list that looks like @code{(lambda (x) (+ 12 x))} and | |
821 | makes it the value (@emph{not} the function definition!) of | |
822 | @code{silly}. | |
823 | ||
824 | Here is how we might call this function: | |
825 | ||
826 | @example | |
827 | @group | |
828 | (funcall silly 1) | |
829 | @result{} 13 | |
830 | @end group | |
831 | @end example | |
832 | ||
833 | @noindent | |
834 | (It does @emph{not} work to write @code{(silly 1)}, because this function | |
835 | is not the @emph{function definition} of @code{silly}. We have not given | |
836 | @code{silly} any function definition, just a value as a variable.) | |
837 | ||
838 | Most of the time, anonymous functions are constants that appear in | |
f9f59935 RS |
839 | your program. For example, you might want to pass one as an argument to |
840 | the function @code{mapcar}, which applies any given function to each | |
841 | element of a list. | |
842 | ||
843 | Here we define a function @code{change-property} which | |
844 | uses a function as its third argument: | |
9c52bf47 KH |
845 | |
846 | @example | |
847 | @group | |
f9f59935 RS |
848 | (defun change-property (symbol prop function) |
849 | (let ((value (get symbol prop))) | |
850 | (put symbol prop (funcall function value)))) | |
9c52bf47 | 851 | @end group |
f9f59935 RS |
852 | @end example |
853 | ||
854 | @noindent | |
855 | Here we define a function that uses @code{change-property}, | |
969fe9b5 | 856 | passing it a function to double a number: |
f9f59935 RS |
857 | |
858 | @example | |
9c52bf47 | 859 | @group |
f9f59935 | 860 | (defun double-property (symbol prop) |
65500a82 | 861 | (change-property symbol prop '(lambda (x) (* 2 x)))) |
9c52bf47 KH |
862 | @end group |
863 | @end example | |
864 | ||
865 | @noindent | |
866 | In such cases, we usually use the special form @code{function} instead | |
f9f59935 | 867 | of simple quotation to quote the anonymous function, like this: |
9c52bf47 KH |
868 | |
869 | @example | |
870 | @group | |
f9f59935 | 871 | (defun double-property (symbol prop) |
a9f0a989 RS |
872 | (change-property symbol prop |
873 | (function (lambda (x) (* 2 x))))) | |
9c52bf47 KH |
874 | @end group |
875 | @end example | |
876 | ||
f9f59935 RS |
877 | Using @code{function} instead of @code{quote} makes a difference if you |
878 | compile the function @code{double-property}. For example, if you | |
879 | compile the second definition of @code{double-property}, the anonymous | |
880 | function is compiled as well. By contrast, if you compile the first | |
881 | definition which uses ordinary @code{quote}, the argument passed to | |
882 | @code{change-property} is the precise list shown: | |
9c52bf47 KH |
883 | |
884 | @example | |
885 | (lambda (x) (* x 2)) | |
886 | @end example | |
887 | ||
888 | @noindent | |
889 | The Lisp compiler cannot assume this list is a function, even though it | |
f9f59935 | 890 | looks like one, since it does not know what @code{change-property} will |
a9f0a989 | 891 | do with the list. Perhaps it will check whether the @sc{car} of the third |
f9f59935 RS |
892 | element is the symbol @code{*}! Using @code{function} tells the |
893 | compiler it is safe to go ahead and compile the constant function. | |
9c52bf47 | 894 | |
65500a82 RS |
895 | Nowadays it is possible to omit @code{function} entirely, like this: |
896 | ||
897 | @example | |
898 | @group | |
899 | (defun double-property (symbol prop) | |
900 | (change-property symbol prop (lambda (x) (* 2 x)))) | |
901 | @end group | |
902 | @end example | |
903 | ||
904 | @noindent | |
905 | This is because @code{lambda} itself implies @code{function}. | |
906 | ||
9c52bf47 KH |
907 | We sometimes write @code{function} instead of @code{quote} when |
908 | quoting the name of a function, but this usage is just a sort of | |
f9f59935 | 909 | comment: |
9c52bf47 KH |
910 | |
911 | @example | |
912 | (function @var{symbol}) @equiv{} (quote @var{symbol}) @equiv{} '@var{symbol} | |
a9f0a989 RS |
913 | @end example |
914 | ||
8241495d | 915 | @cindex @samp{#'} syntax |
a9f0a989 RS |
916 | The read syntax @code{#'} is a short-hand for using @code{function}. |
917 | For example, | |
918 | ||
919 | @example | |
920 | #'(lambda (x) (* x x)) | |
921 | @end example | |
922 | ||
923 | @noindent | |
924 | is equivalent to | |
925 | ||
926 | @example | |
927 | (function (lambda (x) (* x x))) | |
9c52bf47 KH |
928 | @end example |
929 | ||
f9f59935 RS |
930 | @defspec function function-object |
931 | @cindex function quoting | |
932 | This special form returns @var{function-object} without evaluating it. | |
933 | In this, it is equivalent to @code{quote}. However, it serves as a | |
934 | note to the Emacs Lisp compiler that @var{function-object} is intended | |
935 | to be used only as a function, and therefore can safely be compiled. | |
936 | Contrast this with @code{quote}, in @ref{Quoting}. | |
937 | @end defspec | |
938 | ||
9c52bf47 KH |
939 | See @code{documentation} in @ref{Accessing Documentation}, for a |
940 | realistic example using @code{function} and an anonymous function. | |
941 | ||
942 | @node Function Cells | |
943 | @section Accessing Function Cell Contents | |
944 | ||
945 | The @dfn{function definition} of a symbol is the object stored in the | |
946 | function cell of the symbol. The functions described here access, test, | |
947 | and set the function cell of symbols. | |
948 | ||
f25df2ab RS |
949 | See also the function @code{indirect-function} in @ref{Function |
950 | Indirection}. | |
951 | ||
9c52bf47 KH |
952 | @defun symbol-function symbol |
953 | @kindex void-function | |
954 | This returns the object in the function cell of @var{symbol}. If the | |
955 | symbol's function cell is void, a @code{void-function} error is | |
956 | signaled. | |
957 | ||
958 | This function does not check that the returned object is a legitimate | |
959 | function. | |
960 | ||
961 | @example | |
962 | @group | |
963 | (defun bar (n) (+ n 2)) | |
964 | @result{} bar | |
965 | @end group | |
966 | @group | |
967 | (symbol-function 'bar) | |
968 | @result{} (lambda (n) (+ n 2)) | |
969 | @end group | |
970 | @group | |
971 | (fset 'baz 'bar) | |
972 | @result{} bar | |
973 | @end group | |
974 | @group | |
975 | (symbol-function 'baz) | |
976 | @result{} bar | |
977 | @end group | |
978 | @end example | |
979 | @end defun | |
980 | ||
981 | @cindex void function cell | |
982 | If you have never given a symbol any function definition, we say that | |
983 | that symbol's function cell is @dfn{void}. In other words, the function | |
984 | cell does not have any Lisp object in it. If you try to call such a symbol | |
985 | as a function, it signals a @code{void-function} error. | |
986 | ||
987 | Note that void is not the same as @code{nil} or the symbol | |
988 | @code{void}. The symbols @code{nil} and @code{void} are Lisp objects, | |
989 | and can be stored into a function cell just as any other object can be | |
990 | (and they can be valid functions if you define them in turn with | |
f25df2ab | 991 | @code{defun}). A void function cell contains no object whatsoever. |
9c52bf47 KH |
992 | |
993 | You can test the voidness of a symbol's function definition with | |
994 | @code{fboundp}. After you have given a symbol a function definition, you | |
995 | can make it void once more using @code{fmakunbound}. | |
996 | ||
997 | @defun fboundp symbol | |
998 | This function returns @code{t} if the symbol has an object in its | |
999 | function cell, @code{nil} otherwise. It does not check that the object | |
1000 | is a legitimate function. | |
1001 | @end defun | |
1002 | ||
1003 | @defun fmakunbound symbol | |
1004 | This function makes @var{symbol}'s function cell void, so that a | |
1005 | subsequent attempt to access this cell will cause a @code{void-function} | |
a9f0a989 | 1006 | error. (See also @code{makunbound}, in @ref{Void Variables}.) |
9c52bf47 KH |
1007 | |
1008 | @example | |
1009 | @group | |
1010 | (defun foo (x) x) | |
f9f59935 | 1011 | @result{} foo |
9c52bf47 KH |
1012 | @end group |
1013 | @group | |
f25df2ab RS |
1014 | (foo 1) |
1015 | @result{}1 | |
1016 | @end group | |
1017 | @group | |
9c52bf47 | 1018 | (fmakunbound 'foo) |
f9f59935 | 1019 | @result{} foo |
9c52bf47 KH |
1020 | @end group |
1021 | @group | |
1022 | (foo 1) | |
1023 | @error{} Symbol's function definition is void: foo | |
1024 | @end group | |
1025 | @end example | |
1026 | @end defun | |
1027 | ||
baa573a3 | 1028 | @defun fset symbol definition |
f9f59935 RS |
1029 | This function stores @var{definition} in the function cell of |
1030 | @var{symbol}. The result is @var{definition}. Normally | |
1031 | @var{definition} should be a function or the name of a function, but | |
1032 | this is not checked. The argument @var{symbol} is an ordinary evaluated | |
1033 | argument. | |
9c52bf47 KH |
1034 | |
1035 | There are three normal uses of this function: | |
1036 | ||
1037 | @itemize @bullet | |
1038 | @item | |
969fe9b5 RS |
1039 | Copying one symbol's function definition to another---in other words, |
1040 | making an alternate name for a function. (If you think of this as the | |
1041 | definition of the new name, you should use @code{defalias} instead of | |
1042 | @code{fset}; see @ref{Defining Functions}.) | |
9c52bf47 KH |
1043 | |
1044 | @item | |
1045 | Giving a symbol a function definition that is not a list and therefore | |
f25df2ab RS |
1046 | cannot be made with @code{defun}. For example, you can use @code{fset} |
1047 | to give a symbol @code{s1} a function definition which is another symbol | |
1048 | @code{s2}; then @code{s1} serves as an alias for whatever definition | |
969fe9b5 RS |
1049 | @code{s2} presently has. (Once again use @code{defalias} instead of |
1050 | @code{fset} if you think of this as the definition of @code{s1}.) | |
9c52bf47 KH |
1051 | |
1052 | @item | |
1053 | In constructs for defining or altering functions. If @code{defun} | |
1054 | were not a primitive, it could be written in Lisp (as a macro) using | |
1055 | @code{fset}. | |
1056 | @end itemize | |
1057 | ||
969fe9b5 | 1058 | Here are examples of these uses: |
9c52bf47 KH |
1059 | |
1060 | @example | |
1061 | @group | |
969fe9b5 RS |
1062 | ;; @r{Save @code{foo}'s definition in @code{old-foo}.} |
1063 | (fset 'old-foo (symbol-function 'foo)) | |
9c52bf47 KH |
1064 | @end group |
1065 | ||
1066 | @group | |
1067 | ;; @r{Make the symbol @code{car} the function definition of @code{xfirst}.} | |
969fe9b5 | 1068 | ;; @r{(Most likely, @code{defalias} would be better than @code{fset} here.)} |
9c52bf47 KH |
1069 | (fset 'xfirst 'car) |
1070 | @result{} car | |
1071 | @end group | |
1072 | @group | |
1073 | (xfirst '(1 2 3)) | |
1074 | @result{} 1 | |
1075 | @end group | |
1076 | @group | |
1077 | (symbol-function 'xfirst) | |
1078 | @result{} car | |
1079 | @end group | |
1080 | @group | |
1081 | (symbol-function (symbol-function 'xfirst)) | |
1082 | @result{} #<subr car> | |
1083 | @end group | |
1084 | ||
1085 | @group | |
1086 | ;; @r{Define a named keyboard macro.} | |
1087 | (fset 'kill-two-lines "\^u2\^k") | |
1088 | @result{} "\^u2\^k" | |
1089 | @end group | |
f25df2ab | 1090 | |
969fe9b5 RS |
1091 | @group |
1092 | ;; @r{Here is a function that alters other functions.} | |
1093 | (defun copy-function-definition (new old) | |
1094 | "Define NEW with the same function definition as OLD." | |
1095 | (fset new (symbol-function old))) | |
1096 | @end group | |
1097 | @end example | |
9c52bf47 KH |
1098 | @end defun |
1099 | ||
1100 | When writing a function that extends a previously defined function, | |
bfe721d1 | 1101 | the following idiom is sometimes used: |
9c52bf47 KH |
1102 | |
1103 | @example | |
1104 | (fset 'old-foo (symbol-function 'foo)) | |
1105 | (defun foo () | |
1106 | "Just like old-foo, except more so." | |
1107 | @group | |
1108 | (old-foo) | |
1109 | (more-so)) | |
1110 | @end group | |
1111 | @end example | |
1112 | ||
1113 | @noindent | |
1114 | This does not work properly if @code{foo} has been defined to autoload. | |
1115 | In such a case, when @code{foo} calls @code{old-foo}, Lisp attempts | |
1116 | to define @code{old-foo} by loading a file. Since this presumably | |
1117 | defines @code{foo} rather than @code{old-foo}, it does not produce the | |
1118 | proper results. The only way to avoid this problem is to make sure the | |
1119 | file is loaded before moving aside the old definition of @code{foo}. | |
1120 | ||
bfe721d1 | 1121 | But it is unmodular and unclean, in any case, for a Lisp file to |
969fe9b5 RS |
1122 | redefine a function defined elsewhere. It is cleaner to use the advice |
1123 | facility (@pxref{Advising Functions}). | |
bfe721d1 | 1124 | |
9c52bf47 KH |
1125 | @node Inline Functions |
1126 | @section Inline Functions | |
1127 | @cindex inline functions | |
1128 | ||
1129 | @findex defsubst | |
1130 | You can define an @dfn{inline function} by using @code{defsubst} instead | |
1131 | of @code{defun}. An inline function works just like an ordinary | |
1132 | function except for one thing: when you compile a call to the function, | |
1133 | the function's definition is open-coded into the caller. | |
1134 | ||
1135 | Making a function inline makes explicit calls run faster. But it also | |
1136 | has disadvantages. For one thing, it reduces flexibility; if you change | |
1137 | the definition of the function, calls already inlined still use the old | |
1138 | definition until you recompile them. Since the flexibility of | |
1139 | redefining functions is an important feature of Emacs, you should not | |
1140 | make a function inline unless its speed is really crucial. | |
1141 | ||
1142 | Another disadvantage is that making a large function inline can increase | |
1143 | the size of compiled code both in files and in memory. Since the speed | |
1144 | advantage of inline functions is greatest for small functions, you | |
1145 | generally should not make large functions inline. | |
1146 | ||
1147 | It's possible to define a macro to expand into the same code that an | |
969fe9b5 RS |
1148 | inline function would execute. (@xref{Macros}.) But the macro would be |
1149 | limited to direct use in expressions---a macro cannot be called with | |
9c52bf47 | 1150 | @code{apply}, @code{mapcar} and so on. Also, it takes some work to |
969fe9b5 RS |
1151 | convert an ordinary function into a macro. To convert it into an inline |
1152 | function is very easy; simply replace @code{defun} with @code{defsubst}. | |
1153 | Since each argument of an inline function is evaluated exactly once, you | |
1154 | needn't worry about how many times the body uses the arguments, as you | |
1155 | do for macros. (@xref{Argument Evaluation}.) | |
9c52bf47 | 1156 | |
f25df2ab | 1157 | Inline functions can be used and open-coded later on in the same file, |
9c52bf47 KH |
1158 | following the definition, just like macros. |
1159 | ||
bfe721d1 | 1160 | @c Emacs versions prior to 19 did not have inline functions. |
9c52bf47 KH |
1161 | |
1162 | @node Related Topics | |
1163 | @section Other Topics Related to Functions | |
1164 | ||
1165 | Here is a table of several functions that do things related to | |
1166 | function calling and function definitions. They are documented | |
1167 | elsewhere, but we provide cross references here. | |
1168 | ||
1169 | @table @code | |
1170 | @item apply | |
1171 | See @ref{Calling Functions}. | |
1172 | ||
1173 | @item autoload | |
1174 | See @ref{Autoload}. | |
1175 | ||
1176 | @item call-interactively | |
1177 | See @ref{Interactive Call}. | |
1178 | ||
1179 | @item commandp | |
1180 | See @ref{Interactive Call}. | |
1181 | ||
1182 | @item documentation | |
1183 | See @ref{Accessing Documentation}. | |
1184 | ||
1185 | @item eval | |
1186 | See @ref{Eval}. | |
1187 | ||
1188 | @item funcall | |
1189 | See @ref{Calling Functions}. | |
1190 | ||
969fe9b5 RS |
1191 | @item function |
1192 | See @ref{Anonymous Functions}. | |
1193 | ||
9c52bf47 KH |
1194 | @item ignore |
1195 | See @ref{Calling Functions}. | |
1196 | ||
1197 | @item indirect-function | |
1198 | See @ref{Function Indirection}. | |
1199 | ||
1200 | @item interactive | |
1201 | See @ref{Using Interactive}. | |
1202 | ||
1203 | @item interactive-p | |
1204 | See @ref{Interactive Call}. | |
1205 | ||
1206 | @item mapatoms | |
1207 | See @ref{Creating Symbols}. | |
1208 | ||
1209 | @item mapcar | |
1210 | See @ref{Mapping Functions}. | |
1211 | ||
969fe9b5 RS |
1212 | @item map-char-table |
1213 | See @ref{Char-Tables}. | |
1214 | ||
9c52bf47 KH |
1215 | @item mapconcat |
1216 | See @ref{Mapping Functions}. | |
1217 | ||
1218 | @item undefined | |
1219 | See @ref{Key Lookup}. | |
1220 | @end table | |
1221 |