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1 | @c -*-texinfo-*- |
2 | @c This is part of the GNU Emacs Lisp Reference Manual. | |
fd897522 | 3 | @c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999 |
177c0ea7 | 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. | |
177c0ea7 | 21 | * Anonymous Functions:: Lambda expressions are functions with no names. |
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22 | * Function Cells:: Accessing or setting the function definition |
23 | of a symbol. | |
24 | * Inline Functions:: Defining functions that the compiler will open code. | |
7ed9159a | 25 | * Function safety:: Determining whether a function is safe to call. |
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26 | * Related Topics:: Cross-references to specific Lisp primitives |
27 | that have a special bearing on how functions work. | |
28 | @end menu | |
29 | ||
30 | @node What Is a Function | |
31 | @section What Is a Function? | |
32 | ||
33 | In a general sense, a function is a rule for carrying on a computation | |
34 | given several values called @dfn{arguments}. The result of the | |
35 | computation is called the value of the function. The computation can | |
36 | also have side effects: lasting changes in the values of variables or | |
37 | the contents of data structures. | |
38 | ||
39 | Here are important terms for functions in Emacs Lisp and for other | |
40 | function-like objects. | |
41 | ||
42 | @table @dfn | |
43 | @item function | |
44 | @cindex function | |
45 | In Emacs Lisp, a @dfn{function} is anything that can be applied to | |
46 | arguments in a Lisp program. In some cases, we use it more | |
47 | specifically to mean a function written in Lisp. Special forms and | |
48 | macros are not functions. | |
49 | ||
50 | @item primitive | |
51 | @cindex primitive | |
52 | @cindex subr | |
53 | @cindex built-in function | |
54 | A @dfn{primitive} is a function callable from Lisp that is written in C, | |
55 | such as @code{car} or @code{append}. These functions are also called | |
56 | @dfn{built-in} functions or @dfn{subrs}. (Special forms are also | |
57 | considered primitives.) | |
58 | ||
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59 | Usually the reason we implement a function as a primitive is either |
60 | because it is fundamental, because it provides a low-level interface to | |
61 | operating system services, or because it needs to run fast. Primitives | |
62 | can be modified or added only by changing the C sources and recompiling | |
63 | the editor. See @ref{Writing Emacs Primitives}. | |
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64 | |
65 | @item lambda expression | |
66 | A @dfn{lambda expression} is a function written in Lisp. | |
67 | These are described in the following section. | |
37680279 | 68 | @ifnottex |
9c52bf47 | 69 | @xref{Lambda Expressions}. |
37680279 | 70 | @end ifnottex |
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71 | |
72 | @item special form | |
73 | A @dfn{special form} is a primitive that is like a function but does not | |
74 | evaluate all of its arguments in the usual way. It may evaluate only | |
75 | some of the arguments, or may evaluate them in an unusual order, or | |
76 | several times. Many special forms are described in @ref{Control | |
77 | Structures}. | |
78 | ||
79 | @item macro | |
80 | @cindex macro | |
81 | A @dfn{macro} is a construct defined in Lisp by the programmer. It | |
82 | differs from a function in that it translates a Lisp expression that you | |
83 | write into an equivalent expression to be evaluated instead of the | |
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84 | original expression. Macros enable Lisp programmers to do the sorts of |
85 | things that special forms can do. @xref{Macros}, for how to define and | |
86 | use macros. | |
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87 | |
88 | @item command | |
89 | @cindex command | |
90 | A @dfn{command} is an object that @code{command-execute} can invoke; it | |
91 | is a possible definition for a key sequence. Some functions are | |
92 | commands; a function written in Lisp is a command if it contains an | |
93 | interactive declaration (@pxref{Defining Commands}). Such a function | |
94 | can be called from Lisp expressions like other functions; in this case, | |
95 | the fact that the function is a command makes no difference. | |
96 | ||
97 | Keyboard macros (strings and vectors) are commands also, even though | |
98 | they are not functions. A symbol is a command if its function | |
99 | definition is a command; such symbols can be invoked with @kbd{M-x}. | |
100 | The symbol is a function as well if the definition is a function. | |
101 | @xref{Command Overview}. | |
102 | ||
103 | @item keystroke command | |
104 | @cindex keystroke command | |
105 | A @dfn{keystroke command} is a command that is bound to a key sequence | |
106 | (typically one to three keystrokes). The distinction is made here | |
107 | merely to avoid confusion with the meaning of ``command'' in non-Emacs | |
108 | editors; for Lisp programs, the distinction is normally unimportant. | |
109 | ||
110 | @item byte-code function | |
111 | A @dfn{byte-code function} is a function that has been compiled by the | |
112 | byte compiler. @xref{Byte-Code Type}. | |
113 | @end table | |
114 | ||
f9f59935 | 115 | @defun functionp object |
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116 | This function returns @code{t} if @var{object} is any kind of function, |
117 | or a special form or macro. | |
f9f59935 RS |
118 | @end defun |
119 | ||
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120 | @defun subrp object |
121 | This function returns @code{t} if @var{object} is a built-in function | |
122 | (i.e., a Lisp primitive). | |
123 | ||
124 | @example | |
125 | @group | |
126 | (subrp 'message) ; @r{@code{message} is a symbol,} | |
127 | @result{} nil ; @r{not a subr object.} | |
128 | @end group | |
129 | @group | |
130 | (subrp (symbol-function 'message)) | |
131 | @result{} t | |
132 | @end group | |
133 | @end example | |
134 | @end defun | |
135 | ||
136 | @defun byte-code-function-p object | |
137 | This function returns @code{t} if @var{object} is a byte-code | |
138 | function. For example: | |
139 | ||
140 | @example | |
141 | @group | |
142 | (byte-code-function-p (symbol-function 'next-line)) | |
143 | @result{} t | |
144 | @end group | |
145 | @end example | |
146 | @end defun | |
147 | ||
aad52941 DL |
148 | @defun subr-arity subr |
149 | @tindex subr-arity | |
150 | This function provides information about the argument list of a | |
151 | primitive, @var{subr}. The returned value is a pair | |
152 | @code{(@var{min} . @var{max})}. @var{min} is the minimum number of | |
153 | args. @var{max} is the maximum number or the symbol @code{many}, for a | |
154 | function with @code{&rest} arguments, or the symbol @code{unevalled} if | |
155 | @var{subr} is a special form. | |
156 | @end defun | |
157 | ||
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158 | @node Lambda Expressions |
159 | @section Lambda Expressions | |
160 | @cindex lambda expression | |
161 | ||
162 | A function written in Lisp is a list that looks like this: | |
163 | ||
164 | @example | |
165 | (lambda (@var{arg-variables}@dots{}) | |
166 | @r{[}@var{documentation-string}@r{]} | |
167 | @r{[}@var{interactive-declaration}@r{]} | |
168 | @var{body-forms}@dots{}) | |
169 | @end example | |
170 | ||
171 | @noindent | |
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172 | Such a list is called a @dfn{lambda expression}. In Emacs Lisp, it |
173 | actually is valid as an expression---it evaluates to itself. In some | |
174 | other Lisp dialects, a lambda expression is not a valid expression at | |
175 | all. In either case, its main use is not to be evaluated as an | |
176 | expression, but to be called as a function. | |
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177 | |
178 | @menu | |
179 | * Lambda Components:: The parts of a lambda expression. | |
180 | * Simple Lambda:: A simple example. | |
181 | * Argument List:: Details and special features of argument lists. | |
182 | * Function Documentation:: How to put documentation in a function. | |
183 | @end menu | |
184 | ||
185 | @node Lambda Components | |
186 | @subsection Components of a Lambda Expression | |
187 | ||
37680279 | 188 | @ifnottex |
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189 | |
190 | A function written in Lisp (a ``lambda expression'') is a list that | |
191 | looks like this: | |
192 | ||
193 | @example | |
194 | (lambda (@var{arg-variables}@dots{}) | |
195 | [@var{documentation-string}] | |
196 | [@var{interactive-declaration}] | |
197 | @var{body-forms}@dots{}) | |
198 | @end example | |
37680279 | 199 | @end ifnottex |
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200 | |
201 | @cindex lambda list | |
202 | The first element of a lambda expression is always the symbol | |
203 | @code{lambda}. This indicates that the list represents a function. The | |
204 | reason functions are defined to start with @code{lambda} is so that | |
205 | other lists, intended for other uses, will not accidentally be valid as | |
206 | functions. | |
207 | ||
f9f59935 | 208 | The second element is a list of symbols---the argument variable names. |
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209 | This is called the @dfn{lambda list}. When a Lisp function is called, |
210 | the argument values are matched up against the variables in the lambda | |
211 | list, which are given local bindings with the values provided. | |
212 | @xref{Local Variables}. | |
213 | ||
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214 | The documentation string is a Lisp string object placed within the |
215 | function definition to describe the function for the Emacs help | |
216 | facilities. @xref{Function Documentation}. | |
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217 | |
218 | The interactive declaration is a list of the form @code{(interactive | |
219 | @var{code-string})}. This declares how to provide arguments if the | |
220 | function is used interactively. Functions with this declaration are called | |
221 | @dfn{commands}; they can be called using @kbd{M-x} or bound to a key. | |
222 | Functions not intended to be called in this way should not have interactive | |
223 | declarations. @xref{Defining Commands}, for how to write an interactive | |
224 | declaration. | |
225 | ||
226 | @cindex body of function | |
227 | The rest of the elements are the @dfn{body} of the function: the Lisp | |
228 | code to do the work of the function (or, as a Lisp programmer would say, | |
229 | ``a list of Lisp forms to evaluate''). The value returned by the | |
230 | function is the value returned by the last element of the body. | |
231 | ||
232 | @node Simple Lambda | |
233 | @subsection A Simple Lambda-Expression Example | |
234 | ||
235 | Consider for example the following function: | |
236 | ||
237 | @example | |
238 | (lambda (a b c) (+ a b c)) | |
239 | @end example | |
240 | ||
241 | @noindent | |
242 | We can call this function by writing it as the @sc{car} of an | |
243 | expression, like this: | |
244 | ||
245 | @example | |
246 | @group | |
247 | ((lambda (a b c) (+ a b c)) | |
248 | 1 2 3) | |
249 | @end group | |
250 | @end example | |
251 | ||
252 | @noindent | |
253 | This call evaluates the body of the lambda expression with the variable | |
254 | @code{a} bound to 1, @code{b} bound to 2, and @code{c} bound to 3. | |
255 | Evaluation of the body adds these three numbers, producing the result 6; | |
256 | therefore, this call to the function returns the value 6. | |
257 | ||
258 | Note that the arguments can be the results of other function calls, as in | |
259 | this example: | |
260 | ||
261 | @example | |
262 | @group | |
263 | ((lambda (a b c) (+ a b c)) | |
264 | 1 (* 2 3) (- 5 4)) | |
265 | @end group | |
266 | @end example | |
267 | ||
268 | @noindent | |
269 | This evaluates the arguments @code{1}, @code{(* 2 3)}, and @code{(- 5 | |
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270 | 4)} from left to right. Then it applies the lambda expression to the |
271 | argument values 1, 6 and 1 to produce the value 8. | |
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272 | |
273 | It is not often useful to write a lambda expression as the @sc{car} of | |
274 | a form in this way. You can get the same result, of making local | |
275 | variables and giving them values, using the special form @code{let} | |
276 | (@pxref{Local Variables}). And @code{let} is clearer and easier to use. | |
277 | In practice, lambda expressions are either stored as the function | |
278 | definitions of symbols, to produce named functions, or passed as | |
279 | arguments to other functions (@pxref{Anonymous Functions}). | |
280 | ||
281 | However, calls to explicit lambda expressions were very useful in the | |
282 | old days of Lisp, before the special form @code{let} was invented. At | |
283 | that time, they were the only way to bind and initialize local | |
284 | variables. | |
285 | ||
286 | @node Argument List | |
f9f59935 | 287 | @subsection Other Features of Argument Lists |
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288 | @kindex wrong-number-of-arguments |
289 | @cindex argument binding | |
290 | @cindex binding arguments | |
291 | ||
292 | Our simple sample function, @code{(lambda (a b c) (+ a b c))}, | |
293 | specifies three argument variables, so it must be called with three | |
294 | arguments: if you try to call it with only two arguments or four | |
295 | arguments, you get a @code{wrong-number-of-arguments} error. | |
296 | ||
297 | It is often convenient to write a function that allows certain | |
298 | arguments to be omitted. For example, the function @code{substring} | |
299 | accepts three arguments---a string, the start index and the end | |
300 | index---but the third argument defaults to the @var{length} of the | |
301 | string if you omit it. It is also convenient for certain functions to | |
f25df2ab | 302 | accept an indefinite number of arguments, as the functions @code{list} |
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303 | and @code{+} do. |
304 | ||
305 | @cindex optional arguments | |
306 | @cindex rest arguments | |
307 | @kindex &optional | |
308 | @kindex &rest | |
309 | To specify optional arguments that may be omitted when a function | |
310 | is called, simply include the keyword @code{&optional} before the optional | |
311 | arguments. To specify a list of zero or more extra arguments, include the | |
312 | keyword @code{&rest} before one final argument. | |
313 | ||
314 | Thus, the complete syntax for an argument list is as follows: | |
315 | ||
316 | @example | |
317 | @group | |
318 | (@var{required-vars}@dots{} | |
319 | @r{[}&optional @var{optional-vars}@dots{}@r{]} | |
320 | @r{[}&rest @var{rest-var}@r{]}) | |
321 | @end group | |
322 | @end example | |
323 | ||
324 | @noindent | |
325 | The square brackets indicate that the @code{&optional} and @code{&rest} | |
326 | clauses, and the variables that follow them, are optional. | |
327 | ||
328 | A call to the function requires one actual argument for each of the | |
329 | @var{required-vars}. There may be actual arguments for zero or more of | |
330 | the @var{optional-vars}, and there cannot be any actual arguments beyond | |
331 | that unless the lambda list uses @code{&rest}. In that case, there may | |
332 | be any number of extra actual arguments. | |
333 | ||
334 | If actual arguments for the optional and rest variables are omitted, | |
f25df2ab | 335 | then they always default to @code{nil}. There is no way for the |
9c52bf47 | 336 | function to distinguish between an explicit argument of @code{nil} and |
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337 | an omitted argument. However, the body of the function is free to |
338 | consider @code{nil} an abbreviation for some other meaningful value. | |
339 | This is what @code{substring} does; @code{nil} as the third argument to | |
340 | @code{substring} means to use the length of the string supplied. | |
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341 | |
342 | @cindex CL note---default optional arg | |
343 | @quotation | |
344 | @b{Common Lisp note:} Common Lisp allows the function to specify what | |
345 | default value to use when an optional argument is omitted; Emacs Lisp | |
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346 | always uses @code{nil}. Emacs Lisp does not support ``supplied-p'' |
347 | variables that tell you whether an argument was explicitly passed. | |
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348 | @end quotation |
349 | ||
350 | For example, an argument list that looks like this: | |
351 | ||
352 | @example | |
353 | (a b &optional c d &rest e) | |
354 | @end example | |
355 | ||
356 | @noindent | |
357 | binds @code{a} and @code{b} to the first two actual arguments, which are | |
358 | required. If one or two more arguments are provided, @code{c} and | |
359 | @code{d} are bound to them respectively; any arguments after the first | |
360 | four are collected into a list and @code{e} is bound to that list. If | |
361 | there are only two arguments, @code{c} is @code{nil}; if two or three | |
362 | arguments, @code{d} is @code{nil}; if four arguments or fewer, @code{e} | |
363 | is @code{nil}. | |
364 | ||
365 | There is no way to have required arguments following optional | |
366 | ones---it would not make sense. To see why this must be so, suppose | |
367 | that @code{c} in the example were optional and @code{d} were required. | |
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368 | Suppose three actual arguments are given; which variable would the |
369 | third argument be for? Would it be used for the @var{c}, or for | |
370 | @var{d}? One can argue for both possibilities. Similarly, it makes | |
371 | no sense to have any more arguments (either required or optional) | |
372 | after a @code{&rest} argument. | |
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373 | |
374 | Here are some examples of argument lists and proper calls: | |
375 | ||
376 | @smallexample | |
377 | ((lambda (n) (1+ n)) ; @r{One required:} | |
378 | 1) ; @r{requires exactly one argument.} | |
379 | @result{} 2 | |
380 | ((lambda (n &optional n1) ; @r{One required and one optional:} | |
381 | (if n1 (+ n n1) (1+ n))) ; @r{1 or 2 arguments.} | |
382 | 1 2) | |
383 | @result{} 3 | |
384 | ((lambda (n &rest ns) ; @r{One required and one rest:} | |
385 | (+ n (apply '+ ns))) ; @r{1 or more arguments.} | |
386 | 1 2 3 4 5) | |
387 | @result{} 15 | |
388 | @end smallexample | |
389 | ||
390 | @node Function Documentation | |
391 | @subsection Documentation Strings of Functions | |
392 | @cindex documentation of function | |
393 | ||
394 | A lambda expression may optionally have a @dfn{documentation string} just | |
395 | after the lambda list. This string does not affect execution of the | |
396 | function; it is a kind of comment, but a systematized comment which | |
397 | actually appears inside the Lisp world and can be used by the Emacs help | |
398 | facilities. @xref{Documentation}, for how the @var{documentation-string} is | |
399 | accessed. | |
400 | ||
bfe721d1 | 401 | It is a good idea to provide documentation strings for all the |
969fe9b5 | 402 | functions in your program, even those that are called only from within |
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403 | your program. Documentation strings are like comments, except that they |
404 | are easier to access. | |
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405 | |
406 | The first line of the documentation string should stand on its own, | |
407 | because @code{apropos} displays just this first line. It should consist | |
408 | of one or two complete sentences that summarize the function's purpose. | |
409 | ||
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410 | The start of the documentation string is usually indented in the source file, |
411 | but since these spaces come before the starting double-quote, they are not part of | |
9c52bf47 | 412 | the string. Some people make a practice of indenting any additional |
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413 | lines of the string so that the text lines up in the program source. |
414 | @emph{This is a mistake.} The indentation of the following lines is | |
415 | inside the string; what looks nice in the source code will look ugly | |
416 | when displayed by the help commands. | |
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417 | |
418 | You may wonder how the documentation string could be optional, since | |
419 | there are required components of the function that follow it (the body). | |
420 | Since evaluation of a string returns that string, without any side effects, | |
421 | it has no effect if it is not the last form in the body. Thus, in | |
422 | practice, there is no confusion between the first form of the body and the | |
423 | documentation string; if the only body form is a string then it serves both | |
424 | as the return value and as the documentation. | |
425 | ||
426 | @node Function Names | |
427 | @section Naming a Function | |
428 | @cindex function definition | |
429 | @cindex named function | |
430 | @cindex function name | |
431 | ||
432 | In most computer languages, every function has a name; the idea of a | |
433 | function without a name is nonsensical. In Lisp, a function in the | |
434 | strictest sense has no name. It is simply a list whose first element is | |
969fe9b5 | 435 | @code{lambda}, a byte-code function object, or a primitive subr-object. |
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436 | |
437 | However, a symbol can serve as the name of a function. This happens | |
438 | when you put the function in the symbol's @dfn{function cell} | |
439 | (@pxref{Symbol Components}). Then the symbol itself becomes a valid, | |
440 | callable function, equivalent to the list or subr-object that its | |
441 | function cell refers to. The contents of the function cell are also | |
442 | called the symbol's @dfn{function definition}. The procedure of using a | |
443 | symbol's function definition in place of the symbol is called | |
444 | @dfn{symbol function indirection}; see @ref{Function Indirection}. | |
445 | ||
446 | In practice, nearly all functions are given names in this way and | |
447 | referred to through their names. For example, the symbol @code{car} works | |
448 | as a function and does what it does because the primitive subr-object | |
449 | @code{#<subr car>} is stored in its function cell. | |
450 | ||
451 | We give functions names because it is convenient to refer to them by | |
452 | their names in Lisp expressions. For primitive subr-objects such as | |
453 | @code{#<subr car>}, names are the only way you can refer to them: there | |
454 | is no read syntax for such objects. For functions written in Lisp, the | |
455 | name is more convenient to use in a call than an explicit lambda | |
456 | expression. Also, a function with a name can refer to itself---it can | |
457 | be recursive. Writing the function's name in its own definition is much | |
458 | more convenient than making the function definition point to itself | |
459 | (something that is not impossible but that has various disadvantages in | |
460 | practice). | |
461 | ||
462 | We often identify functions with the symbols used to name them. For | |
463 | example, we often speak of ``the function @code{car}'', not | |
464 | distinguishing between the symbol @code{car} and the primitive | |
465 | subr-object that is its function definition. For most purposes, there | |
466 | is no need to distinguish. | |
467 | ||
468 | Even so, keep in mind that a function need not have a unique name. While | |
469 | a given function object @emph{usually} appears in the function cell of only | |
470 | one symbol, this is just a matter of convenience. It is easy to store | |
471 | it in several symbols using @code{fset}; then each of the symbols is | |
472 | equally well a name for the same function. | |
473 | ||
a9f0a989 RS |
474 | A symbol used as a function name may also be used as a variable; these |
475 | two uses of a symbol are independent and do not conflict. (Some Lisp | |
476 | dialects, such as Scheme, do not distinguish between a symbol's value | |
477 | and its function definition; a symbol's value as a variable is also its | |
478 | function definition.) If you have not given a symbol a function | |
479 | definition, you cannot use it as a function; whether the symbol has a | |
480 | value as a variable makes no difference to this. | |
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481 | |
482 | @node Defining Functions | |
05fd2b65 | 483 | @section Defining Functions |
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484 | @cindex defining a function |
485 | ||
486 | We usually give a name to a function when it is first created. This | |
487 | is called @dfn{defining a function}, and it is done with the | |
488 | @code{defun} special form. | |
489 | ||
490 | @defspec defun name argument-list body-forms | |
491 | @code{defun} is the usual way to define new Lisp functions. It | |
492 | defines the symbol @var{name} as a function that looks like this: | |
493 | ||
494 | @example | |
495 | (lambda @var{argument-list} . @var{body-forms}) | |
496 | @end example | |
497 | ||
498 | @code{defun} stores this lambda expression in the function cell of | |
499 | @var{name}. It returns the value @var{name}, but usually we ignore this | |
500 | value. | |
501 | ||
502 | As described previously (@pxref{Lambda Expressions}), | |
503 | @var{argument-list} is a list of argument names and may include the | |
f9f59935 RS |
504 | keywords @code{&optional} and @code{&rest}. Also, the first two of the |
505 | @var{body-forms} may be a documentation string and an interactive | |
9c52bf47 KH |
506 | declaration. |
507 | ||
508 | There is no conflict if the same symbol @var{name} is also used as a | |
509 | variable, since the symbol's value cell is independent of the function | |
510 | cell. @xref{Symbol Components}. | |
511 | ||
512 | Here are some examples: | |
513 | ||
514 | @example | |
515 | @group | |
516 | (defun foo () 5) | |
517 | @result{} foo | |
518 | @end group | |
519 | @group | |
520 | (foo) | |
521 | @result{} 5 | |
522 | @end group | |
523 | ||
524 | @group | |
525 | (defun bar (a &optional b &rest c) | |
526 | (list a b c)) | |
527 | @result{} bar | |
528 | @end group | |
529 | @group | |
530 | (bar 1 2 3 4 5) | |
531 | @result{} (1 2 (3 4 5)) | |
532 | @end group | |
533 | @group | |
534 | (bar 1) | |
535 | @result{} (1 nil nil) | |
536 | @end group | |
537 | @group | |
538 | (bar) | |
539 | @error{} Wrong number of arguments. | |
540 | @end group | |
541 | ||
542 | @group | |
543 | (defun capitalize-backwards () | |
544 | "Upcase the last letter of a word." | |
545 | (interactive) | |
546 | (backward-word 1) | |
547 | (forward-word 1) | |
548 | (backward-char 1) | |
549 | (capitalize-word 1)) | |
550 | @result{} capitalize-backwards | |
551 | @end group | |
552 | @end example | |
553 | ||
554 | Be careful not to redefine existing functions unintentionally. | |
555 | @code{defun} redefines even primitive functions such as @code{car} | |
556 | without any hesitation or notification. Redefining a function already | |
557 | defined is often done deliberately, and there is no way to distinguish | |
558 | deliberate redefinition from unintentional redefinition. | |
559 | @end defspec | |
560 | ||
561 | @defun defalias name definition | |
562 | This special form defines the symbol @var{name} as a function, with | |
f25df2ab | 563 | definition @var{definition} (which can be any valid Lisp function). |
bfe721d1 KH |
564 | |
565 | The proper place to use @code{defalias} is where a specific function | |
566 | name is being defined---especially where that name appears explicitly in | |
567 | the source file being loaded. This is because @code{defalias} records | |
568 | which file defined the function, just like @code{defun} | |
569 | (@pxref{Unloading}). | |
570 | ||
571 | By contrast, in programs that manipulate function definitions for other | |
572 | purposes, it is better to use @code{fset}, which does not keep such | |
573 | records. | |
9c52bf47 KH |
574 | @end defun |
575 | ||
bfe721d1 KH |
576 | See also @code{defsubst}, which defines a function like @code{defun} |
577 | and tells the Lisp compiler to open-code it. @xref{Inline Functions}. | |
578 | ||
9c52bf47 KH |
579 | @node Calling Functions |
580 | @section Calling Functions | |
581 | @cindex function invocation | |
582 | @cindex calling a function | |
583 | ||
584 | Defining functions is only half the battle. Functions don't do | |
585 | anything until you @dfn{call} them, i.e., tell them to run. Calling a | |
586 | function is also known as @dfn{invocation}. | |
587 | ||
f25df2ab RS |
588 | The most common way of invoking a function is by evaluating a list. |
589 | For example, evaluating the list @code{(concat "a" "b")} calls the | |
590 | function @code{concat} with arguments @code{"a"} and @code{"b"}. | |
591 | @xref{Evaluation}, for a description of evaluation. | |
9c52bf47 KH |
592 | |
593 | When you write a list as an expression in your program, the function | |
f9f59935 RS |
594 | name it calls is written in your program. This means that you choose |
595 | which function to call, and how many arguments to give it, when you | |
596 | write the program. Usually that's just what you want. Occasionally you | |
597 | need to compute at run time which function to call. To do that, use the | |
969fe9b5 | 598 | function @code{funcall}. When you also need to determine at run time |
a9f0a989 | 599 | how many arguments to pass, use @code{apply}. |
9c52bf47 KH |
600 | |
601 | @defun funcall function &rest arguments | |
602 | @code{funcall} calls @var{function} with @var{arguments}, and returns | |
603 | whatever @var{function} returns. | |
604 | ||
605 | Since @code{funcall} is a function, all of its arguments, including | |
606 | @var{function}, are evaluated before @code{funcall} is called. This | |
607 | means that you can use any expression to obtain the function to be | |
608 | called. It also means that @code{funcall} does not see the expressions | |
609 | you write for the @var{arguments}, only their values. These values are | |
610 | @emph{not} evaluated a second time in the act of calling @var{function}; | |
611 | @code{funcall} enters the normal procedure for calling a function at the | |
612 | place where the arguments have already been evaluated. | |
613 | ||
614 | The argument @var{function} must be either a Lisp function or a | |
615 | primitive function. Special forms and macros are not allowed, because | |
616 | they make sense only when given the ``unevaluated'' argument | |
617 | expressions. @code{funcall} cannot provide these because, as we saw | |
618 | above, it never knows them in the first place. | |
619 | ||
620 | @example | |
621 | @group | |
622 | (setq f 'list) | |
623 | @result{} list | |
624 | @end group | |
625 | @group | |
626 | (funcall f 'x 'y 'z) | |
627 | @result{} (x y z) | |
628 | @end group | |
629 | @group | |
630 | (funcall f 'x 'y '(z)) | |
631 | @result{} (x y (z)) | |
632 | @end group | |
633 | @group | |
634 | (funcall 'and t nil) | |
635 | @error{} Invalid function: #<subr and> | |
636 | @end group | |
637 | @end example | |
638 | ||
7f785b50 | 639 | Compare these examples with the examples of @code{apply}. |
9c52bf47 KH |
640 | @end defun |
641 | ||
642 | @defun apply function &rest arguments | |
643 | @code{apply} calls @var{function} with @var{arguments}, just like | |
644 | @code{funcall} but with one difference: the last of @var{arguments} is a | |
f9f59935 RS |
645 | list of objects, which are passed to @var{function} as separate |
646 | arguments, rather than a single list. We say that @code{apply} | |
647 | @dfn{spreads} this list so that each individual element becomes an | |
648 | argument. | |
9c52bf47 KH |
649 | |
650 | @code{apply} returns the result of calling @var{function}. As with | |
651 | @code{funcall}, @var{function} must either be a Lisp function or a | |
652 | primitive function; special forms and macros do not make sense in | |
653 | @code{apply}. | |
654 | ||
655 | @example | |
656 | @group | |
657 | (setq f 'list) | |
658 | @result{} list | |
659 | @end group | |
660 | @group | |
661 | (apply f 'x 'y 'z) | |
662 | @error{} Wrong type argument: listp, z | |
663 | @end group | |
664 | @group | |
665 | (apply '+ 1 2 '(3 4)) | |
666 | @result{} 10 | |
667 | @end group | |
668 | @group | |
669 | (apply '+ '(1 2 3 4)) | |
670 | @result{} 10 | |
671 | @end group | |
672 | ||
673 | @group | |
674 | (apply 'append '((a b c) nil (x y z) nil)) | |
675 | @result{} (a b c x y z) | |
676 | @end group | |
677 | @end example | |
678 | ||
679 | For an interesting example of using @code{apply}, see the description of | |
680 | @code{mapcar}, in @ref{Mapping Functions}. | |
681 | @end defun | |
682 | ||
683 | @cindex functionals | |
684 | It is common for Lisp functions to accept functions as arguments or | |
685 | find them in data structures (especially in hook variables and property | |
686 | lists) and call them using @code{funcall} or @code{apply}. Functions | |
687 | that accept function arguments are often called @dfn{functionals}. | |
688 | ||
bfe721d1 KH |
689 | Sometimes, when you call a functional, it is useful to supply a no-op |
690 | function as the argument. Here are two different kinds of no-op | |
9c52bf47 KH |
691 | function: |
692 | ||
693 | @defun identity arg | |
694 | This function returns @var{arg} and has no side effects. | |
695 | @end defun | |
696 | ||
697 | @defun ignore &rest args | |
698 | This function ignores any arguments and returns @code{nil}. | |
699 | @end defun | |
700 | ||
701 | @node Mapping Functions | |
702 | @section Mapping Functions | |
703 | @cindex mapping functions | |
704 | ||
705 | A @dfn{mapping function} applies a given function to each element of a | |
f9f59935 | 706 | list or other collection. Emacs Lisp has several such functions; |
9c52bf47 | 707 | @code{mapcar} and @code{mapconcat}, which scan a list, are described |
f9f59935 | 708 | here. @xref{Creating Symbols}, for the function @code{mapatoms} which |
7f785b50 GM |
709 | maps over the symbols in an obarray. @xref{Hash Access}, for the |
710 | function @code{maphash} which maps over key/value associations in a | |
711 | hash table. | |
969fe9b5 RS |
712 | |
713 | These mapping functions do not allow char-tables because a char-table | |
714 | is a sparse array whose nominal range of indices is very large. To map | |
715 | over a char-table in a way that deals properly with its sparse nature, | |
716 | use the function @code{map-char-table} (@pxref{Char-Tables}). | |
9c52bf47 KH |
717 | |
718 | @defun mapcar function sequence | |
f25df2ab RS |
719 | @code{mapcar} applies @var{function} to each element of @var{sequence} |
720 | in turn, and returns a list of the results. | |
9c52bf47 | 721 | |
969fe9b5 RS |
722 | The argument @var{sequence} can be any kind of sequence except a |
723 | char-table; that is, a list, a vector, a bool-vector, or a string. The | |
9c52bf47 KH |
724 | result is always a list. The length of the result is the same as the |
725 | length of @var{sequence}. | |
726 | ||
727 | @smallexample | |
728 | @group | |
729 | @exdent @r{For example:} | |
730 | ||
731 | (mapcar 'car '((a b) (c d) (e f))) | |
732 | @result{} (a c e) | |
733 | (mapcar '1+ [1 2 3]) | |
734 | @result{} (2 3 4) | |
735 | (mapcar 'char-to-string "abc") | |
736 | @result{} ("a" "b" "c") | |
737 | @end group | |
738 | ||
739 | @group | |
740 | ;; @r{Call each function in @code{my-hooks}.} | |
741 | (mapcar 'funcall my-hooks) | |
742 | @end group | |
743 | ||
744 | @group | |
969fe9b5 | 745 | (defun mapcar* (function &rest args) |
9c52bf47 KH |
746 | "Apply FUNCTION to successive cars of all ARGS. |
747 | Return the list of results." | |
748 | ;; @r{If no list is exhausted,} | |
177c0ea7 | 749 | (if (not (memq 'nil args)) |
969fe9b5 | 750 | ;; @r{apply function to @sc{car}s.} |
177c0ea7 JB |
751 | (cons (apply function (mapcar 'car args)) |
752 | (apply 'mapcar* function | |
9c52bf47 KH |
753 | ;; @r{Recurse for rest of elements.} |
754 | (mapcar 'cdr args))))) | |
755 | @end group | |
756 | ||
757 | @group | |
758 | (mapcar* 'cons '(a b c) '(1 2 3 4)) | |
759 | @result{} ((a . 1) (b . 2) (c . 3)) | |
760 | @end group | |
761 | @end smallexample | |
762 | @end defun | |
763 | ||
3c30cb6e DL |
764 | @defun mapc function sequence |
765 | @tindex mapc | |
766 | @code{mapc} is like @code{mapcar} except that @var{function} is used for | |
767 | side-effects only---the values it returns are ignored, not collected | |
768 | into a list. @code{mapc} always returns @var{sequence}. | |
769 | @end defun | |
770 | ||
9c52bf47 KH |
771 | @defun mapconcat function sequence separator |
772 | @code{mapconcat} applies @var{function} to each element of | |
773 | @var{sequence}: the results, which must be strings, are concatenated. | |
774 | Between each pair of result strings, @code{mapconcat} inserts the string | |
775 | @var{separator}. Usually @var{separator} contains a space or comma or | |
776 | other suitable punctuation. | |
777 | ||
778 | The argument @var{function} must be a function that can take one | |
969fe9b5 RS |
779 | argument and return a string. The argument @var{sequence} can be any |
780 | kind of sequence except a char-table; that is, a list, a vector, a | |
781 | bool-vector, or a string. | |
177c0ea7 | 782 | |
9c52bf47 KH |
783 | @smallexample |
784 | @group | |
785 | (mapconcat 'symbol-name | |
786 | '(The cat in the hat) | |
787 | " ") | |
788 | @result{} "The cat in the hat" | |
789 | @end group | |
790 | ||
791 | @group | |
792 | (mapconcat (function (lambda (x) (format "%c" (1+ x)))) | |
793 | "HAL-8000" | |
794 | "") | |
795 | @result{} "IBM.9111" | |
796 | @end group | |
797 | @end smallexample | |
798 | @end defun | |
799 | ||
800 | @node Anonymous Functions | |
801 | @section Anonymous Functions | |
802 | @cindex anonymous function | |
803 | ||
804 | In Lisp, a function is a list that starts with @code{lambda}, a | |
805 | byte-code function compiled from such a list, or alternatively a | |
806 | primitive subr-object; names are ``extra''. Although usually functions | |
807 | are defined with @code{defun} and given names at the same time, it is | |
808 | occasionally more concise to use an explicit lambda expression---an | |
809 | anonymous function. Such a list is valid wherever a function name is. | |
810 | ||
811 | Any method of creating such a list makes a valid function. Even this: | |
812 | ||
813 | @smallexample | |
814 | @group | |
ba3dafc8 | 815 | (setq silly (append '(lambda (x)) (list (list '+ (* 3 4) 'x)))) |
9c52bf47 KH |
816 | @result{} (lambda (x) (+ 12 x)) |
817 | @end group | |
818 | @end smallexample | |
819 | ||
820 | @noindent | |
821 | This computes a list that looks like @code{(lambda (x) (+ 12 x))} and | |
822 | makes it the value (@emph{not} the function definition!) of | |
823 | @code{silly}. | |
824 | ||
825 | Here is how we might call this function: | |
826 | ||
827 | @example | |
828 | @group | |
829 | (funcall silly 1) | |
830 | @result{} 13 | |
831 | @end group | |
832 | @end example | |
833 | ||
834 | @noindent | |
835 | (It does @emph{not} work to write @code{(silly 1)}, because this function | |
836 | is not the @emph{function definition} of @code{silly}. We have not given | |
837 | @code{silly} any function definition, just a value as a variable.) | |
838 | ||
839 | Most of the time, anonymous functions are constants that appear in | |
f9f59935 RS |
840 | your program. For example, you might want to pass one as an argument to |
841 | the function @code{mapcar}, which applies any given function to each | |
842 | element of a list. | |
843 | ||
177c0ea7 | 844 | Here we define a function @code{change-property} which |
f9f59935 | 845 | uses a function as its third argument: |
9c52bf47 KH |
846 | |
847 | @example | |
848 | @group | |
f9f59935 RS |
849 | (defun change-property (symbol prop function) |
850 | (let ((value (get symbol prop))) | |
851 | (put symbol prop (funcall function value)))) | |
9c52bf47 | 852 | @end group |
f9f59935 RS |
853 | @end example |
854 | ||
855 | @noindent | |
856 | Here we define a function that uses @code{change-property}, | |
969fe9b5 | 857 | passing it a function to double a number: |
f9f59935 RS |
858 | |
859 | @example | |
9c52bf47 | 860 | @group |
f9f59935 | 861 | (defun double-property (symbol prop) |
65500a82 | 862 | (change-property symbol prop '(lambda (x) (* 2 x)))) |
9c52bf47 KH |
863 | @end group |
864 | @end example | |
865 | ||
866 | @noindent | |
867 | In such cases, we usually use the special form @code{function} instead | |
f9f59935 | 868 | of simple quotation to quote the anonymous function, like this: |
9c52bf47 KH |
869 | |
870 | @example | |
871 | @group | |
f9f59935 | 872 | (defun double-property (symbol prop) |
a9f0a989 RS |
873 | (change-property symbol prop |
874 | (function (lambda (x) (* 2 x))))) | |
9c52bf47 KH |
875 | @end group |
876 | @end example | |
877 | ||
f9f59935 RS |
878 | Using @code{function} instead of @code{quote} makes a difference if you |
879 | compile the function @code{double-property}. For example, if you | |
880 | compile the second definition of @code{double-property}, the anonymous | |
881 | function is compiled as well. By contrast, if you compile the first | |
882 | definition which uses ordinary @code{quote}, the argument passed to | |
883 | @code{change-property} is the precise list shown: | |
9c52bf47 KH |
884 | |
885 | @example | |
886 | (lambda (x) (* x 2)) | |
887 | @end example | |
888 | ||
889 | @noindent | |
890 | The Lisp compiler cannot assume this list is a function, even though it | |
f9f59935 | 891 | looks like one, since it does not know what @code{change-property} will |
a9f0a989 | 892 | do with the list. Perhaps it will check whether the @sc{car} of the third |
f9f59935 RS |
893 | element is the symbol @code{*}! Using @code{function} tells the |
894 | compiler it is safe to go ahead and compile the constant function. | |
9c52bf47 | 895 | |
65500a82 RS |
896 | Nowadays it is possible to omit @code{function} entirely, like this: |
897 | ||
898 | @example | |
899 | @group | |
900 | (defun double-property (symbol prop) | |
901 | (change-property symbol prop (lambda (x) (* 2 x)))) | |
902 | @end group | |
903 | @end example | |
904 | ||
905 | @noindent | |
906 | This is because @code{lambda} itself implies @code{function}. | |
907 | ||
9c52bf47 KH |
908 | We sometimes write @code{function} instead of @code{quote} when |
909 | quoting the name of a function, but this usage is just a sort of | |
f9f59935 | 910 | comment: |
9c52bf47 KH |
911 | |
912 | @example | |
913 | (function @var{symbol}) @equiv{} (quote @var{symbol}) @equiv{} '@var{symbol} | |
a9f0a989 RS |
914 | @end example |
915 | ||
8241495d | 916 | @cindex @samp{#'} syntax |
a9f0a989 | 917 | The read syntax @code{#'} is a short-hand for using @code{function}. |
177c0ea7 | 918 | For example, |
a9f0a989 RS |
919 | |
920 | @example | |
921 | #'(lambda (x) (* x x)) | |
922 | @end example | |
923 | ||
924 | @noindent | |
925 | is equivalent to | |
926 | ||
927 | @example | |
928 | (function (lambda (x) (* x x))) | |
9c52bf47 KH |
929 | @end example |
930 | ||
f9f59935 RS |
931 | @defspec function function-object |
932 | @cindex function quoting | |
933 | This special form returns @var{function-object} without evaluating it. | |
934 | In this, it is equivalent to @code{quote}. However, it serves as a | |
935 | note to the Emacs Lisp compiler that @var{function-object} is intended | |
936 | to be used only as a function, and therefore can safely be compiled. | |
937 | Contrast this with @code{quote}, in @ref{Quoting}. | |
938 | @end defspec | |
939 | ||
9c52bf47 KH |
940 | See @code{documentation} in @ref{Accessing Documentation}, for a |
941 | realistic example using @code{function} and an anonymous function. | |
942 | ||
943 | @node Function Cells | |
944 | @section Accessing Function Cell Contents | |
945 | ||
946 | The @dfn{function definition} of a symbol is the object stored in the | |
947 | function cell of the symbol. The functions described here access, test, | |
948 | and set the function cell of symbols. | |
949 | ||
f25df2ab RS |
950 | See also the function @code{indirect-function} in @ref{Function |
951 | Indirection}. | |
952 | ||
9c52bf47 KH |
953 | @defun symbol-function symbol |
954 | @kindex void-function | |
955 | This returns the object in the function cell of @var{symbol}. If the | |
956 | symbol's function cell is void, a @code{void-function} error is | |
957 | signaled. | |
958 | ||
959 | This function does not check that the returned object is a legitimate | |
960 | function. | |
961 | ||
962 | @example | |
963 | @group | |
964 | (defun bar (n) (+ n 2)) | |
965 | @result{} bar | |
966 | @end group | |
967 | @group | |
968 | (symbol-function 'bar) | |
969 | @result{} (lambda (n) (+ n 2)) | |
970 | @end group | |
971 | @group | |
972 | (fset 'baz 'bar) | |
973 | @result{} bar | |
974 | @end group | |
975 | @group | |
976 | (symbol-function 'baz) | |
977 | @result{} bar | |
978 | @end group | |
979 | @end example | |
980 | @end defun | |
981 | ||
982 | @cindex void function cell | |
983 | If you have never given a symbol any function definition, we say that | |
984 | that symbol's function cell is @dfn{void}. In other words, the function | |
985 | cell does not have any Lisp object in it. If you try to call such a symbol | |
986 | as a function, it signals a @code{void-function} error. | |
987 | ||
988 | Note that void is not the same as @code{nil} or the symbol | |
989 | @code{void}. The symbols @code{nil} and @code{void} are Lisp objects, | |
990 | and can be stored into a function cell just as any other object can be | |
991 | (and they can be valid functions if you define them in turn with | |
f25df2ab | 992 | @code{defun}). A void function cell contains no object whatsoever. |
9c52bf47 KH |
993 | |
994 | You can test the voidness of a symbol's function definition with | |
995 | @code{fboundp}. After you have given a symbol a function definition, you | |
996 | can make it void once more using @code{fmakunbound}. | |
997 | ||
998 | @defun fboundp symbol | |
999 | This function returns @code{t} if the symbol has an object in its | |
1000 | function cell, @code{nil} otherwise. It does not check that the object | |
1001 | is a legitimate function. | |
1002 | @end defun | |
1003 | ||
1004 | @defun fmakunbound symbol | |
1005 | This function makes @var{symbol}'s function cell void, so that a | |
1006 | subsequent attempt to access this cell will cause a @code{void-function} | |
a9f0a989 | 1007 | error. (See also @code{makunbound}, in @ref{Void Variables}.) |
9c52bf47 KH |
1008 | |
1009 | @example | |
1010 | @group | |
1011 | (defun foo (x) x) | |
f9f59935 | 1012 | @result{} foo |
9c52bf47 KH |
1013 | @end group |
1014 | @group | |
f25df2ab RS |
1015 | (foo 1) |
1016 | @result{}1 | |
1017 | @end group | |
1018 | @group | |
9c52bf47 | 1019 | (fmakunbound 'foo) |
f9f59935 | 1020 | @result{} foo |
9c52bf47 KH |
1021 | @end group |
1022 | @group | |
1023 | (foo 1) | |
1024 | @error{} Symbol's function definition is void: foo | |
1025 | @end group | |
1026 | @end example | |
1027 | @end defun | |
1028 | ||
baa573a3 | 1029 | @defun fset symbol definition |
f9f59935 RS |
1030 | This function stores @var{definition} in the function cell of |
1031 | @var{symbol}. The result is @var{definition}. Normally | |
1032 | @var{definition} should be a function or the name of a function, but | |
1033 | this is not checked. The argument @var{symbol} is an ordinary evaluated | |
1034 | argument. | |
9c52bf47 KH |
1035 | |
1036 | There are three normal uses of this function: | |
1037 | ||
1038 | @itemize @bullet | |
1039 | @item | |
969fe9b5 RS |
1040 | Copying one symbol's function definition to another---in other words, |
1041 | making an alternate name for a function. (If you think of this as the | |
1042 | definition of the new name, you should use @code{defalias} instead of | |
1043 | @code{fset}; see @ref{Defining Functions}.) | |
9c52bf47 KH |
1044 | |
1045 | @item | |
1046 | Giving a symbol a function definition that is not a list and therefore | |
f25df2ab RS |
1047 | cannot be made with @code{defun}. For example, you can use @code{fset} |
1048 | to give a symbol @code{s1} a function definition which is another symbol | |
1049 | @code{s2}; then @code{s1} serves as an alias for whatever definition | |
969fe9b5 RS |
1050 | @code{s2} presently has. (Once again use @code{defalias} instead of |
1051 | @code{fset} if you think of this as the definition of @code{s1}.) | |
9c52bf47 KH |
1052 | |
1053 | @item | |
1054 | In constructs for defining or altering functions. If @code{defun} | |
1055 | were not a primitive, it could be written in Lisp (as a macro) using | |
1056 | @code{fset}. | |
1057 | @end itemize | |
1058 | ||
969fe9b5 | 1059 | Here are examples of these uses: |
9c52bf47 KH |
1060 | |
1061 | @example | |
1062 | @group | |
969fe9b5 RS |
1063 | ;; @r{Save @code{foo}'s definition in @code{old-foo}.} |
1064 | (fset 'old-foo (symbol-function 'foo)) | |
9c52bf47 KH |
1065 | @end group |
1066 | ||
1067 | @group | |
1068 | ;; @r{Make the symbol @code{car} the function definition of @code{xfirst}.} | |
969fe9b5 | 1069 | ;; @r{(Most likely, @code{defalias} would be better than @code{fset} here.)} |
9c52bf47 KH |
1070 | (fset 'xfirst 'car) |
1071 | @result{} car | |
1072 | @end group | |
1073 | @group | |
1074 | (xfirst '(1 2 3)) | |
1075 | @result{} 1 | |
1076 | @end group | |
1077 | @group | |
1078 | (symbol-function 'xfirst) | |
1079 | @result{} car | |
1080 | @end group | |
1081 | @group | |
1082 | (symbol-function (symbol-function 'xfirst)) | |
1083 | @result{} #<subr car> | |
1084 | @end group | |
1085 | ||
1086 | @group | |
1087 | ;; @r{Define a named keyboard macro.} | |
1088 | (fset 'kill-two-lines "\^u2\^k") | |
1089 | @result{} "\^u2\^k" | |
1090 | @end group | |
f25df2ab | 1091 | |
969fe9b5 RS |
1092 | @group |
1093 | ;; @r{Here is a function that alters other functions.} | |
1094 | (defun copy-function-definition (new old) | |
1095 | "Define NEW with the same function definition as OLD." | |
1096 | (fset new (symbol-function old))) | |
1097 | @end group | |
1098 | @end example | |
9c52bf47 KH |
1099 | @end defun |
1100 | ||
1101 | When writing a function that extends a previously defined function, | |
bfe721d1 | 1102 | the following idiom is sometimes used: |
9c52bf47 KH |
1103 | |
1104 | @example | |
1105 | (fset 'old-foo (symbol-function 'foo)) | |
1106 | (defun foo () | |
1107 | "Just like old-foo, except more so." | |
1108 | @group | |
1109 | (old-foo) | |
1110 | (more-so)) | |
1111 | @end group | |
1112 | @end example | |
1113 | ||
1114 | @noindent | |
1115 | This does not work properly if @code{foo} has been defined to autoload. | |
1116 | In such a case, when @code{foo} calls @code{old-foo}, Lisp attempts | |
1117 | to define @code{old-foo} by loading a file. Since this presumably | |
1118 | defines @code{foo} rather than @code{old-foo}, it does not produce the | |
1119 | proper results. The only way to avoid this problem is to make sure the | |
1120 | file is loaded before moving aside the old definition of @code{foo}. | |
1121 | ||
bfe721d1 | 1122 | But it is unmodular and unclean, in any case, for a Lisp file to |
969fe9b5 RS |
1123 | redefine a function defined elsewhere. It is cleaner to use the advice |
1124 | facility (@pxref{Advising Functions}). | |
bfe721d1 | 1125 | |
9c52bf47 KH |
1126 | @node Inline Functions |
1127 | @section Inline Functions | |
1128 | @cindex inline functions | |
1129 | ||
1130 | @findex defsubst | |
1131 | You can define an @dfn{inline function} by using @code{defsubst} instead | |
1132 | of @code{defun}. An inline function works just like an ordinary | |
1133 | function except for one thing: when you compile a call to the function, | |
1134 | the function's definition is open-coded into the caller. | |
1135 | ||
1136 | Making a function inline makes explicit calls run faster. But it also | |
1137 | has disadvantages. For one thing, it reduces flexibility; if you change | |
1138 | the definition of the function, calls already inlined still use the old | |
1139 | definition until you recompile them. Since the flexibility of | |
1140 | redefining functions is an important feature of Emacs, you should not | |
1141 | make a function inline unless its speed is really crucial. | |
1142 | ||
1143 | Another disadvantage is that making a large function inline can increase | |
1144 | the size of compiled code both in files and in memory. Since the speed | |
1145 | advantage of inline functions is greatest for small functions, you | |
1146 | generally should not make large functions inline. | |
1147 | ||
1148 | It's possible to define a macro to expand into the same code that an | |
969fe9b5 RS |
1149 | inline function would execute. (@xref{Macros}.) But the macro would be |
1150 | limited to direct use in expressions---a macro cannot be called with | |
9c52bf47 | 1151 | @code{apply}, @code{mapcar} and so on. Also, it takes some work to |
969fe9b5 RS |
1152 | convert an ordinary function into a macro. To convert it into an inline |
1153 | function is very easy; simply replace @code{defun} with @code{defsubst}. | |
1154 | Since each argument of an inline function is evaluated exactly once, you | |
1155 | needn't worry about how many times the body uses the arguments, as you | |
1156 | do for macros. (@xref{Argument Evaluation}.) | |
9c52bf47 | 1157 | |
f25df2ab | 1158 | Inline functions can be used and open-coded later on in the same file, |
9c52bf47 KH |
1159 | following the definition, just like macros. |
1160 | ||
7ed9159a JY |
1161 | @node Function safety |
1162 | @section Determining whether a function is safe to call | |
1163 | @cindex function safety | |
1164 | @cindex safety of functions | |
1165 | @cindex virus detection | |
1166 | @cindex Trojan-horse detection | |
1167 | @cindex DDoS attacks | |
1168 | ||
1169 | Some major modes such as SES (see @pxref{Top,,,ses}) will call | |
1170 | functions that are stored in user files. User files sometimes have | |
1171 | poor pedigrees---you can get a spreadsheet from someone you've just | |
1172 | met, or you can get one through email from someone you've never met. | |
1173 | Such files can contain viruses and other Trojan horses that could | |
1174 | corrupt your operating system environment, delete your files, or even | |
1175 | turn your computer into a DDoS zombie! To avoid this terrible fate, | |
1176 | you should not call a function whose source code is stored in a user | |
1177 | file until you have determined that it is safe. | |
1178 | ||
1179 | @defun unsafep form &optional unsafep-vars | |
1180 | Returns nil if @var{form} is a @dfn{safe} lisp expression, or returns | |
1181 | a list that describes why it might be unsafe. The argument | |
1182 | @var{unsafep-vars} is a list of symbols known to have temporary | |
1183 | bindings at this point; it is mainly used for internal recursive | |
1184 | calls. The current buffer is an implicit argument, which provides a | |
1185 | list of buffer-local bindings. | |
1186 | @end defun | |
1187 | ||
1188 | Being quick and simple, @code{unsafep} does a very light analysis and | |
1189 | rejects many Lisp expressions that are actually safe. There are no | |
1190 | known cases where @code{unsafep} returns nil for an unsafe expression. | |
1191 | However, a ``safe'' Lisp expression can return a string with a | |
1192 | @code{display} property, containing an associated Lisp expression to | |
1193 | be executed after the string is inserted into a buffer. This | |
1194 | associated expression can be a virus. In order to be safe, you must | |
1195 | delete properties from all strings calculated by user code before | |
1196 | inserting them into buffers. | |
1197 | ||
1198 | What is a safe Lisp expression? Basically, it's an expression that | |
1199 | calls only built-in functions with no side effects (or only innocuous | |
1200 | ones). Innocuous side effects include displaying messages and | |
1201 | altering non-risky buffer-local variables (but not global variables). | |
1202 | ||
1203 | @table @dfn | |
1204 | @item Safe expression | |
1205 | @itemize | |
1206 | @item | |
1207 | An atom or quoted thing. | |
1208 | @item | |
1209 | A call to a safe function (see below), if all its arguments are | |
1210 | safe expressions. | |
1211 | @item | |
1212 | One of the special forms [and, catch, cond, if, or, prog1, prog2, | |
1213 | progn, while, unwind-protect], if all its arguments are safe. | |
1214 | @item | |
1215 | A form that creates temporary bindings [condition-case, dolist, | |
1216 | dotimes, lambda, let, let*], if all args are safe and the symbols to | |
1217 | be bound are not explicitly risky (see @pxref{File Local Variables}). | |
1218 | @item | |
1219 | An assignment [add-to-list, setq, push, pop], if all args are safe and | |
1220 | the symbols to be assigned are not explicitly risky and they already | |
1221 | have temporary or buffer-local bindings. | |
1222 | @item | |
1223 | One of [apply, mapc, mapcar, mapconcat] if the first argument is a | |
1224 | safe explicit lambda and the other args are safe expressions. | |
1225 | @end itemize | |
1226 | ||
1227 | @item Safe function | |
1228 | @itemize | |
1229 | @item | |
1230 | A lambda containing safe expressions. | |
1231 | @item | |
1232 | A symbol on the list @code{safe-functions}, so the user says it's safe. | |
1233 | @item | |
1234 | A symbol with a non-nil @code{side-effect-free} property. | |
1235 | @item | |
1236 | A symbol with a non-nil @code{safe-function} property. Value t | |
1237 | indicates a function that is safe but has innocuous side effects. | |
1238 | Other values will someday indicate functions with classes of side | |
1239 | effects that are not always safe. | |
1240 | @end itemize | |
1241 | ||
1242 | The @code{side-effect-free} and @code{safe-function} properties are | |
1243 | provided for built-in functions and for low-level functions and macros | |
1244 | defined in @file{subr.el}. You can assign these properties for the | |
1245 | functions you write. | |
1246 | ||
1247 | @end table | |
1248 | ||
1249 | ||
bfe721d1 | 1250 | @c Emacs versions prior to 19 did not have inline functions. |
9c52bf47 KH |
1251 | |
1252 | @node Related Topics | |
1253 | @section Other Topics Related to Functions | |
1254 | ||
1255 | Here is a table of several functions that do things related to | |
1256 | function calling and function definitions. They are documented | |
1257 | elsewhere, but we provide cross references here. | |
1258 | ||
1259 | @table @code | |
1260 | @item apply | |
1261 | See @ref{Calling Functions}. | |
1262 | ||
1263 | @item autoload | |
1264 | See @ref{Autoload}. | |
1265 | ||
1266 | @item call-interactively | |
1267 | See @ref{Interactive Call}. | |
1268 | ||
1269 | @item commandp | |
1270 | See @ref{Interactive Call}. | |
1271 | ||
1272 | @item documentation | |
1273 | See @ref{Accessing Documentation}. | |
1274 | ||
1275 | @item eval | |
1276 | See @ref{Eval}. | |
1277 | ||
1278 | @item funcall | |
1279 | See @ref{Calling Functions}. | |
1280 | ||
969fe9b5 RS |
1281 | @item function |
1282 | See @ref{Anonymous Functions}. | |
1283 | ||
9c52bf47 KH |
1284 | @item ignore |
1285 | See @ref{Calling Functions}. | |
1286 | ||
1287 | @item indirect-function | |
1288 | See @ref{Function Indirection}. | |
1289 | ||
1290 | @item interactive | |
1291 | See @ref{Using Interactive}. | |
1292 | ||
1293 | @item interactive-p | |
1294 | See @ref{Interactive Call}. | |
1295 | ||
1296 | @item mapatoms | |
1297 | See @ref{Creating Symbols}. | |
1298 | ||
1299 | @item mapcar | |
1300 | See @ref{Mapping Functions}. | |
1301 | ||
969fe9b5 RS |
1302 | @item map-char-table |
1303 | See @ref{Char-Tables}. | |
1304 | ||
9c52bf47 KH |
1305 | @item mapconcat |
1306 | See @ref{Mapping Functions}. | |
1307 | ||
1308 | @item undefined | |
1309 | See @ref{Key Lookup}. | |
1310 | @end table | |
1311 |