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1 | @c -*-texinfo-*- |
2 | @c This is part of the GNU Emacs Lisp Reference Manual. | |
7baeca0c | 3 | @c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999, 2004 |
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. | |
a68defff | 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 | |
67bce69d | 56 | @dfn{built-in functions}, or @dfn{subrs}. (Special forms are also |
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57 | considered primitives.) |
58 | ||
a9f0a989 | 59 | Usually the reason we implement a function as a primitive is either |
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60 | because it is fundamental, because it provides a low-level interface |
61 | to operating system services, or because it needs to run fast. | |
62 | Primitives can be modified or added only by changing the C sources and | |
63 | recompiling 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 |
117 | function, or a special form, or, recursively, a symbol whose function | |
118 | definition is a function or special form. (This does not include | |
119 | macros.) | |
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120 | @end defun |
121 | ||
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122 | Unlike @code{functionp}, the next three functions do @emph{not} |
123 | treat a symbol as its function definition. | |
124 | ||
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125 | @defun subrp object |
126 | This function returns @code{t} if @var{object} is a built-in function | |
127 | (i.e., a Lisp primitive). | |
128 | ||
129 | @example | |
130 | @group | |
131 | (subrp 'message) ; @r{@code{message} is a symbol,} | |
132 | @result{} nil ; @r{not a subr object.} | |
133 | @end group | |
134 | @group | |
135 | (subrp (symbol-function 'message)) | |
136 | @result{} t | |
137 | @end group | |
138 | @end example | |
139 | @end defun | |
140 | ||
141 | @defun byte-code-function-p object | |
142 | This function returns @code{t} if @var{object} is a byte-code | |
143 | function. For example: | |
144 | ||
145 | @example | |
146 | @group | |
147 | (byte-code-function-p (symbol-function 'next-line)) | |
148 | @result{} t | |
149 | @end group | |
150 | @end example | |
151 | @end defun | |
152 | ||
aad52941 DL |
153 | @defun subr-arity subr |
154 | @tindex subr-arity | |
155 | This function provides information about the argument list of a | |
156 | primitive, @var{subr}. The returned value is a pair | |
157 | @code{(@var{min} . @var{max})}. @var{min} is the minimum number of | |
158 | args. @var{max} is the maximum number or the symbol @code{many}, for a | |
159 | function with @code{&rest} arguments, or the symbol @code{unevalled} if | |
160 | @var{subr} is a special form. | |
161 | @end defun | |
162 | ||
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163 | @node Lambda Expressions |
164 | @section Lambda Expressions | |
165 | @cindex lambda expression | |
166 | ||
167 | A function written in Lisp is a list that looks like this: | |
168 | ||
169 | @example | |
170 | (lambda (@var{arg-variables}@dots{}) | |
171 | @r{[}@var{documentation-string}@r{]} | |
172 | @r{[}@var{interactive-declaration}@r{]} | |
173 | @var{body-forms}@dots{}) | |
174 | @end example | |
175 | ||
176 | @noindent | |
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177 | Such a list is called a @dfn{lambda expression}. In Emacs Lisp, it |
178 | actually is valid as an expression---it evaluates to itself. In some | |
179 | other Lisp dialects, a lambda expression is not a valid expression at | |
180 | all. In either case, its main use is not to be evaluated as an | |
181 | expression, but to be called as a function. | |
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182 | |
183 | @menu | |
184 | * Lambda Components:: The parts of a lambda expression. | |
185 | * Simple Lambda:: A simple example. | |
186 | * Argument List:: Details and special features of argument lists. | |
187 | * Function Documentation:: How to put documentation in a function. | |
188 | @end menu | |
189 | ||
190 | @node Lambda Components | |
191 | @subsection Components of a Lambda Expression | |
192 | ||
37680279 | 193 | @ifnottex |
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194 | |
195 | A function written in Lisp (a ``lambda expression'') is a list that | |
196 | looks like this: | |
197 | ||
198 | @example | |
199 | (lambda (@var{arg-variables}@dots{}) | |
200 | [@var{documentation-string}] | |
201 | [@var{interactive-declaration}] | |
202 | @var{body-forms}@dots{}) | |
203 | @end example | |
37680279 | 204 | @end ifnottex |
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205 | |
206 | @cindex lambda list | |
207 | The first element of a lambda expression is always the symbol | |
208 | @code{lambda}. This indicates that the list represents a function. The | |
209 | reason functions are defined to start with @code{lambda} is so that | |
210 | other lists, intended for other uses, will not accidentally be valid as | |
211 | functions. | |
212 | ||
f9f59935 | 213 | The second element is a list of symbols---the argument variable names. |
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214 | This is called the @dfn{lambda list}. When a Lisp function is called, |
215 | the argument values are matched up against the variables in the lambda | |
216 | list, which are given local bindings with the values provided. | |
217 | @xref{Local Variables}. | |
218 | ||
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219 | The documentation string is a Lisp string object placed within the |
220 | function definition to describe the function for the Emacs help | |
221 | facilities. @xref{Function Documentation}. | |
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222 | |
223 | The interactive declaration is a list of the form @code{(interactive | |
224 | @var{code-string})}. This declares how to provide arguments if the | |
225 | function is used interactively. Functions with this declaration are called | |
226 | @dfn{commands}; they can be called using @kbd{M-x} or bound to a key. | |
227 | Functions not intended to be called in this way should not have interactive | |
228 | declarations. @xref{Defining Commands}, for how to write an interactive | |
229 | declaration. | |
230 | ||
231 | @cindex body of function | |
232 | The rest of the elements are the @dfn{body} of the function: the Lisp | |
233 | code to do the work of the function (or, as a Lisp programmer would say, | |
234 | ``a list of Lisp forms to evaluate''). The value returned by the | |
235 | function is the value returned by the last element of the body. | |
236 | ||
237 | @node Simple Lambda | |
238 | @subsection A Simple Lambda-Expression Example | |
239 | ||
240 | Consider for example the following function: | |
241 | ||
242 | @example | |
243 | (lambda (a b c) (+ a b c)) | |
244 | @end example | |
245 | ||
246 | @noindent | |
247 | We can call this function by writing it as the @sc{car} of an | |
248 | expression, like this: | |
249 | ||
250 | @example | |
251 | @group | |
252 | ((lambda (a b c) (+ a b c)) | |
253 | 1 2 3) | |
254 | @end group | |
255 | @end example | |
256 | ||
257 | @noindent | |
258 | This call evaluates the body of the lambda expression with the variable | |
259 | @code{a} bound to 1, @code{b} bound to 2, and @code{c} bound to 3. | |
260 | Evaluation of the body adds these three numbers, producing the result 6; | |
261 | therefore, this call to the function returns the value 6. | |
262 | ||
263 | Note that the arguments can be the results of other function calls, as in | |
264 | this example: | |
265 | ||
266 | @example | |
267 | @group | |
268 | ((lambda (a b c) (+ a b c)) | |
269 | 1 (* 2 3) (- 5 4)) | |
270 | @end group | |
271 | @end example | |
272 | ||
273 | @noindent | |
274 | This evaluates the arguments @code{1}, @code{(* 2 3)}, and @code{(- 5 | |
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275 | 4)} from left to right. Then it applies the lambda expression to the |
276 | argument values 1, 6 and 1 to produce the value 8. | |
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277 | |
278 | It is not often useful to write a lambda expression as the @sc{car} of | |
279 | a form in this way. You can get the same result, of making local | |
280 | variables and giving them values, using the special form @code{let} | |
281 | (@pxref{Local Variables}). And @code{let} is clearer and easier to use. | |
282 | In practice, lambda expressions are either stored as the function | |
283 | definitions of symbols, to produce named functions, or passed as | |
284 | arguments to other functions (@pxref{Anonymous Functions}). | |
285 | ||
286 | However, calls to explicit lambda expressions were very useful in the | |
287 | old days of Lisp, before the special form @code{let} was invented. At | |
288 | that time, they were the only way to bind and initialize local | |
289 | variables. | |
290 | ||
291 | @node Argument List | |
f9f59935 | 292 | @subsection Other Features of Argument Lists |
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293 | @kindex wrong-number-of-arguments |
294 | @cindex argument binding | |
295 | @cindex binding arguments | |
296 | ||
297 | Our simple sample function, @code{(lambda (a b c) (+ a b c))}, | |
298 | specifies three argument variables, so it must be called with three | |
299 | arguments: if you try to call it with only two arguments or four | |
300 | arguments, you get a @code{wrong-number-of-arguments} error. | |
301 | ||
302 | It is often convenient to write a function that allows certain | |
303 | arguments to be omitted. For example, the function @code{substring} | |
304 | accepts three arguments---a string, the start index and the end | |
305 | index---but the third argument defaults to the @var{length} of the | |
306 | string if you omit it. It is also convenient for certain functions to | |
f25df2ab | 307 | accept an indefinite number of arguments, as the functions @code{list} |
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308 | and @code{+} do. |
309 | ||
310 | @cindex optional arguments | |
311 | @cindex rest arguments | |
312 | @kindex &optional | |
313 | @kindex &rest | |
314 | To specify optional arguments that may be omitted when a function | |
315 | is called, simply include the keyword @code{&optional} before the optional | |
316 | arguments. To specify a list of zero or more extra arguments, include the | |
317 | keyword @code{&rest} before one final argument. | |
318 | ||
319 | Thus, the complete syntax for an argument list is as follows: | |
320 | ||
321 | @example | |
322 | @group | |
323 | (@var{required-vars}@dots{} | |
324 | @r{[}&optional @var{optional-vars}@dots{}@r{]} | |
325 | @r{[}&rest @var{rest-var}@r{]}) | |
326 | @end group | |
327 | @end example | |
328 | ||
329 | @noindent | |
330 | The square brackets indicate that the @code{&optional} and @code{&rest} | |
331 | clauses, and the variables that follow them, are optional. | |
332 | ||
333 | A call to the function requires one actual argument for each of the | |
334 | @var{required-vars}. There may be actual arguments for zero or more of | |
335 | the @var{optional-vars}, and there cannot be any actual arguments beyond | |
336 | that unless the lambda list uses @code{&rest}. In that case, there may | |
337 | be any number of extra actual arguments. | |
338 | ||
339 | If actual arguments for the optional and rest variables are omitted, | |
f25df2ab | 340 | then they always default to @code{nil}. There is no way for the |
9c52bf47 | 341 | function to distinguish between an explicit argument of @code{nil} and |
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342 | an omitted argument. However, the body of the function is free to |
343 | consider @code{nil} an abbreviation for some other meaningful value. | |
344 | This is what @code{substring} does; @code{nil} as the third argument to | |
345 | @code{substring} means to use the length of the string supplied. | |
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346 | |
347 | @cindex CL note---default optional arg | |
348 | @quotation | |
349 | @b{Common Lisp note:} Common Lisp allows the function to specify what | |
350 | default value to use when an optional argument is omitted; Emacs Lisp | |
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351 | always uses @code{nil}. Emacs Lisp does not support ``supplied-p'' |
352 | variables that tell you whether an argument was explicitly passed. | |
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353 | @end quotation |
354 | ||
355 | For example, an argument list that looks like this: | |
356 | ||
357 | @example | |
358 | (a b &optional c d &rest e) | |
359 | @end example | |
360 | ||
361 | @noindent | |
362 | binds @code{a} and @code{b} to the first two actual arguments, which are | |
363 | required. If one or two more arguments are provided, @code{c} and | |
364 | @code{d} are bound to them respectively; any arguments after the first | |
365 | four are collected into a list and @code{e} is bound to that list. If | |
366 | there are only two arguments, @code{c} is @code{nil}; if two or three | |
367 | arguments, @code{d} is @code{nil}; if four arguments or fewer, @code{e} | |
368 | is @code{nil}. | |
369 | ||
370 | There is no way to have required arguments following optional | |
371 | ones---it would not make sense. To see why this must be so, suppose | |
372 | that @code{c} in the example were optional and @code{d} were required. | |
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373 | Suppose three actual arguments are given; which variable would the |
374 | third argument be for? Would it be used for the @var{c}, or for | |
375 | @var{d}? One can argue for both possibilities. Similarly, it makes | |
376 | no sense to have any more arguments (either required or optional) | |
377 | after a @code{&rest} argument. | |
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378 | |
379 | Here are some examples of argument lists and proper calls: | |
380 | ||
381 | @smallexample | |
382 | ((lambda (n) (1+ n)) ; @r{One required:} | |
383 | 1) ; @r{requires exactly one argument.} | |
384 | @result{} 2 | |
385 | ((lambda (n &optional n1) ; @r{One required and one optional:} | |
386 | (if n1 (+ n n1) (1+ n))) ; @r{1 or 2 arguments.} | |
387 | 1 2) | |
388 | @result{} 3 | |
389 | ((lambda (n &rest ns) ; @r{One required and one rest:} | |
390 | (+ n (apply '+ ns))) ; @r{1 or more arguments.} | |
391 | 1 2 3 4 5) | |
392 | @result{} 15 | |
393 | @end smallexample | |
394 | ||
395 | @node Function Documentation | |
396 | @subsection Documentation Strings of Functions | |
397 | @cindex documentation of function | |
398 | ||
399 | A lambda expression may optionally have a @dfn{documentation string} just | |
400 | after the lambda list. This string does not affect execution of the | |
401 | function; it is a kind of comment, but a systematized comment which | |
402 | actually appears inside the Lisp world and can be used by the Emacs help | |
403 | facilities. @xref{Documentation}, for how the @var{documentation-string} is | |
404 | accessed. | |
405 | ||
bfe721d1 | 406 | It is a good idea to provide documentation strings for all the |
969fe9b5 | 407 | functions in your program, even those that are called only from within |
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408 | your program. Documentation strings are like comments, except that they |
409 | are easier to access. | |
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410 | |
411 | The first line of the documentation string should stand on its own, | |
412 | because @code{apropos} displays just this first line. It should consist | |
413 | of one or two complete sentences that summarize the function's purpose. | |
414 | ||
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415 | The start of the documentation string is usually indented in the |
416 | source file, but since these spaces come before the starting | |
417 | double-quote, they are not part of the string. Some people make a | |
418 | practice of indenting any additional lines of the string so that the | |
419 | text lines up in the program source. @emph{That is a mistake.} The | |
420 | indentation of the following lines is inside the string; what looks | |
421 | nice in the source code will look ugly when displayed by the help | |
422 | commands. | |
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423 | |
424 | You may wonder how the documentation string could be optional, since | |
425 | there are required components of the function that follow it (the body). | |
426 | Since evaluation of a string returns that string, without any side effects, | |
427 | it has no effect if it is not the last form in the body. Thus, in | |
428 | practice, there is no confusion between the first form of the body and the | |
429 | documentation string; if the only body form is a string then it serves both | |
430 | as the return value and as the documentation. | |
431 | ||
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432 | The last line of the documentation string can specify calling |
433 | conventions different from the actual function arguments. Write | |
434 | text like this: | |
435 | ||
436 | @example | |
a0e91642 | 437 | \(fn @var{arglist}) |
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438 | @end example |
439 | ||
440 | @noindent | |
a0e91642 | 441 | following a blank line, at the beginning of the line, with no newline |
67bce69d RS |
442 | following it inside the documentation string. (The @samp{\} is used |
443 | to avoid confusing the Emacs motion commands.) The calling convention | |
444 | specified in this way appears in help messages in place of the one | |
445 | derived from the actual arguments of the function. | |
446 | ||
447 | This feature is particularly useful for macro definitions, since the | |
448 | arguments written in a macro definition often do not correspond to the | |
449 | way users think of the parts of the macro call. | |
13a105af | 450 | |
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451 | @node Function Names |
452 | @section Naming a Function | |
453 | @cindex function definition | |
454 | @cindex named function | |
455 | @cindex function name | |
456 | ||
457 | In most computer languages, every function has a name; the idea of a | |
458 | function without a name is nonsensical. In Lisp, a function in the | |
459 | strictest sense has no name. It is simply a list whose first element is | |
969fe9b5 | 460 | @code{lambda}, a byte-code function object, or a primitive subr-object. |
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461 | |
462 | However, a symbol can serve as the name of a function. This happens | |
463 | when you put the function in the symbol's @dfn{function cell} | |
464 | (@pxref{Symbol Components}). Then the symbol itself becomes a valid, | |
465 | callable function, equivalent to the list or subr-object that its | |
466 | function cell refers to. The contents of the function cell are also | |
467 | called the symbol's @dfn{function definition}. The procedure of using a | |
468 | symbol's function definition in place of the symbol is called | |
469 | @dfn{symbol function indirection}; see @ref{Function Indirection}. | |
470 | ||
471 | In practice, nearly all functions are given names in this way and | |
472 | referred to through their names. For example, the symbol @code{car} works | |
473 | as a function and does what it does because the primitive subr-object | |
474 | @code{#<subr car>} is stored in its function cell. | |
475 | ||
476 | We give functions names because it is convenient to refer to them by | |
477 | their names in Lisp expressions. For primitive subr-objects such as | |
478 | @code{#<subr car>}, names are the only way you can refer to them: there | |
479 | is no read syntax for such objects. For functions written in Lisp, the | |
480 | name is more convenient to use in a call than an explicit lambda | |
481 | expression. Also, a function with a name can refer to itself---it can | |
482 | be recursive. Writing the function's name in its own definition is much | |
483 | more convenient than making the function definition point to itself | |
484 | (something that is not impossible but that has various disadvantages in | |
485 | practice). | |
486 | ||
487 | We often identify functions with the symbols used to name them. For | |
488 | example, we often speak of ``the function @code{car}'', not | |
489 | distinguishing between the symbol @code{car} and the primitive | |
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490 | subr-object that is its function definition. For most purposes, the |
491 | distinction is not important. | |
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492 | |
493 | Even so, keep in mind that a function need not have a unique name. While | |
494 | a given function object @emph{usually} appears in the function cell of only | |
495 | one symbol, this is just a matter of convenience. It is easy to store | |
496 | it in several symbols using @code{fset}; then each of the symbols is | |
497 | equally well a name for the same function. | |
498 | ||
a9f0a989 RS |
499 | A symbol used as a function name may also be used as a variable; these |
500 | two uses of a symbol are independent and do not conflict. (Some Lisp | |
501 | dialects, such as Scheme, do not distinguish between a symbol's value | |
502 | and its function definition; a symbol's value as a variable is also its | |
503 | function definition.) If you have not given a symbol a function | |
504 | definition, you cannot use it as a function; whether the symbol has a | |
505 | value as a variable makes no difference to this. | |
9c52bf47 KH |
506 | |
507 | @node Defining Functions | |
05fd2b65 | 508 | @section Defining Functions |
9c52bf47 KH |
509 | @cindex defining a function |
510 | ||
511 | We usually give a name to a function when it is first created. This | |
512 | is called @dfn{defining a function}, and it is done with the | |
513 | @code{defun} special form. | |
514 | ||
515 | @defspec defun name argument-list body-forms | |
516 | @code{defun} is the usual way to define new Lisp functions. It | |
517 | defines the symbol @var{name} as a function that looks like this: | |
518 | ||
519 | @example | |
520 | (lambda @var{argument-list} . @var{body-forms}) | |
521 | @end example | |
522 | ||
523 | @code{defun} stores this lambda expression in the function cell of | |
524 | @var{name}. It returns the value @var{name}, but usually we ignore this | |
525 | value. | |
526 | ||
527 | As described previously (@pxref{Lambda Expressions}), | |
528 | @var{argument-list} is a list of argument names and may include the | |
f9f59935 RS |
529 | keywords @code{&optional} and @code{&rest}. Also, the first two of the |
530 | @var{body-forms} may be a documentation string and an interactive | |
9c52bf47 KH |
531 | declaration. |
532 | ||
533 | There is no conflict if the same symbol @var{name} is also used as a | |
534 | variable, since the symbol's value cell is independent of the function | |
535 | cell. @xref{Symbol Components}. | |
536 | ||
537 | Here are some examples: | |
538 | ||
539 | @example | |
540 | @group | |
541 | (defun foo () 5) | |
542 | @result{} foo | |
543 | @end group | |
544 | @group | |
545 | (foo) | |
546 | @result{} 5 | |
547 | @end group | |
548 | ||
549 | @group | |
550 | (defun bar (a &optional b &rest c) | |
551 | (list a b c)) | |
552 | @result{} bar | |
553 | @end group | |
554 | @group | |
555 | (bar 1 2 3 4 5) | |
556 | @result{} (1 2 (3 4 5)) | |
557 | @end group | |
558 | @group | |
559 | (bar 1) | |
560 | @result{} (1 nil nil) | |
561 | @end group | |
562 | @group | |
563 | (bar) | |
564 | @error{} Wrong number of arguments. | |
565 | @end group | |
566 | ||
567 | @group | |
568 | (defun capitalize-backwards () | |
569 | "Upcase the last letter of a word." | |
570 | (interactive) | |
571 | (backward-word 1) | |
572 | (forward-word 1) | |
573 | (backward-char 1) | |
574 | (capitalize-word 1)) | |
575 | @result{} capitalize-backwards | |
576 | @end group | |
577 | @end example | |
578 | ||
579 | Be careful not to redefine existing functions unintentionally. | |
580 | @code{defun} redefines even primitive functions such as @code{car} | |
581 | without any hesitation or notification. Redefining a function already | |
582 | defined is often done deliberately, and there is no way to distinguish | |
583 | deliberate redefinition from unintentional redefinition. | |
584 | @end defspec | |
585 | ||
a0e91642 | 586 | @defun defalias name definition &optional docstring |
7baeca0c | 587 | @anchor{Definition of defalias} |
9c52bf47 | 588 | This special form defines the symbol @var{name} as a function, with |
f25df2ab | 589 | definition @var{definition} (which can be any valid Lisp function). |
a0e91642 LT |
590 | It returns @var{definition}. |
591 | ||
592 | If @var{docstring} is non-@code{nil}, it becomes the function | |
593 | documentation of @var{name}. Otherwise, any documentation provided by | |
594 | @var{definition} is used. | |
bfe721d1 KH |
595 | |
596 | The proper place to use @code{defalias} is where a specific function | |
597 | name is being defined---especially where that name appears explicitly in | |
598 | the source file being loaded. This is because @code{defalias} records | |
599 | which file defined the function, just like @code{defun} | |
600 | (@pxref{Unloading}). | |
601 | ||
602 | By contrast, in programs that manipulate function definitions for other | |
603 | purposes, it is better to use @code{fset}, which does not keep such | |
604 | records. | |
9c52bf47 KH |
605 | @end defun |
606 | ||
a68defff | 607 | You cannot create a new primitive function with @code{defun} or |
a0e91642 | 608 | @code{defalias}, but you can use them to change the function definition of |
a68defff RS |
609 | any symbol, even one such as @code{car} or @code{x-popup-menu} whose |
610 | normal definition is a primitive. However, this is risky: for | |
611 | instance, it is next to impossible to redefine @code{car} without | |
612 | breaking Lisp completely. Redefining an obscure function such as | |
613 | @code{x-popup-menu} is less dangerous, but it still may not work as | |
614 | you expect. If there are calls to the primitive from C code, they | |
615 | call the primitive's C definition directly, so changing the symbol's | |
616 | definition will have no effect on them. | |
617 | ||
bfe721d1 KH |
618 | See also @code{defsubst}, which defines a function like @code{defun} |
619 | and tells the Lisp compiler to open-code it. @xref{Inline Functions}. | |
620 | ||
9c52bf47 KH |
621 | @node Calling Functions |
622 | @section Calling Functions | |
623 | @cindex function invocation | |
624 | @cindex calling a function | |
625 | ||
626 | Defining functions is only half the battle. Functions don't do | |
627 | anything until you @dfn{call} them, i.e., tell them to run. Calling a | |
628 | function is also known as @dfn{invocation}. | |
629 | ||
f25df2ab RS |
630 | The most common way of invoking a function is by evaluating a list. |
631 | For example, evaluating the list @code{(concat "a" "b")} calls the | |
632 | function @code{concat} with arguments @code{"a"} and @code{"b"}. | |
633 | @xref{Evaluation}, for a description of evaluation. | |
9c52bf47 | 634 | |
67bce69d RS |
635 | When you write a list as an expression in your program, you specify |
636 | which function to call, and how many arguments to give it, in the text | |
637 | of the program. Usually that's just what you want. Occasionally you | |
638 | need to compute at run time which function to call. To do that, use | |
639 | the function @code{funcall}. When you also need to determine at run | |
640 | time how many arguments to pass, use @code{apply}. | |
9c52bf47 KH |
641 | |
642 | @defun funcall function &rest arguments | |
643 | @code{funcall} calls @var{function} with @var{arguments}, and returns | |
644 | whatever @var{function} returns. | |
645 | ||
646 | Since @code{funcall} is a function, all of its arguments, including | |
647 | @var{function}, are evaluated before @code{funcall} is called. This | |
648 | means that you can use any expression to obtain the function to be | |
67bce69d RS |
649 | called. It also means that @code{funcall} does not see the |
650 | expressions you write for the @var{arguments}, only their values. | |
651 | These values are @emph{not} evaluated a second time in the act of | |
652 | calling @var{function}; the operation of @code{funcall} is like the | |
653 | normal procedure for calling a function, once its arguments have | |
654 | already been evaluated. | |
9c52bf47 KH |
655 | |
656 | The argument @var{function} must be either a Lisp function or a | |
657 | primitive function. Special forms and macros are not allowed, because | |
658 | they make sense only when given the ``unevaluated'' argument | |
659 | expressions. @code{funcall} cannot provide these because, as we saw | |
660 | above, it never knows them in the first place. | |
661 | ||
662 | @example | |
663 | @group | |
664 | (setq f 'list) | |
665 | @result{} list | |
666 | @end group | |
667 | @group | |
668 | (funcall f 'x 'y 'z) | |
669 | @result{} (x y z) | |
670 | @end group | |
671 | @group | |
672 | (funcall f 'x 'y '(z)) | |
673 | @result{} (x y (z)) | |
674 | @end group | |
675 | @group | |
676 | (funcall 'and t nil) | |
677 | @error{} Invalid function: #<subr and> | |
678 | @end group | |
679 | @end example | |
680 | ||
7f785b50 | 681 | Compare these examples with the examples of @code{apply}. |
9c52bf47 KH |
682 | @end defun |
683 | ||
684 | @defun apply function &rest arguments | |
685 | @code{apply} calls @var{function} with @var{arguments}, just like | |
686 | @code{funcall} but with one difference: the last of @var{arguments} is a | |
f9f59935 RS |
687 | list of objects, which are passed to @var{function} as separate |
688 | arguments, rather than a single list. We say that @code{apply} | |
689 | @dfn{spreads} this list so that each individual element becomes an | |
690 | argument. | |
9c52bf47 KH |
691 | |
692 | @code{apply} returns the result of calling @var{function}. As with | |
693 | @code{funcall}, @var{function} must either be a Lisp function or a | |
694 | primitive function; special forms and macros do not make sense in | |
695 | @code{apply}. | |
696 | ||
697 | @example | |
698 | @group | |
699 | (setq f 'list) | |
700 | @result{} list | |
701 | @end group | |
702 | @group | |
703 | (apply f 'x 'y 'z) | |
704 | @error{} Wrong type argument: listp, z | |
705 | @end group | |
706 | @group | |
707 | (apply '+ 1 2 '(3 4)) | |
708 | @result{} 10 | |
709 | @end group | |
710 | @group | |
711 | (apply '+ '(1 2 3 4)) | |
712 | @result{} 10 | |
713 | @end group | |
714 | ||
715 | @group | |
716 | (apply 'append '((a b c) nil (x y z) nil)) | |
717 | @result{} (a b c x y z) | |
718 | @end group | |
719 | @end example | |
720 | ||
a0e91642 LT |
721 | For an interesting example of using @code{apply}, see @ref{Definition |
722 | of mapcar}. | |
9c52bf47 KH |
723 | @end defun |
724 | ||
725 | @cindex functionals | |
726 | It is common for Lisp functions to accept functions as arguments or | |
727 | find them in data structures (especially in hook variables and property | |
728 | lists) and call them using @code{funcall} or @code{apply}. Functions | |
729 | that accept function arguments are often called @dfn{functionals}. | |
730 | ||
bfe721d1 KH |
731 | Sometimes, when you call a functional, it is useful to supply a no-op |
732 | function as the argument. Here are two different kinds of no-op | |
9c52bf47 KH |
733 | function: |
734 | ||
735 | @defun identity arg | |
736 | This function returns @var{arg} and has no side effects. | |
737 | @end defun | |
738 | ||
739 | @defun ignore &rest args | |
740 | This function ignores any arguments and returns @code{nil}. | |
741 | @end defun | |
742 | ||
743 | @node Mapping Functions | |
744 | @section Mapping Functions | |
745 | @cindex mapping functions | |
746 | ||
a0e91642 LT |
747 | A @dfn{mapping function} applies a given function (@emph{not} a |
748 | special form or macro) to each element of a list or other collection. | |
749 | Emacs Lisp has several such functions; @code{mapcar} and | |
750 | @code{mapconcat}, which scan a list, are described here. | |
751 | @xref{Definition of mapatoms}, for the function @code{mapatoms} which | |
752 | maps over the symbols in an obarray. @xref{Definition of maphash}, | |
753 | for the function @code{maphash} which maps over key/value associations | |
754 | in a hash table. | |
969fe9b5 RS |
755 | |
756 | These mapping functions do not allow char-tables because a char-table | |
757 | is a sparse array whose nominal range of indices is very large. To map | |
758 | over a char-table in a way that deals properly with its sparse nature, | |
759 | use the function @code{map-char-table} (@pxref{Char-Tables}). | |
9c52bf47 KH |
760 | |
761 | @defun mapcar function sequence | |
7baeca0c | 762 | @anchor{Definition of mapcar} |
f25df2ab RS |
763 | @code{mapcar} applies @var{function} to each element of @var{sequence} |
764 | in turn, and returns a list of the results. | |
9c52bf47 | 765 | |
969fe9b5 RS |
766 | The argument @var{sequence} can be any kind of sequence except a |
767 | char-table; that is, a list, a vector, a bool-vector, or a string. The | |
9c52bf47 KH |
768 | result is always a list. The length of the result is the same as the |
769 | length of @var{sequence}. | |
770 | ||
771 | @smallexample | |
772 | @group | |
773 | @exdent @r{For example:} | |
774 | ||
775 | (mapcar 'car '((a b) (c d) (e f))) | |
776 | @result{} (a c e) | |
777 | (mapcar '1+ [1 2 3]) | |
778 | @result{} (2 3 4) | |
779 | (mapcar 'char-to-string "abc") | |
780 | @result{} ("a" "b" "c") | |
781 | @end group | |
782 | ||
783 | @group | |
784 | ;; @r{Call each function in @code{my-hooks}.} | |
785 | (mapcar 'funcall my-hooks) | |
786 | @end group | |
787 | ||
788 | @group | |
969fe9b5 | 789 | (defun mapcar* (function &rest args) |
9c52bf47 KH |
790 | "Apply FUNCTION to successive cars of all ARGS. |
791 | Return the list of results." | |
792 | ;; @r{If no list is exhausted,} | |
a0e91642 | 793 | (if (not (memq nil args)) |
969fe9b5 | 794 | ;; @r{apply function to @sc{car}s.} |
177c0ea7 JB |
795 | (cons (apply function (mapcar 'car args)) |
796 | (apply 'mapcar* function | |
9c52bf47 KH |
797 | ;; @r{Recurse for rest of elements.} |
798 | (mapcar 'cdr args))))) | |
799 | @end group | |
800 | ||
801 | @group | |
802 | (mapcar* 'cons '(a b c) '(1 2 3 4)) | |
803 | @result{} ((a . 1) (b . 2) (c . 3)) | |
804 | @end group | |
805 | @end smallexample | |
806 | @end defun | |
807 | ||
3c30cb6e DL |
808 | @defun mapc function sequence |
809 | @tindex mapc | |
810 | @code{mapc} is like @code{mapcar} except that @var{function} is used for | |
811 | side-effects only---the values it returns are ignored, not collected | |
812 | into a list. @code{mapc} always returns @var{sequence}. | |
813 | @end defun | |
814 | ||
9c52bf47 KH |
815 | @defun mapconcat function sequence separator |
816 | @code{mapconcat} applies @var{function} to each element of | |
817 | @var{sequence}: the results, which must be strings, are concatenated. | |
818 | Between each pair of result strings, @code{mapconcat} inserts the string | |
819 | @var{separator}. Usually @var{separator} contains a space or comma or | |
820 | other suitable punctuation. | |
821 | ||
822 | The argument @var{function} must be a function that can take one | |
969fe9b5 RS |
823 | argument and return a string. The argument @var{sequence} can be any |
824 | kind of sequence except a char-table; that is, a list, a vector, a | |
825 | bool-vector, or a string. | |
177c0ea7 | 826 | |
9c52bf47 KH |
827 | @smallexample |
828 | @group | |
829 | (mapconcat 'symbol-name | |
830 | '(The cat in the hat) | |
831 | " ") | |
832 | @result{} "The cat in the hat" | |
833 | @end group | |
834 | ||
835 | @group | |
836 | (mapconcat (function (lambda (x) (format "%c" (1+ x)))) | |
837 | "HAL-8000" | |
838 | "") | |
839 | @result{} "IBM.9111" | |
840 | @end group | |
841 | @end smallexample | |
842 | @end defun | |
843 | ||
844 | @node Anonymous Functions | |
845 | @section Anonymous Functions | |
846 | @cindex anonymous function | |
847 | ||
848 | In Lisp, a function is a list that starts with @code{lambda}, a | |
849 | byte-code function compiled from such a list, or alternatively a | |
850 | primitive subr-object; names are ``extra''. Although usually functions | |
851 | are defined with @code{defun} and given names at the same time, it is | |
852 | occasionally more concise to use an explicit lambda expression---an | |
853 | anonymous function. Such a list is valid wherever a function name is. | |
854 | ||
855 | Any method of creating such a list makes a valid function. Even this: | |
856 | ||
857 | @smallexample | |
858 | @group | |
ba3dafc8 | 859 | (setq silly (append '(lambda (x)) (list (list '+ (* 3 4) 'x)))) |
9c52bf47 KH |
860 | @result{} (lambda (x) (+ 12 x)) |
861 | @end group | |
862 | @end smallexample | |
863 | ||
864 | @noindent | |
865 | This computes a list that looks like @code{(lambda (x) (+ 12 x))} and | |
866 | makes it the value (@emph{not} the function definition!) of | |
867 | @code{silly}. | |
868 | ||
869 | Here is how we might call this function: | |
870 | ||
871 | @example | |
872 | @group | |
873 | (funcall silly 1) | |
874 | @result{} 13 | |
875 | @end group | |
876 | @end example | |
877 | ||
878 | @noindent | |
879 | (It does @emph{not} work to write @code{(silly 1)}, because this function | |
880 | is not the @emph{function definition} of @code{silly}. We have not given | |
881 | @code{silly} any function definition, just a value as a variable.) | |
882 | ||
883 | Most of the time, anonymous functions are constants that appear in | |
f9f59935 RS |
884 | your program. For example, you might want to pass one as an argument to |
885 | the function @code{mapcar}, which applies any given function to each | |
886 | element of a list. | |
887 | ||
177c0ea7 | 888 | Here we define a function @code{change-property} which |
f9f59935 | 889 | uses a function as its third argument: |
9c52bf47 KH |
890 | |
891 | @example | |
892 | @group | |
f9f59935 RS |
893 | (defun change-property (symbol prop function) |
894 | (let ((value (get symbol prop))) | |
895 | (put symbol prop (funcall function value)))) | |
9c52bf47 | 896 | @end group |
f9f59935 RS |
897 | @end example |
898 | ||
899 | @noindent | |
900 | Here we define a function that uses @code{change-property}, | |
969fe9b5 | 901 | passing it a function to double a number: |
f9f59935 RS |
902 | |
903 | @example | |
9c52bf47 | 904 | @group |
f9f59935 | 905 | (defun double-property (symbol prop) |
65500a82 | 906 | (change-property symbol prop '(lambda (x) (* 2 x)))) |
9c52bf47 KH |
907 | @end group |
908 | @end example | |
909 | ||
910 | @noindent | |
911 | In such cases, we usually use the special form @code{function} instead | |
f9f59935 | 912 | of simple quotation to quote the anonymous function, like this: |
9c52bf47 KH |
913 | |
914 | @example | |
915 | @group | |
f9f59935 | 916 | (defun double-property (symbol prop) |
a9f0a989 RS |
917 | (change-property symbol prop |
918 | (function (lambda (x) (* 2 x))))) | |
9c52bf47 KH |
919 | @end group |
920 | @end example | |
921 | ||
f9f59935 RS |
922 | Using @code{function} instead of @code{quote} makes a difference if you |
923 | compile the function @code{double-property}. For example, if you | |
924 | compile the second definition of @code{double-property}, the anonymous | |
925 | function is compiled as well. By contrast, if you compile the first | |
926 | definition which uses ordinary @code{quote}, the argument passed to | |
927 | @code{change-property} is the precise list shown: | |
9c52bf47 KH |
928 | |
929 | @example | |
930 | (lambda (x) (* x 2)) | |
931 | @end example | |
932 | ||
933 | @noindent | |
934 | The Lisp compiler cannot assume this list is a function, even though it | |
f9f59935 | 935 | looks like one, since it does not know what @code{change-property} will |
a9f0a989 | 936 | do with the list. Perhaps it will check whether the @sc{car} of the third |
f9f59935 RS |
937 | element is the symbol @code{*}! Using @code{function} tells the |
938 | compiler it is safe to go ahead and compile the constant function. | |
9c52bf47 | 939 | |
65500a82 RS |
940 | Nowadays it is possible to omit @code{function} entirely, like this: |
941 | ||
942 | @example | |
943 | @group | |
944 | (defun double-property (symbol prop) | |
945 | (change-property symbol prop (lambda (x) (* 2 x)))) | |
946 | @end group | |
947 | @end example | |
948 | ||
949 | @noindent | |
950 | This is because @code{lambda} itself implies @code{function}. | |
951 | ||
9c52bf47 KH |
952 | We sometimes write @code{function} instead of @code{quote} when |
953 | quoting the name of a function, but this usage is just a sort of | |
f9f59935 | 954 | comment: |
9c52bf47 KH |
955 | |
956 | @example | |
957 | (function @var{symbol}) @equiv{} (quote @var{symbol}) @equiv{} '@var{symbol} | |
a9f0a989 RS |
958 | @end example |
959 | ||
8241495d | 960 | @cindex @samp{#'} syntax |
a9f0a989 | 961 | The read syntax @code{#'} is a short-hand for using @code{function}. |
177c0ea7 | 962 | For example, |
a9f0a989 RS |
963 | |
964 | @example | |
965 | #'(lambda (x) (* x x)) | |
966 | @end example | |
967 | ||
968 | @noindent | |
969 | is equivalent to | |
970 | ||
971 | @example | |
972 | (function (lambda (x) (* x x))) | |
9c52bf47 KH |
973 | @end example |
974 | ||
f9f59935 RS |
975 | @defspec function function-object |
976 | @cindex function quoting | |
977 | This special form returns @var{function-object} without evaluating it. | |
978 | In this, it is equivalent to @code{quote}. However, it serves as a | |
979 | note to the Emacs Lisp compiler that @var{function-object} is intended | |
980 | to be used only as a function, and therefore can safely be compiled. | |
981 | Contrast this with @code{quote}, in @ref{Quoting}. | |
982 | @end defspec | |
983 | ||
a0e91642 LT |
984 | @xref{describe-symbols example}, for a realistic example using |
985 | @code{function} and an anonymous function. | |
9c52bf47 KH |
986 | |
987 | @node Function Cells | |
988 | @section Accessing Function Cell Contents | |
989 | ||
990 | The @dfn{function definition} of a symbol is the object stored in the | |
991 | function cell of the symbol. The functions described here access, test, | |
992 | and set the function cell of symbols. | |
993 | ||
a0e91642 LT |
994 | See also the function @code{indirect-function}. @xref{Definition of |
995 | indirect-function}. | |
f25df2ab | 996 | |
9c52bf47 KH |
997 | @defun symbol-function symbol |
998 | @kindex void-function | |
999 | This returns the object in the function cell of @var{symbol}. If the | |
1000 | symbol's function cell is void, a @code{void-function} error is | |
1001 | signaled. | |
1002 | ||
1003 | This function does not check that the returned object is a legitimate | |
1004 | function. | |
1005 | ||
1006 | @example | |
1007 | @group | |
1008 | (defun bar (n) (+ n 2)) | |
1009 | @result{} bar | |
1010 | @end group | |
1011 | @group | |
1012 | (symbol-function 'bar) | |
1013 | @result{} (lambda (n) (+ n 2)) | |
1014 | @end group | |
1015 | @group | |
1016 | (fset 'baz 'bar) | |
1017 | @result{} bar | |
1018 | @end group | |
1019 | @group | |
1020 | (symbol-function 'baz) | |
1021 | @result{} bar | |
1022 | @end group | |
1023 | @end example | |
1024 | @end defun | |
1025 | ||
1026 | @cindex void function cell | |
1027 | If you have never given a symbol any function definition, we say that | |
1028 | that symbol's function cell is @dfn{void}. In other words, the function | |
1029 | cell does not have any Lisp object in it. If you try to call such a symbol | |
1030 | as a function, it signals a @code{void-function} error. | |
1031 | ||
1032 | Note that void is not the same as @code{nil} or the symbol | |
1033 | @code{void}. The symbols @code{nil} and @code{void} are Lisp objects, | |
1034 | and can be stored into a function cell just as any other object can be | |
1035 | (and they can be valid functions if you define them in turn with | |
f25df2ab | 1036 | @code{defun}). A void function cell contains no object whatsoever. |
9c52bf47 KH |
1037 | |
1038 | You can test the voidness of a symbol's function definition with | |
1039 | @code{fboundp}. After you have given a symbol a function definition, you | |
1040 | can make it void once more using @code{fmakunbound}. | |
1041 | ||
1042 | @defun fboundp symbol | |
1043 | This function returns @code{t} if the symbol has an object in its | |
1044 | function cell, @code{nil} otherwise. It does not check that the object | |
1045 | is a legitimate function. | |
1046 | @end defun | |
1047 | ||
1048 | @defun fmakunbound symbol | |
1049 | This function makes @var{symbol}'s function cell void, so that a | |
a0e91642 LT |
1050 | subsequent attempt to access this cell will cause a |
1051 | @code{void-function} error. It returns @var{symbol}. (See also | |
1052 | @code{makunbound}, in @ref{Void Variables}.) | |
9c52bf47 KH |
1053 | |
1054 | @example | |
1055 | @group | |
1056 | (defun foo (x) x) | |
f9f59935 | 1057 | @result{} foo |
9c52bf47 KH |
1058 | @end group |
1059 | @group | |
f25df2ab RS |
1060 | (foo 1) |
1061 | @result{}1 | |
1062 | @end group | |
1063 | @group | |
9c52bf47 | 1064 | (fmakunbound 'foo) |
f9f59935 | 1065 | @result{} foo |
9c52bf47 KH |
1066 | @end group |
1067 | @group | |
1068 | (foo 1) | |
1069 | @error{} Symbol's function definition is void: foo | |
1070 | @end group | |
1071 | @end example | |
1072 | @end defun | |
1073 | ||
baa573a3 | 1074 | @defun fset symbol definition |
f9f59935 RS |
1075 | This function stores @var{definition} in the function cell of |
1076 | @var{symbol}. The result is @var{definition}. Normally | |
1077 | @var{definition} should be a function or the name of a function, but | |
1078 | this is not checked. The argument @var{symbol} is an ordinary evaluated | |
1079 | argument. | |
9c52bf47 KH |
1080 | |
1081 | There are three normal uses of this function: | |
1082 | ||
1083 | @itemize @bullet | |
1084 | @item | |
969fe9b5 RS |
1085 | Copying one symbol's function definition to another---in other words, |
1086 | making an alternate name for a function. (If you think of this as the | |
1087 | definition of the new name, you should use @code{defalias} instead of | |
a0e91642 | 1088 | @code{fset}; see @ref{Definition of defalias}.) |
9c52bf47 KH |
1089 | |
1090 | @item | |
1091 | Giving a symbol a function definition that is not a list and therefore | |
f25df2ab RS |
1092 | cannot be made with @code{defun}. For example, you can use @code{fset} |
1093 | to give a symbol @code{s1} a function definition which is another symbol | |
1094 | @code{s2}; then @code{s1} serves as an alias for whatever definition | |
969fe9b5 RS |
1095 | @code{s2} presently has. (Once again use @code{defalias} instead of |
1096 | @code{fset} if you think of this as the definition of @code{s1}.) | |
9c52bf47 KH |
1097 | |
1098 | @item | |
1099 | In constructs for defining or altering functions. If @code{defun} | |
1100 | were not a primitive, it could be written in Lisp (as a macro) using | |
1101 | @code{fset}. | |
1102 | @end itemize | |
1103 | ||
969fe9b5 | 1104 | Here are examples of these uses: |
9c52bf47 KH |
1105 | |
1106 | @example | |
1107 | @group | |
969fe9b5 RS |
1108 | ;; @r{Save @code{foo}'s definition in @code{old-foo}.} |
1109 | (fset 'old-foo (symbol-function 'foo)) | |
9c52bf47 KH |
1110 | @end group |
1111 | ||
1112 | @group | |
1113 | ;; @r{Make the symbol @code{car} the function definition of @code{xfirst}.} | |
969fe9b5 | 1114 | ;; @r{(Most likely, @code{defalias} would be better than @code{fset} here.)} |
9c52bf47 KH |
1115 | (fset 'xfirst 'car) |
1116 | @result{} car | |
1117 | @end group | |
1118 | @group | |
1119 | (xfirst '(1 2 3)) | |
1120 | @result{} 1 | |
1121 | @end group | |
1122 | @group | |
1123 | (symbol-function 'xfirst) | |
1124 | @result{} car | |
1125 | @end group | |
1126 | @group | |
1127 | (symbol-function (symbol-function 'xfirst)) | |
1128 | @result{} #<subr car> | |
1129 | @end group | |
1130 | ||
1131 | @group | |
1132 | ;; @r{Define a named keyboard macro.} | |
1133 | (fset 'kill-two-lines "\^u2\^k") | |
1134 | @result{} "\^u2\^k" | |
1135 | @end group | |
f25df2ab | 1136 | |
969fe9b5 RS |
1137 | @group |
1138 | ;; @r{Here is a function that alters other functions.} | |
1139 | (defun copy-function-definition (new old) | |
1140 | "Define NEW with the same function definition as OLD." | |
1141 | (fset new (symbol-function old))) | |
1142 | @end group | |
1143 | @end example | |
9c52bf47 KH |
1144 | @end defun |
1145 | ||
67bce69d RS |
1146 | @code{fset} is sometimes used to save the old definition of a |
1147 | function before redefining it. That permits the new definition to | |
1148 | invoke the old definition. But it is unmodular and unclean for a Lisp | |
1149 | file to redefine a function defined elsewhere. If you want to modify | |
1150 | a function defined by another package, it is cleaner to use | |
1151 | @code{defadvice} (@pxref{Advising Functions}). | |
bfe721d1 | 1152 | |
9c52bf47 KH |
1153 | @node Inline Functions |
1154 | @section Inline Functions | |
1155 | @cindex inline functions | |
1156 | ||
1157 | @findex defsubst | |
1158 | You can define an @dfn{inline function} by using @code{defsubst} instead | |
1159 | of @code{defun}. An inline function works just like an ordinary | |
1160 | function except for one thing: when you compile a call to the function, | |
1161 | the function's definition is open-coded into the caller. | |
1162 | ||
1163 | Making a function inline makes explicit calls run faster. But it also | |
1164 | has disadvantages. For one thing, it reduces flexibility; if you change | |
1165 | the definition of the function, calls already inlined still use the old | |
1166 | definition until you recompile them. Since the flexibility of | |
1167 | redefining functions is an important feature of Emacs, you should not | |
1168 | make a function inline unless its speed is really crucial. | |
1169 | ||
1170 | Another disadvantage is that making a large function inline can increase | |
1171 | the size of compiled code both in files and in memory. Since the speed | |
1172 | advantage of inline functions is greatest for small functions, you | |
1173 | generally should not make large functions inline. | |
1174 | ||
1175 | It's possible to define a macro to expand into the same code that an | |
969fe9b5 RS |
1176 | inline function would execute. (@xref{Macros}.) But the macro would be |
1177 | limited to direct use in expressions---a macro cannot be called with | |
9c52bf47 | 1178 | @code{apply}, @code{mapcar} and so on. Also, it takes some work to |
969fe9b5 RS |
1179 | convert an ordinary function into a macro. To convert it into an inline |
1180 | function is very easy; simply replace @code{defun} with @code{defsubst}. | |
1181 | Since each argument of an inline function is evaluated exactly once, you | |
1182 | needn't worry about how many times the body uses the arguments, as you | |
1183 | do for macros. (@xref{Argument Evaluation}.) | |
9c52bf47 | 1184 | |
f25df2ab | 1185 | Inline functions can be used and open-coded later on in the same file, |
9c52bf47 KH |
1186 | following the definition, just like macros. |
1187 | ||
a68defff | 1188 | @node Function Safety |
7ed9159a JY |
1189 | @section Determining whether a function is safe to call |
1190 | @cindex function safety | |
1191 | @cindex safety of functions | |
a68defff | 1192 | |
0c6b7a1f LT |
1193 | Some major modes such as SES call functions that are stored in user |
1194 | files. (@inforef{Top, ,ses}, for more information on SES.) User | |
1195 | files sometimes have poor pedigrees---you can get a spreadsheet from | |
1196 | someone you've just met, or you can get one through email from someone | |
1197 | you've never met. So it is risky to call a function whose source code | |
1198 | is stored in a user file until you have determined that it is safe. | |
7ed9159a JY |
1199 | |
1200 | @defun unsafep form &optional unsafep-vars | |
bb3edd15 | 1201 | Returns @code{nil} if @var{form} is a @dfn{safe} Lisp expression, or |
a68defff | 1202 | returns a list that describes why it might be unsafe. The argument |
7ed9159a JY |
1203 | @var{unsafep-vars} is a list of symbols known to have temporary |
1204 | bindings at this point; it is mainly used for internal recursive | |
1205 | calls. The current buffer is an implicit argument, which provides a | |
1206 | list of buffer-local bindings. | |
1207 | @end defun | |
1208 | ||
1209 | Being quick and simple, @code{unsafep} does a very light analysis and | |
1210 | rejects many Lisp expressions that are actually safe. There are no | |
a68defff RS |
1211 | known cases where @code{unsafep} returns @code{nil} for an unsafe |
1212 | expression. However, a ``safe'' Lisp expression can return a string | |
1213 | with a @code{display} property, containing an associated Lisp | |
1214 | expression to be executed after the string is inserted into a buffer. | |
1215 | This associated expression can be a virus. In order to be safe, you | |
1216 | must delete properties from all strings calculated by user code before | |
7ed9159a JY |
1217 | inserting them into buffers. |
1218 | ||
a68defff | 1219 | @ignore |
7ed9159a JY |
1220 | What is a safe Lisp expression? Basically, it's an expression that |
1221 | calls only built-in functions with no side effects (or only innocuous | |
1222 | ones). Innocuous side effects include displaying messages and | |
1223 | altering non-risky buffer-local variables (but not global variables). | |
1224 | ||
1225 | @table @dfn | |
1226 | @item Safe expression | |
1227 | @itemize | |
1228 | @item | |
1229 | An atom or quoted thing. | |
1230 | @item | |
1231 | A call to a safe function (see below), if all its arguments are | |
1232 | safe expressions. | |
1233 | @item | |
a68defff RS |
1234 | One of the special forms @code{and}, @code{catch}, @code{cond}, |
1235 | @code{if}, @code{or}, @code{prog1}, @code{prog2}, @code{progn}, | |
1236 | @code{while}, and @code{unwind-protect}], if all its arguments are | |
1237 | safe. | |
7ed9159a | 1238 | @item |
a68defff RS |
1239 | A form that creates temporary bindings (@code{condition-case}, |
1240 | @code{dolist}, @code{dotimes}, @code{lambda}, @code{let}, or | |
1241 | @code{let*}), if all args are safe and the symbols to be bound are not | |
1242 | explicitly risky (see @pxref{File Local Variables}). | |
7ed9159a | 1243 | @item |
a68defff RS |
1244 | An assignment using @code{add-to-list}, @code{setq}, @code{push}, or |
1245 | @code{pop}, if all args are safe and the symbols to be assigned are | |
1246 | not explicitly risky and they already have temporary or buffer-local | |
1247 | bindings. | |
7ed9159a JY |
1248 | @item |
1249 | One of [apply, mapc, mapcar, mapconcat] if the first argument is a | |
1250 | safe explicit lambda and the other args are safe expressions. | |
1251 | @end itemize | |
1252 | ||
1253 | @item Safe function | |
1254 | @itemize | |
1255 | @item | |
1256 | A lambda containing safe expressions. | |
1257 | @item | |
1258 | A symbol on the list @code{safe-functions}, so the user says it's safe. | |
1259 | @item | |
a68defff | 1260 | A symbol with a non-@code{nil} @code{side-effect-free} property. |
7ed9159a | 1261 | @item |
a68defff | 1262 | A symbol with a non-@code{nil} @code{safe-function} property. Value t |
7ed9159a JY |
1263 | indicates a function that is safe but has innocuous side effects. |
1264 | Other values will someday indicate functions with classes of side | |
1265 | effects that are not always safe. | |
1266 | @end itemize | |
1267 | ||
1268 | The @code{side-effect-free} and @code{safe-function} properties are | |
1269 | provided for built-in functions and for low-level functions and macros | |
1270 | defined in @file{subr.el}. You can assign these properties for the | |
1271 | functions you write. | |
7ed9159a | 1272 | @end table |
a68defff | 1273 | @end ignore |
9c52bf47 KH |
1274 | |
1275 | @node Related Topics | |
1276 | @section Other Topics Related to Functions | |
1277 | ||
1278 | Here is a table of several functions that do things related to | |
1279 | function calling and function definitions. They are documented | |
1280 | elsewhere, but we provide cross references here. | |
1281 | ||
1282 | @table @code | |
1283 | @item apply | |
1284 | See @ref{Calling Functions}. | |
1285 | ||
1286 | @item autoload | |
1287 | See @ref{Autoload}. | |
1288 | ||
1289 | @item call-interactively | |
1290 | See @ref{Interactive Call}. | |
1291 | ||
1292 | @item commandp | |
1293 | See @ref{Interactive Call}. | |
1294 | ||
1295 | @item documentation | |
1296 | See @ref{Accessing Documentation}. | |
1297 | ||
1298 | @item eval | |
1299 | See @ref{Eval}. | |
1300 | ||
1301 | @item funcall | |
1302 | See @ref{Calling Functions}. | |
1303 | ||
969fe9b5 RS |
1304 | @item function |
1305 | See @ref{Anonymous Functions}. | |
1306 | ||
9c52bf47 KH |
1307 | @item ignore |
1308 | See @ref{Calling Functions}. | |
1309 | ||
1310 | @item indirect-function | |
0a9e14dd | 1311 | See @ref{Function Indirection}. |
9c52bf47 KH |
1312 | |
1313 | @item interactive | |
1314 | See @ref{Using Interactive}. | |
1315 | ||
1316 | @item interactive-p | |
1317 | See @ref{Interactive Call}. | |
1318 | ||
1319 | @item mapatoms | |
1320 | See @ref{Creating Symbols}. | |
1321 | ||
1322 | @item mapcar | |
1323 | See @ref{Mapping Functions}. | |
1324 | ||
969fe9b5 RS |
1325 | @item map-char-table |
1326 | See @ref{Char-Tables}. | |
1327 | ||
9c52bf47 KH |
1328 | @item mapconcat |
1329 | See @ref{Mapping Functions}. | |
1330 | ||
1331 | @item undefined | |
0a9e14dd | 1332 | See @ref{Functions for Key Lookup}. |
9c52bf47 KH |
1333 | @end table |
1334 | ||
ab5796a9 MB |
1335 | @ignore |
1336 | arch-tag: 39100cdf-8a55-4898-acba-595db619e8e2 | |
1337 | @end ignore |