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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. | |
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 | |
56 | @dfn{built-in} functions or @dfn{subrs}. (Special forms are also | |
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 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 |
f25df2ab | 413 | lines of the string so that the text lines up in the program source. |
13a105af | 414 | @emph{That is a mistake.} The indentation of the following lines is |
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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 | ||
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426 | The last line of the documentation string can specify calling |
427 | conventions different from the actual function arguments. Write | |
428 | text like this: | |
429 | ||
430 | @example | |
431 | (fn @var{arglist}) | |
432 | @end example | |
433 | ||
434 | @noindent | |
435 | following a blank line, with no newline following it inside the | |
436 | documentation string. This feature is particularly useful for | |
437 | macro definitions. | |
438 | ||
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439 | @node Function Names |
440 | @section Naming a Function | |
441 | @cindex function definition | |
442 | @cindex named function | |
443 | @cindex function name | |
444 | ||
445 | In most computer languages, every function has a name; the idea of a | |
446 | function without a name is nonsensical. In Lisp, a function in the | |
447 | strictest sense has no name. It is simply a list whose first element is | |
969fe9b5 | 448 | @code{lambda}, a byte-code function object, or a primitive subr-object. |
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449 | |
450 | However, a symbol can serve as the name of a function. This happens | |
451 | when you put the function in the symbol's @dfn{function cell} | |
452 | (@pxref{Symbol Components}). Then the symbol itself becomes a valid, | |
453 | callable function, equivalent to the list or subr-object that its | |
454 | function cell refers to. The contents of the function cell are also | |
455 | called the symbol's @dfn{function definition}. The procedure of using a | |
456 | symbol's function definition in place of the symbol is called | |
457 | @dfn{symbol function indirection}; see @ref{Function Indirection}. | |
458 | ||
459 | In practice, nearly all functions are given names in this way and | |
460 | referred to through their names. For example, the symbol @code{car} works | |
461 | as a function and does what it does because the primitive subr-object | |
462 | @code{#<subr car>} is stored in its function cell. | |
463 | ||
464 | We give functions names because it is convenient to refer to them by | |
465 | their names in Lisp expressions. For primitive subr-objects such as | |
466 | @code{#<subr car>}, names are the only way you can refer to them: there | |
467 | is no read syntax for such objects. For functions written in Lisp, the | |
468 | name is more convenient to use in a call than an explicit lambda | |
469 | expression. Also, a function with a name can refer to itself---it can | |
470 | be recursive. Writing the function's name in its own definition is much | |
471 | more convenient than making the function definition point to itself | |
472 | (something that is not impossible but that has various disadvantages in | |
473 | practice). | |
474 | ||
475 | We often identify functions with the symbols used to name them. For | |
476 | example, we often speak of ``the function @code{car}'', not | |
477 | distinguishing between the symbol @code{car} and the primitive | |
478 | subr-object that is its function definition. For most purposes, there | |
479 | is no need to distinguish. | |
480 | ||
481 | Even so, keep in mind that a function need not have a unique name. While | |
482 | a given function object @emph{usually} appears in the function cell of only | |
483 | one symbol, this is just a matter of convenience. It is easy to store | |
484 | it in several symbols using @code{fset}; then each of the symbols is | |
485 | equally well a name for the same function. | |
486 | ||
a9f0a989 RS |
487 | A symbol used as a function name may also be used as a variable; these |
488 | two uses of a symbol are independent and do not conflict. (Some Lisp | |
489 | dialects, such as Scheme, do not distinguish between a symbol's value | |
490 | and its function definition; a symbol's value as a variable is also its | |
491 | function definition.) If you have not given a symbol a function | |
492 | definition, you cannot use it as a function; whether the symbol has a | |
493 | value as a variable makes no difference to this. | |
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494 | |
495 | @node Defining Functions | |
05fd2b65 | 496 | @section Defining Functions |
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497 | @cindex defining a function |
498 | ||
499 | We usually give a name to a function when it is first created. This | |
500 | is called @dfn{defining a function}, and it is done with the | |
501 | @code{defun} special form. | |
502 | ||
503 | @defspec defun name argument-list body-forms | |
504 | @code{defun} is the usual way to define new Lisp functions. It | |
505 | defines the symbol @var{name} as a function that looks like this: | |
506 | ||
507 | @example | |
508 | (lambda @var{argument-list} . @var{body-forms}) | |
509 | @end example | |
510 | ||
511 | @code{defun} stores this lambda expression in the function cell of | |
512 | @var{name}. It returns the value @var{name}, but usually we ignore this | |
513 | value. | |
514 | ||
515 | As described previously (@pxref{Lambda Expressions}), | |
516 | @var{argument-list} is a list of argument names and may include the | |
f9f59935 RS |
517 | keywords @code{&optional} and @code{&rest}. Also, the first two of the |
518 | @var{body-forms} may be a documentation string and an interactive | |
9c52bf47 KH |
519 | declaration. |
520 | ||
521 | There is no conflict if the same symbol @var{name} is also used as a | |
522 | variable, since the symbol's value cell is independent of the function | |
523 | cell. @xref{Symbol Components}. | |
524 | ||
525 | Here are some examples: | |
526 | ||
527 | @example | |
528 | @group | |
529 | (defun foo () 5) | |
530 | @result{} foo | |
531 | @end group | |
532 | @group | |
533 | (foo) | |
534 | @result{} 5 | |
535 | @end group | |
536 | ||
537 | @group | |
538 | (defun bar (a &optional b &rest c) | |
539 | (list a b c)) | |
540 | @result{} bar | |
541 | @end group | |
542 | @group | |
543 | (bar 1 2 3 4 5) | |
544 | @result{} (1 2 (3 4 5)) | |
545 | @end group | |
546 | @group | |
547 | (bar 1) | |
548 | @result{} (1 nil nil) | |
549 | @end group | |
550 | @group | |
551 | (bar) | |
552 | @error{} Wrong number of arguments. | |
553 | @end group | |
554 | ||
555 | @group | |
556 | (defun capitalize-backwards () | |
557 | "Upcase the last letter of a word." | |
558 | (interactive) | |
559 | (backward-word 1) | |
560 | (forward-word 1) | |
561 | (backward-char 1) | |
562 | (capitalize-word 1)) | |
563 | @result{} capitalize-backwards | |
564 | @end group | |
565 | @end example | |
566 | ||
567 | Be careful not to redefine existing functions unintentionally. | |
568 | @code{defun} redefines even primitive functions such as @code{car} | |
569 | without any hesitation or notification. Redefining a function already | |
570 | defined is often done deliberately, and there is no way to distinguish | |
571 | deliberate redefinition from unintentional redefinition. | |
572 | @end defspec | |
573 | ||
574 | @defun defalias name definition | |
575 | This special form defines the symbol @var{name} as a function, with | |
f25df2ab | 576 | definition @var{definition} (which can be any valid Lisp function). |
bfe721d1 KH |
577 | |
578 | The proper place to use @code{defalias} is where a specific function | |
579 | name is being defined---especially where that name appears explicitly in | |
580 | the source file being loaded. This is because @code{defalias} records | |
581 | which file defined the function, just like @code{defun} | |
582 | (@pxref{Unloading}). | |
583 | ||
584 | By contrast, in programs that manipulate function definitions for other | |
585 | purposes, it is better to use @code{fset}, which does not keep such | |
586 | records. | |
9c52bf47 KH |
587 | @end defun |
588 | ||
a68defff RS |
589 | You cannot create a new primitive function with @code{defun} or |
590 | @code{defalias}, but you use them to change the function definition of | |
591 | any symbol, even one such as @code{car} or @code{x-popup-menu} whose | |
592 | normal definition is a primitive. However, this is risky: for | |
593 | instance, it is next to impossible to redefine @code{car} without | |
594 | breaking Lisp completely. Redefining an obscure function such as | |
595 | @code{x-popup-menu} is less dangerous, but it still may not work as | |
596 | you expect. If there are calls to the primitive from C code, they | |
597 | call the primitive's C definition directly, so changing the symbol's | |
598 | definition will have no effect on them. | |
599 | ||
bfe721d1 KH |
600 | See also @code{defsubst}, which defines a function like @code{defun} |
601 | and tells the Lisp compiler to open-code it. @xref{Inline Functions}. | |
602 | ||
9c52bf47 KH |
603 | @node Calling Functions |
604 | @section Calling Functions | |
605 | @cindex function invocation | |
606 | @cindex calling a function | |
607 | ||
608 | Defining functions is only half the battle. Functions don't do | |
609 | anything until you @dfn{call} them, i.e., tell them to run. Calling a | |
610 | function is also known as @dfn{invocation}. | |
611 | ||
f25df2ab RS |
612 | The most common way of invoking a function is by evaluating a list. |
613 | For example, evaluating the list @code{(concat "a" "b")} calls the | |
614 | function @code{concat} with arguments @code{"a"} and @code{"b"}. | |
615 | @xref{Evaluation}, for a description of evaluation. | |
9c52bf47 KH |
616 | |
617 | When you write a list as an expression in your program, the function | |
f9f59935 RS |
618 | name it calls is written in your program. This means that you choose |
619 | which function to call, and how many arguments to give it, when you | |
620 | write the program. Usually that's just what you want. Occasionally you | |
621 | need to compute at run time which function to call. To do that, use the | |
969fe9b5 | 622 | function @code{funcall}. When you also need to determine at run time |
a9f0a989 | 623 | how many arguments to pass, use @code{apply}. |
9c52bf47 KH |
624 | |
625 | @defun funcall function &rest arguments | |
626 | @code{funcall} calls @var{function} with @var{arguments}, and returns | |
627 | whatever @var{function} returns. | |
628 | ||
629 | Since @code{funcall} is a function, all of its arguments, including | |
630 | @var{function}, are evaluated before @code{funcall} is called. This | |
631 | means that you can use any expression to obtain the function to be | |
632 | called. It also means that @code{funcall} does not see the expressions | |
633 | you write for the @var{arguments}, only their values. These values are | |
634 | @emph{not} evaluated a second time in the act of calling @var{function}; | |
635 | @code{funcall} enters the normal procedure for calling a function at the | |
636 | place where the arguments have already been evaluated. | |
637 | ||
638 | The argument @var{function} must be either a Lisp function or a | |
639 | primitive function. Special forms and macros are not allowed, because | |
640 | they make sense only when given the ``unevaluated'' argument | |
641 | expressions. @code{funcall} cannot provide these because, as we saw | |
642 | above, it never knows them in the first place. | |
643 | ||
644 | @example | |
645 | @group | |
646 | (setq f 'list) | |
647 | @result{} list | |
648 | @end group | |
649 | @group | |
650 | (funcall f 'x 'y 'z) | |
651 | @result{} (x y z) | |
652 | @end group | |
653 | @group | |
654 | (funcall f 'x 'y '(z)) | |
655 | @result{} (x y (z)) | |
656 | @end group | |
657 | @group | |
658 | (funcall 'and t nil) | |
659 | @error{} Invalid function: #<subr and> | |
660 | @end group | |
661 | @end example | |
662 | ||
7f785b50 | 663 | Compare these examples with the examples of @code{apply}. |
9c52bf47 KH |
664 | @end defun |
665 | ||
666 | @defun apply function &rest arguments | |
667 | @code{apply} calls @var{function} with @var{arguments}, just like | |
668 | @code{funcall} but with one difference: the last of @var{arguments} is a | |
f9f59935 RS |
669 | list of objects, which are passed to @var{function} as separate |
670 | arguments, rather than a single list. We say that @code{apply} | |
671 | @dfn{spreads} this list so that each individual element becomes an | |
672 | argument. | |
9c52bf47 KH |
673 | |
674 | @code{apply} returns the result of calling @var{function}. As with | |
675 | @code{funcall}, @var{function} must either be a Lisp function or a | |
676 | primitive function; special forms and macros do not make sense in | |
677 | @code{apply}. | |
678 | ||
679 | @example | |
680 | @group | |
681 | (setq f 'list) | |
682 | @result{} list | |
683 | @end group | |
684 | @group | |
685 | (apply f 'x 'y 'z) | |
686 | @error{} Wrong type argument: listp, z | |
687 | @end group | |
688 | @group | |
689 | (apply '+ 1 2 '(3 4)) | |
690 | @result{} 10 | |
691 | @end group | |
692 | @group | |
693 | (apply '+ '(1 2 3 4)) | |
694 | @result{} 10 | |
695 | @end group | |
696 | ||
697 | @group | |
698 | (apply 'append '((a b c) nil (x y z) nil)) | |
699 | @result{} (a b c x y z) | |
700 | @end group | |
701 | @end example | |
702 | ||
703 | For an interesting example of using @code{apply}, see the description of | |
704 | @code{mapcar}, in @ref{Mapping Functions}. | |
705 | @end defun | |
706 | ||
707 | @cindex functionals | |
708 | It is common for Lisp functions to accept functions as arguments or | |
709 | find them in data structures (especially in hook variables and property | |
710 | lists) and call them using @code{funcall} or @code{apply}. Functions | |
711 | that accept function arguments are often called @dfn{functionals}. | |
712 | ||
bfe721d1 KH |
713 | Sometimes, when you call a functional, it is useful to supply a no-op |
714 | function as the argument. Here are two different kinds of no-op | |
9c52bf47 KH |
715 | function: |
716 | ||
717 | @defun identity arg | |
718 | This function returns @var{arg} and has no side effects. | |
719 | @end defun | |
720 | ||
721 | @defun ignore &rest args | |
722 | This function ignores any arguments and returns @code{nil}. | |
723 | @end defun | |
724 | ||
725 | @node Mapping Functions | |
726 | @section Mapping Functions | |
727 | @cindex mapping functions | |
728 | ||
729 | A @dfn{mapping function} applies a given function to each element of a | |
f9f59935 | 730 | list or other collection. Emacs Lisp has several such functions; |
9c52bf47 | 731 | @code{mapcar} and @code{mapconcat}, which scan a list, are described |
f9f59935 | 732 | here. @xref{Creating Symbols}, for the function @code{mapatoms} which |
7f785b50 GM |
733 | maps over the symbols in an obarray. @xref{Hash Access}, for the |
734 | function @code{maphash} which maps over key/value associations in a | |
735 | hash table. | |
969fe9b5 RS |
736 | |
737 | These mapping functions do not allow char-tables because a char-table | |
738 | is a sparse array whose nominal range of indices is very large. To map | |
739 | over a char-table in a way that deals properly with its sparse nature, | |
740 | use the function @code{map-char-table} (@pxref{Char-Tables}). | |
9c52bf47 KH |
741 | |
742 | @defun mapcar function sequence | |
f25df2ab RS |
743 | @code{mapcar} applies @var{function} to each element of @var{sequence} |
744 | in turn, and returns a list of the results. | |
9c52bf47 | 745 | |
969fe9b5 RS |
746 | The argument @var{sequence} can be any kind of sequence except a |
747 | char-table; that is, a list, a vector, a bool-vector, or a string. The | |
9c52bf47 KH |
748 | result is always a list. The length of the result is the same as the |
749 | length of @var{sequence}. | |
750 | ||
751 | @smallexample | |
752 | @group | |
753 | @exdent @r{For example:} | |
754 | ||
755 | (mapcar 'car '((a b) (c d) (e f))) | |
756 | @result{} (a c e) | |
757 | (mapcar '1+ [1 2 3]) | |
758 | @result{} (2 3 4) | |
759 | (mapcar 'char-to-string "abc") | |
760 | @result{} ("a" "b" "c") | |
761 | @end group | |
762 | ||
763 | @group | |
764 | ;; @r{Call each function in @code{my-hooks}.} | |
765 | (mapcar 'funcall my-hooks) | |
766 | @end group | |
767 | ||
768 | @group | |
969fe9b5 | 769 | (defun mapcar* (function &rest args) |
9c52bf47 KH |
770 | "Apply FUNCTION to successive cars of all ARGS. |
771 | Return the list of results." | |
772 | ;; @r{If no list is exhausted,} | |
177c0ea7 | 773 | (if (not (memq 'nil args)) |
969fe9b5 | 774 | ;; @r{apply function to @sc{car}s.} |
177c0ea7 JB |
775 | (cons (apply function (mapcar 'car args)) |
776 | (apply 'mapcar* function | |
9c52bf47 KH |
777 | ;; @r{Recurse for rest of elements.} |
778 | (mapcar 'cdr args))))) | |
779 | @end group | |
780 | ||
781 | @group | |
782 | (mapcar* 'cons '(a b c) '(1 2 3 4)) | |
783 | @result{} ((a . 1) (b . 2) (c . 3)) | |
784 | @end group | |
785 | @end smallexample | |
786 | @end defun | |
787 | ||
3c30cb6e DL |
788 | @defun mapc function sequence |
789 | @tindex mapc | |
790 | @code{mapc} is like @code{mapcar} except that @var{function} is used for | |
791 | side-effects only---the values it returns are ignored, not collected | |
792 | into a list. @code{mapc} always returns @var{sequence}. | |
793 | @end defun | |
794 | ||
9c52bf47 KH |
795 | @defun mapconcat function sequence separator |
796 | @code{mapconcat} applies @var{function} to each element of | |
797 | @var{sequence}: the results, which must be strings, are concatenated. | |
798 | Between each pair of result strings, @code{mapconcat} inserts the string | |
799 | @var{separator}. Usually @var{separator} contains a space or comma or | |
800 | other suitable punctuation. | |
801 | ||
802 | The argument @var{function} must be a function that can take one | |
969fe9b5 RS |
803 | argument and return a string. The argument @var{sequence} can be any |
804 | kind of sequence except a char-table; that is, a list, a vector, a | |
805 | bool-vector, or a string. | |
177c0ea7 | 806 | |
9c52bf47 KH |
807 | @smallexample |
808 | @group | |
809 | (mapconcat 'symbol-name | |
810 | '(The cat in the hat) | |
811 | " ") | |
812 | @result{} "The cat in the hat" | |
813 | @end group | |
814 | ||
815 | @group | |
816 | (mapconcat (function (lambda (x) (format "%c" (1+ x)))) | |
817 | "HAL-8000" | |
818 | "") | |
819 | @result{} "IBM.9111" | |
820 | @end group | |
821 | @end smallexample | |
822 | @end defun | |
823 | ||
824 | @node Anonymous Functions | |
825 | @section Anonymous Functions | |
826 | @cindex anonymous function | |
827 | ||
828 | In Lisp, a function is a list that starts with @code{lambda}, a | |
829 | byte-code function compiled from such a list, or alternatively a | |
830 | primitive subr-object; names are ``extra''. Although usually functions | |
831 | are defined with @code{defun} and given names at the same time, it is | |
832 | occasionally more concise to use an explicit lambda expression---an | |
833 | anonymous function. Such a list is valid wherever a function name is. | |
834 | ||
835 | Any method of creating such a list makes a valid function. Even this: | |
836 | ||
837 | @smallexample | |
838 | @group | |
ba3dafc8 | 839 | (setq silly (append '(lambda (x)) (list (list '+ (* 3 4) 'x)))) |
9c52bf47 KH |
840 | @result{} (lambda (x) (+ 12 x)) |
841 | @end group | |
842 | @end smallexample | |
843 | ||
844 | @noindent | |
845 | This computes a list that looks like @code{(lambda (x) (+ 12 x))} and | |
846 | makes it the value (@emph{not} the function definition!) of | |
847 | @code{silly}. | |
848 | ||
849 | Here is how we might call this function: | |
850 | ||
851 | @example | |
852 | @group | |
853 | (funcall silly 1) | |
854 | @result{} 13 | |
855 | @end group | |
856 | @end example | |
857 | ||
858 | @noindent | |
859 | (It does @emph{not} work to write @code{(silly 1)}, because this function | |
860 | is not the @emph{function definition} of @code{silly}. We have not given | |
861 | @code{silly} any function definition, just a value as a variable.) | |
862 | ||
863 | Most of the time, anonymous functions are constants that appear in | |
f9f59935 RS |
864 | your program. For example, you might want to pass one as an argument to |
865 | the function @code{mapcar}, which applies any given function to each | |
866 | element of a list. | |
867 | ||
177c0ea7 | 868 | Here we define a function @code{change-property} which |
f9f59935 | 869 | uses a function as its third argument: |
9c52bf47 KH |
870 | |
871 | @example | |
872 | @group | |
f9f59935 RS |
873 | (defun change-property (symbol prop function) |
874 | (let ((value (get symbol prop))) | |
875 | (put symbol prop (funcall function value)))) | |
9c52bf47 | 876 | @end group |
f9f59935 RS |
877 | @end example |
878 | ||
879 | @noindent | |
880 | Here we define a function that uses @code{change-property}, | |
969fe9b5 | 881 | passing it a function to double a number: |
f9f59935 RS |
882 | |
883 | @example | |
9c52bf47 | 884 | @group |
f9f59935 | 885 | (defun double-property (symbol prop) |
65500a82 | 886 | (change-property symbol prop '(lambda (x) (* 2 x)))) |
9c52bf47 KH |
887 | @end group |
888 | @end example | |
889 | ||
890 | @noindent | |
891 | In such cases, we usually use the special form @code{function} instead | |
f9f59935 | 892 | of simple quotation to quote the anonymous function, like this: |
9c52bf47 KH |
893 | |
894 | @example | |
895 | @group | |
f9f59935 | 896 | (defun double-property (symbol prop) |
a9f0a989 RS |
897 | (change-property symbol prop |
898 | (function (lambda (x) (* 2 x))))) | |
9c52bf47 KH |
899 | @end group |
900 | @end example | |
901 | ||
f9f59935 RS |
902 | Using @code{function} instead of @code{quote} makes a difference if you |
903 | compile the function @code{double-property}. For example, if you | |
904 | compile the second definition of @code{double-property}, the anonymous | |
905 | function is compiled as well. By contrast, if you compile the first | |
906 | definition which uses ordinary @code{quote}, the argument passed to | |
907 | @code{change-property} is the precise list shown: | |
9c52bf47 KH |
908 | |
909 | @example | |
910 | (lambda (x) (* x 2)) | |
911 | @end example | |
912 | ||
913 | @noindent | |
914 | The Lisp compiler cannot assume this list is a function, even though it | |
f9f59935 | 915 | looks like one, since it does not know what @code{change-property} will |
a9f0a989 | 916 | do with the list. Perhaps it will check whether the @sc{car} of the third |
f9f59935 RS |
917 | element is the symbol @code{*}! Using @code{function} tells the |
918 | compiler it is safe to go ahead and compile the constant function. | |
9c52bf47 | 919 | |
65500a82 RS |
920 | Nowadays it is possible to omit @code{function} entirely, like this: |
921 | ||
922 | @example | |
923 | @group | |
924 | (defun double-property (symbol prop) | |
925 | (change-property symbol prop (lambda (x) (* 2 x)))) | |
926 | @end group | |
927 | @end example | |
928 | ||
929 | @noindent | |
930 | This is because @code{lambda} itself implies @code{function}. | |
931 | ||
9c52bf47 KH |
932 | We sometimes write @code{function} instead of @code{quote} when |
933 | quoting the name of a function, but this usage is just a sort of | |
f9f59935 | 934 | comment: |
9c52bf47 KH |
935 | |
936 | @example | |
937 | (function @var{symbol}) @equiv{} (quote @var{symbol}) @equiv{} '@var{symbol} | |
a9f0a989 RS |
938 | @end example |
939 | ||
8241495d | 940 | @cindex @samp{#'} syntax |
a9f0a989 | 941 | The read syntax @code{#'} is a short-hand for using @code{function}. |
177c0ea7 | 942 | For example, |
a9f0a989 RS |
943 | |
944 | @example | |
945 | #'(lambda (x) (* x x)) | |
946 | @end example | |
947 | ||
948 | @noindent | |
949 | is equivalent to | |
950 | ||
951 | @example | |
952 | (function (lambda (x) (* x x))) | |
9c52bf47 KH |
953 | @end example |
954 | ||
f9f59935 RS |
955 | @defspec function function-object |
956 | @cindex function quoting | |
957 | This special form returns @var{function-object} without evaluating it. | |
958 | In this, it is equivalent to @code{quote}. However, it serves as a | |
959 | note to the Emacs Lisp compiler that @var{function-object} is intended | |
960 | to be used only as a function, and therefore can safely be compiled. | |
961 | Contrast this with @code{quote}, in @ref{Quoting}. | |
962 | @end defspec | |
963 | ||
9c52bf47 KH |
964 | See @code{documentation} in @ref{Accessing Documentation}, for a |
965 | realistic example using @code{function} and an anonymous function. | |
966 | ||
967 | @node Function Cells | |
968 | @section Accessing Function Cell Contents | |
969 | ||
970 | The @dfn{function definition} of a symbol is the object stored in the | |
971 | function cell of the symbol. The functions described here access, test, | |
972 | and set the function cell of symbols. | |
973 | ||
f25df2ab RS |
974 | See also the function @code{indirect-function} in @ref{Function |
975 | Indirection}. | |
976 | ||
9c52bf47 KH |
977 | @defun symbol-function symbol |
978 | @kindex void-function | |
979 | This returns the object in the function cell of @var{symbol}. If the | |
980 | symbol's function cell is void, a @code{void-function} error is | |
981 | signaled. | |
982 | ||
983 | This function does not check that the returned object is a legitimate | |
984 | function. | |
985 | ||
986 | @example | |
987 | @group | |
988 | (defun bar (n) (+ n 2)) | |
989 | @result{} bar | |
990 | @end group | |
991 | @group | |
992 | (symbol-function 'bar) | |
993 | @result{} (lambda (n) (+ n 2)) | |
994 | @end group | |
995 | @group | |
996 | (fset 'baz 'bar) | |
997 | @result{} bar | |
998 | @end group | |
999 | @group | |
1000 | (symbol-function 'baz) | |
1001 | @result{} bar | |
1002 | @end group | |
1003 | @end example | |
1004 | @end defun | |
1005 | ||
1006 | @cindex void function cell | |
1007 | If you have never given a symbol any function definition, we say that | |
1008 | that symbol's function cell is @dfn{void}. In other words, the function | |
1009 | cell does not have any Lisp object in it. If you try to call such a symbol | |
1010 | as a function, it signals a @code{void-function} error. | |
1011 | ||
1012 | Note that void is not the same as @code{nil} or the symbol | |
1013 | @code{void}. The symbols @code{nil} and @code{void} are Lisp objects, | |
1014 | and can be stored into a function cell just as any other object can be | |
1015 | (and they can be valid functions if you define them in turn with | |
f25df2ab | 1016 | @code{defun}). A void function cell contains no object whatsoever. |
9c52bf47 KH |
1017 | |
1018 | You can test the voidness of a symbol's function definition with | |
1019 | @code{fboundp}. After you have given a symbol a function definition, you | |
1020 | can make it void once more using @code{fmakunbound}. | |
1021 | ||
1022 | @defun fboundp symbol | |
1023 | This function returns @code{t} if the symbol has an object in its | |
1024 | function cell, @code{nil} otherwise. It does not check that the object | |
1025 | is a legitimate function. | |
1026 | @end defun | |
1027 | ||
1028 | @defun fmakunbound symbol | |
1029 | This function makes @var{symbol}'s function cell void, so that a | |
1030 | subsequent attempt to access this cell will cause a @code{void-function} | |
a9f0a989 | 1031 | error. (See also @code{makunbound}, in @ref{Void Variables}.) |
9c52bf47 KH |
1032 | |
1033 | @example | |
1034 | @group | |
1035 | (defun foo (x) x) | |
f9f59935 | 1036 | @result{} foo |
9c52bf47 KH |
1037 | @end group |
1038 | @group | |
f25df2ab RS |
1039 | (foo 1) |
1040 | @result{}1 | |
1041 | @end group | |
1042 | @group | |
9c52bf47 | 1043 | (fmakunbound 'foo) |
f9f59935 | 1044 | @result{} foo |
9c52bf47 KH |
1045 | @end group |
1046 | @group | |
1047 | (foo 1) | |
1048 | @error{} Symbol's function definition is void: foo | |
1049 | @end group | |
1050 | @end example | |
1051 | @end defun | |
1052 | ||
baa573a3 | 1053 | @defun fset symbol definition |
f9f59935 RS |
1054 | This function stores @var{definition} in the function cell of |
1055 | @var{symbol}. The result is @var{definition}. Normally | |
1056 | @var{definition} should be a function or the name of a function, but | |
1057 | this is not checked. The argument @var{symbol} is an ordinary evaluated | |
1058 | argument. | |
9c52bf47 KH |
1059 | |
1060 | There are three normal uses of this function: | |
1061 | ||
1062 | @itemize @bullet | |
1063 | @item | |
969fe9b5 RS |
1064 | Copying one symbol's function definition to another---in other words, |
1065 | making an alternate name for a function. (If you think of this as the | |
1066 | definition of the new name, you should use @code{defalias} instead of | |
1067 | @code{fset}; see @ref{Defining Functions}.) | |
9c52bf47 KH |
1068 | |
1069 | @item | |
1070 | Giving a symbol a function definition that is not a list and therefore | |
f25df2ab RS |
1071 | cannot be made with @code{defun}. For example, you can use @code{fset} |
1072 | to give a symbol @code{s1} a function definition which is another symbol | |
1073 | @code{s2}; then @code{s1} serves as an alias for whatever definition | |
969fe9b5 RS |
1074 | @code{s2} presently has. (Once again use @code{defalias} instead of |
1075 | @code{fset} if you think of this as the definition of @code{s1}.) | |
9c52bf47 KH |
1076 | |
1077 | @item | |
1078 | In constructs for defining or altering functions. If @code{defun} | |
1079 | were not a primitive, it could be written in Lisp (as a macro) using | |
1080 | @code{fset}. | |
1081 | @end itemize | |
1082 | ||
969fe9b5 | 1083 | Here are examples of these uses: |
9c52bf47 KH |
1084 | |
1085 | @example | |
1086 | @group | |
969fe9b5 RS |
1087 | ;; @r{Save @code{foo}'s definition in @code{old-foo}.} |
1088 | (fset 'old-foo (symbol-function 'foo)) | |
9c52bf47 KH |
1089 | @end group |
1090 | ||
1091 | @group | |
1092 | ;; @r{Make the symbol @code{car} the function definition of @code{xfirst}.} | |
969fe9b5 | 1093 | ;; @r{(Most likely, @code{defalias} would be better than @code{fset} here.)} |
9c52bf47 KH |
1094 | (fset 'xfirst 'car) |
1095 | @result{} car | |
1096 | @end group | |
1097 | @group | |
1098 | (xfirst '(1 2 3)) | |
1099 | @result{} 1 | |
1100 | @end group | |
1101 | @group | |
1102 | (symbol-function 'xfirst) | |
1103 | @result{} car | |
1104 | @end group | |
1105 | @group | |
1106 | (symbol-function (symbol-function 'xfirst)) | |
1107 | @result{} #<subr car> | |
1108 | @end group | |
1109 | ||
1110 | @group | |
1111 | ;; @r{Define a named keyboard macro.} | |
1112 | (fset 'kill-two-lines "\^u2\^k") | |
1113 | @result{} "\^u2\^k" | |
1114 | @end group | |
f25df2ab | 1115 | |
969fe9b5 RS |
1116 | @group |
1117 | ;; @r{Here is a function that alters other functions.} | |
1118 | (defun copy-function-definition (new old) | |
1119 | "Define NEW with the same function definition as OLD." | |
1120 | (fset new (symbol-function old))) | |
1121 | @end group | |
1122 | @end example | |
9c52bf47 KH |
1123 | @end defun |
1124 | ||
1125 | When writing a function that extends a previously defined function, | |
bfe721d1 | 1126 | the following idiom is sometimes used: |
9c52bf47 KH |
1127 | |
1128 | @example | |
1129 | (fset 'old-foo (symbol-function 'foo)) | |
1130 | (defun foo () | |
1131 | "Just like old-foo, except more so." | |
1132 | @group | |
1133 | (old-foo) | |
1134 | (more-so)) | |
1135 | @end group | |
1136 | @end example | |
1137 | ||
1138 | @noindent | |
1139 | This does not work properly if @code{foo} has been defined to autoload. | |
1140 | In such a case, when @code{foo} calls @code{old-foo}, Lisp attempts | |
1141 | to define @code{old-foo} by loading a file. Since this presumably | |
1142 | defines @code{foo} rather than @code{old-foo}, it does not produce the | |
1143 | proper results. The only way to avoid this problem is to make sure the | |
1144 | file is loaded before moving aside the old definition of @code{foo}. | |
1145 | ||
bfe721d1 | 1146 | But it is unmodular and unclean, in any case, for a Lisp file to |
969fe9b5 RS |
1147 | redefine a function defined elsewhere. It is cleaner to use the advice |
1148 | facility (@pxref{Advising Functions}). | |
bfe721d1 | 1149 | |
9c52bf47 KH |
1150 | @node Inline Functions |
1151 | @section Inline Functions | |
1152 | @cindex inline functions | |
1153 | ||
1154 | @findex defsubst | |
1155 | You can define an @dfn{inline function} by using @code{defsubst} instead | |
1156 | of @code{defun}. An inline function works just like an ordinary | |
1157 | function except for one thing: when you compile a call to the function, | |
1158 | the function's definition is open-coded into the caller. | |
1159 | ||
1160 | Making a function inline makes explicit calls run faster. But it also | |
1161 | has disadvantages. For one thing, it reduces flexibility; if you change | |
1162 | the definition of the function, calls already inlined still use the old | |
1163 | definition until you recompile them. Since the flexibility of | |
1164 | redefining functions is an important feature of Emacs, you should not | |
1165 | make a function inline unless its speed is really crucial. | |
1166 | ||
1167 | Another disadvantage is that making a large function inline can increase | |
1168 | the size of compiled code both in files and in memory. Since the speed | |
1169 | advantage of inline functions is greatest for small functions, you | |
1170 | generally should not make large functions inline. | |
1171 | ||
1172 | It's possible to define a macro to expand into the same code that an | |
969fe9b5 RS |
1173 | inline function would execute. (@xref{Macros}.) But the macro would be |
1174 | limited to direct use in expressions---a macro cannot be called with | |
9c52bf47 | 1175 | @code{apply}, @code{mapcar} and so on. Also, it takes some work to |
969fe9b5 RS |
1176 | convert an ordinary function into a macro. To convert it into an inline |
1177 | function is very easy; simply replace @code{defun} with @code{defsubst}. | |
1178 | Since each argument of an inline function is evaluated exactly once, you | |
1179 | needn't worry about how many times the body uses the arguments, as you | |
1180 | do for macros. (@xref{Argument Evaluation}.) | |
9c52bf47 | 1181 | |
f25df2ab | 1182 | Inline functions can be used and open-coded later on in the same file, |
9c52bf47 KH |
1183 | following the definition, just like macros. |
1184 | ||
a68defff | 1185 | @node Function Safety |
7ed9159a JY |
1186 | @section Determining whether a function is safe to call |
1187 | @cindex function safety | |
1188 | @cindex safety of functions | |
a68defff RS |
1189 | |
1190 | Some major modes such as SES (@pxref{Top,,,ses}) call functions that | |
1191 | are stored in user files. User files sometimes have poor | |
1192 | pedigrees---you can get a spreadsheet from someone you've just met, or | |
1193 | you can get one through email from someone you've never met. So it is | |
1194 | risky to call a function whose source code is stored in a user file | |
1195 | until you have determined that it is safe. | |
7ed9159a JY |
1196 | |
1197 | @defun unsafep form &optional unsafep-vars | |
a68defff RS |
1198 | Returns @code{nil} if @var{form} is a @dfn{safe} lisp expression, or |
1199 | returns a list that describes why it might be unsafe. The argument | |
7ed9159a JY |
1200 | @var{unsafep-vars} is a list of symbols known to have temporary |
1201 | bindings at this point; it is mainly used for internal recursive | |
1202 | calls. The current buffer is an implicit argument, which provides a | |
1203 | list of buffer-local bindings. | |
1204 | @end defun | |
1205 | ||
1206 | Being quick and simple, @code{unsafep} does a very light analysis and | |
1207 | rejects many Lisp expressions that are actually safe. There are no | |
a68defff RS |
1208 | known cases where @code{unsafep} returns @code{nil} for an unsafe |
1209 | expression. However, a ``safe'' Lisp expression can return a string | |
1210 | with a @code{display} property, containing an associated Lisp | |
1211 | expression to be executed after the string is inserted into a buffer. | |
1212 | This associated expression can be a virus. In order to be safe, you | |
1213 | must delete properties from all strings calculated by user code before | |
7ed9159a JY |
1214 | inserting them into buffers. |
1215 | ||
a68defff | 1216 | @ignore |
7ed9159a JY |
1217 | What is a safe Lisp expression? Basically, it's an expression that |
1218 | calls only built-in functions with no side effects (or only innocuous | |
1219 | ones). Innocuous side effects include displaying messages and | |
1220 | altering non-risky buffer-local variables (but not global variables). | |
1221 | ||
1222 | @table @dfn | |
1223 | @item Safe expression | |
1224 | @itemize | |
1225 | @item | |
1226 | An atom or quoted thing. | |
1227 | @item | |
1228 | A call to a safe function (see below), if all its arguments are | |
1229 | safe expressions. | |
1230 | @item | |
a68defff RS |
1231 | One of the special forms @code{and}, @code{catch}, @code{cond}, |
1232 | @code{if}, @code{or}, @code{prog1}, @code{prog2}, @code{progn}, | |
1233 | @code{while}, and @code{unwind-protect}], if all its arguments are | |
1234 | safe. | |
7ed9159a | 1235 | @item |
a68defff RS |
1236 | A form that creates temporary bindings (@code{condition-case}, |
1237 | @code{dolist}, @code{dotimes}, @code{lambda}, @code{let}, or | |
1238 | @code{let*}), if all args are safe and the symbols to be bound are not | |
1239 | explicitly risky (see @pxref{File Local Variables}). | |
7ed9159a | 1240 | @item |
a68defff RS |
1241 | An assignment using @code{add-to-list}, @code{setq}, @code{push}, or |
1242 | @code{pop}, if all args are safe and the symbols to be assigned are | |
1243 | not explicitly risky and they already have temporary or buffer-local | |
1244 | bindings. | |
7ed9159a JY |
1245 | @item |
1246 | One of [apply, mapc, mapcar, mapconcat] if the first argument is a | |
1247 | safe explicit lambda and the other args are safe expressions. | |
1248 | @end itemize | |
1249 | ||
1250 | @item Safe function | |
1251 | @itemize | |
1252 | @item | |
1253 | A lambda containing safe expressions. | |
1254 | @item | |
1255 | A symbol on the list @code{safe-functions}, so the user says it's safe. | |
1256 | @item | |
a68defff | 1257 | A symbol with a non-@code{nil} @code{side-effect-free} property. |
7ed9159a | 1258 | @item |
a68defff | 1259 | A symbol with a non-@code{nil} @code{safe-function} property. Value t |
7ed9159a JY |
1260 | indicates a function that is safe but has innocuous side effects. |
1261 | Other values will someday indicate functions with classes of side | |
1262 | effects that are not always safe. | |
1263 | @end itemize | |
1264 | ||
1265 | The @code{side-effect-free} and @code{safe-function} properties are | |
1266 | provided for built-in functions and for low-level functions and macros | |
1267 | defined in @file{subr.el}. You can assign these properties for the | |
1268 | functions you write. | |
7ed9159a | 1269 | @end table |
a68defff | 1270 | @end ignore |
9c52bf47 KH |
1271 | |
1272 | @node Related Topics | |
1273 | @section Other Topics Related to Functions | |
1274 | ||
1275 | Here is a table of several functions that do things related to | |
1276 | function calling and function definitions. They are documented | |
1277 | elsewhere, but we provide cross references here. | |
1278 | ||
1279 | @table @code | |
1280 | @item apply | |
1281 | See @ref{Calling Functions}. | |
1282 | ||
1283 | @item autoload | |
1284 | See @ref{Autoload}. | |
1285 | ||
1286 | @item call-interactively | |
1287 | See @ref{Interactive Call}. | |
1288 | ||
1289 | @item commandp | |
1290 | See @ref{Interactive Call}. | |
1291 | ||
1292 | @item documentation | |
1293 | See @ref{Accessing Documentation}. | |
1294 | ||
1295 | @item eval | |
1296 | See @ref{Eval}. | |
1297 | ||
1298 | @item funcall | |
1299 | See @ref{Calling Functions}. | |
1300 | ||
969fe9b5 RS |
1301 | @item function |
1302 | See @ref{Anonymous Functions}. | |
1303 | ||
9c52bf47 KH |
1304 | @item ignore |
1305 | See @ref{Calling Functions}. | |
1306 | ||
1307 | @item indirect-function | |
1308 | See @ref{Function Indirection}. | |
1309 | ||
1310 | @item interactive | |
1311 | See @ref{Using Interactive}. | |
1312 | ||
1313 | @item interactive-p | |
1314 | See @ref{Interactive Call}. | |
1315 | ||
1316 | @item mapatoms | |
1317 | See @ref{Creating Symbols}. | |
1318 | ||
1319 | @item mapcar | |
1320 | See @ref{Mapping Functions}. | |
1321 | ||
969fe9b5 RS |
1322 | @item map-char-table |
1323 | See @ref{Char-Tables}. | |
1324 | ||
9c52bf47 KH |
1325 | @item mapconcat |
1326 | See @ref{Mapping Functions}. | |
1327 | ||
1328 | @item undefined | |
1329 | See @ref{Key Lookup}. | |
1330 | @end table | |
1331 | ||
ab5796a9 MB |
1332 | @ignore |
1333 | arch-tag: 39100cdf-8a55-4898-acba-595db619e8e2 | |
1334 | @end ignore |