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1 | @c -*-texinfo-*- |
2 | @c This is part of the GNU Emacs Lisp Reference Manual. | |
3 | @c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999, 2001, | |
5df4f04c | 4 | @c 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 |
d24880de | 5 | @c Free Software Foundation, Inc. |
b8d4c8d0 | 6 | @c See the file elisp.texi for copying conditions. |
6336d8c3 | 7 | @setfilename ../../info/functions |
b8d4c8d0 GM |
8 | @node Functions, Macros, Variables, Top |
9 | @chapter Functions | |
10 | ||
11 | A Lisp program is composed mainly of Lisp functions. This chapter | |
12 | explains what functions are, how they accept arguments, and how to | |
13 | define them. | |
14 | ||
15 | @menu | |
16 | * What Is a Function:: Lisp functions vs. primitives; terminology. | |
17 | * Lambda Expressions:: How functions are expressed as Lisp objects. | |
18 | * Function Names:: A symbol can serve as the name of a function. | |
19 | * Defining Functions:: Lisp expressions for defining functions. | |
20 | * Calling Functions:: How to use an existing function. | |
21 | * Mapping Functions:: Applying a function to each element of a list, etc. | |
22 | * Anonymous Functions:: Lambda expressions are functions with no names. | |
23 | * Function Cells:: Accessing or setting the function definition | |
24 | of a symbol. | |
25 | * Obsolete Functions:: Declaring functions obsolete. | |
d24880de | 26 | * Inline Functions:: Defining functions that the compiler will open code. |
e31dfb12 | 27 | * Declaring Functions:: Telling the compiler that a function is defined. |
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28 | * Function Safety:: Determining whether a function is safe to call. |
29 | * Related Topics:: Cross-references to specific Lisp primitives | |
30 | that have a special bearing on how functions work. | |
31 | @end menu | |
32 | ||
33 | @node What Is a Function | |
34 | @section What Is a Function? | |
35 | ||
36 | In a general sense, a function is a rule for carrying on a computation | |
37 | given several values called @dfn{arguments}. The result of the | |
38 | computation is called the value of the function. The computation can | |
39 | also have side effects: lasting changes in the values of variables or | |
40 | the contents of data structures. | |
41 | ||
42 | Here are important terms for functions in Emacs Lisp and for other | |
43 | function-like objects. | |
44 | ||
45 | @table @dfn | |
46 | @item function | |
47 | @cindex function | |
48 | In Emacs Lisp, a @dfn{function} is anything that can be applied to | |
49 | arguments in a Lisp program. In some cases, we use it more | |
50 | specifically to mean a function written in Lisp. Special forms and | |
51 | macros are not functions. | |
52 | ||
53 | @item primitive | |
54 | @cindex primitive | |
55 | @cindex subr | |
56 | @cindex built-in function | |
57 | A @dfn{primitive} is a function callable from Lisp that is written in C, | |
58 | such as @code{car} or @code{append}. These functions are also called | |
59 | @dfn{built-in functions}, or @dfn{subrs}. (Special forms are also | |
60 | considered primitives.) | |
61 | ||
62 | Usually the reason we implement a function as a primitive is either | |
63 | because it is fundamental, because it provides a low-level interface | |
64 | to operating system services, or because it needs to run fast. | |
65 | Primitives can be modified or added only by changing the C sources and | |
66 | recompiling the editor. See @ref{Writing Emacs Primitives}. | |
67 | ||
68 | @item lambda expression | |
69 | A @dfn{lambda expression} is a function written in Lisp. | |
70 | These are described in the following section. | |
71 | @ifnottex | |
72 | @xref{Lambda Expressions}. | |
73 | @end ifnottex | |
74 | ||
75 | @item special form | |
76 | A @dfn{special form} is a primitive that is like a function but does not | |
77 | evaluate all of its arguments in the usual way. It may evaluate only | |
78 | some of the arguments, or may evaluate them in an unusual order, or | |
79 | several times. Many special forms are described in @ref{Control | |
80 | Structures}. | |
81 | ||
82 | @item macro | |
83 | @cindex macro | |
84 | A @dfn{macro} is a construct defined in Lisp by the programmer. It | |
85 | differs from a function in that it translates a Lisp expression that you | |
86 | write into an equivalent expression to be evaluated instead of the | |
87 | original expression. Macros enable Lisp programmers to do the sorts of | |
88 | things that special forms can do. @xref{Macros}, for how to define and | |
89 | use macros. | |
90 | ||
91 | @item command | |
92 | @cindex command | |
93 | A @dfn{command} is an object that @code{command-execute} can invoke; it | |
94 | is a possible definition for a key sequence. Some functions are | |
95 | commands; a function written in Lisp is a command if it contains an | |
96 | interactive declaration (@pxref{Defining Commands}). Such a function | |
97 | can be called from Lisp expressions like other functions; in this case, | |
98 | the fact that the function is a command makes no difference. | |
99 | ||
100 | Keyboard macros (strings and vectors) are commands also, even though | |
101 | they are not functions. A symbol is a command if its function | |
102 | definition is a command; such symbols can be invoked with @kbd{M-x}. | |
103 | The symbol is a function as well if the definition is a function. | |
104 | @xref{Interactive Call}. | |
105 | ||
106 | @item keystroke command | |
107 | @cindex keystroke command | |
108 | A @dfn{keystroke command} is a command that is bound to a key sequence | |
109 | (typically one to three keystrokes). The distinction is made here | |
110 | merely to avoid confusion with the meaning of ``command'' in non-Emacs | |
111 | editors; for Lisp programs, the distinction is normally unimportant. | |
112 | ||
113 | @item byte-code function | |
114 | A @dfn{byte-code function} is a function that has been compiled by the | |
115 | byte compiler. @xref{Byte-Code Type}. | |
116 | @end table | |
117 | ||
118 | @defun functionp object | |
119 | This function returns @code{t} if @var{object} is any kind of | |
9b240644 EZ |
120 | function, i.e.@: can be passed to @code{funcall}. Note that |
121 | @code{functionp} returns @code{nil} for special forms (@pxref{Special | |
122 | Forms}). | |
b8d4c8d0 GM |
123 | @end defun |
124 | ||
125 | Unlike @code{functionp}, the next three functions do @emph{not} | |
126 | treat a symbol as its function definition. | |
127 | ||
128 | @defun subrp object | |
129 | This function returns @code{t} if @var{object} is a built-in function | |
130 | (i.e., a Lisp primitive). | |
131 | ||
132 | @example | |
133 | @group | |
134 | (subrp 'message) ; @r{@code{message} is a symbol,} | |
135 | @result{} nil ; @r{not a subr object.} | |
136 | @end group | |
137 | @group | |
138 | (subrp (symbol-function 'message)) | |
139 | @result{} t | |
140 | @end group | |
141 | @end example | |
142 | @end defun | |
143 | ||
144 | @defun byte-code-function-p object | |
145 | This function returns @code{t} if @var{object} is a byte-code | |
146 | function. For example: | |
147 | ||
148 | @example | |
149 | @group | |
150 | (byte-code-function-p (symbol-function 'next-line)) | |
151 | @result{} t | |
152 | @end group | |
153 | @end example | |
154 | @end defun | |
155 | ||
156 | @defun subr-arity subr | |
157 | This function provides information about the argument list of a | |
158 | primitive, @var{subr}. The returned value is a pair | |
159 | @code{(@var{min} . @var{max})}. @var{min} is the minimum number of | |
160 | args. @var{max} is the maximum number or the symbol @code{many}, for a | |
161 | function with @code{&rest} arguments, or the symbol @code{unevalled} if | |
162 | @var{subr} is a special form. | |
163 | @end defun | |
164 | ||
165 | @node Lambda Expressions | |
166 | @section Lambda Expressions | |
167 | @cindex lambda expression | |
168 | ||
169 | A function written in Lisp is a list that looks like this: | |
170 | ||
171 | @example | |
172 | (lambda (@var{arg-variables}@dots{}) | |
173 | @r{[}@var{documentation-string}@r{]} | |
174 | @r{[}@var{interactive-declaration}@r{]} | |
175 | @var{body-forms}@dots{}) | |
176 | @end example | |
177 | ||
178 | @noindent | |
179 | Such a list is called a @dfn{lambda expression}. In Emacs Lisp, it | |
180 | actually is valid as an expression---it evaluates to itself. In some | |
181 | other Lisp dialects, a lambda expression is not a valid expression at | |
182 | all. In either case, its main use is not to be evaluated as an | |
183 | expression, but to be called as a function. | |
184 | ||
185 | @menu | |
186 | * Lambda Components:: The parts of a lambda expression. | |
187 | * Simple Lambda:: A simple example. | |
188 | * Argument List:: Details and special features of argument lists. | |
189 | * Function Documentation:: How to put documentation in a function. | |
190 | @end menu | |
191 | ||
192 | @node Lambda Components | |
193 | @subsection Components of a Lambda Expression | |
194 | ||
195 | @ifnottex | |
196 | ||
197 | A function written in Lisp (a ``lambda expression'') is a list that | |
198 | looks like this: | |
199 | ||
200 | @example | |
201 | (lambda (@var{arg-variables}@dots{}) | |
202 | [@var{documentation-string}] | |
203 | [@var{interactive-declaration}] | |
204 | @var{body-forms}@dots{}) | |
205 | @end example | |
206 | @end ifnottex | |
207 | ||
208 | @cindex lambda list | |
209 | The first element of a lambda expression is always the symbol | |
210 | @code{lambda}. This indicates that the list represents a function. The | |
211 | reason functions are defined to start with @code{lambda} is so that | |
212 | other lists, intended for other uses, will not accidentally be valid as | |
213 | functions. | |
214 | ||
215 | The second element is a list of symbols---the argument variable names. | |
216 | This is called the @dfn{lambda list}. When a Lisp function is called, | |
217 | the argument values are matched up against the variables in the lambda | |
218 | list, which are given local bindings with the values provided. | |
219 | @xref{Local Variables}. | |
220 | ||
221 | The documentation string is a Lisp string object placed within the | |
222 | function definition to describe the function for the Emacs help | |
223 | facilities. @xref{Function Documentation}. | |
224 | ||
225 | The interactive declaration is a list of the form @code{(interactive | |
226 | @var{code-string})}. This declares how to provide arguments if the | |
227 | function is used interactively. Functions with this declaration are called | |
228 | @dfn{commands}; they can be called using @kbd{M-x} or bound to a key. | |
229 | Functions not intended to be called in this way should not have interactive | |
230 | declarations. @xref{Defining Commands}, for how to write an interactive | |
231 | declaration. | |
232 | ||
233 | @cindex body of function | |
234 | The rest of the elements are the @dfn{body} of the function: the Lisp | |
235 | code to do the work of the function (or, as a Lisp programmer would say, | |
236 | ``a list of Lisp forms to evaluate''). The value returned by the | |
237 | function is the value returned by the last element of the body. | |
238 | ||
239 | @node Simple Lambda | |
240 | @subsection A Simple Lambda-Expression Example | |
241 | ||
242 | Consider for example the following function: | |
243 | ||
244 | @example | |
245 | (lambda (a b c) (+ a b c)) | |
246 | @end example | |
247 | ||
248 | @noindent | |
249 | We can call this function by writing it as the @sc{car} of an | |
250 | expression, like this: | |
251 | ||
252 | @example | |
253 | @group | |
254 | ((lambda (a b c) (+ a b c)) | |
255 | 1 2 3) | |
256 | @end group | |
257 | @end example | |
258 | ||
259 | @noindent | |
260 | This call evaluates the body of the lambda expression with the variable | |
261 | @code{a} bound to 1, @code{b} bound to 2, and @code{c} bound to 3. | |
262 | Evaluation of the body adds these three numbers, producing the result 6; | |
263 | therefore, this call to the function returns the value 6. | |
264 | ||
265 | Note that the arguments can be the results of other function calls, as in | |
266 | this example: | |
267 | ||
268 | @example | |
269 | @group | |
270 | ((lambda (a b c) (+ a b c)) | |
271 | 1 (* 2 3) (- 5 4)) | |
272 | @end group | |
273 | @end example | |
274 | ||
275 | @noindent | |
276 | This evaluates the arguments @code{1}, @code{(* 2 3)}, and @code{(- 5 | |
277 | 4)} from left to right. Then it applies the lambda expression to the | |
278 | argument values 1, 6 and 1 to produce the value 8. | |
279 | ||
280 | It is not often useful to write a lambda expression as the @sc{car} of | |
281 | a form in this way. You can get the same result, of making local | |
282 | variables and giving them values, using the special form @code{let} | |
283 | (@pxref{Local Variables}). And @code{let} is clearer and easier to use. | |
284 | In practice, lambda expressions are either stored as the function | |
285 | definitions of symbols, to produce named functions, or passed as | |
286 | arguments to other functions (@pxref{Anonymous Functions}). | |
287 | ||
288 | However, calls to explicit lambda expressions were very useful in the | |
289 | old days of Lisp, before the special form @code{let} was invented. At | |
290 | that time, they were the only way to bind and initialize local | |
291 | variables. | |
292 | ||
293 | @node Argument List | |
294 | @subsection Other Features of Argument Lists | |
295 | @kindex wrong-number-of-arguments | |
296 | @cindex argument binding | |
297 | @cindex binding arguments | |
298 | @cindex argument lists, features | |
299 | ||
300 | Our simple sample function, @code{(lambda (a b c) (+ a b c))}, | |
301 | specifies three argument variables, so it must be called with three | |
302 | arguments: if you try to call it with only two arguments or four | |
303 | arguments, you get a @code{wrong-number-of-arguments} error. | |
304 | ||
305 | It is often convenient to write a function that allows certain | |
306 | arguments to be omitted. For example, the function @code{substring} | |
307 | accepts three arguments---a string, the start index and the end | |
308 | index---but the third argument defaults to the @var{length} of the | |
309 | string if you omit it. It is also convenient for certain functions to | |
310 | accept an indefinite number of arguments, as the functions @code{list} | |
311 | and @code{+} do. | |
312 | ||
313 | @cindex optional arguments | |
314 | @cindex rest arguments | |
315 | @kindex &optional | |
316 | @kindex &rest | |
317 | To specify optional arguments that may be omitted when a function | |
318 | is called, simply include the keyword @code{&optional} before the optional | |
319 | arguments. To specify a list of zero or more extra arguments, include the | |
320 | keyword @code{&rest} before one final argument. | |
321 | ||
322 | Thus, the complete syntax for an argument list is as follows: | |
323 | ||
324 | @example | |
325 | @group | |
326 | (@var{required-vars}@dots{} | |
327 | @r{[}&optional @var{optional-vars}@dots{}@r{]} | |
328 | @r{[}&rest @var{rest-var}@r{]}) | |
329 | @end group | |
330 | @end example | |
331 | ||
332 | @noindent | |
333 | The square brackets indicate that the @code{&optional} and @code{&rest} | |
334 | clauses, and the variables that follow them, are optional. | |
335 | ||
336 | A call to the function requires one actual argument for each of the | |
337 | @var{required-vars}. There may be actual arguments for zero or more of | |
338 | the @var{optional-vars}, and there cannot be any actual arguments beyond | |
339 | that unless the lambda list uses @code{&rest}. In that case, there may | |
340 | be any number of extra actual arguments. | |
341 | ||
342 | If actual arguments for the optional and rest variables are omitted, | |
343 | then they always default to @code{nil}. There is no way for the | |
344 | function to distinguish between an explicit argument of @code{nil} and | |
345 | an omitted argument. However, the body of the function is free to | |
346 | consider @code{nil} an abbreviation for some other meaningful value. | |
347 | This is what @code{substring} does; @code{nil} as the third argument to | |
348 | @code{substring} means to use the length of the string supplied. | |
349 | ||
350 | @cindex CL note---default optional arg | |
351 | @quotation | |
352 | @b{Common Lisp note:} Common Lisp allows the function to specify what | |
353 | default value to use when an optional argument is omitted; Emacs Lisp | |
354 | always uses @code{nil}. Emacs Lisp does not support ``supplied-p'' | |
355 | variables that tell you whether an argument was explicitly passed. | |
356 | @end quotation | |
357 | ||
358 | For example, an argument list that looks like this: | |
359 | ||
360 | @example | |
361 | (a b &optional c d &rest e) | |
362 | @end example | |
363 | ||
364 | @noindent | |
365 | binds @code{a} and @code{b} to the first two actual arguments, which are | |
366 | required. If one or two more arguments are provided, @code{c} and | |
367 | @code{d} are bound to them respectively; any arguments after the first | |
368 | four are collected into a list and @code{e} is bound to that list. If | |
369 | there are only two arguments, @code{c} is @code{nil}; if two or three | |
370 | arguments, @code{d} is @code{nil}; if four arguments or fewer, @code{e} | |
371 | is @code{nil}. | |
372 | ||
373 | There is no way to have required arguments following optional | |
374 | ones---it would not make sense. To see why this must be so, suppose | |
375 | that @code{c} in the example were optional and @code{d} were required. | |
376 | Suppose three actual arguments are given; which variable would the | |
377 | third argument be for? Would it be used for the @var{c}, or for | |
378 | @var{d}? One can argue for both possibilities. Similarly, it makes | |
379 | no sense to have any more arguments (either required or optional) | |
380 | after a @code{&rest} argument. | |
381 | ||
382 | Here are some examples of argument lists and proper calls: | |
383 | ||
384 | @smallexample | |
385 | ((lambda (n) (1+ n)) ; @r{One required:} | |
386 | 1) ; @r{requires exactly one argument.} | |
387 | @result{} 2 | |
388 | ((lambda (n &optional n1) ; @r{One required and one optional:} | |
389 | (if n1 (+ n n1) (1+ n))) ; @r{1 or 2 arguments.} | |
390 | 1 2) | |
391 | @result{} 3 | |
392 | ((lambda (n &rest ns) ; @r{One required and one rest:} | |
393 | (+ n (apply '+ ns))) ; @r{1 or more arguments.} | |
394 | 1 2 3 4 5) | |
395 | @result{} 15 | |
396 | @end smallexample | |
397 | ||
398 | @node Function Documentation | |
399 | @subsection Documentation Strings of Functions | |
400 | @cindex documentation of function | |
401 | ||
402 | A lambda expression may optionally have a @dfn{documentation string} just | |
403 | after the lambda list. This string does not affect execution of the | |
404 | function; it is a kind of comment, but a systematized comment which | |
405 | actually appears inside the Lisp world and can be used by the Emacs help | |
406 | facilities. @xref{Documentation}, for how the @var{documentation-string} is | |
407 | accessed. | |
408 | ||
409 | It is a good idea to provide documentation strings for all the | |
410 | functions in your program, even those that are called only from within | |
411 | your program. Documentation strings are like comments, except that they | |
412 | are easier to access. | |
413 | ||
414 | The first line of the documentation string should stand on its own, | |
415 | because @code{apropos} displays just this first line. It should consist | |
416 | of one or two complete sentences that summarize the function's purpose. | |
417 | ||
418 | The start of the documentation string is usually indented in the | |
419 | source file, but since these spaces come before the starting | |
420 | double-quote, they are not part of the string. Some people make a | |
421 | practice of indenting any additional lines of the string so that the | |
422 | text lines up in the program source. @emph{That is a mistake.} The | |
423 | indentation of the following lines is inside the string; what looks | |
424 | nice in the source code will look ugly when displayed by the help | |
425 | commands. | |
426 | ||
427 | You may wonder how the documentation string could be optional, since | |
428 | there are required components of the function that follow it (the body). | |
429 | Since evaluation of a string returns that string, without any side effects, | |
430 | it has no effect if it is not the last form in the body. Thus, in | |
431 | practice, there is no confusion between the first form of the body and the | |
432 | documentation string; if the only body form is a string then it serves both | |
433 | as the return value and as the documentation. | |
434 | ||
435 | The last line of the documentation string can specify calling | |
436 | conventions different from the actual function arguments. Write | |
437 | text like this: | |
438 | ||
439 | @example | |
440 | \(fn @var{arglist}) | |
441 | @end example | |
442 | ||
443 | @noindent | |
444 | following a blank line, at the beginning of the line, with no newline | |
445 | following it inside the documentation string. (The @samp{\} is used | |
446 | to avoid confusing the Emacs motion commands.) The calling convention | |
447 | specified in this way appears in help messages in place of the one | |
448 | derived from the actual arguments of the function. | |
449 | ||
450 | This feature is particularly useful for macro definitions, since the | |
451 | arguments written in a macro definition often do not correspond to the | |
452 | way users think of the parts of the macro call. | |
453 | ||
454 | @node Function Names | |
455 | @section Naming a Function | |
456 | @cindex function definition | |
457 | @cindex named function | |
458 | @cindex function name | |
459 | ||
460 | In most computer languages, every function has a name; the idea of a | |
461 | function without a name is nonsensical. In Lisp, a function in the | |
462 | strictest sense has no name. It is simply a list whose first element is | |
463 | @code{lambda}, a byte-code function object, or a primitive subr-object. | |
464 | ||
465 | However, a symbol can serve as the name of a function. This happens | |
466 | when you put the function in the symbol's @dfn{function cell} | |
467 | (@pxref{Symbol Components}). Then the symbol itself becomes a valid, | |
468 | callable function, equivalent to the list or subr-object that its | |
469 | function cell refers to. The contents of the function cell are also | |
470 | called the symbol's @dfn{function definition}. The procedure of using a | |
471 | symbol's function definition in place of the symbol is called | |
472 | @dfn{symbol function indirection}; see @ref{Function Indirection}. | |
473 | ||
474 | In practice, nearly all functions are given names in this way and | |
475 | referred to through their names. For example, the symbol @code{car} works | |
476 | as a function and does what it does because the primitive subr-object | |
477 | @code{#<subr car>} is stored in its function cell. | |
478 | ||
479 | We give functions names because it is convenient to refer to them by | |
480 | their names in Lisp expressions. For primitive subr-objects such as | |
481 | @code{#<subr car>}, names are the only way you can refer to them: there | |
482 | is no read syntax for such objects. For functions written in Lisp, the | |
483 | name is more convenient to use in a call than an explicit lambda | |
484 | expression. Also, a function with a name can refer to itself---it can | |
485 | be recursive. Writing the function's name in its own definition is much | |
486 | more convenient than making the function definition point to itself | |
487 | (something that is not impossible but that has various disadvantages in | |
488 | practice). | |
489 | ||
490 | We often identify functions with the symbols used to name them. For | |
491 | example, we often speak of ``the function @code{car},'' not | |
492 | distinguishing between the symbol @code{car} and the primitive | |
493 | subr-object that is its function definition. For most purposes, the | |
494 | distinction is not important. | |
495 | ||
496 | Even so, keep in mind that a function need not have a unique name. While | |
497 | a given function object @emph{usually} appears in the function cell of only | |
498 | one symbol, this is just a matter of convenience. It is easy to store | |
499 | it in several symbols using @code{fset}; then each of the symbols is | |
500 | equally well a name for the same function. | |
501 | ||
502 | A symbol used as a function name may also be used as a variable; these | |
503 | two uses of a symbol are independent and do not conflict. (Some Lisp | |
504 | dialects, such as Scheme, do not distinguish between a symbol's value | |
505 | and its function definition; a symbol's value as a variable is also its | |
506 | function definition.) If you have not given a symbol a function | |
507 | definition, you cannot use it as a function; whether the symbol has a | |
508 | value as a variable makes no difference to this. | |
509 | ||
510 | @node Defining Functions | |
511 | @section Defining Functions | |
512 | @cindex defining a function | |
513 | ||
514 | We usually give a name to a function when it is first created. This | |
515 | is called @dfn{defining a function}, and it is done with the | |
516 | @code{defun} special form. | |
517 | ||
518 | @defspec defun name argument-list body-forms | |
519 | @code{defun} is the usual way to define new Lisp functions. It | |
520 | defines the symbol @var{name} as a function that looks like this: | |
521 | ||
522 | @example | |
523 | (lambda @var{argument-list} . @var{body-forms}) | |
524 | @end example | |
525 | ||
526 | @code{defun} stores this lambda expression in the function cell of | |
527 | @var{name}. It returns the value @var{name}, but usually we ignore this | |
528 | value. | |
529 | ||
530 | As described previously, @var{argument-list} is a list of argument | |
531 | names and may include the keywords @code{&optional} and @code{&rest} | |
532 | (@pxref{Lambda Expressions}). Also, the first two of the | |
533 | @var{body-forms} may be a documentation string and an interactive | |
534 | declaration. | |
535 | ||
536 | There is no conflict if the same symbol @var{name} is also used as a | |
537 | variable, since the symbol's value cell is independent of the function | |
538 | cell. @xref{Symbol Components}. | |
539 | ||
540 | Here are some examples: | |
541 | ||
542 | @example | |
543 | @group | |
544 | (defun foo () 5) | |
545 | @result{} foo | |
546 | @end group | |
547 | @group | |
548 | (foo) | |
549 | @result{} 5 | |
550 | @end group | |
551 | ||
552 | @group | |
553 | (defun bar (a &optional b &rest c) | |
554 | (list a b c)) | |
555 | @result{} bar | |
556 | @end group | |
557 | @group | |
558 | (bar 1 2 3 4 5) | |
559 | @result{} (1 2 (3 4 5)) | |
560 | @end group | |
561 | @group | |
562 | (bar 1) | |
563 | @result{} (1 nil nil) | |
564 | @end group | |
565 | @group | |
566 | (bar) | |
567 | @error{} Wrong number of arguments. | |
568 | @end group | |
569 | ||
570 | @group | |
571 | (defun capitalize-backwards () | |
572 | "Upcase the last letter of a word." | |
573 | (interactive) | |
574 | (backward-word 1) | |
575 | (forward-word 1) | |
576 | (backward-char 1) | |
577 | (capitalize-word 1)) | |
578 | @result{} capitalize-backwards | |
579 | @end group | |
580 | @end example | |
581 | ||
582 | Be careful not to redefine existing functions unintentionally. | |
583 | @code{defun} redefines even primitive functions such as @code{car} | |
584 | without any hesitation or notification. Redefining a function already | |
585 | defined is often done deliberately, and there is no way to distinguish | |
586 | deliberate redefinition from unintentional redefinition. | |
587 | @end defspec | |
588 | ||
589 | @cindex function aliases | |
590 | @defun defalias name definition &optional docstring | |
591 | @anchor{Definition of defalias} | |
592 | This special form defines the symbol @var{name} as a function, with | |
593 | definition @var{definition} (which can be any valid Lisp function). | |
594 | It returns @var{definition}. | |
595 | ||
596 | If @var{docstring} is non-@code{nil}, it becomes the function | |
597 | documentation of @var{name}. Otherwise, any documentation provided by | |
598 | @var{definition} is used. | |
599 | ||
600 | The proper place to use @code{defalias} is where a specific function | |
601 | name is being defined---especially where that name appears explicitly in | |
602 | the source file being loaded. This is because @code{defalias} records | |
603 | which file defined the function, just like @code{defun} | |
604 | (@pxref{Unloading}). | |
605 | ||
606 | By contrast, in programs that manipulate function definitions for other | |
607 | purposes, it is better to use @code{fset}, which does not keep such | |
608 | records. @xref{Function Cells}. | |
609 | @end defun | |
610 | ||
611 | You cannot create a new primitive function with @code{defun} or | |
612 | @code{defalias}, but you can use them to change the function definition of | |
613 | any symbol, even one such as @code{car} or @code{x-popup-menu} whose | |
614 | normal definition is a primitive. However, this is risky: for | |
615 | instance, it is next to impossible to redefine @code{car} without | |
616 | breaking Lisp completely. Redefining an obscure function such as | |
617 | @code{x-popup-menu} is less dangerous, but it still may not work as | |
618 | you expect. If there are calls to the primitive from C code, they | |
619 | call the primitive's C definition directly, so changing the symbol's | |
620 | definition will have no effect on them. | |
621 | ||
622 | See also @code{defsubst}, which defines a function like @code{defun} | |
623 | and tells the Lisp compiler to open-code it. @xref{Inline Functions}. | |
624 | ||
625 | @node Calling Functions | |
626 | @section Calling Functions | |
627 | @cindex function invocation | |
628 | @cindex calling a function | |
629 | ||
630 | Defining functions is only half the battle. Functions don't do | |
631 | anything until you @dfn{call} them, i.e., tell them to run. Calling a | |
632 | function is also known as @dfn{invocation}. | |
633 | ||
634 | The most common way of invoking a function is by evaluating a list. | |
635 | For example, evaluating the list @code{(concat "a" "b")} calls the | |
636 | function @code{concat} with arguments @code{"a"} and @code{"b"}. | |
637 | @xref{Evaluation}, for a description of evaluation. | |
638 | ||
639 | When you write a list as an expression in your program, you specify | |
640 | which function to call, and how many arguments to give it, in the text | |
641 | of the program. Usually that's just what you want. Occasionally you | |
642 | need to compute at run time which function to call. To do that, use | |
643 | the function @code{funcall}. When you also need to determine at run | |
644 | time how many arguments to pass, use @code{apply}. | |
645 | ||
646 | @defun funcall function &rest arguments | |
647 | @code{funcall} calls @var{function} with @var{arguments}, and returns | |
648 | whatever @var{function} returns. | |
649 | ||
650 | Since @code{funcall} is a function, all of its arguments, including | |
651 | @var{function}, are evaluated before @code{funcall} is called. This | |
652 | means that you can use any expression to obtain the function to be | |
653 | called. It also means that @code{funcall} does not see the | |
654 | expressions you write for the @var{arguments}, only their values. | |
655 | These values are @emph{not} evaluated a second time in the act of | |
656 | calling @var{function}; the operation of @code{funcall} is like the | |
657 | normal procedure for calling a function, once its arguments have | |
658 | already been evaluated. | |
659 | ||
660 | The argument @var{function} must be either a Lisp function or a | |
661 | primitive function. Special forms and macros are not allowed, because | |
662 | they make sense only when given the ``unevaluated'' argument | |
663 | expressions. @code{funcall} cannot provide these because, as we saw | |
664 | above, it never knows them in the first place. | |
665 | ||
666 | @example | |
667 | @group | |
668 | (setq f 'list) | |
669 | @result{} list | |
670 | @end group | |
671 | @group | |
672 | (funcall f 'x 'y 'z) | |
673 | @result{} (x y z) | |
674 | @end group | |
675 | @group | |
676 | (funcall f 'x 'y '(z)) | |
677 | @result{} (x y (z)) | |
678 | @end group | |
679 | @group | |
680 | (funcall 'and t nil) | |
681 | @error{} Invalid function: #<subr and> | |
682 | @end group | |
683 | @end example | |
684 | ||
685 | Compare these examples with the examples of @code{apply}. | |
686 | @end defun | |
687 | ||
688 | @defun apply function &rest arguments | |
689 | @code{apply} calls @var{function} with @var{arguments}, just like | |
690 | @code{funcall} but with one difference: the last of @var{arguments} is a | |
691 | list of objects, which are passed to @var{function} as separate | |
692 | arguments, rather than a single list. We say that @code{apply} | |
693 | @dfn{spreads} this list so that each individual element becomes an | |
694 | argument. | |
695 | ||
696 | @code{apply} returns the result of calling @var{function}. As with | |
697 | @code{funcall}, @var{function} must either be a Lisp function or a | |
698 | primitive function; special forms and macros do not make sense in | |
699 | @code{apply}. | |
700 | ||
701 | @example | |
702 | @group | |
703 | (setq f 'list) | |
704 | @result{} list | |
705 | @end group | |
706 | @group | |
707 | (apply f 'x 'y 'z) | |
708 | @error{} Wrong type argument: listp, z | |
709 | @end group | |
710 | @group | |
711 | (apply '+ 1 2 '(3 4)) | |
712 | @result{} 10 | |
713 | @end group | |
714 | @group | |
715 | (apply '+ '(1 2 3 4)) | |
716 | @result{} 10 | |
717 | @end group | |
718 | ||
719 | @group | |
720 | (apply 'append '((a b c) nil (x y z) nil)) | |
721 | @result{} (a b c x y z) | |
722 | @end group | |
723 | @end example | |
724 | ||
725 | For an interesting example of using @code{apply}, see @ref{Definition | |
726 | of mapcar}. | |
727 | @end defun | |
728 | ||
80f85d7c EZ |
729 | @cindex partial application of functions |
730 | @cindex currying | |
a18a6d49 | 731 | Sometimes it is useful to fix some of the function's arguments at |
80f85d7c EZ |
732 | certain values, and leave the rest of arguments for when the function |
733 | is actually called. The act of fixing some of the function's | |
734 | arguments is called @dfn{partial application} of the function@footnote{ | |
735 | This is related to, but different from @dfn{currying}, which | |
736 | transforms a function that takes multiple arguments in such a way that | |
737 | it can be called as a chain of functions, each one with a single | |
738 | argument.}. | |
739 | The result is a new function that accepts the rest of | |
740 | arguments and calls the original function with all the arguments | |
a18a6d49 EZ |
741 | combined. |
742 | ||
743 | Here's how to do partial application in Emacs Lisp: | |
80f85d7c EZ |
744 | |
745 | @defun apply-partially func &rest args | |
746 | This function returns a new function which, when called, will call | |
747 | @var{func} with the list of arguments composed from @var{args} and | |
748 | additional arguments specified at the time of the call. If @var{func} | |
749 | accepts @var{n} arguments, then a call to @code{apply-partially} with | |
750 | @w{@code{@var{m} < @var{n}}} arguments will produce a new function of | |
751 | @w{@code{@var{n} - @var{m}}} arguments. | |
752 | ||
834b5485 EZ |
753 | Here's how we could define the built-in function @code{1+}, if it |
754 | didn't exist, using @code{apply-partially} and @code{+}, another | |
755 | built-in function: | |
80f85d7c EZ |
756 | |
757 | @example | |
80f85d7c | 758 | @group |
834b5485 EZ |
759 | (defalias '1+ (apply-partially '+ 1) |
760 | "Increment argument by one.") | |
761 | @end group | |
762 | @group | |
763 | (1+ 10) | |
80f85d7c EZ |
764 | @result{} 11 |
765 | @end group | |
766 | @end example | |
767 | @end defun | |
768 | ||
b8d4c8d0 GM |
769 | @cindex functionals |
770 | It is common for Lisp functions to accept functions as arguments or | |
771 | find them in data structures (especially in hook variables and property | |
772 | lists) and call them using @code{funcall} or @code{apply}. Functions | |
773 | that accept function arguments are often called @dfn{functionals}. | |
774 | ||
775 | Sometimes, when you call a functional, it is useful to supply a no-op | |
776 | function as the argument. Here are two different kinds of no-op | |
777 | function: | |
778 | ||
779 | @defun identity arg | |
780 | This function returns @var{arg} and has no side effects. | |
781 | @end defun | |
782 | ||
783 | @defun ignore &rest args | |
784 | This function ignores any arguments and returns @code{nil}. | |
785 | @end defun | |
786 | ||
787 | @node Mapping Functions | |
788 | @section Mapping Functions | |
789 | @cindex mapping functions | |
790 | ||
791 | A @dfn{mapping function} applies a given function (@emph{not} a | |
792 | special form or macro) to each element of a list or other collection. | |
793 | Emacs Lisp has several such functions; @code{mapcar} and | |
794 | @code{mapconcat}, which scan a list, are described here. | |
795 | @xref{Definition of mapatoms}, for the function @code{mapatoms} which | |
796 | maps over the symbols in an obarray. @xref{Definition of maphash}, | |
797 | for the function @code{maphash} which maps over key/value associations | |
798 | in a hash table. | |
799 | ||
800 | These mapping functions do not allow char-tables because a char-table | |
801 | is a sparse array whose nominal range of indices is very large. To map | |
802 | over a char-table in a way that deals properly with its sparse nature, | |
803 | use the function @code{map-char-table} (@pxref{Char-Tables}). | |
804 | ||
805 | @defun mapcar function sequence | |
806 | @anchor{Definition of mapcar} | |
807 | @code{mapcar} applies @var{function} to each element of @var{sequence} | |
808 | in turn, and returns a list of the results. | |
809 | ||
810 | The argument @var{sequence} can be any kind of sequence except a | |
811 | char-table; that is, a list, a vector, a bool-vector, or a string. The | |
812 | result is always a list. The length of the result is the same as the | |
813 | length of @var{sequence}. For example: | |
814 | ||
815 | @smallexample | |
816 | @group | |
817 | (mapcar 'car '((a b) (c d) (e f))) | |
818 | @result{} (a c e) | |
819 | (mapcar '1+ [1 2 3]) | |
820 | @result{} (2 3 4) | |
3e99b825 | 821 | (mapcar 'string "abc") |
b8d4c8d0 GM |
822 | @result{} ("a" "b" "c") |
823 | @end group | |
824 | ||
825 | @group | |
826 | ;; @r{Call each function in @code{my-hooks}.} | |
827 | (mapcar 'funcall my-hooks) | |
828 | @end group | |
829 | ||
830 | @group | |
831 | (defun mapcar* (function &rest args) | |
832 | "Apply FUNCTION to successive cars of all ARGS. | |
833 | Return the list of results." | |
834 | ;; @r{If no list is exhausted,} | |
835 | (if (not (memq nil args)) | |
836 | ;; @r{apply function to @sc{car}s.} | |
837 | (cons (apply function (mapcar 'car args)) | |
838 | (apply 'mapcar* function | |
839 | ;; @r{Recurse for rest of elements.} | |
840 | (mapcar 'cdr args))))) | |
841 | @end group | |
842 | ||
843 | @group | |
844 | (mapcar* 'cons '(a b c) '(1 2 3 4)) | |
845 | @result{} ((a . 1) (b . 2) (c . 3)) | |
846 | @end group | |
847 | @end smallexample | |
848 | @end defun | |
849 | ||
850 | @defun mapc function sequence | |
851 | @code{mapc} is like @code{mapcar} except that @var{function} is used for | |
852 | side-effects only---the values it returns are ignored, not collected | |
853 | into a list. @code{mapc} always returns @var{sequence}. | |
854 | @end defun | |
855 | ||
856 | @defun mapconcat function sequence separator | |
857 | @code{mapconcat} applies @var{function} to each element of | |
858 | @var{sequence}: the results, which must be strings, are concatenated. | |
859 | Between each pair of result strings, @code{mapconcat} inserts the string | |
860 | @var{separator}. Usually @var{separator} contains a space or comma or | |
861 | other suitable punctuation. | |
862 | ||
863 | The argument @var{function} must be a function that can take one | |
864 | argument and return a string. The argument @var{sequence} can be any | |
865 | kind of sequence except a char-table; that is, a list, a vector, a | |
866 | bool-vector, or a string. | |
867 | ||
868 | @smallexample | |
869 | @group | |
870 | (mapconcat 'symbol-name | |
871 | '(The cat in the hat) | |
872 | " ") | |
873 | @result{} "The cat in the hat" | |
874 | @end group | |
875 | ||
876 | @group | |
877 | (mapconcat (function (lambda (x) (format "%c" (1+ x)))) | |
878 | "HAL-8000" | |
879 | "") | |
880 | @result{} "IBM.9111" | |
881 | @end group | |
882 | @end smallexample | |
883 | @end defun | |
884 | ||
885 | @node Anonymous Functions | |
886 | @section Anonymous Functions | |
887 | @cindex anonymous function | |
888 | ||
889 | In Lisp, a function is a list that starts with @code{lambda}, a | |
890 | byte-code function compiled from such a list, or alternatively a | |
5d6ab672 CY |
891 | primitive subr-object; names are ``extra.'' Although functions are |
892 | usually defined with @code{defun} and given names at the same time, it | |
893 | is occasionally more concise to use an explicit lambda expression---an | |
b8d4c8d0 GM |
894 | anonymous function. Such a list is valid wherever a function name is. |
895 | ||
896 | Any method of creating such a list makes a valid function. Even this: | |
897 | ||
898 | @smallexample | |
899 | @group | |
900 | (setq silly (append '(lambda (x)) (list (list '+ (* 3 4) 'x)))) | |
901 | @result{} (lambda (x) (+ 12 x)) | |
902 | @end group | |
903 | @end smallexample | |
904 | ||
905 | @noindent | |
906 | This computes a list that looks like @code{(lambda (x) (+ 12 x))} and | |
907 | makes it the value (@emph{not} the function definition!) of | |
908 | @code{silly}. | |
909 | ||
910 | Here is how we might call this function: | |
911 | ||
912 | @example | |
913 | @group | |
914 | (funcall silly 1) | |
915 | @result{} 13 | |
916 | @end group | |
917 | @end example | |
918 | ||
919 | @noindent | |
5d6ab672 CY |
920 | It does @emph{not} work to write @code{(silly 1)}, because this |
921 | function is not the @emph{function definition} of @code{silly}. We | |
922 | have not given @code{silly} any function definition, just a value as a | |
923 | variable. | |
b8d4c8d0 GM |
924 | |
925 | Most of the time, anonymous functions are constants that appear in | |
5d6ab672 CY |
926 | your program. For instance, you might want to pass one as an argument |
927 | to the function @code{mapcar}, which applies any given function to | |
928 | each element of a list (@pxref{Mapping Functions}). | |
929 | @xref{describe-symbols example}, for a realistic example of this. | |
b8d4c8d0 | 930 | |
5d6ab672 CY |
931 | In the following example, we define a @code{change-property} |
932 | function that takes a function as its third argument, followed by a | |
933 | @code{double-property} function that makes use of | |
934 | @code{change-property} by passing it an anonymous function: | |
b8d4c8d0 GM |
935 | |
936 | @example | |
937 | @group | |
938 | (defun change-property (symbol prop function) | |
939 | (let ((value (get symbol prop))) | |
940 | (put symbol prop (funcall function value)))) | |
941 | @end group | |
b8d4c8d0 | 942 | |
b8d4c8d0 GM |
943 | @group |
944 | (defun double-property (symbol prop) | |
5d6ab672 | 945 | (change-property symbol prop (lambda (x) (* 2 x)))) |
b8d4c8d0 GM |
946 | @end group |
947 | @end example | |
948 | ||
b8d4c8d0 | 949 | @noindent |
5d6ab672 CY |
950 | In the @code{double-property} function, we did not quote the |
951 | @code{lambda} form. This is permissible, because a @code{lambda} form | |
952 | is @dfn{self-quoting}: evaluating the form yields the form itself. | |
b8d4c8d0 | 953 | |
5d6ab672 CY |
954 | Whether or not you quote a @code{lambda} form makes a difference if |
955 | you compile the code (@pxref{Byte Compilation}). If the @code{lambda} | |
956 | form is unquoted, as in the above example, the anonymous function is | |
957 | also compiled. Suppose, however, that we quoted the @code{lambda} | |
958 | form: | |
b8d4c8d0 GM |
959 | |
960 | @example | |
961 | @group | |
962 | (defun double-property (symbol prop) | |
5d6ab672 | 963 | (change-property symbol prop '(lambda (x) (* 2 x)))) |
b8d4c8d0 GM |
964 | @end group |
965 | @end example | |
966 | ||
967 | @noindent | |
5d6ab672 CY |
968 | If you compile this, the argument passed to @code{change-property} is |
969 | the precise list shown: | |
b8d4c8d0 GM |
970 | |
971 | @example | |
5d6ab672 | 972 | (lambda (x) (* x 2)) |
b8d4c8d0 GM |
973 | @end example |
974 | ||
975 | @noindent | |
5d6ab672 CY |
976 | The Lisp compiler cannot assume this list is a function, even though |
977 | it looks like one, since it does not know what @code{change-property} | |
978 | will do with the list. Perhaps it will check whether the @sc{car} of | |
979 | the third element is the symbol @code{*}! | |
b8d4c8d0 | 980 | |
5d6ab672 CY |
981 | @findex function |
982 | The @code{function} special form explicitly tells the byte-compiler | |
983 | that its argument is a function: | |
b8d4c8d0 GM |
984 | |
985 | @defspec function function-object | |
986 | @cindex function quoting | |
987 | This special form returns @var{function-object} without evaluating it. | |
988 | In this, it is equivalent to @code{quote}. However, it serves as a | |
989 | note to the Emacs Lisp compiler that @var{function-object} is intended | |
990 | to be used only as a function, and therefore can safely be compiled. | |
991 | Contrast this with @code{quote}, in @ref{Quoting}. | |
992 | @end defspec | |
993 | ||
5d6ab672 CY |
994 | @cindex @samp{#'} syntax |
995 | The read syntax @code{#'} is a short-hand for using @code{function}. | |
996 | Generally, it is not necessary to use either @code{#'} or | |
997 | @code{function}; just use an unquoted @code{lambda} form instead. | |
998 | (Actually, @code{lambda} is a macro defined using @code{function}.) | |
999 | The following forms are all equivalent: | |
1000 | ||
1001 | @example | |
1002 | #'(lambda (x) (* x x)) | |
1003 | (function (lambda (x) (* x x))) | |
1004 | (lambda (x) (* x x)) | |
1005 | @end example | |
1006 | ||
1007 | We sometimes write @code{function} instead of @code{quote} when | |
1008 | quoting the name of a function, but this usage is just a sort of | |
1009 | comment: | |
1010 | ||
1011 | @example | |
1012 | (function @var{symbol}) @equiv{} (quote @var{symbol}) @equiv{} '@var{symbol} | |
1013 | @end example | |
b8d4c8d0 GM |
1014 | |
1015 | @node Function Cells | |
1016 | @section Accessing Function Cell Contents | |
1017 | ||
1018 | The @dfn{function definition} of a symbol is the object stored in the | |
1019 | function cell of the symbol. The functions described here access, test, | |
1020 | and set the function cell of symbols. | |
1021 | ||
1022 | See also the function @code{indirect-function}. @xref{Definition of | |
1023 | indirect-function}. | |
1024 | ||
1025 | @defun symbol-function symbol | |
1026 | @kindex void-function | |
1027 | This returns the object in the function cell of @var{symbol}. If the | |
1028 | symbol's function cell is void, a @code{void-function} error is | |
1029 | signaled. | |
1030 | ||
1031 | This function does not check that the returned object is a legitimate | |
1032 | function. | |
1033 | ||
1034 | @example | |
1035 | @group | |
1036 | (defun bar (n) (+ n 2)) | |
1037 | @result{} bar | |
1038 | @end group | |
1039 | @group | |
1040 | (symbol-function 'bar) | |
1041 | @result{} (lambda (n) (+ n 2)) | |
1042 | @end group | |
1043 | @group | |
1044 | (fset 'baz 'bar) | |
1045 | @result{} bar | |
1046 | @end group | |
1047 | @group | |
1048 | (symbol-function 'baz) | |
1049 | @result{} bar | |
1050 | @end group | |
1051 | @end example | |
1052 | @end defun | |
1053 | ||
1054 | @cindex void function cell | |
1055 | If you have never given a symbol any function definition, we say that | |
1056 | that symbol's function cell is @dfn{void}. In other words, the function | |
1057 | cell does not have any Lisp object in it. If you try to call such a symbol | |
1058 | as a function, it signals a @code{void-function} error. | |
1059 | ||
1060 | Note that void is not the same as @code{nil} or the symbol | |
1061 | @code{void}. The symbols @code{nil} and @code{void} are Lisp objects, | |
1062 | and can be stored into a function cell just as any other object can be | |
1063 | (and they can be valid functions if you define them in turn with | |
1064 | @code{defun}). A void function cell contains no object whatsoever. | |
1065 | ||
1066 | You can test the voidness of a symbol's function definition with | |
1067 | @code{fboundp}. After you have given a symbol a function definition, you | |
1068 | can make it void once more using @code{fmakunbound}. | |
1069 | ||
1070 | @defun fboundp symbol | |
1071 | This function returns @code{t} if the symbol has an object in its | |
1072 | function cell, @code{nil} otherwise. It does not check that the object | |
1073 | is a legitimate function. | |
1074 | @end defun | |
1075 | ||
1076 | @defun fmakunbound symbol | |
1077 | This function makes @var{symbol}'s function cell void, so that a | |
1078 | subsequent attempt to access this cell will cause a | |
1079 | @code{void-function} error. It returns @var{symbol}. (See also | |
1080 | @code{makunbound}, in @ref{Void Variables}.) | |
1081 | ||
1082 | @example | |
1083 | @group | |
1084 | (defun foo (x) x) | |
1085 | @result{} foo | |
1086 | @end group | |
1087 | @group | |
1088 | (foo 1) | |
1089 | @result{}1 | |
1090 | @end group | |
1091 | @group | |
1092 | (fmakunbound 'foo) | |
1093 | @result{} foo | |
1094 | @end group | |
1095 | @group | |
1096 | (foo 1) | |
1097 | @error{} Symbol's function definition is void: foo | |
1098 | @end group | |
1099 | @end example | |
1100 | @end defun | |
1101 | ||
1102 | @defun fset symbol definition | |
1103 | This function stores @var{definition} in the function cell of | |
1104 | @var{symbol}. The result is @var{definition}. Normally | |
1105 | @var{definition} should be a function or the name of a function, but | |
1106 | this is not checked. The argument @var{symbol} is an ordinary evaluated | |
1107 | argument. | |
1108 | ||
1109 | There are three normal uses of this function: | |
1110 | ||
1111 | @itemize @bullet | |
1112 | @item | |
1113 | Copying one symbol's function definition to another---in other words, | |
1114 | making an alternate name for a function. (If you think of this as the | |
1115 | definition of the new name, you should use @code{defalias} instead of | |
1116 | @code{fset}; see @ref{Definition of defalias}.) | |
1117 | ||
1118 | @item | |
1119 | Giving a symbol a function definition that is not a list and therefore | |
1120 | cannot be made with @code{defun}. For example, you can use @code{fset} | |
1121 | to give a symbol @code{s1} a function definition which is another symbol | |
1122 | @code{s2}; then @code{s1} serves as an alias for whatever definition | |
1123 | @code{s2} presently has. (Once again use @code{defalias} instead of | |
1124 | @code{fset} if you think of this as the definition of @code{s1}.) | |
1125 | ||
1126 | @item | |
1127 | In constructs for defining or altering functions. If @code{defun} | |
1128 | were not a primitive, it could be written in Lisp (as a macro) using | |
1129 | @code{fset}. | |
1130 | @end itemize | |
1131 | ||
1132 | Here are examples of these uses: | |
1133 | ||
1134 | @example | |
1135 | @group | |
1136 | ;; @r{Save @code{foo}'s definition in @code{old-foo}.} | |
1137 | (fset 'old-foo (symbol-function 'foo)) | |
1138 | @end group | |
1139 | ||
1140 | @group | |
1141 | ;; @r{Make the symbol @code{car} the function definition of @code{xfirst}.} | |
1142 | ;; @r{(Most likely, @code{defalias} would be better than @code{fset} here.)} | |
1143 | (fset 'xfirst 'car) | |
1144 | @result{} car | |
1145 | @end group | |
1146 | @group | |
1147 | (xfirst '(1 2 3)) | |
1148 | @result{} 1 | |
1149 | @end group | |
1150 | @group | |
1151 | (symbol-function 'xfirst) | |
1152 | @result{} car | |
1153 | @end group | |
1154 | @group | |
1155 | (symbol-function (symbol-function 'xfirst)) | |
1156 | @result{} #<subr car> | |
1157 | @end group | |
1158 | ||
1159 | @group | |
1160 | ;; @r{Define a named keyboard macro.} | |
1161 | (fset 'kill-two-lines "\^u2\^k") | |
1162 | @result{} "\^u2\^k" | |
1163 | @end group | |
1164 | ||
1165 | @group | |
1166 | ;; @r{Here is a function that alters other functions.} | |
1167 | (defun copy-function-definition (new old) | |
1168 | "Define NEW with the same function definition as OLD." | |
1169 | (fset new (symbol-function old))) | |
1170 | @end group | |
1171 | @end example | |
1172 | @end defun | |
1173 | ||
1174 | @code{fset} is sometimes used to save the old definition of a | |
1175 | function before redefining it. That permits the new definition to | |
1176 | invoke the old definition. But it is unmodular and unclean for a Lisp | |
1177 | file to redefine a function defined elsewhere. If you want to modify | |
1178 | a function defined by another package, it is cleaner to use | |
1179 | @code{defadvice} (@pxref{Advising Functions}). | |
1180 | ||
1181 | @node Obsolete Functions | |
1182 | @section Declaring Functions Obsolete | |
1183 | ||
1184 | You can use @code{make-obsolete} to declare a function obsolete. This | |
1185 | indicates that the function may be removed at some stage in the future. | |
1186 | ||
1187 | @defun make-obsolete obsolete-name current-name &optional when | |
1188 | This function makes the byte compiler warn that the function | |
1189 | @var{obsolete-name} is obsolete. If @var{current-name} is a symbol, the | |
1190 | warning message says to use @var{current-name} instead of | |
1191 | @var{obsolete-name}. @var{current-name} does not need to be an alias for | |
1192 | @var{obsolete-name}; it can be a different function with similar | |
1193 | functionality. If @var{current-name} is a string, it is the warning | |
1194 | message. | |
1195 | ||
1196 | If provided, @var{when} should be a string indicating when the function | |
1197 | was first made obsolete---for example, a date or a release number. | |
1198 | @end defun | |
1199 | ||
1200 | You can define a function as an alias and declare it obsolete at the | |
eb5ed549 | 1201 | same time using the macro @code{define-obsolete-function-alias}: |
b8d4c8d0 GM |
1202 | |
1203 | @defmac define-obsolete-function-alias obsolete-name current-name &optional when docstring | |
1204 | This macro marks the function @var{obsolete-name} obsolete and also | |
1205 | defines it as an alias for the function @var{current-name}. It is | |
1206 | equivalent to the following: | |
1207 | ||
1208 | @example | |
1209 | (defalias @var{obsolete-name} @var{current-name} @var{docstring}) | |
1210 | (make-obsolete @var{obsolete-name} @var{current-name} @var{when}) | |
1211 | @end example | |
1212 | @end defmac | |
1213 | ||
eb5ed549 CY |
1214 | In addition, you can mark a certain a particular calling convention |
1215 | for a function as obsolete: | |
1216 | ||
1217 | @defun set-advertised-calling-convention function signature | |
1218 | This function specifies the argument list @var{signature} as the | |
1219 | correct way to call @var{function}. This causes the Emacs byte | |
1220 | compiler to issue a warning whenever it comes across an Emacs Lisp | |
1221 | program that calls @var{function} any other way (however, it will | |
1222 | still allow the code to be byte compiled). | |
1223 | ||
1224 | For instance, in old versions of Emacs the @code{sit-for} function | |
1225 | accepted three arguments, like this | |
1226 | ||
1227 | @smallexample | |
1228 | (sit-for seconds milliseconds nodisp) | |
1229 | @end smallexample | |
1230 | ||
1231 | However, calling @code{sit-for} this way is considered obsolete | |
1232 | (@pxref{Waiting}). The old calling convention is deprecated like | |
1233 | this: | |
1234 | ||
1235 | @smallexample | |
1236 | (set-advertised-calling-convention | |
1237 | 'sit-for '(seconds &optional nodisp)) | |
1238 | @end smallexample | |
1239 | @end defun | |
1240 | ||
b8d4c8d0 GM |
1241 | @node Inline Functions |
1242 | @section Inline Functions | |
1243 | @cindex inline functions | |
1244 | ||
1245 | @findex defsubst | |
1246 | You can define an @dfn{inline function} by using @code{defsubst} instead | |
1247 | of @code{defun}. An inline function works just like an ordinary | |
1248 | function except for one thing: when you compile a call to the function, | |
1249 | the function's definition is open-coded into the caller. | |
1250 | ||
1251 | Making a function inline makes explicit calls run faster. But it also | |
1252 | has disadvantages. For one thing, it reduces flexibility; if you | |
1253 | change the definition of the function, calls already inlined still use | |
1254 | the old definition until you recompile them. | |
1255 | ||
1256 | Another disadvantage is that making a large function inline can increase | |
1257 | the size of compiled code both in files and in memory. Since the speed | |
1258 | advantage of inline functions is greatest for small functions, you | |
1259 | generally should not make large functions inline. | |
1260 | ||
1261 | Also, inline functions do not behave well with respect to debugging, | |
1262 | tracing, and advising (@pxref{Advising Functions}). Since ease of | |
1263 | debugging and the flexibility of redefining functions are important | |
1264 | features of Emacs, you should not make a function inline, even if it's | |
1265 | small, unless its speed is really crucial, and you've timed the code | |
1266 | to verify that using @code{defun} actually has performance problems. | |
1267 | ||
1268 | It's possible to define a macro to expand into the same code that an | |
1269 | inline function would execute. (@xref{Macros}.) But the macro would be | |
1270 | limited to direct use in expressions---a macro cannot be called with | |
1271 | @code{apply}, @code{mapcar} and so on. Also, it takes some work to | |
1272 | convert an ordinary function into a macro. To convert it into an inline | |
1273 | function is very easy; simply replace @code{defun} with @code{defsubst}. | |
1274 | Since each argument of an inline function is evaluated exactly once, you | |
1275 | needn't worry about how many times the body uses the arguments, as you | |
1276 | do for macros. (@xref{Argument Evaluation}.) | |
1277 | ||
1278 | Inline functions can be used and open-coded later on in the same file, | |
1279 | following the definition, just like macros. | |
1280 | ||
e31dfb12 GM |
1281 | @node Declaring Functions |
1282 | @section Telling the Compiler that a Function is Defined | |
1283 | @cindex function declaration | |
1284 | @cindex declaring functions | |
c4540067 | 1285 | @findex declare-function |
e31dfb12 | 1286 | |
a0925923 RS |
1287 | Byte-compiling a file often produces warnings about functions that the |
1288 | compiler doesn't know about (@pxref{Compiler Errors}). Sometimes this | |
1289 | indicates a real problem, but usually the functions in question are | |
1290 | defined in other files which would be loaded if that code is run. For | |
1291 | example, byte-compiling @file{fortran.el} used to warn: | |
e31dfb12 | 1292 | |
a0925923 | 1293 | @smallexample |
e31dfb12 | 1294 | In end of data: |
bfd0fe7b GM |
1295 | fortran.el:2152:1:Warning: the function `gud-find-c-expr' is not known |
1296 | to be defined. | |
a0925923 | 1297 | @end smallexample |
e31dfb12 | 1298 | |
a0925923 RS |
1299 | In fact, @code{gud-find-c-expr} is only used in the function that |
1300 | Fortran mode uses for the local value of | |
1301 | @code{gud-find-expr-function}, which is a callback from GUD; if it is | |
1302 | called, the GUD functions will be loaded. When you know that such a | |
1303 | warning does not indicate a real problem, it is good to suppress the | |
1304 | warning. That makes new warnings which might mean real problems more | |
1305 | visible. You do that with @code{declare-function}. | |
e31dfb12 GM |
1306 | |
1307 | All you need to do is add a @code{declare-function} statement before the | |
1308 | first use of the function in question: | |
1309 | ||
a0925923 | 1310 | @smallexample |
e31dfb12 | 1311 | (declare-function gud-find-c-expr "gud.el" nil) |
7a6a1728 | 1312 | @end smallexample |
e31dfb12 GM |
1313 | |
1314 | This says that @code{gud-find-c-expr} is defined in @file{gud.el} (the | |
a0925923 RS |
1315 | @samp{.el} can be omitted). The compiler takes for granted that that file |
1316 | really defines the function, and does not check. | |
7a6a1728 | 1317 | |
a0925923 RS |
1318 | The optional third argument specifies the argument list of |
1319 | @code{gud-find-c-expr}. In this case, it takes no arguments | |
1320 | (@code{nil} is different from not specifying a value). In other | |
1321 | cases, this might be something like @code{(file &optional overwrite)}. | |
1322 | You don't have to specify the argument list, but if you do the | |
1323 | byte compiler can check that the calls match the declaration. | |
1324 | ||
8f4b37d8 | 1325 | @defmac declare-function function file &optional arglist fileonly |
a0925923 | 1326 | Tell the byte compiler to assume that @var{function} is defined, with |
b0fbc500 CY |
1327 | arguments @var{arglist}, and that the definition should come from the |
1328 | file @var{file}. @var{fileonly} non-@code{nil} means only check that | |
8f4b37d8 | 1329 | @var{file} exists, not that it actually defines @var{function}. |
a0925923 RS |
1330 | @end defmac |
1331 | ||
1332 | To verify that these functions really are declared where | |
1333 | @code{declare-function} says they are, use @code{check-declare-file} | |
1334 | to check all @code{declare-function} calls in one source file, or use | |
1335 | @code{check-declare-directory} check all the files in and under a | |
1336 | certain directory. | |
1337 | ||
1338 | These commands find the file that ought to contain a function's | |
1339 | definition using @code{locate-library}; if that finds no file, they | |
1340 | expand the definition file name relative to the directory of the file | |
1341 | that contains the @code{declare-function} call. | |
1342 | ||
3d9842c6 GM |
1343 | You can also say that a function is defined by C code by specifying a |
1344 | file name ending in @samp{.c} or @samp{.m}. @code{check-declare-file} | |
1345 | looks for these files in the C source code directory. This is useful | |
1346 | only when you call a function that is defined only on certain systems. | |
1347 | Most of the primitive functions of Emacs are always defined so they will | |
a0925923 | 1348 | never give you a warning. |
e31dfb12 | 1349 | |
c4540067 GM |
1350 | Sometimes a file will optionally use functions from an external package. |
1351 | If you prefix the filename in the @code{declare-function} statement with | |
1352 | @samp{ext:}, then it will be checked if it is found, otherwise skipped | |
1353 | without error. | |
1354 | ||
8f4b37d8 GM |
1355 | There are some function definitions that @samp{check-declare} does not |
1356 | understand (e.g. @code{defstruct} and some other macros). In such cases, | |
6297397b GM |
1357 | you can pass a non-@code{nil} @var{fileonly} argument to |
1358 | @code{declare-function}, meaning to only check that the file exists, not | |
1359 | that it actually defines the function. Note that to do this without | |
1360 | having to specify an argument list, you should set the @var{arglist} | |
1361 | argument to @code{t} (because @code{nil} means an empty argument list, as | |
1362 | opposed to an unspecified one). | |
8f4b37d8 | 1363 | |
b8d4c8d0 GM |
1364 | @node Function Safety |
1365 | @section Determining whether a Function is Safe to Call | |
1366 | @cindex function safety | |
1367 | @cindex safety of functions | |
1368 | ||
1369 | Some major modes such as SES call functions that are stored in user | |
1370 | files. (@inforef{Top, ,ses}, for more information on SES.) User | |
1371 | files sometimes have poor pedigrees---you can get a spreadsheet from | |
1372 | someone you've just met, or you can get one through email from someone | |
1373 | you've never met. So it is risky to call a function whose source code | |
1374 | is stored in a user file until you have determined that it is safe. | |
1375 | ||
1376 | @defun unsafep form &optional unsafep-vars | |
1377 | Returns @code{nil} if @var{form} is a @dfn{safe} Lisp expression, or | |
1378 | returns a list that describes why it might be unsafe. The argument | |
1379 | @var{unsafep-vars} is a list of symbols known to have temporary | |
1380 | bindings at this point; it is mainly used for internal recursive | |
1381 | calls. The current buffer is an implicit argument, which provides a | |
1382 | list of buffer-local bindings. | |
1383 | @end defun | |
1384 | ||
1385 | Being quick and simple, @code{unsafep} does a very light analysis and | |
1386 | rejects many Lisp expressions that are actually safe. There are no | |
1387 | known cases where @code{unsafep} returns @code{nil} for an unsafe | |
1388 | expression. However, a ``safe'' Lisp expression can return a string | |
1389 | with a @code{display} property, containing an associated Lisp | |
1390 | expression to be executed after the string is inserted into a buffer. | |
1391 | This associated expression can be a virus. In order to be safe, you | |
1392 | must delete properties from all strings calculated by user code before | |
1393 | inserting them into buffers. | |
1394 | ||
1395 | @ignore | |
1396 | What is a safe Lisp expression? Basically, it's an expression that | |
1397 | calls only built-in functions with no side effects (or only innocuous | |
1398 | ones). Innocuous side effects include displaying messages and | |
1399 | altering non-risky buffer-local variables (but not global variables). | |
1400 | ||
1401 | @table @dfn | |
1402 | @item Safe expression | |
1403 | @itemize | |
1404 | @item | |
1405 | An atom or quoted thing. | |
1406 | @item | |
1407 | A call to a safe function (see below), if all its arguments are | |
1408 | safe expressions. | |
1409 | @item | |
1410 | One of the special forms @code{and}, @code{catch}, @code{cond}, | |
1411 | @code{if}, @code{or}, @code{prog1}, @code{prog2}, @code{progn}, | |
1412 | @code{while}, and @code{unwind-protect}], if all its arguments are | |
1413 | safe. | |
1414 | @item | |
1415 | A form that creates temporary bindings (@code{condition-case}, | |
1416 | @code{dolist}, @code{dotimes}, @code{lambda}, @code{let}, or | |
1417 | @code{let*}), if all args are safe and the symbols to be bound are not | |
1418 | explicitly risky (see @pxref{File Local Variables}). | |
1419 | @item | |
1420 | An assignment using @code{add-to-list}, @code{setq}, @code{push}, or | |
1421 | @code{pop}, if all args are safe and the symbols to be assigned are | |
1422 | not explicitly risky and they already have temporary or buffer-local | |
1423 | bindings. | |
1424 | @item | |
1425 | One of [apply, mapc, mapcar, mapconcat] if the first argument is a | |
1426 | safe explicit lambda and the other args are safe expressions. | |
1427 | @end itemize | |
1428 | ||
1429 | @item Safe function | |
1430 | @itemize | |
1431 | @item | |
1432 | A lambda containing safe expressions. | |
1433 | @item | |
1434 | A symbol on the list @code{safe-functions}, so the user says it's safe. | |
1435 | @item | |
1436 | A symbol with a non-@code{nil} @code{side-effect-free} property. | |
1437 | @item | |
27610f35 RS |
1438 | A symbol with a non-@code{nil} @code{safe-function} property. The |
1439 | value @code{t} indicates a function that is safe but has innocuous | |
1440 | side effects. Other values will someday indicate functions with | |
1441 | classes of side effects that are not always safe. | |
b8d4c8d0 GM |
1442 | @end itemize |
1443 | ||
1444 | The @code{side-effect-free} and @code{safe-function} properties are | |
1445 | provided for built-in functions and for low-level functions and macros | |
1446 | defined in @file{subr.el}. You can assign these properties for the | |
1447 | functions you write. | |
1448 | @end table | |
1449 | @end ignore | |
1450 | ||
1451 | @node Related Topics | |
1452 | @section Other Topics Related to Functions | |
1453 | ||
1454 | Here is a table of several functions that do things related to | |
1455 | function calling and function definitions. They are documented | |
1456 | elsewhere, but we provide cross references here. | |
1457 | ||
1458 | @table @code | |
1459 | @item apply | |
1460 | See @ref{Calling Functions}. | |
1461 | ||
1462 | @item autoload | |
1463 | See @ref{Autoload}. | |
1464 | ||
1465 | @item call-interactively | |
1466 | See @ref{Interactive Call}. | |
1467 | ||
39dc0d57 RS |
1468 | @item called-interactively-p |
1469 | See @ref{Distinguish Interactive}. | |
1470 | ||
b8d4c8d0 GM |
1471 | @item commandp |
1472 | See @ref{Interactive Call}. | |
1473 | ||
1474 | @item documentation | |
1475 | See @ref{Accessing Documentation}. | |
1476 | ||
1477 | @item eval | |
1478 | See @ref{Eval}. | |
1479 | ||
1480 | @item funcall | |
1481 | See @ref{Calling Functions}. | |
1482 | ||
1483 | @item function | |
1484 | See @ref{Anonymous Functions}. | |
1485 | ||
1486 | @item ignore | |
1487 | See @ref{Calling Functions}. | |
1488 | ||
1489 | @item indirect-function | |
1490 | See @ref{Function Indirection}. | |
1491 | ||
1492 | @item interactive | |
1493 | See @ref{Using Interactive}. | |
1494 | ||
1495 | @item interactive-p | |
39dc0d57 | 1496 | See @ref{Distinguish Interactive}. |
b8d4c8d0 GM |
1497 | |
1498 | @item mapatoms | |
1499 | See @ref{Creating Symbols}. | |
1500 | ||
1501 | @item mapcar | |
1502 | See @ref{Mapping Functions}. | |
1503 | ||
1504 | @item map-char-table | |
1505 | See @ref{Char-Tables}. | |
1506 | ||
1507 | @item mapconcat | |
1508 | See @ref{Mapping Functions}. | |
1509 | ||
1510 | @item undefined | |
1511 | See @ref{Functions for Key Lookup}. | |
1512 | @end table | |
1513 | ||
1514 | @ignore | |
1515 | arch-tag: 39100cdf-8a55-4898-acba-595db619e8e2 | |
1516 | @end ignore |