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1 | @c -*-texinfo-*- |
2 | @c This is part of the GNU Emacs Lisp Reference Manual. | |
acaf905b | 3 | @c Copyright (C) 1990-1995, 1998-1999, 2001-2012 |
d24880de | 4 | @c Free Software Foundation, Inc. |
b8d4c8d0 | 5 | @c See the file elisp.texi for copying conditions. |
6336d8c3 | 6 | @setfilename ../../info/functions |
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7 | @node Functions, Macros, Variables, Top |
8 | @chapter Functions | |
9 | ||
10 | A Lisp program is composed mainly of Lisp functions. This chapter | |
11 | explains what functions are, how they accept arguments, and how to | |
12 | define them. | |
13 | ||
14 | @menu | |
15 | * What Is a Function:: Lisp functions vs. primitives; terminology. | |
16 | * Lambda Expressions:: How functions are expressed as Lisp objects. | |
17 | * Function Names:: A symbol can serve as the name of a function. | |
18 | * Defining Functions:: Lisp expressions for defining functions. | |
19 | * Calling Functions:: How to use an existing function. | |
20 | * Mapping Functions:: Applying a function to each element of a list, etc. | |
21 | * Anonymous Functions:: Lambda expressions are functions with no names. | |
22 | * Function Cells:: Accessing or setting the function definition | |
23 | of a symbol. | |
735cc5ca | 24 | * Closures:: Functions that enclose a lexical environment. |
b8d4c8d0 | 25 | * Obsolete Functions:: Declaring functions obsolete. |
735cc5ca | 26 | * Inline Functions:: Functions that the compiler will expand inline. |
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 | ||
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36 | @cindex return value |
37 | @cindex value of function | |
38 | @cindex argument | |
39 | In a general sense, a function is a rule for carrying out a | |
40 | computation given input values called @dfn{arguments}. The result of | |
41 | the computation is called the @dfn{value} or @dfn{return value} of the | |
42 | function. The computation can also have side effects, such as lasting | |
43 | changes in the values of variables or the contents of data structures. | |
44 | ||
45 | In most computer languages, every function has a name. But in Lisp, | |
46 | a function in the strictest sense has no name: it is an object which | |
47 | can @emph{optionally} be associated with a symbol (e.g.@: @code{car}) | |
48 | that serves as the function name. @xref{Function Names}. When a | |
49 | function has been given a name, we usually also refer to that symbol | |
50 | as a ``function'' (e.g.@: we refer to ``the function @code{car}''). | |
51 | In this manual, the distinction between a function name and the | |
52 | function object itself is usually unimportant, but we will take note | |
53 | wherever it is relevant. | |
54 | ||
55 | Certain function-like objects, called @dfn{special forms} and | |
56 | @dfn{macros}, also accept arguments to carry out computations. | |
57 | However, as explained below, these are not considered functions in | |
58 | Emacs Lisp. | |
59 | ||
60 | Here are important terms for functions and function-like objects: | |
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61 | |
62 | @table @dfn | |
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63 | @item lambda expression |
64 | A function (in the strict sense, i.e.@: a function object) which is | |
65 | written in Lisp. These are described in the following section. | |
66 | @ifnottex | |
67 | @xref{Lambda Expressions}. | |
68 | @end ifnottex | |
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69 | |
70 | @item primitive | |
71 | @cindex primitive | |
72 | @cindex subr | |
73 | @cindex built-in function | |
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74 | A function which is callable from Lisp but is actually written in C. |
75 | Primitives are also called @dfn{built-in functions}, or @dfn{subrs}. | |
76 | Examples include functions like @code{car} and @code{append}. In | |
77 | addition, all special forms (see below) are also considered | |
78 | primitives. | |
79 | ||
80 | Usually, a function is implemented as a primitive because it is a | |
81 | fundamental part of Lisp (e.g.@: @code{car}), or because it provides a | |
82 | low-level interface to operating system services, or because it needs | |
83 | to run fast. Unlike functions defined in Lisp, primitives can be | |
84 | modified or added only by changing the C sources and recompiling | |
85 | Emacs. See @ref{Writing Emacs Primitives}. | |
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86 | |
87 | @item special form | |
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88 | A primitive that is like a function but does not evaluate all of its |
89 | arguments in the usual way. It may evaluate only some of the | |
90 | arguments, or may evaluate them in an unusual order, or several times. | |
91 | Examples include @code{if}, @code{and}, and @code{while}. | |
92 | @xref{Special Forms}. | |
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93 | |
94 | @item macro | |
95 | @cindex macro | |
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96 | A construct defined in Lisp, which differs from a function in that it |
97 | translates a Lisp expression into another expression which is to be | |
98 | evaluated instead of the original expression. Macros enable Lisp | |
99 | programmers to do the sorts of things that special forms can do. | |
100 | @xref{Macros}. | |
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101 | |
102 | @item command | |
103 | @cindex command | |
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104 | An object which can be invoked via the @code{command-execute} |
105 | primitive, usually due to the user typing in a key sequence | |
106 | @dfn{bound} to that command. @xref{Interactive Call}. A command is | |
107 | usually a function; if the function is written in Lisp, it is made | |
108 | into a command by an @code{interactive} form in the function | |
109 | definition (@pxref{Defining Commands}). Commands that are functions | |
110 | can also be called from Lisp expressions, just like other functions. | |
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111 | |
112 | Keyboard macros (strings and vectors) are commands also, even though | |
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113 | they are not functions. @xref{Keyboard Macros}. We say that a symbol |
114 | is a command if its function cell contains a command (@pxref{Symbol | |
115 | Components}); such a @dfn{named command} can be invoked with | |
116 | @kbd{M-x}. | |
117 | ||
118 | @item closure | |
119 | A function object that is much like a lambda expression, except that | |
120 | it also encloses an ``environment'' of lexical variable bindings. | |
121 | @xref{Closures}. | |
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122 | |
123 | @item byte-code function | |
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124 | A function that has been compiled by the byte compiler. |
125 | @xref{Byte-Code Type}. | |
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126 | |
127 | @item autoload object | |
128 | @cindex autoload object | |
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129 | A place-holder for a real function. If the autoload object is called, |
130 | Emacs loads the file containing the definition of the real function, | |
131 | and then calls the real function. @xref{Autoload}. | |
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132 | @end table |
133 | ||
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134 | You can use the function @code{functionp} to test if an object is a |
135 | function: | |
136 | ||
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137 | @defun functionp object |
138 | This function returns @code{t} if @var{object} is any kind of | |
9b240644 | 139 | function, i.e.@: can be passed to @code{funcall}. Note that |
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140 | @code{functionp} returns @code{t} for symbols that are function names, |
141 | and returns @code{nil} for special forms. | |
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142 | @end defun |
143 | ||
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144 | @noindent |
145 | Unlike @code{functionp}, the next three functions do @emph{not} treat | |
146 | a symbol as its function definition. | |
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147 | |
148 | @defun subrp object | |
149 | This function returns @code{t} if @var{object} is a built-in function | |
150 | (i.e., a Lisp primitive). | |
151 | ||
152 | @example | |
153 | @group | |
154 | (subrp 'message) ; @r{@code{message} is a symbol,} | |
155 | @result{} nil ; @r{not a subr object.} | |
156 | @end group | |
157 | @group | |
158 | (subrp (symbol-function 'message)) | |
159 | @result{} t | |
160 | @end group | |
161 | @end example | |
162 | @end defun | |
163 | ||
164 | @defun byte-code-function-p object | |
165 | This function returns @code{t} if @var{object} is a byte-code | |
166 | function. For example: | |
167 | ||
168 | @example | |
169 | @group | |
170 | (byte-code-function-p (symbol-function 'next-line)) | |
171 | @result{} t | |
172 | @end group | |
173 | @end example | |
174 | @end defun | |
175 | ||
176 | @defun subr-arity subr | |
177 | This function provides information about the argument list of a | |
178 | primitive, @var{subr}. The returned value is a pair | |
179 | @code{(@var{min} . @var{max})}. @var{min} is the minimum number of | |
180 | args. @var{max} is the maximum number or the symbol @code{many}, for a | |
181 | function with @code{&rest} arguments, or the symbol @code{unevalled} if | |
182 | @var{subr} is a special form. | |
183 | @end defun | |
184 | ||
185 | @node Lambda Expressions | |
186 | @section Lambda Expressions | |
187 | @cindex lambda expression | |
188 | ||
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189 | A lambda expression is a function object written in Lisp. Here is |
190 | an example: | |
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191 | |
192 | @example | |
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193 | (lambda (x) |
194 | "Return the hyperbolic cosine of X." | |
195 | (* 0.5 (+ (exp x) (exp (- x))))) | |
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196 | @end example |
197 | ||
198 | @noindent | |
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199 | In Emacs Lisp, such a list is valid as an expression---it evaluates to |
200 | itself. But its main use is not to be evaluated as an expression, but | |
201 | to be called as a function. | |
202 | ||
203 | A lambda expression, by itself, has no name; it is an @dfn{anonymous | |
204 | function}. Although lambda expressions can be used this way | |
205 | (@pxref{Anonymous Functions}), they are more commonly associated with | |
206 | symbols to make @dfn{named functions} (@pxref{Function Names}). | |
207 | Before going into these details, the following subsections describe | |
208 | the components of a lambda expression and what they do. | |
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209 | |
210 | @menu | |
211 | * Lambda Components:: The parts of a lambda expression. | |
212 | * Simple Lambda:: A simple example. | |
213 | * Argument List:: Details and special features of argument lists. | |
214 | * Function Documentation:: How to put documentation in a function. | |
215 | @end menu | |
216 | ||
217 | @node Lambda Components | |
218 | @subsection Components of a Lambda Expression | |
219 | ||
735cc5ca | 220 | A lambda expression is a list that looks like this: |
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221 | |
222 | @example | |
223 | (lambda (@var{arg-variables}@dots{}) | |
224 | [@var{documentation-string}] | |
225 | [@var{interactive-declaration}] | |
226 | @var{body-forms}@dots{}) | |
227 | @end example | |
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228 | |
229 | @cindex lambda list | |
230 | The first element of a lambda expression is always the symbol | |
231 | @code{lambda}. This indicates that the list represents a function. The | |
232 | reason functions are defined to start with @code{lambda} is so that | |
233 | other lists, intended for other uses, will not accidentally be valid as | |
234 | functions. | |
235 | ||
236 | The second element is a list of symbols---the argument variable names. | |
237 | This is called the @dfn{lambda list}. When a Lisp function is called, | |
238 | the argument values are matched up against the variables in the lambda | |
239 | list, which are given local bindings with the values provided. | |
240 | @xref{Local Variables}. | |
241 | ||
242 | The documentation string is a Lisp string object placed within the | |
243 | function definition to describe the function for the Emacs help | |
244 | facilities. @xref{Function Documentation}. | |
245 | ||
246 | The interactive declaration is a list of the form @code{(interactive | |
247 | @var{code-string})}. This declares how to provide arguments if the | |
248 | function is used interactively. Functions with this declaration are called | |
249 | @dfn{commands}; they can be called using @kbd{M-x} or bound to a key. | |
250 | Functions not intended to be called in this way should not have interactive | |
251 | declarations. @xref{Defining Commands}, for how to write an interactive | |
252 | declaration. | |
253 | ||
254 | @cindex body of function | |
255 | The rest of the elements are the @dfn{body} of the function: the Lisp | |
256 | code to do the work of the function (or, as a Lisp programmer would say, | |
257 | ``a list of Lisp forms to evaluate''). The value returned by the | |
258 | function is the value returned by the last element of the body. | |
259 | ||
260 | @node Simple Lambda | |
735cc5ca | 261 | @subsection A Simple Lambda Expression Example |
b8d4c8d0 | 262 | |
735cc5ca | 263 | Consider the following example: |
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264 | |
265 | @example | |
266 | (lambda (a b c) (+ a b c)) | |
267 | @end example | |
268 | ||
269 | @noindent | |
270 | We can call this function by writing it as the @sc{car} of an | |
271 | expression, like this: | |
272 | ||
273 | @example | |
274 | @group | |
275 | ((lambda (a b c) (+ a b c)) | |
276 | 1 2 3) | |
277 | @end group | |
278 | @end example | |
279 | ||
280 | @noindent | |
281 | This call evaluates the body of the lambda expression with the variable | |
282 | @code{a} bound to 1, @code{b} bound to 2, and @code{c} bound to 3. | |
283 | Evaluation of the body adds these three numbers, producing the result 6; | |
284 | therefore, this call to the function returns the value 6. | |
285 | ||
286 | Note that the arguments can be the results of other function calls, as in | |
287 | this example: | |
288 | ||
289 | @example | |
290 | @group | |
291 | ((lambda (a b c) (+ a b c)) | |
292 | 1 (* 2 3) (- 5 4)) | |
293 | @end group | |
294 | @end example | |
295 | ||
296 | @noindent | |
297 | This evaluates the arguments @code{1}, @code{(* 2 3)}, and @code{(- 5 | |
298 | 4)} from left to right. Then it applies the lambda expression to the | |
299 | argument values 1, 6 and 1 to produce the value 8. | |
300 | ||
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301 | As these examples show, you can use a form with a lambda expression |
302 | as its @sc{car} to make local variables and give them values. In the | |
303 | old days of Lisp, this technique was the only way to bind and | |
304 | initialize local variables. But nowadays, it is clearer to use the | |
305 | special form @code{let} for this purpose (@pxref{Local Variables}). | |
306 | Lambda expressions are mainly used as anonymous functions for passing | |
307 | as arguments to other functions (@pxref{Anonymous Functions}), or | |
308 | stored as symbol function definitions to produce named functions | |
309 | (@pxref{Function Names}). | |
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310 | |
311 | @node Argument List | |
312 | @subsection Other Features of Argument Lists | |
313 | @kindex wrong-number-of-arguments | |
314 | @cindex argument binding | |
315 | @cindex binding arguments | |
316 | @cindex argument lists, features | |
317 | ||
318 | Our simple sample function, @code{(lambda (a b c) (+ a b c))}, | |
319 | specifies three argument variables, so it must be called with three | |
320 | arguments: if you try to call it with only two arguments or four | |
321 | arguments, you get a @code{wrong-number-of-arguments} error. | |
322 | ||
323 | It is often convenient to write a function that allows certain | |
324 | arguments to be omitted. For example, the function @code{substring} | |
325 | accepts three arguments---a string, the start index and the end | |
326 | index---but the third argument defaults to the @var{length} of the | |
327 | string if you omit it. It is also convenient for certain functions to | |
328 | accept an indefinite number of arguments, as the functions @code{list} | |
329 | and @code{+} do. | |
330 | ||
331 | @cindex optional arguments | |
332 | @cindex rest arguments | |
333 | @kindex &optional | |
334 | @kindex &rest | |
335 | To specify optional arguments that may be omitted when a function | |
336 | is called, simply include the keyword @code{&optional} before the optional | |
337 | arguments. To specify a list of zero or more extra arguments, include the | |
338 | keyword @code{&rest} before one final argument. | |
339 | ||
340 | Thus, the complete syntax for an argument list is as follows: | |
341 | ||
342 | @example | |
343 | @group | |
344 | (@var{required-vars}@dots{} | |
345 | @r{[}&optional @var{optional-vars}@dots{}@r{]} | |
346 | @r{[}&rest @var{rest-var}@r{]}) | |
347 | @end group | |
348 | @end example | |
349 | ||
350 | @noindent | |
351 | The square brackets indicate that the @code{&optional} and @code{&rest} | |
352 | clauses, and the variables that follow them, are optional. | |
353 | ||
354 | A call to the function requires one actual argument for each of the | |
355 | @var{required-vars}. There may be actual arguments for zero or more of | |
356 | the @var{optional-vars}, and there cannot be any actual arguments beyond | |
357 | that unless the lambda list uses @code{&rest}. In that case, there may | |
358 | be any number of extra actual arguments. | |
359 | ||
360 | If actual arguments for the optional and rest variables are omitted, | |
361 | then they always default to @code{nil}. There is no way for the | |
362 | function to distinguish between an explicit argument of @code{nil} and | |
363 | an omitted argument. However, the body of the function is free to | |
364 | consider @code{nil} an abbreviation for some other meaningful value. | |
365 | This is what @code{substring} does; @code{nil} as the third argument to | |
366 | @code{substring} means to use the length of the string supplied. | |
367 | ||
368 | @cindex CL note---default optional arg | |
369 | @quotation | |
370 | @b{Common Lisp note:} Common Lisp allows the function to specify what | |
371 | default value to use when an optional argument is omitted; Emacs Lisp | |
372 | always uses @code{nil}. Emacs Lisp does not support ``supplied-p'' | |
373 | variables that tell you whether an argument was explicitly passed. | |
374 | @end quotation | |
375 | ||
376 | For example, an argument list that looks like this: | |
377 | ||
378 | @example | |
379 | (a b &optional c d &rest e) | |
380 | @end example | |
381 | ||
382 | @noindent | |
383 | binds @code{a} and @code{b} to the first two actual arguments, which are | |
384 | required. If one or two more arguments are provided, @code{c} and | |
385 | @code{d} are bound to them respectively; any arguments after the first | |
386 | four are collected into a list and @code{e} is bound to that list. If | |
387 | there are only two arguments, @code{c} is @code{nil}; if two or three | |
388 | arguments, @code{d} is @code{nil}; if four arguments or fewer, @code{e} | |
389 | is @code{nil}. | |
390 | ||
391 | There is no way to have required arguments following optional | |
392 | ones---it would not make sense. To see why this must be so, suppose | |
393 | that @code{c} in the example were optional and @code{d} were required. | |
394 | Suppose three actual arguments are given; which variable would the | |
395 | third argument be for? Would it be used for the @var{c}, or for | |
396 | @var{d}? One can argue for both possibilities. Similarly, it makes | |
397 | no sense to have any more arguments (either required or optional) | |
398 | after a @code{&rest} argument. | |
399 | ||
400 | Here are some examples of argument lists and proper calls: | |
401 | ||
402 | @smallexample | |
403 | ((lambda (n) (1+ n)) ; @r{One required:} | |
404 | 1) ; @r{requires exactly one argument.} | |
405 | @result{} 2 | |
406 | ((lambda (n &optional n1) ; @r{One required and one optional:} | |
407 | (if n1 (+ n n1) (1+ n))) ; @r{1 or 2 arguments.} | |
408 | 1 2) | |
409 | @result{} 3 | |
410 | ((lambda (n &rest ns) ; @r{One required and one rest:} | |
411 | (+ n (apply '+ ns))) ; @r{1 or more arguments.} | |
412 | 1 2 3 4 5) | |
413 | @result{} 15 | |
414 | @end smallexample | |
415 | ||
416 | @node Function Documentation | |
417 | @subsection Documentation Strings of Functions | |
418 | @cindex documentation of function | |
419 | ||
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420 | A lambda expression may optionally have a @dfn{documentation string} |
421 | just after the lambda list. This string does not affect execution of | |
422 | the function; it is a kind of comment, but a systematized comment | |
423 | which actually appears inside the Lisp world and can be used by the | |
424 | Emacs help facilities. @xref{Documentation}, for how the | |
425 | documentation string is accessed. | |
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426 | |
427 | It is a good idea to provide documentation strings for all the | |
428 | functions in your program, even those that are called only from within | |
429 | your program. Documentation strings are like comments, except that they | |
430 | are easier to access. | |
431 | ||
432 | The first line of the documentation string should stand on its own, | |
433 | because @code{apropos} displays just this first line. It should consist | |
434 | of one or two complete sentences that summarize the function's purpose. | |
435 | ||
436 | The start of the documentation string is usually indented in the | |
437 | source file, but since these spaces come before the starting | |
438 | double-quote, they are not part of the string. Some people make a | |
439 | practice of indenting any additional lines of the string so that the | |
440 | text lines up in the program source. @emph{That is a mistake.} The | |
441 | indentation of the following lines is inside the string; what looks | |
442 | nice in the source code will look ugly when displayed by the help | |
443 | commands. | |
444 | ||
445 | You may wonder how the documentation string could be optional, since | |
446 | there are required components of the function that follow it (the body). | |
447 | Since evaluation of a string returns that string, without any side effects, | |
448 | it has no effect if it is not the last form in the body. Thus, in | |
449 | practice, there is no confusion between the first form of the body and the | |
450 | documentation string; if the only body form is a string then it serves both | |
451 | as the return value and as the documentation. | |
452 | ||
453 | The last line of the documentation string can specify calling | |
454 | conventions different from the actual function arguments. Write | |
455 | text like this: | |
456 | ||
457 | @example | |
458 | \(fn @var{arglist}) | |
459 | @end example | |
460 | ||
461 | @noindent | |
462 | following a blank line, at the beginning of the line, with no newline | |
463 | following it inside the documentation string. (The @samp{\} is used | |
464 | to avoid confusing the Emacs motion commands.) The calling convention | |
465 | specified in this way appears in help messages in place of the one | |
466 | derived from the actual arguments of the function. | |
467 | ||
468 | This feature is particularly useful for macro definitions, since the | |
469 | arguments written in a macro definition often do not correspond to the | |
470 | way users think of the parts of the macro call. | |
471 | ||
472 | @node Function Names | |
473 | @section Naming a Function | |
474 | @cindex function definition | |
475 | @cindex named function | |
476 | @cindex function name | |
477 | ||
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478 | A symbol can serve as the name of a function. This happens when the |
479 | symbol's @dfn{function cell} (@pxref{Symbol Components}) contains a | |
480 | function object (e.g.@: a lambda expression). Then the symbol itself | |
481 | becomes a valid, callable function, equivalent to the function object | |
482 | in its function cell. | |
483 | ||
484 | The contents of the function cell are also called the symbol's | |
485 | @dfn{function definition}. The procedure of using a symbol's function | |
486 | definition in place of the symbol is called @dfn{symbol function | |
487 | indirection}; see @ref{Function Indirection}. If you have not given a | |
488 | symbol a function definition, its function cell is said to be | |
489 | @dfn{void}, and it cannot be used as a function. | |
490 | ||
491 | In practice, nearly all functions have names, and are referred to by | |
492 | their names. You can create a named Lisp function by defining a | |
493 | lambda expression and putting it in a function cell (@pxref{Function | |
494 | Cells}). However, it is more common to use the @code{defun} special | |
495 | form, described in the next section. | |
496 | @ifnottex | |
497 | @xref{Defining Functions}. | |
498 | @end ifnottex | |
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499 | |
500 | We give functions names because it is convenient to refer to them by | |
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501 | their names in Lisp expressions. Also, a named Lisp function can |
502 | easily refer to itself---it can be recursive. Furthermore, primitives | |
503 | can only be referred to textually by their names, since primitive | |
504 | function objects (@pxref{Primitive Function Type}) have no read | |
505 | syntax. | |
506 | ||
507 | A function need not have a unique name. A given function object | |
508 | @emph{usually} appears in the function cell of only one symbol, but | |
509 | this is just a convention. It is easy to store it in several symbols | |
510 | using @code{fset}; then each of the symbols is a valid name for the | |
511 | same function. | |
512 | ||
513 | Note that a symbol used as a function name may also be used as a | |
514 | variable; these two uses of a symbol are independent and do not | |
515 | conflict. (This is not the case in some dialects of Lisp, like | |
516 | Scheme.) | |
b8d4c8d0 GM |
517 | |
518 | @node Defining Functions | |
519 | @section Defining Functions | |
520 | @cindex defining a function | |
521 | ||
522 | We usually give a name to a function when it is first created. This | |
523 | is called @dfn{defining a function}, and it is done with the | |
524 | @code{defun} special form. | |
525 | ||
735cc5ca | 526 | @defspec defun name argument-list body-forms... |
b8d4c8d0 GM |
527 | @code{defun} is the usual way to define new Lisp functions. It |
528 | defines the symbol @var{name} as a function that looks like this: | |
529 | ||
530 | @example | |
531 | (lambda @var{argument-list} . @var{body-forms}) | |
532 | @end example | |
533 | ||
534 | @code{defun} stores this lambda expression in the function cell of | |
535 | @var{name}. It returns the value @var{name}, but usually we ignore this | |
536 | value. | |
537 | ||
538 | As described previously, @var{argument-list} is a list of argument | |
735cc5ca CY |
539 | names and may include the keywords @code{&optional} and @code{&rest}. |
540 | Also, the first two of the @var{body-forms} may be a documentation | |
541 | string and an interactive declaration. @xref{Lambda Components}. | |
b8d4c8d0 GM |
542 | |
543 | Here are some examples: | |
544 | ||
545 | @example | |
546 | @group | |
547 | (defun foo () 5) | |
548 | @result{} foo | |
549 | @end group | |
550 | @group | |
551 | (foo) | |
552 | @result{} 5 | |
553 | @end group | |
554 | ||
555 | @group | |
556 | (defun bar (a &optional b &rest c) | |
557 | (list a b c)) | |
558 | @result{} bar | |
559 | @end group | |
560 | @group | |
561 | (bar 1 2 3 4 5) | |
562 | @result{} (1 2 (3 4 5)) | |
563 | @end group | |
564 | @group | |
565 | (bar 1) | |
566 | @result{} (1 nil nil) | |
567 | @end group | |
568 | @group | |
569 | (bar) | |
570 | @error{} Wrong number of arguments. | |
571 | @end group | |
572 | ||
573 | @group | |
574 | (defun capitalize-backwards () | |
735cc5ca | 575 | "Upcase the last letter of the word at point." |
b8d4c8d0 GM |
576 | (interactive) |
577 | (backward-word 1) | |
578 | (forward-word 1) | |
579 | (backward-char 1) | |
580 | (capitalize-word 1)) | |
581 | @result{} capitalize-backwards | |
582 | @end group | |
583 | @end example | |
584 | ||
585 | Be careful not to redefine existing functions unintentionally. | |
586 | @code{defun} redefines even primitive functions such as @code{car} | |
735cc5ca CY |
587 | without any hesitation or notification. Emacs does not prevent you |
588 | from doing this, because redefining a function is sometimes done | |
589 | deliberately, and there is no way to distinguish deliberate | |
590 | redefinition from unintentional redefinition. | |
b8d4c8d0 GM |
591 | @end defspec |
592 | ||
593 | @cindex function aliases | |
594 | @defun defalias name definition &optional docstring | |
595 | @anchor{Definition of defalias} | |
596 | This special form defines the symbol @var{name} as a function, with | |
597 | definition @var{definition} (which can be any valid Lisp function). | |
598 | It returns @var{definition}. | |
599 | ||
600 | If @var{docstring} is non-@code{nil}, it becomes the function | |
601 | documentation of @var{name}. Otherwise, any documentation provided by | |
602 | @var{definition} is used. | |
603 | ||
604 | The proper place to use @code{defalias} is where a specific function | |
605 | name is being defined---especially where that name appears explicitly in | |
606 | the source file being loaded. This is because @code{defalias} records | |
607 | which file defined the function, just like @code{defun} | |
608 | (@pxref{Unloading}). | |
609 | ||
610 | By contrast, in programs that manipulate function definitions for other | |
611 | purposes, it is better to use @code{fset}, which does not keep such | |
612 | records. @xref{Function Cells}. | |
613 | @end defun | |
614 | ||
615 | You cannot create a new primitive function with @code{defun} or | |
616 | @code{defalias}, but you can use them to change the function definition of | |
617 | any symbol, even one such as @code{car} or @code{x-popup-menu} whose | |
618 | normal definition is a primitive. However, this is risky: for | |
619 | instance, it is next to impossible to redefine @code{car} without | |
620 | breaking Lisp completely. Redefining an obscure function such as | |
621 | @code{x-popup-menu} is less dangerous, but it still may not work as | |
622 | you expect. If there are calls to the primitive from C code, they | |
623 | call the primitive's C definition directly, so changing the symbol's | |
624 | definition will have no effect on them. | |
625 | ||
626 | See also @code{defsubst}, which defines a function like @code{defun} | |
735cc5ca CY |
627 | and tells the Lisp compiler to perform inline expansion on it. |
628 | @xref{Inline Functions}. | |
b8d4c8d0 GM |
629 | |
630 | @node Calling Functions | |
631 | @section Calling Functions | |
632 | @cindex function invocation | |
633 | @cindex calling a function | |
634 | ||
635 | Defining functions is only half the battle. Functions don't do | |
636 | anything until you @dfn{call} them, i.e., tell them to run. Calling a | |
637 | function is also known as @dfn{invocation}. | |
638 | ||
639 | The most common way of invoking a function is by evaluating a list. | |
640 | For example, evaluating the list @code{(concat "a" "b")} calls the | |
641 | function @code{concat} with arguments @code{"a"} and @code{"b"}. | |
642 | @xref{Evaluation}, for a description of evaluation. | |
643 | ||
644 | When you write a list as an expression in your program, you specify | |
645 | which function to call, and how many arguments to give it, in the text | |
646 | of the program. Usually that's just what you want. Occasionally you | |
647 | need to compute at run time which function to call. To do that, use | |
648 | the function @code{funcall}. When you also need to determine at run | |
649 | time how many arguments to pass, use @code{apply}. | |
650 | ||
651 | @defun funcall function &rest arguments | |
652 | @code{funcall} calls @var{function} with @var{arguments}, and returns | |
653 | whatever @var{function} returns. | |
654 | ||
655 | Since @code{funcall} is a function, all of its arguments, including | |
656 | @var{function}, are evaluated before @code{funcall} is called. This | |
657 | means that you can use any expression to obtain the function to be | |
658 | called. It also means that @code{funcall} does not see the | |
659 | expressions you write for the @var{arguments}, only their values. | |
660 | These values are @emph{not} evaluated a second time in the act of | |
661 | calling @var{function}; the operation of @code{funcall} is like the | |
662 | normal procedure for calling a function, once its arguments have | |
663 | already been evaluated. | |
664 | ||
665 | The argument @var{function} must be either a Lisp function or a | |
666 | primitive function. Special forms and macros are not allowed, because | |
667 | they make sense only when given the ``unevaluated'' argument | |
668 | expressions. @code{funcall} cannot provide these because, as we saw | |
669 | above, it never knows them in the first place. | |
670 | ||
671 | @example | |
672 | @group | |
673 | (setq f 'list) | |
674 | @result{} list | |
675 | @end group | |
676 | @group | |
677 | (funcall f 'x 'y 'z) | |
678 | @result{} (x y z) | |
679 | @end group | |
680 | @group | |
681 | (funcall f 'x 'y '(z)) | |
682 | @result{} (x y (z)) | |
683 | @end group | |
684 | @group | |
685 | (funcall 'and t nil) | |
686 | @error{} Invalid function: #<subr and> | |
687 | @end group | |
688 | @end example | |
689 | ||
690 | Compare these examples with the examples of @code{apply}. | |
691 | @end defun | |
692 | ||
693 | @defun apply function &rest arguments | |
694 | @code{apply} calls @var{function} with @var{arguments}, just like | |
695 | @code{funcall} but with one difference: the last of @var{arguments} is a | |
696 | list of objects, which are passed to @var{function} as separate | |
697 | arguments, rather than a single list. We say that @code{apply} | |
698 | @dfn{spreads} this list so that each individual element becomes an | |
699 | argument. | |
700 | ||
701 | @code{apply} returns the result of calling @var{function}. As with | |
702 | @code{funcall}, @var{function} must either be a Lisp function or a | |
703 | primitive function; special forms and macros do not make sense in | |
704 | @code{apply}. | |
705 | ||
706 | @example | |
707 | @group | |
708 | (setq f 'list) | |
709 | @result{} list | |
710 | @end group | |
711 | @group | |
712 | (apply f 'x 'y 'z) | |
713 | @error{} Wrong type argument: listp, z | |
714 | @end group | |
715 | @group | |
716 | (apply '+ 1 2 '(3 4)) | |
717 | @result{} 10 | |
718 | @end group | |
719 | @group | |
720 | (apply '+ '(1 2 3 4)) | |
721 | @result{} 10 | |
722 | @end group | |
723 | ||
724 | @group | |
725 | (apply 'append '((a b c) nil (x y z) nil)) | |
726 | @result{} (a b c x y z) | |
727 | @end group | |
728 | @end example | |
729 | ||
730 | For an interesting example of using @code{apply}, see @ref{Definition | |
731 | of mapcar}. | |
732 | @end defun | |
733 | ||
80f85d7c EZ |
734 | @cindex partial application of functions |
735 | @cindex currying | |
a18a6d49 | 736 | Sometimes it is useful to fix some of the function's arguments at |
80f85d7c EZ |
737 | certain values, and leave the rest of arguments for when the function |
738 | is actually called. The act of fixing some of the function's | |
739 | arguments is called @dfn{partial application} of the function@footnote{ | |
740 | This is related to, but different from @dfn{currying}, which | |
741 | transforms a function that takes multiple arguments in such a way that | |
742 | it can be called as a chain of functions, each one with a single | |
743 | argument.}. | |
744 | The result is a new function that accepts the rest of | |
745 | arguments and calls the original function with all the arguments | |
a18a6d49 EZ |
746 | combined. |
747 | ||
748 | Here's how to do partial application in Emacs Lisp: | |
80f85d7c EZ |
749 | |
750 | @defun apply-partially func &rest args | |
751 | This function returns a new function which, when called, will call | |
752 | @var{func} with the list of arguments composed from @var{args} and | |
753 | additional arguments specified at the time of the call. If @var{func} | |
754 | accepts @var{n} arguments, then a call to @code{apply-partially} with | |
755 | @w{@code{@var{m} < @var{n}}} arguments will produce a new function of | |
756 | @w{@code{@var{n} - @var{m}}} arguments. | |
757 | ||
834b5485 EZ |
758 | Here's how we could define the built-in function @code{1+}, if it |
759 | didn't exist, using @code{apply-partially} and @code{+}, another | |
760 | built-in function: | |
80f85d7c EZ |
761 | |
762 | @example | |
80f85d7c | 763 | @group |
834b5485 EZ |
764 | (defalias '1+ (apply-partially '+ 1) |
765 | "Increment argument by one.") | |
766 | @end group | |
767 | @group | |
768 | (1+ 10) | |
80f85d7c EZ |
769 | @result{} 11 |
770 | @end group | |
771 | @end example | |
772 | @end defun | |
773 | ||
b8d4c8d0 GM |
774 | @cindex functionals |
775 | It is common for Lisp functions to accept functions as arguments or | |
776 | find them in data structures (especially in hook variables and property | |
777 | lists) and call them using @code{funcall} or @code{apply}. Functions | |
778 | that accept function arguments are often called @dfn{functionals}. | |
779 | ||
780 | Sometimes, when you call a functional, it is useful to supply a no-op | |
781 | function as the argument. Here are two different kinds of no-op | |
782 | function: | |
783 | ||
784 | @defun identity arg | |
785 | This function returns @var{arg} and has no side effects. | |
786 | @end defun | |
787 | ||
788 | @defun ignore &rest args | |
789 | This function ignores any arguments and returns @code{nil}. | |
790 | @end defun | |
791 | ||
735cc5ca CY |
792 | Some functions are user-visible @dfn{commands}, which can be called |
793 | interactively (usually by a key sequence). It is possible to invoke | |
794 | such a command exactly as though it was called interactively, by using | |
795 | the @code{call-interactively} function. @xref{Interactive Call}. | |
413c488d | 796 | |
b8d4c8d0 GM |
797 | @node Mapping Functions |
798 | @section Mapping Functions | |
799 | @cindex mapping functions | |
800 | ||
801 | A @dfn{mapping function} applies a given function (@emph{not} a | |
802 | special form or macro) to each element of a list or other collection. | |
735cc5ca CY |
803 | Emacs Lisp has several such functions; this section describes |
804 | @code{mapcar}, @code{mapc}, and @code{mapconcat}, which map over a | |
805 | list. @xref{Definition of mapatoms}, for the function @code{mapatoms} | |
806 | which maps over the symbols in an obarray. @xref{Definition of | |
807 | maphash}, for the function @code{maphash} which maps over key/value | |
808 | associations in a hash table. | |
b8d4c8d0 GM |
809 | |
810 | These mapping functions do not allow char-tables because a char-table | |
811 | is a sparse array whose nominal range of indices is very large. To map | |
812 | over a char-table in a way that deals properly with its sparse nature, | |
813 | use the function @code{map-char-table} (@pxref{Char-Tables}). | |
814 | ||
815 | @defun mapcar function sequence | |
816 | @anchor{Definition of mapcar} | |
817 | @code{mapcar} applies @var{function} to each element of @var{sequence} | |
818 | in turn, and returns a list of the results. | |
819 | ||
820 | The argument @var{sequence} can be any kind of sequence except a | |
821 | char-table; that is, a list, a vector, a bool-vector, or a string. The | |
822 | result is always a list. The length of the result is the same as the | |
823 | length of @var{sequence}. For example: | |
824 | ||
825 | @smallexample | |
826 | @group | |
827 | (mapcar 'car '((a b) (c d) (e f))) | |
828 | @result{} (a c e) | |
829 | (mapcar '1+ [1 2 3]) | |
830 | @result{} (2 3 4) | |
3e99b825 | 831 | (mapcar 'string "abc") |
b8d4c8d0 GM |
832 | @result{} ("a" "b" "c") |
833 | @end group | |
834 | ||
835 | @group | |
836 | ;; @r{Call each function in @code{my-hooks}.} | |
837 | (mapcar 'funcall my-hooks) | |
838 | @end group | |
839 | ||
840 | @group | |
841 | (defun mapcar* (function &rest args) | |
842 | "Apply FUNCTION to successive cars of all ARGS. | |
843 | Return the list of results." | |
844 | ;; @r{If no list is exhausted,} | |
845 | (if (not (memq nil args)) | |
846 | ;; @r{apply function to @sc{car}s.} | |
847 | (cons (apply function (mapcar 'car args)) | |
848 | (apply 'mapcar* function | |
849 | ;; @r{Recurse for rest of elements.} | |
850 | (mapcar 'cdr args))))) | |
851 | @end group | |
852 | ||
853 | @group | |
854 | (mapcar* 'cons '(a b c) '(1 2 3 4)) | |
855 | @result{} ((a . 1) (b . 2) (c . 3)) | |
856 | @end group | |
857 | @end smallexample | |
858 | @end defun | |
859 | ||
860 | @defun mapc function sequence | |
861 | @code{mapc} is like @code{mapcar} except that @var{function} is used for | |
862 | side-effects only---the values it returns are ignored, not collected | |
863 | into a list. @code{mapc} always returns @var{sequence}. | |
864 | @end defun | |
865 | ||
866 | @defun mapconcat function sequence separator | |
867 | @code{mapconcat} applies @var{function} to each element of | |
868 | @var{sequence}: the results, which must be strings, are concatenated. | |
869 | Between each pair of result strings, @code{mapconcat} inserts the string | |
870 | @var{separator}. Usually @var{separator} contains a space or comma or | |
871 | other suitable punctuation. | |
872 | ||
873 | The argument @var{function} must be a function that can take one | |
874 | argument and return a string. The argument @var{sequence} can be any | |
875 | kind of sequence except a char-table; that is, a list, a vector, a | |
876 | bool-vector, or a string. | |
877 | ||
878 | @smallexample | |
879 | @group | |
880 | (mapconcat 'symbol-name | |
881 | '(The cat in the hat) | |
882 | " ") | |
883 | @result{} "The cat in the hat" | |
884 | @end group | |
885 | ||
886 | @group | |
887 | (mapconcat (function (lambda (x) (format "%c" (1+ x)))) | |
888 | "HAL-8000" | |
889 | "") | |
890 | @result{} "IBM.9111" | |
891 | @end group | |
892 | @end smallexample | |
893 | @end defun | |
894 | ||
895 | @node Anonymous Functions | |
896 | @section Anonymous Functions | |
897 | @cindex anonymous function | |
898 | ||
735cc5ca CY |
899 | Although functions are usually defined with @code{defun} and given |
900 | names at the same time, it is sometimes convenient to use an explicit | |
901 | lambda expression---an @dfn{anonymous function}. Anonymous functions | |
902 | are valid wherever function names are. They are often assigned as | |
903 | variable values, or as arguments to functions; for instance, you might | |
904 | pass one as the @var{function} argument to @code{mapcar}, which | |
905 | applies that function to each element of a list (@pxref{Mapping | |
906 | Functions}). @xref{describe-symbols example}, for a realistic example | |
907 | of this. | |
908 | ||
909 | When defining a lambda expression that is to be used as an anonymous | |
910 | function, you can in principle use any method to construct the list. | |
911 | But typically you should use the @code{lambda} macro, or the | |
912 | @code{function} special form, or the @code{#'} read syntax: | |
913 | ||
914 | @defmac lambda args body... | |
915 | This macro returns an anonymous function with argument list @var{args} | |
916 | and body forms given by @var{body}. In effect, this macro makes | |
917 | @code{lambda} forms ``self-quoting'': evaluating a form whose @sc{car} | |
918 | is @code{lambda} yields the form itself: | |
b8d4c8d0 | 919 | |
735cc5ca CY |
920 | @example |
921 | (lambda (x) (* x x)) | |
922 | @result{} (lambda (x) (* x x)) | |
923 | @end example | |
b8d4c8d0 | 924 | |
735cc5ca CY |
925 | The @code{lambda} form has one other effect: it tells the Emacs |
926 | evaluator and byte-compiler that its argument is a function, by using | |
927 | @code{function} as a subroutine (see below). | |
928 | @end defmac | |
b8d4c8d0 | 929 | |
735cc5ca CY |
930 | @defspec function function-object |
931 | @cindex function quoting | |
932 | This special form returns @var{function-object} without evaluating it. | |
933 | In this, it is similar to @code{quote} (@pxref{Quoting}). But unlike | |
934 | @code{quote}, it also serves as a note to the Emacs evaluator and | |
935 | byte-compiler that @var{function-object} is intended to be used as a | |
936 | function. Assuming @var{function-object} is a valid lambda | |
937 | expression, this has two effects: | |
b8d4c8d0 | 938 | |
735cc5ca CY |
939 | @itemize |
940 | @item | |
941 | When the code is byte-compiled, @var{function-object} is compiled into | |
942 | a byte-code function object (@pxref{Byte Compilation}). | |
b8d4c8d0 | 943 | |
735cc5ca CY |
944 | @item |
945 | When lexical binding is enabled, @var{function-object} is converted | |
946 | into a closure. @xref{Closures}. | |
947 | @end itemize | |
948 | @end defspec | |
b8d4c8d0 | 949 | |
735cc5ca CY |
950 | @cindex @samp{#'} syntax |
951 | The read syntax @code{#'} is a short-hand for using @code{function}. | |
952 | The following forms are all equivalent: | |
b8d4c8d0 | 953 | |
735cc5ca CY |
954 | @example |
955 | (lambda (x) (* x x)) | |
956 | (function (lambda (x) (* x x))) | |
957 | #'(lambda (x) (* x x)) | |
958 | @end example | |
b8d4c8d0 | 959 | |
5d6ab672 CY |
960 | In the following example, we define a @code{change-property} |
961 | function that takes a function as its third argument, followed by a | |
962 | @code{double-property} function that makes use of | |
963 | @code{change-property} by passing it an anonymous function: | |
b8d4c8d0 GM |
964 | |
965 | @example | |
966 | @group | |
967 | (defun change-property (symbol prop function) | |
968 | (let ((value (get symbol prop))) | |
969 | (put symbol prop (funcall function value)))) | |
970 | @end group | |
b8d4c8d0 | 971 | |
b8d4c8d0 GM |
972 | @group |
973 | (defun double-property (symbol prop) | |
5d6ab672 | 974 | (change-property symbol prop (lambda (x) (* 2 x)))) |
b8d4c8d0 GM |
975 | @end group |
976 | @end example | |
977 | ||
b8d4c8d0 | 978 | @noindent |
735cc5ca | 979 | Note that we do not quote the @code{lambda} form. |
b8d4c8d0 | 980 | |
735cc5ca CY |
981 | If you compile the above code, the anonymous function is also |
982 | compiled. This would not happen if, say, you had constructed the | |
983 | anonymous function by quoting it as a list: | |
b8d4c8d0 GM |
984 | |
985 | @example | |
986 | @group | |
987 | (defun double-property (symbol prop) | |
5d6ab672 | 988 | (change-property symbol prop '(lambda (x) (* 2 x)))) |
b8d4c8d0 GM |
989 | @end group |
990 | @end example | |
991 | ||
992 | @noindent | |
735cc5ca CY |
993 | In that case, the anonymous function is kept as a lambda expression in |
994 | the compiled code. The byte-compiler cannot assume this list is a | |
995 | function, even though it looks like one, since it does not know that | |
996 | @code{change-property} intends to use it as a function. | |
b8d4c8d0 GM |
997 | |
998 | @node Function Cells | |
999 | @section Accessing Function Cell Contents | |
1000 | ||
1001 | The @dfn{function definition} of a symbol is the object stored in the | |
1002 | function cell of the symbol. The functions described here access, test, | |
1003 | and set the function cell of symbols. | |
1004 | ||
1005 | See also the function @code{indirect-function}. @xref{Definition of | |
1006 | indirect-function}. | |
1007 | ||
1008 | @defun symbol-function symbol | |
1009 | @kindex void-function | |
1010 | This returns the object in the function cell of @var{symbol}. If the | |
1011 | symbol's function cell is void, a @code{void-function} error is | |
1012 | signaled. | |
1013 | ||
1014 | This function does not check that the returned object is a legitimate | |
1015 | function. | |
1016 | ||
1017 | @example | |
1018 | @group | |
1019 | (defun bar (n) (+ n 2)) | |
1020 | @result{} bar | |
1021 | @end group | |
1022 | @group | |
1023 | (symbol-function 'bar) | |
1024 | @result{} (lambda (n) (+ n 2)) | |
1025 | @end group | |
1026 | @group | |
1027 | (fset 'baz 'bar) | |
1028 | @result{} bar | |
1029 | @end group | |
1030 | @group | |
1031 | (symbol-function 'baz) | |
1032 | @result{} bar | |
1033 | @end group | |
1034 | @end example | |
1035 | @end defun | |
1036 | ||
1037 | @cindex void function cell | |
1038 | If you have never given a symbol any function definition, we say that | |
1039 | that symbol's function cell is @dfn{void}. In other words, the function | |
1040 | cell does not have any Lisp object in it. If you try to call such a symbol | |
1041 | as a function, it signals a @code{void-function} error. | |
1042 | ||
1043 | Note that void is not the same as @code{nil} or the symbol | |
1044 | @code{void}. The symbols @code{nil} and @code{void} are Lisp objects, | |
1045 | and can be stored into a function cell just as any other object can be | |
1046 | (and they can be valid functions if you define them in turn with | |
1047 | @code{defun}). A void function cell contains no object whatsoever. | |
1048 | ||
1049 | You can test the voidness of a symbol's function definition with | |
1050 | @code{fboundp}. After you have given a symbol a function definition, you | |
1051 | can make it void once more using @code{fmakunbound}. | |
1052 | ||
1053 | @defun fboundp symbol | |
1054 | This function returns @code{t} if the symbol has an object in its | |
1055 | function cell, @code{nil} otherwise. It does not check that the object | |
1056 | is a legitimate function. | |
1057 | @end defun | |
1058 | ||
1059 | @defun fmakunbound symbol | |
1060 | This function makes @var{symbol}'s function cell void, so that a | |
1061 | subsequent attempt to access this cell will cause a | |
1062 | @code{void-function} error. It returns @var{symbol}. (See also | |
1063 | @code{makunbound}, in @ref{Void Variables}.) | |
1064 | ||
1065 | @example | |
1066 | @group | |
1067 | (defun foo (x) x) | |
1068 | @result{} foo | |
1069 | @end group | |
1070 | @group | |
1071 | (foo 1) | |
1072 | @result{}1 | |
1073 | @end group | |
1074 | @group | |
1075 | (fmakunbound 'foo) | |
1076 | @result{} foo | |
1077 | @end group | |
1078 | @group | |
1079 | (foo 1) | |
1080 | @error{} Symbol's function definition is void: foo | |
1081 | @end group | |
1082 | @end example | |
1083 | @end defun | |
1084 | ||
1085 | @defun fset symbol definition | |
1086 | This function stores @var{definition} in the function cell of | |
1087 | @var{symbol}. The result is @var{definition}. Normally | |
1088 | @var{definition} should be a function or the name of a function, but | |
1089 | this is not checked. The argument @var{symbol} is an ordinary evaluated | |
1090 | argument. | |
1091 | ||
735cc5ca CY |
1092 | The primary use of this function is as a subroutine by constructs that |
1093 | define or alter functions, like @code{defadvice} (@pxref{Advising | |
1094 | Functions}). (If @code{defun} were not a primitive, it could be | |
1095 | written as a Lisp macro using @code{fset}.) You can also use it to | |
1096 | give a symbol a function definition that is not a list, e.g.@: a | |
1097 | keyboard macro (@pxref{Keyboard Macros}): | |
b8d4c8d0 | 1098 | |
735cc5ca CY |
1099 | @example |
1100 | ;; @r{Define a named keyboard macro.} | |
1101 | (fset 'kill-two-lines "\^u2\^k") | |
1102 | @result{} "\^u2\^k" | |
1103 | @end example | |
b8d4c8d0 | 1104 | |
735cc5ca CY |
1105 | It you wish to use @code{fset} to make an alternate name for a |
1106 | function, consider using @code{defalias} instead. @xref{Definition of | |
1107 | defalias}. | |
1108 | @end defun | |
b8d4c8d0 | 1109 | |
735cc5ca CY |
1110 | @node Closures |
1111 | @section Closures | |
b8d4c8d0 | 1112 | |
735cc5ca CY |
1113 | As explained in @ref{Variable Scoping}, Emacs can optionally enable |
1114 | lexical binding of variables. When lexical binding is enabled, any | |
1115 | named function that you create (e.g.@: with @code{defun}), as well as | |
1116 | any anonymous function that you create using the @code{lambda} macro | |
1117 | or the @code{function} special form or the @code{#'} syntax | |
1118 | (@pxref{Anonymous Functions}), is automatically converted into a | |
1119 | closure. | |
b8d4c8d0 | 1120 | |
735cc5ca CY |
1121 | A closure is a function that also carries a record of the lexical |
1122 | environment that existed when the function was defined. When it is | |
1123 | invoked, any lexical variable references within its definition use the | |
1124 | retained lexical environment. In all other respects, closures behave | |
1125 | much like ordinary functions; in particular, they can be called in the | |
1126 | same way as ordinary functions. | |
b8d4c8d0 | 1127 | |
735cc5ca | 1128 | @xref{Lexical Binding}, for an example of using a closure. |
b8d4c8d0 | 1129 | |
735cc5ca CY |
1130 | Currently, an Emacs Lisp closure object is represented by a list |
1131 | with the symbol @code{closure} as the first element, a list | |
1132 | representing the lexical environment as the second element, and the | |
1133 | argument list and body forms as the remaining elements: | |
b8d4c8d0 | 1134 | |
735cc5ca CY |
1135 | @example |
1136 | ;; @r{lexical binding is enabled.} | |
1137 | (lambda (x) (* x x)) | |
1138 | @result{} (closure (t) (x) (* x x)) | |
b8d4c8d0 | 1139 | @end example |
b8d4c8d0 | 1140 | |
735cc5ca CY |
1141 | @noindent |
1142 | However, the fact that the internal structure of a closure is | |
1143 | ``exposed'' to the rest of the Lisp world is considered an internal | |
1144 | implementation detail. For this reason, we recommend against directly | |
1145 | examining or altering the structure of closure objects. | |
b8d4c8d0 GM |
1146 | |
1147 | @node Obsolete Functions | |
1148 | @section Declaring Functions Obsolete | |
1149 | ||
1150 | You can use @code{make-obsolete} to declare a function obsolete. This | |
1151 | indicates that the function may be removed at some stage in the future. | |
1152 | ||
1153 | @defun make-obsolete obsolete-name current-name &optional when | |
1154 | This function makes the byte compiler warn that the function | |
1155 | @var{obsolete-name} is obsolete. If @var{current-name} is a symbol, the | |
1156 | warning message says to use @var{current-name} instead of | |
1157 | @var{obsolete-name}. @var{current-name} does not need to be an alias for | |
1158 | @var{obsolete-name}; it can be a different function with similar | |
1159 | functionality. If @var{current-name} is a string, it is the warning | |
1160 | message. | |
1161 | ||
1162 | If provided, @var{when} should be a string indicating when the function | |
1163 | was first made obsolete---for example, a date or a release number. | |
1164 | @end defun | |
1165 | ||
1166 | You can define a function as an alias and declare it obsolete at the | |
eb5ed549 | 1167 | same time using the macro @code{define-obsolete-function-alias}: |
b8d4c8d0 GM |
1168 | |
1169 | @defmac define-obsolete-function-alias obsolete-name current-name &optional when docstring | |
1170 | This macro marks the function @var{obsolete-name} obsolete and also | |
1171 | defines it as an alias for the function @var{current-name}. It is | |
1172 | equivalent to the following: | |
1173 | ||
1174 | @example | |
1175 | (defalias @var{obsolete-name} @var{current-name} @var{docstring}) | |
1176 | (make-obsolete @var{obsolete-name} @var{current-name} @var{when}) | |
1177 | @end example | |
1178 | @end defmac | |
1179 | ||
eb5ed549 CY |
1180 | In addition, you can mark a certain a particular calling convention |
1181 | for a function as obsolete: | |
1182 | ||
1183 | @defun set-advertised-calling-convention function signature | |
1184 | This function specifies the argument list @var{signature} as the | |
1185 | correct way to call @var{function}. This causes the Emacs byte | |
1186 | compiler to issue a warning whenever it comes across an Emacs Lisp | |
1187 | program that calls @var{function} any other way (however, it will | |
1188 | still allow the code to be byte compiled). | |
1189 | ||
1190 | For instance, in old versions of Emacs the @code{sit-for} function | |
1191 | accepted three arguments, like this | |
1192 | ||
1193 | @smallexample | |
1194 | (sit-for seconds milliseconds nodisp) | |
1195 | @end smallexample | |
1196 | ||
1197 | However, calling @code{sit-for} this way is considered obsolete | |
1198 | (@pxref{Waiting}). The old calling convention is deprecated like | |
1199 | this: | |
1200 | ||
1201 | @smallexample | |
1202 | (set-advertised-calling-convention | |
1203 | 'sit-for '(seconds &optional nodisp)) | |
1204 | @end smallexample | |
1205 | @end defun | |
1206 | ||
b8d4c8d0 GM |
1207 | @node Inline Functions |
1208 | @section Inline Functions | |
1209 | @cindex inline functions | |
1210 | ||
735cc5ca CY |
1211 | @defmac defsubst name argument-list body-forms... |
1212 | Define an inline function. The syntax is exactly the same as | |
1213 | @code{defun} (@pxref{Defining Functions}). | |
1214 | @end defmac | |
1215 | ||
1216 | You can define an @dfn{inline function} by using @code{defsubst} | |
1217 | instead of @code{defun}. An inline function works just like an | |
1218 | ordinary function except for one thing: when you byte-compile a call | |
1219 | to the function (@pxref{Byte Compilation}), the function's definition | |
1220 | is expanded into the caller. | |
b8d4c8d0 | 1221 | |
735cc5ca CY |
1222 | Making a function inline often makes its function calls run faster. |
1223 | But it also has disadvantages. For one thing, it reduces flexibility; | |
1224 | if you change the definition of the function, calls already inlined | |
1225 | still use the old definition until you recompile them. | |
b8d4c8d0 | 1226 | |
735cc5ca CY |
1227 | Another disadvantage is that making a large function inline can |
1228 | increase the size of compiled code both in files and in memory. Since | |
1229 | the speed advantage of inline functions is greatest for small | |
1230 | functions, you generally should not make large functions inline. | |
b8d4c8d0 | 1231 | |
735cc5ca | 1232 | Also, inline functions do not behave well with respect to debugging, |
b8d4c8d0 GM |
1233 | tracing, and advising (@pxref{Advising Functions}). Since ease of |
1234 | debugging and the flexibility of redefining functions are important | |
1235 | features of Emacs, you should not make a function inline, even if it's | |
1236 | small, unless its speed is really crucial, and you've timed the code | |
1237 | to verify that using @code{defun} actually has performance problems. | |
1238 | ||
735cc5ca CY |
1239 | It's possible to define a macro to expand into the same code that an |
1240 | inline function would execute (@pxref{Macros}). But the macro would | |
1241 | be limited to direct use in expressions---a macro cannot be called | |
1242 | with @code{apply}, @code{mapcar} and so on. Also, it takes some work | |
1243 | to convert an ordinary function into a macro. To convert it into an | |
1244 | inline function is easy; just replace @code{defun} with | |
1245 | @code{defsubst}. Since each argument of an inline function is | |
1246 | evaluated exactly once, you needn't worry about how many times the | |
1247 | body uses the arguments, as you do for macros. | |
b8d4c8d0 | 1248 | |
735cc5ca CY |
1249 | After an inline function is defined, its inline expansion can be |
1250 | performed later on in the same file, just like macros. | |
b8d4c8d0 | 1251 | |
e31dfb12 GM |
1252 | @node Declaring Functions |
1253 | @section Telling the Compiler that a Function is Defined | |
1254 | @cindex function declaration | |
1255 | @cindex declaring functions | |
c4540067 | 1256 | @findex declare-function |
e31dfb12 | 1257 | |
a0925923 RS |
1258 | Byte-compiling a file often produces warnings about functions that the |
1259 | compiler doesn't know about (@pxref{Compiler Errors}). Sometimes this | |
1260 | indicates a real problem, but usually the functions in question are | |
1261 | defined in other files which would be loaded if that code is run. For | |
1262 | example, byte-compiling @file{fortran.el} used to warn: | |
e31dfb12 | 1263 | |
a0925923 | 1264 | @smallexample |
e31dfb12 | 1265 | In end of data: |
bfd0fe7b GM |
1266 | fortran.el:2152:1:Warning: the function `gud-find-c-expr' is not known |
1267 | to be defined. | |
a0925923 | 1268 | @end smallexample |
e31dfb12 | 1269 | |
a0925923 RS |
1270 | In fact, @code{gud-find-c-expr} is only used in the function that |
1271 | Fortran mode uses for the local value of | |
1272 | @code{gud-find-expr-function}, which is a callback from GUD; if it is | |
1273 | called, the GUD functions will be loaded. When you know that such a | |
1274 | warning does not indicate a real problem, it is good to suppress the | |
1275 | warning. That makes new warnings which might mean real problems more | |
1276 | visible. You do that with @code{declare-function}. | |
e31dfb12 GM |
1277 | |
1278 | All you need to do is add a @code{declare-function} statement before the | |
1279 | first use of the function in question: | |
1280 | ||
a0925923 | 1281 | @smallexample |
e31dfb12 | 1282 | (declare-function gud-find-c-expr "gud.el" nil) |
7a6a1728 | 1283 | @end smallexample |
e31dfb12 GM |
1284 | |
1285 | This says that @code{gud-find-c-expr} is defined in @file{gud.el} (the | |
a0925923 RS |
1286 | @samp{.el} can be omitted). The compiler takes for granted that that file |
1287 | really defines the function, and does not check. | |
7a6a1728 | 1288 | |
a0925923 RS |
1289 | The optional third argument specifies the argument list of |
1290 | @code{gud-find-c-expr}. In this case, it takes no arguments | |
1291 | (@code{nil} is different from not specifying a value). In other | |
1292 | cases, this might be something like @code{(file &optional overwrite)}. | |
1293 | You don't have to specify the argument list, but if you do the | |
1294 | byte compiler can check that the calls match the declaration. | |
1295 | ||
8f4b37d8 | 1296 | @defmac declare-function function file &optional arglist fileonly |
a0925923 | 1297 | Tell the byte compiler to assume that @var{function} is defined, with |
b0fbc500 CY |
1298 | arguments @var{arglist}, and that the definition should come from the |
1299 | file @var{file}. @var{fileonly} non-@code{nil} means only check that | |
8f4b37d8 | 1300 | @var{file} exists, not that it actually defines @var{function}. |
a0925923 RS |
1301 | @end defmac |
1302 | ||
1303 | To verify that these functions really are declared where | |
1304 | @code{declare-function} says they are, use @code{check-declare-file} | |
1305 | to check all @code{declare-function} calls in one source file, or use | |
1306 | @code{check-declare-directory} check all the files in and under a | |
1307 | certain directory. | |
1308 | ||
1309 | These commands find the file that ought to contain a function's | |
1310 | definition using @code{locate-library}; if that finds no file, they | |
1311 | expand the definition file name relative to the directory of the file | |
1312 | that contains the @code{declare-function} call. | |
1313 | ||
735cc5ca CY |
1314 | You can also say that a function is a primitive by specifying a file |
1315 | name ending in @samp{.c} or @samp{.m}. This is useful only when you | |
1316 | call a primitive that is defined only on certain systems. Most | |
1317 | primitives are always defined, so they will never give you a warning. | |
e31dfb12 | 1318 | |
c4540067 GM |
1319 | Sometimes a file will optionally use functions from an external package. |
1320 | If you prefix the filename in the @code{declare-function} statement with | |
1321 | @samp{ext:}, then it will be checked if it is found, otherwise skipped | |
1322 | without error. | |
1323 | ||
8f4b37d8 GM |
1324 | There are some function definitions that @samp{check-declare} does not |
1325 | understand (e.g. @code{defstruct} and some other macros). In such cases, | |
6297397b GM |
1326 | you can pass a non-@code{nil} @var{fileonly} argument to |
1327 | @code{declare-function}, meaning to only check that the file exists, not | |
1328 | that it actually defines the function. Note that to do this without | |
1329 | having to specify an argument list, you should set the @var{arglist} | |
1330 | argument to @code{t} (because @code{nil} means an empty argument list, as | |
1331 | opposed to an unspecified one). | |
8f4b37d8 | 1332 | |
b8d4c8d0 GM |
1333 | @node Function Safety |
1334 | @section Determining whether a Function is Safe to Call | |
1335 | @cindex function safety | |
1336 | @cindex safety of functions | |
1337 | ||
1338 | Some major modes such as SES call functions that are stored in user | |
1339 | files. (@inforef{Top, ,ses}, for more information on SES.) User | |
1340 | files sometimes have poor pedigrees---you can get a spreadsheet from | |
1341 | someone you've just met, or you can get one through email from someone | |
1342 | you've never met. So it is risky to call a function whose source code | |
1343 | is stored in a user file until you have determined that it is safe. | |
1344 | ||
1345 | @defun unsafep form &optional unsafep-vars | |
1346 | Returns @code{nil} if @var{form} is a @dfn{safe} Lisp expression, or | |
1347 | returns a list that describes why it might be unsafe. The argument | |
1348 | @var{unsafep-vars} is a list of symbols known to have temporary | |
1349 | bindings at this point; it is mainly used for internal recursive | |
1350 | calls. The current buffer is an implicit argument, which provides a | |
1351 | list of buffer-local bindings. | |
1352 | @end defun | |
1353 | ||
1354 | Being quick and simple, @code{unsafep} does a very light analysis and | |
1355 | rejects many Lisp expressions that are actually safe. There are no | |
1356 | known cases where @code{unsafep} returns @code{nil} for an unsafe | |
1357 | expression. However, a ``safe'' Lisp expression can return a string | |
1358 | with a @code{display} property, containing an associated Lisp | |
1359 | expression to be executed after the string is inserted into a buffer. | |
1360 | This associated expression can be a virus. In order to be safe, you | |
1361 | must delete properties from all strings calculated by user code before | |
1362 | inserting them into buffers. | |
1363 | ||
1364 | @ignore | |
1365 | What is a safe Lisp expression? Basically, it's an expression that | |
1366 | calls only built-in functions with no side effects (or only innocuous | |
1367 | ones). Innocuous side effects include displaying messages and | |
1368 | altering non-risky buffer-local variables (but not global variables). | |
1369 | ||
1370 | @table @dfn | |
1371 | @item Safe expression | |
1372 | @itemize | |
1373 | @item | |
1374 | An atom or quoted thing. | |
1375 | @item | |
1376 | A call to a safe function (see below), if all its arguments are | |
1377 | safe expressions. | |
1378 | @item | |
1379 | One of the special forms @code{and}, @code{catch}, @code{cond}, | |
1380 | @code{if}, @code{or}, @code{prog1}, @code{prog2}, @code{progn}, | |
1381 | @code{while}, and @code{unwind-protect}], if all its arguments are | |
1382 | safe. | |
1383 | @item | |
1384 | A form that creates temporary bindings (@code{condition-case}, | |
1385 | @code{dolist}, @code{dotimes}, @code{lambda}, @code{let}, or | |
1386 | @code{let*}), if all args are safe and the symbols to be bound are not | |
1387 | explicitly risky (see @pxref{File Local Variables}). | |
1388 | @item | |
1389 | An assignment using @code{add-to-list}, @code{setq}, @code{push}, or | |
1390 | @code{pop}, if all args are safe and the symbols to be assigned are | |
1391 | not explicitly risky and they already have temporary or buffer-local | |
1392 | bindings. | |
1393 | @item | |
1394 | One of [apply, mapc, mapcar, mapconcat] if the first argument is a | |
1395 | safe explicit lambda and the other args are safe expressions. | |
1396 | @end itemize | |
1397 | ||
1398 | @item Safe function | |
1399 | @itemize | |
1400 | @item | |
1401 | A lambda containing safe expressions. | |
1402 | @item | |
1403 | A symbol on the list @code{safe-functions}, so the user says it's safe. | |
1404 | @item | |
1405 | A symbol with a non-@code{nil} @code{side-effect-free} property. | |
1406 | @item | |
27610f35 RS |
1407 | A symbol with a non-@code{nil} @code{safe-function} property. The |
1408 | value @code{t} indicates a function that is safe but has innocuous | |
1409 | side effects. Other values will someday indicate functions with | |
1410 | classes of side effects that are not always safe. | |
b8d4c8d0 GM |
1411 | @end itemize |
1412 | ||
1413 | The @code{side-effect-free} and @code{safe-function} properties are | |
1414 | provided for built-in functions and for low-level functions and macros | |
1415 | defined in @file{subr.el}. You can assign these properties for the | |
1416 | functions you write. | |
1417 | @end table | |
1418 | @end ignore | |
1419 | ||
1420 | @node Related Topics | |
1421 | @section Other Topics Related to Functions | |
1422 | ||
1423 | Here is a table of several functions that do things related to | |
1424 | function calling and function definitions. They are documented | |
1425 | elsewhere, but we provide cross references here. | |
1426 | ||
1427 | @table @code | |
1428 | @item apply | |
1429 | See @ref{Calling Functions}. | |
1430 | ||
1431 | @item autoload | |
1432 | See @ref{Autoload}. | |
1433 | ||
1434 | @item call-interactively | |
1435 | See @ref{Interactive Call}. | |
1436 | ||
39dc0d57 RS |
1437 | @item called-interactively-p |
1438 | See @ref{Distinguish Interactive}. | |
1439 | ||
b8d4c8d0 GM |
1440 | @item commandp |
1441 | See @ref{Interactive Call}. | |
1442 | ||
1443 | @item documentation | |
1444 | See @ref{Accessing Documentation}. | |
1445 | ||
1446 | @item eval | |
1447 | See @ref{Eval}. | |
1448 | ||
1449 | @item funcall | |
1450 | See @ref{Calling Functions}. | |
1451 | ||
1452 | @item function | |
1453 | See @ref{Anonymous Functions}. | |
1454 | ||
1455 | @item ignore | |
1456 | See @ref{Calling Functions}. | |
1457 | ||
1458 | @item indirect-function | |
1459 | See @ref{Function Indirection}. | |
1460 | ||
1461 | @item interactive | |
1462 | See @ref{Using Interactive}. | |
1463 | ||
1464 | @item interactive-p | |
39dc0d57 | 1465 | See @ref{Distinguish Interactive}. |
b8d4c8d0 GM |
1466 | |
1467 | @item mapatoms | |
1468 | See @ref{Creating Symbols}. | |
1469 | ||
1470 | @item mapcar | |
1471 | See @ref{Mapping Functions}. | |
1472 | ||
1473 | @item map-char-table | |
1474 | See @ref{Char-Tables}. | |
1475 | ||
1476 | @item mapconcat | |
1477 | See @ref{Mapping Functions}. | |
1478 | ||
1479 | @item undefined | |
1480 | See @ref{Functions for Key Lookup}. | |
1481 | @end table |