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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. |
b8d4c8d0 GM |
6 | @node Functions, Macros, Variables, Top |
7 | @chapter Functions | |
8 | ||
9 | A Lisp program is composed mainly of Lisp functions. This chapter | |
10 | explains what functions are, how they accept arguments, and how to | |
11 | define them. | |
12 | ||
13 | @menu | |
14 | * What Is a Function:: Lisp functions vs. primitives; terminology. | |
15 | * Lambda Expressions:: How functions are expressed as Lisp objects. | |
16 | * Function Names:: A symbol can serve as the name of a function. | |
17 | * Defining Functions:: Lisp expressions for defining functions. | |
18 | * Calling Functions:: How to use an existing function. | |
19 | * Mapping Functions:: Applying a function to each element of a list, etc. | |
20 | * Anonymous Functions:: Lambda expressions are functions with no names. | |
21 | * Function Cells:: Accessing or setting the function definition | |
22 | of a symbol. | |
735cc5ca | 23 | * Closures:: Functions that enclose a lexical environment. |
b8d4c8d0 | 24 | * Obsolete Functions:: Declaring functions obsolete. |
735cc5ca | 25 | * Inline Functions:: Functions that the compiler will expand inline. |
e31dfb12 | 26 | * Declaring Functions:: Telling the compiler that a function is defined. |
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27 | * Function Safety:: Determining whether a function is safe to call. |
28 | * Related Topics:: Cross-references to specific Lisp primitives | |
29 | that have a special bearing on how functions work. | |
30 | @end menu | |
31 | ||
32 | @node What Is a Function | |
33 | @section What Is a Function? | |
34 | ||
735cc5ca CY |
35 | @cindex return value |
36 | @cindex value of function | |
37 | @cindex argument | |
38 | In a general sense, a function is a rule for carrying out a | |
39 | computation given input values called @dfn{arguments}. The result of | |
40 | the computation is called the @dfn{value} or @dfn{return value} of the | |
41 | function. The computation can also have side effects, such as lasting | |
42 | changes in the values of variables or the contents of data structures. | |
43 | ||
44 | In most computer languages, every function has a name. But in Lisp, | |
45 | a function in the strictest sense has no name: it is an object which | |
46 | can @emph{optionally} be associated with a symbol (e.g.@: @code{car}) | |
47 | that serves as the function name. @xref{Function Names}. When a | |
48 | function has been given a name, we usually also refer to that symbol | |
49 | as a ``function'' (e.g.@: we refer to ``the function @code{car}''). | |
50 | In this manual, the distinction between a function name and the | |
51 | function object itself is usually unimportant, but we will take note | |
52 | wherever it is relevant. | |
53 | ||
54 | Certain function-like objects, called @dfn{special forms} and | |
55 | @dfn{macros}, also accept arguments to carry out computations. | |
56 | However, as explained below, these are not considered functions in | |
57 | Emacs Lisp. | |
58 | ||
59 | Here are important terms for functions and function-like objects: | |
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60 | |
61 | @table @dfn | |
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62 | @item lambda expression |
63 | A function (in the strict sense, i.e.@: a function object) which is | |
64 | written in Lisp. These are described in the following section. | |
65 | @ifnottex | |
66 | @xref{Lambda Expressions}. | |
67 | @end ifnottex | |
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68 | |
69 | @item primitive | |
70 | @cindex primitive | |
71 | @cindex subr | |
72 | @cindex built-in function | |
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73 | A function which is callable from Lisp but is actually written in C. |
74 | Primitives are also called @dfn{built-in functions}, or @dfn{subrs}. | |
75 | Examples include functions like @code{car} and @code{append}. In | |
76 | addition, all special forms (see below) are also considered | |
77 | primitives. | |
78 | ||
79 | Usually, a function is implemented as a primitive because it is a | |
80 | fundamental part of Lisp (e.g.@: @code{car}), or because it provides a | |
81 | low-level interface to operating system services, or because it needs | |
82 | to run fast. Unlike functions defined in Lisp, primitives can be | |
83 | modified or added only by changing the C sources and recompiling | |
84 | Emacs. See @ref{Writing Emacs Primitives}. | |
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85 | |
86 | @item special form | |
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87 | A primitive that is like a function but does not evaluate all of its |
88 | arguments in the usual way. It may evaluate only some of the | |
89 | arguments, or may evaluate them in an unusual order, or several times. | |
90 | Examples include @code{if}, @code{and}, and @code{while}. | |
91 | @xref{Special Forms}. | |
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92 | |
93 | @item macro | |
94 | @cindex macro | |
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95 | A construct defined in Lisp, which differs from a function in that it |
96 | translates a Lisp expression into another expression which is to be | |
97 | evaluated instead of the original expression. Macros enable Lisp | |
98 | programmers to do the sorts of things that special forms can do. | |
99 | @xref{Macros}. | |
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100 | |
101 | @item command | |
102 | @cindex command | |
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103 | An object which can be invoked via the @code{command-execute} |
104 | primitive, usually due to the user typing in a key sequence | |
105 | @dfn{bound} to that command. @xref{Interactive Call}. A command is | |
106 | usually a function; if the function is written in Lisp, it is made | |
107 | into a command by an @code{interactive} form in the function | |
108 | definition (@pxref{Defining Commands}). Commands that are functions | |
109 | can also be called from Lisp expressions, just like other functions. | |
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110 | |
111 | Keyboard macros (strings and vectors) are commands also, even though | |
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112 | they are not functions. @xref{Keyboard Macros}. We say that a symbol |
113 | is a command if its function cell contains a command (@pxref{Symbol | |
114 | Components}); such a @dfn{named command} can be invoked with | |
115 | @kbd{M-x}. | |
116 | ||
117 | @item closure | |
118 | A function object that is much like a lambda expression, except that | |
119 | it also encloses an ``environment'' of lexical variable bindings. | |
120 | @xref{Closures}. | |
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121 | |
122 | @item byte-code function | |
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123 | A function that has been compiled by the byte compiler. |
124 | @xref{Byte-Code Type}. | |
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125 | |
126 | @item autoload object | |
127 | @cindex autoload object | |
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128 | A place-holder for a real function. If the autoload object is called, |
129 | Emacs loads the file containing the definition of the real function, | |
130 | and then calls the real function. @xref{Autoload}. | |
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131 | @end table |
132 | ||
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133 | You can use the function @code{functionp} to test if an object is a |
134 | function: | |
135 | ||
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136 | @defun functionp object |
137 | This function returns @code{t} if @var{object} is any kind of | |
9b240644 | 138 | function, i.e.@: can be passed to @code{funcall}. Note that |
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139 | @code{functionp} returns @code{t} for symbols that are function names, |
140 | and returns @code{nil} for special forms. | |
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141 | @end defun |
142 | ||
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143 | @noindent |
144 | Unlike @code{functionp}, the next three functions do @emph{not} treat | |
145 | a symbol as its function definition. | |
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146 | |
147 | @defun subrp object | |
148 | This function returns @code{t} if @var{object} is a built-in function | |
149 | (i.e., a Lisp primitive). | |
150 | ||
151 | @example | |
152 | @group | |
153 | (subrp 'message) ; @r{@code{message} is a symbol,} | |
154 | @result{} nil ; @r{not a subr object.} | |
155 | @end group | |
156 | @group | |
157 | (subrp (symbol-function 'message)) | |
158 | @result{} t | |
159 | @end group | |
160 | @end example | |
161 | @end defun | |
162 | ||
163 | @defun byte-code-function-p object | |
164 | This function returns @code{t} if @var{object} is a byte-code | |
165 | function. For example: | |
166 | ||
167 | @example | |
168 | @group | |
169 | (byte-code-function-p (symbol-function 'next-line)) | |
170 | @result{} t | |
171 | @end group | |
172 | @end example | |
173 | @end defun | |
174 | ||
175 | @defun subr-arity subr | |
176 | This function provides information about the argument list of a | |
177 | primitive, @var{subr}. The returned value is a pair | |
178 | @code{(@var{min} . @var{max})}. @var{min} is the minimum number of | |
179 | args. @var{max} is the maximum number or the symbol @code{many}, for a | |
180 | function with @code{&rest} arguments, or the symbol @code{unevalled} if | |
181 | @var{subr} is a special form. | |
182 | @end defun | |
183 | ||
184 | @node Lambda Expressions | |
185 | @section Lambda Expressions | |
186 | @cindex lambda expression | |
187 | ||
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188 | A lambda expression is a function object written in Lisp. Here is |
189 | an example: | |
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190 | |
191 | @example | |
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192 | (lambda (x) |
193 | "Return the hyperbolic cosine of X." | |
194 | (* 0.5 (+ (exp x) (exp (- x))))) | |
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195 | @end example |
196 | ||
197 | @noindent | |
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198 | In Emacs Lisp, such a list is valid as an expression---it evaluates to |
199 | itself. But its main use is not to be evaluated as an expression, but | |
200 | to be called as a function. | |
201 | ||
202 | A lambda expression, by itself, has no name; it is an @dfn{anonymous | |
203 | function}. Although lambda expressions can be used this way | |
204 | (@pxref{Anonymous Functions}), they are more commonly associated with | |
205 | symbols to make @dfn{named functions} (@pxref{Function Names}). | |
206 | Before going into these details, the following subsections describe | |
207 | the components of a lambda expression and what they do. | |
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208 | |
209 | @menu | |
210 | * Lambda Components:: The parts of a lambda expression. | |
211 | * Simple Lambda:: A simple example. | |
212 | * Argument List:: Details and special features of argument lists. | |
213 | * Function Documentation:: How to put documentation in a function. | |
214 | @end menu | |
215 | ||
216 | @node Lambda Components | |
217 | @subsection Components of a Lambda Expression | |
218 | ||
735cc5ca | 219 | A lambda expression is a list that looks like this: |
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220 | |
221 | @example | |
222 | (lambda (@var{arg-variables}@dots{}) | |
223 | [@var{documentation-string}] | |
224 | [@var{interactive-declaration}] | |
225 | @var{body-forms}@dots{}) | |
226 | @end example | |
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227 | |
228 | @cindex lambda list | |
229 | The first element of a lambda expression is always the symbol | |
230 | @code{lambda}. This indicates that the list represents a function. The | |
231 | reason functions are defined to start with @code{lambda} is so that | |
232 | other lists, intended for other uses, will not accidentally be valid as | |
233 | functions. | |
234 | ||
235 | The second element is a list of symbols---the argument variable names. | |
236 | This is called the @dfn{lambda list}. When a Lisp function is called, | |
237 | the argument values are matched up against the variables in the lambda | |
238 | list, which are given local bindings with the values provided. | |
239 | @xref{Local Variables}. | |
240 | ||
241 | The documentation string is a Lisp string object placed within the | |
242 | function definition to describe the function for the Emacs help | |
243 | facilities. @xref{Function Documentation}. | |
244 | ||
245 | The interactive declaration is a list of the form @code{(interactive | |
246 | @var{code-string})}. This declares how to provide arguments if the | |
247 | function is used interactively. Functions with this declaration are called | |
248 | @dfn{commands}; they can be called using @kbd{M-x} or bound to a key. | |
249 | Functions not intended to be called in this way should not have interactive | |
250 | declarations. @xref{Defining Commands}, for how to write an interactive | |
251 | declaration. | |
252 | ||
253 | @cindex body of function | |
254 | The rest of the elements are the @dfn{body} of the function: the Lisp | |
255 | code to do the work of the function (or, as a Lisp programmer would say, | |
256 | ``a list of Lisp forms to evaluate''). The value returned by the | |
257 | function is the value returned by the last element of the body. | |
258 | ||
259 | @node Simple Lambda | |
735cc5ca | 260 | @subsection A Simple Lambda Expression Example |
b8d4c8d0 | 261 | |
735cc5ca | 262 | Consider the following example: |
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263 | |
264 | @example | |
265 | (lambda (a b c) (+ a b c)) | |
266 | @end example | |
267 | ||
268 | @noindent | |
88ed9e87 | 269 | We can call this function by passing it to @code{funcall}, like this: |
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270 | |
271 | @example | |
272 | @group | |
88ed9e87 SM |
273 | (funcall (lambda (a b c) (+ a b c)) |
274 | 1 2 3) | |
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275 | @end group |
276 | @end example | |
277 | ||
278 | @noindent | |
279 | This call evaluates the body of the lambda expression with the variable | |
280 | @code{a} bound to 1, @code{b} bound to 2, and @code{c} bound to 3. | |
281 | Evaluation of the body adds these three numbers, producing the result 6; | |
282 | therefore, this call to the function returns the value 6. | |
283 | ||
284 | Note that the arguments can be the results of other function calls, as in | |
285 | this example: | |
286 | ||
287 | @example | |
288 | @group | |
88ed9e87 SM |
289 | (funcall (lambda (a b c) (+ a b c)) |
290 | 1 (* 2 3) (- 5 4)) | |
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291 | @end group |
292 | @end example | |
293 | ||
294 | @noindent | |
295 | This evaluates the arguments @code{1}, @code{(* 2 3)}, and @code{(- 5 | |
296 | 4)} from left to right. Then it applies the lambda expression to the | |
297 | argument values 1, 6 and 1 to produce the value 8. | |
298 | ||
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299 | As these examples show, you can use a form with a lambda expression |
300 | as its @sc{car} to make local variables and give them values. In the | |
301 | old days of Lisp, this technique was the only way to bind and | |
302 | initialize local variables. But nowadays, it is clearer to use the | |
303 | special form @code{let} for this purpose (@pxref{Local Variables}). | |
304 | Lambda expressions are mainly used as anonymous functions for passing | |
305 | as arguments to other functions (@pxref{Anonymous Functions}), or | |
306 | stored as symbol function definitions to produce named functions | |
307 | (@pxref{Function Names}). | |
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308 | |
309 | @node Argument List | |
310 | @subsection Other Features of Argument Lists | |
311 | @kindex wrong-number-of-arguments | |
312 | @cindex argument binding | |
313 | @cindex binding arguments | |
314 | @cindex argument lists, features | |
315 | ||
316 | Our simple sample function, @code{(lambda (a b c) (+ a b c))}, | |
317 | specifies three argument variables, so it must be called with three | |
318 | arguments: if you try to call it with only two arguments or four | |
319 | arguments, you get a @code{wrong-number-of-arguments} error. | |
320 | ||
321 | It is often convenient to write a function that allows certain | |
322 | arguments to be omitted. For example, the function @code{substring} | |
323 | accepts three arguments---a string, the start index and the end | |
324 | index---but the third argument defaults to the @var{length} of the | |
325 | string if you omit it. It is also convenient for certain functions to | |
326 | accept an indefinite number of arguments, as the functions @code{list} | |
327 | and @code{+} do. | |
328 | ||
329 | @cindex optional arguments | |
330 | @cindex rest arguments | |
331 | @kindex &optional | |
332 | @kindex &rest | |
333 | To specify optional arguments that may be omitted when a function | |
334 | is called, simply include the keyword @code{&optional} before the optional | |
335 | arguments. To specify a list of zero or more extra arguments, include the | |
336 | keyword @code{&rest} before one final argument. | |
337 | ||
338 | Thus, the complete syntax for an argument list is as follows: | |
339 | ||
340 | @example | |
341 | @group | |
342 | (@var{required-vars}@dots{} | |
343 | @r{[}&optional @var{optional-vars}@dots{}@r{]} | |
344 | @r{[}&rest @var{rest-var}@r{]}) | |
345 | @end group | |
346 | @end example | |
347 | ||
348 | @noindent | |
349 | The square brackets indicate that the @code{&optional} and @code{&rest} | |
350 | clauses, and the variables that follow them, are optional. | |
351 | ||
352 | A call to the function requires one actual argument for each of the | |
353 | @var{required-vars}. There may be actual arguments for zero or more of | |
354 | the @var{optional-vars}, and there cannot be any actual arguments beyond | |
355 | that unless the lambda list uses @code{&rest}. In that case, there may | |
356 | be any number of extra actual arguments. | |
357 | ||
358 | If actual arguments for the optional and rest variables are omitted, | |
359 | then they always default to @code{nil}. There is no way for the | |
360 | function to distinguish between an explicit argument of @code{nil} and | |
361 | an omitted argument. However, the body of the function is free to | |
362 | consider @code{nil} an abbreviation for some other meaningful value. | |
363 | This is what @code{substring} does; @code{nil} as the third argument to | |
364 | @code{substring} means to use the length of the string supplied. | |
365 | ||
366 | @cindex CL note---default optional arg | |
367 | @quotation | |
368 | @b{Common Lisp note:} Common Lisp allows the function to specify what | |
369 | default value to use when an optional argument is omitted; Emacs Lisp | |
370 | always uses @code{nil}. Emacs Lisp does not support ``supplied-p'' | |
371 | variables that tell you whether an argument was explicitly passed. | |
372 | @end quotation | |
373 | ||
374 | For example, an argument list that looks like this: | |
375 | ||
376 | @example | |
377 | (a b &optional c d &rest e) | |
378 | @end example | |
379 | ||
380 | @noindent | |
381 | binds @code{a} and @code{b} to the first two actual arguments, which are | |
382 | required. If one or two more arguments are provided, @code{c} and | |
383 | @code{d} are bound to them respectively; any arguments after the first | |
384 | four are collected into a list and @code{e} is bound to that list. If | |
385 | there are only two arguments, @code{c} is @code{nil}; if two or three | |
386 | arguments, @code{d} is @code{nil}; if four arguments or fewer, @code{e} | |
387 | is @code{nil}. | |
388 | ||
389 | There is no way to have required arguments following optional | |
390 | ones---it would not make sense. To see why this must be so, suppose | |
391 | that @code{c} in the example were optional and @code{d} were required. | |
392 | Suppose three actual arguments are given; which variable would the | |
393 | third argument be for? Would it be used for the @var{c}, or for | |
394 | @var{d}? One can argue for both possibilities. Similarly, it makes | |
395 | no sense to have any more arguments (either required or optional) | |
396 | after a @code{&rest} argument. | |
397 | ||
398 | Here are some examples of argument lists and proper calls: | |
399 | ||
ddff3351 | 400 | @example |
88ed9e87 SM |
401 | (funcall (lambda (n) (1+ n)) ; @r{One required:} |
402 | 1) ; @r{requires exactly one argument.} | |
b8d4c8d0 | 403 | @result{} 2 |
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404 | (funcall (lambda (n &optional n1) ; @r{One required and one optional:} |
405 | (if n1 (+ n n1) (1+ n))) ; @r{1 or 2 arguments.} | |
406 | 1 2) | |
b8d4c8d0 | 407 | @result{} 3 |
88ed9e87 SM |
408 | (funcall (lambda (n &rest ns) ; @r{One required and one rest:} |
409 | (+ n (apply '+ ns))) ; @r{1 or more arguments.} | |
410 | 1 2 3 4 5) | |
b8d4c8d0 | 411 | @result{} 15 |
ddff3351 | 412 | @end example |
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413 | |
414 | @node Function Documentation | |
415 | @subsection Documentation Strings of Functions | |
416 | @cindex documentation of function | |
417 | ||
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418 | A lambda expression may optionally have a @dfn{documentation string} |
419 | just after the lambda list. This string does not affect execution of | |
420 | the function; it is a kind of comment, but a systematized comment | |
421 | which actually appears inside the Lisp world and can be used by the | |
422 | Emacs help facilities. @xref{Documentation}, for how the | |
423 | documentation string is accessed. | |
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424 | |
425 | It is a good idea to provide documentation strings for all the | |
426 | functions in your program, even those that are called only from within | |
427 | your program. Documentation strings are like comments, except that they | |
428 | are easier to access. | |
429 | ||
430 | The first line of the documentation string should stand on its own, | |
431 | because @code{apropos} displays just this first line. It should consist | |
432 | of one or two complete sentences that summarize the function's purpose. | |
433 | ||
434 | The start of the documentation string is usually indented in the | |
435 | source file, but since these spaces come before the starting | |
436 | double-quote, they are not part of the string. Some people make a | |
437 | practice of indenting any additional lines of the string so that the | |
438 | text lines up in the program source. @emph{That is a mistake.} The | |
439 | indentation of the following lines is inside the string; what looks | |
440 | nice in the source code will look ugly when displayed by the help | |
441 | commands. | |
442 | ||
443 | You may wonder how the documentation string could be optional, since | |
444 | there are required components of the function that follow it (the body). | |
445 | Since evaluation of a string returns that string, without any side effects, | |
446 | it has no effect if it is not the last form in the body. Thus, in | |
447 | practice, there is no confusion between the first form of the body and the | |
448 | documentation string; if the only body form is a string then it serves both | |
449 | as the return value and as the documentation. | |
450 | ||
451 | The last line of the documentation string can specify calling | |
452 | conventions different from the actual function arguments. Write | |
453 | text like this: | |
454 | ||
455 | @example | |
456 | \(fn @var{arglist}) | |
457 | @end example | |
458 | ||
459 | @noindent | |
460 | following a blank line, at the beginning of the line, with no newline | |
461 | following it inside the documentation string. (The @samp{\} is used | |
462 | to avoid confusing the Emacs motion commands.) The calling convention | |
463 | specified in this way appears in help messages in place of the one | |
464 | derived from the actual arguments of the function. | |
465 | ||
466 | This feature is particularly useful for macro definitions, since the | |
467 | arguments written in a macro definition often do not correspond to the | |
468 | way users think of the parts of the macro call. | |
469 | ||
470 | @node Function Names | |
471 | @section Naming a Function | |
472 | @cindex function definition | |
473 | @cindex named function | |
474 | @cindex function name | |
475 | ||
735cc5ca CY |
476 | A symbol can serve as the name of a function. This happens when the |
477 | symbol's @dfn{function cell} (@pxref{Symbol Components}) contains a | |
478 | function object (e.g.@: a lambda expression). Then the symbol itself | |
479 | becomes a valid, callable function, equivalent to the function object | |
480 | in its function cell. | |
481 | ||
482 | The contents of the function cell are also called the symbol's | |
483 | @dfn{function definition}. The procedure of using a symbol's function | |
484 | definition in place of the symbol is called @dfn{symbol function | |
485 | indirection}; see @ref{Function Indirection}. If you have not given a | |
486 | symbol a function definition, its function cell is said to be | |
487 | @dfn{void}, and it cannot be used as a function. | |
488 | ||
489 | In practice, nearly all functions have names, and are referred to by | |
490 | their names. You can create a named Lisp function by defining a | |
491 | lambda expression and putting it in a function cell (@pxref{Function | |
492 | Cells}). However, it is more common to use the @code{defun} special | |
493 | form, described in the next section. | |
494 | @ifnottex | |
495 | @xref{Defining Functions}. | |
496 | @end ifnottex | |
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497 | |
498 | We give functions names because it is convenient to refer to them by | |
735cc5ca CY |
499 | their names in Lisp expressions. Also, a named Lisp function can |
500 | easily refer to itself---it can be recursive. Furthermore, primitives | |
501 | can only be referred to textually by their names, since primitive | |
502 | function objects (@pxref{Primitive Function Type}) have no read | |
503 | syntax. | |
504 | ||
505 | A function need not have a unique name. A given function object | |
506 | @emph{usually} appears in the function cell of only one symbol, but | |
507 | this is just a convention. It is easy to store it in several symbols | |
508 | using @code{fset}; then each of the symbols is a valid name for the | |
509 | same function. | |
510 | ||
511 | Note that a symbol used as a function name may also be used as a | |
512 | variable; these two uses of a symbol are independent and do not | |
513 | conflict. (This is not the case in some dialects of Lisp, like | |
514 | Scheme.) | |
b8d4c8d0 GM |
515 | |
516 | @node Defining Functions | |
517 | @section Defining Functions | |
518 | @cindex defining a function | |
519 | ||
520 | We usually give a name to a function when it is first created. This | |
521 | is called @dfn{defining a function}, and it is done with the | |
522 | @code{defun} special form. | |
523 | ||
735cc5ca | 524 | @defspec defun name argument-list body-forms... |
b8d4c8d0 GM |
525 | @code{defun} is the usual way to define new Lisp functions. It |
526 | defines the symbol @var{name} as a function that looks like this: | |
527 | ||
528 | @example | |
529 | (lambda @var{argument-list} . @var{body-forms}) | |
530 | @end example | |
531 | ||
532 | @code{defun} stores this lambda expression in the function cell of | |
533 | @var{name}. It returns the value @var{name}, but usually we ignore this | |
534 | value. | |
535 | ||
536 | As described previously, @var{argument-list} is a list of argument | |
735cc5ca CY |
537 | names and may include the keywords @code{&optional} and @code{&rest}. |
538 | Also, the first two of the @var{body-forms} may be a documentation | |
539 | string and an interactive declaration. @xref{Lambda Components}. | |
b8d4c8d0 GM |
540 | |
541 | Here are some examples: | |
542 | ||
543 | @example | |
544 | @group | |
545 | (defun foo () 5) | |
546 | @result{} foo | |
547 | @end group | |
548 | @group | |
549 | (foo) | |
550 | @result{} 5 | |
551 | @end group | |
552 | ||
553 | @group | |
554 | (defun bar (a &optional b &rest c) | |
555 | (list a b c)) | |
556 | @result{} bar | |
557 | @end group | |
558 | @group | |
559 | (bar 1 2 3 4 5) | |
560 | @result{} (1 2 (3 4 5)) | |
561 | @end group | |
562 | @group | |
563 | (bar 1) | |
564 | @result{} (1 nil nil) | |
565 | @end group | |
566 | @group | |
567 | (bar) | |
568 | @error{} Wrong number of arguments. | |
569 | @end group | |
570 | ||
571 | @group | |
572 | (defun capitalize-backwards () | |
735cc5ca | 573 | "Upcase the last letter of the word at point." |
b8d4c8d0 GM |
574 | (interactive) |
575 | (backward-word 1) | |
576 | (forward-word 1) | |
577 | (backward-char 1) | |
578 | (capitalize-word 1)) | |
579 | @result{} capitalize-backwards | |
580 | @end group | |
581 | @end example | |
582 | ||
583 | Be careful not to redefine existing functions unintentionally. | |
584 | @code{defun} redefines even primitive functions such as @code{car} | |
735cc5ca CY |
585 | without any hesitation or notification. Emacs does not prevent you |
586 | from doing this, because redefining a function is sometimes done | |
587 | deliberately, and there is no way to distinguish deliberate | |
588 | redefinition from unintentional redefinition. | |
b8d4c8d0 GM |
589 | @end defspec |
590 | ||
591 | @cindex function aliases | |
592 | @defun defalias name definition &optional docstring | |
593 | @anchor{Definition of defalias} | |
594 | This special form defines the symbol @var{name} as a function, with | |
595 | definition @var{definition} (which can be any valid Lisp function). | |
596 | It returns @var{definition}. | |
597 | ||
598 | If @var{docstring} is non-@code{nil}, it becomes the function | |
599 | documentation of @var{name}. Otherwise, any documentation provided by | |
600 | @var{definition} is used. | |
601 | ||
602 | The proper place to use @code{defalias} is where a specific function | |
603 | name is being defined---especially where that name appears explicitly in | |
604 | the source file being loaded. This is because @code{defalias} records | |
605 | which file defined the function, just like @code{defun} | |
606 | (@pxref{Unloading}). | |
607 | ||
608 | By contrast, in programs that manipulate function definitions for other | |
609 | purposes, it is better to use @code{fset}, which does not keep such | |
610 | records. @xref{Function Cells}. | |
611 | @end defun | |
612 | ||
613 | You cannot create a new primitive function with @code{defun} or | |
614 | @code{defalias}, but you can use them to change the function definition of | |
615 | any symbol, even one such as @code{car} or @code{x-popup-menu} whose | |
616 | normal definition is a primitive. However, this is risky: for | |
617 | instance, it is next to impossible to redefine @code{car} without | |
618 | breaking Lisp completely. Redefining an obscure function such as | |
619 | @code{x-popup-menu} is less dangerous, but it still may not work as | |
620 | you expect. If there are calls to the primitive from C code, they | |
621 | call the primitive's C definition directly, so changing the symbol's | |
622 | definition will have no effect on them. | |
623 | ||
624 | See also @code{defsubst}, which defines a function like @code{defun} | |
735cc5ca CY |
625 | and tells the Lisp compiler to perform inline expansion on it. |
626 | @xref{Inline Functions}. | |
b8d4c8d0 GM |
627 | |
628 | @node Calling Functions | |
629 | @section Calling Functions | |
630 | @cindex function invocation | |
631 | @cindex calling a function | |
632 | ||
633 | Defining functions is only half the battle. Functions don't do | |
634 | anything until you @dfn{call} them, i.e., tell them to run. Calling a | |
635 | function is also known as @dfn{invocation}. | |
636 | ||
637 | The most common way of invoking a function is by evaluating a list. | |
638 | For example, evaluating the list @code{(concat "a" "b")} calls the | |
639 | function @code{concat} with arguments @code{"a"} and @code{"b"}. | |
640 | @xref{Evaluation}, for a description of evaluation. | |
641 | ||
642 | When you write a list as an expression in your program, you specify | |
643 | which function to call, and how many arguments to give it, in the text | |
644 | of the program. Usually that's just what you want. Occasionally you | |
645 | need to compute at run time which function to call. To do that, use | |
646 | the function @code{funcall}. When you also need to determine at run | |
647 | time how many arguments to pass, use @code{apply}. | |
648 | ||
649 | @defun funcall function &rest arguments | |
650 | @code{funcall} calls @var{function} with @var{arguments}, and returns | |
651 | whatever @var{function} returns. | |
652 | ||
653 | Since @code{funcall} is a function, all of its arguments, including | |
654 | @var{function}, are evaluated before @code{funcall} is called. This | |
655 | means that you can use any expression to obtain the function to be | |
656 | called. It also means that @code{funcall} does not see the | |
657 | expressions you write for the @var{arguments}, only their values. | |
658 | These values are @emph{not} evaluated a second time in the act of | |
659 | calling @var{function}; the operation of @code{funcall} is like the | |
660 | normal procedure for calling a function, once its arguments have | |
661 | already been evaluated. | |
662 | ||
663 | The argument @var{function} must be either a Lisp function or a | |
664 | primitive function. Special forms and macros are not allowed, because | |
665 | they make sense only when given the ``unevaluated'' argument | |
666 | expressions. @code{funcall} cannot provide these because, as we saw | |
667 | above, it never knows them in the first place. | |
668 | ||
669 | @example | |
670 | @group | |
671 | (setq f 'list) | |
672 | @result{} list | |
673 | @end group | |
674 | @group | |
675 | (funcall f 'x 'y 'z) | |
676 | @result{} (x y z) | |
677 | @end group | |
678 | @group | |
679 | (funcall f 'x 'y '(z)) | |
680 | @result{} (x y (z)) | |
681 | @end group | |
682 | @group | |
683 | (funcall 'and t nil) | |
684 | @error{} Invalid function: #<subr and> | |
685 | @end group | |
686 | @end example | |
687 | ||
688 | Compare these examples with the examples of @code{apply}. | |
689 | @end defun | |
690 | ||
691 | @defun apply function &rest arguments | |
692 | @code{apply} calls @var{function} with @var{arguments}, just like | |
693 | @code{funcall} but with one difference: the last of @var{arguments} is a | |
694 | list of objects, which are passed to @var{function} as separate | |
695 | arguments, rather than a single list. We say that @code{apply} | |
696 | @dfn{spreads} this list so that each individual element becomes an | |
697 | argument. | |
698 | ||
699 | @code{apply} returns the result of calling @var{function}. As with | |
700 | @code{funcall}, @var{function} must either be a Lisp function or a | |
701 | primitive function; special forms and macros do not make sense in | |
702 | @code{apply}. | |
703 | ||
704 | @example | |
705 | @group | |
706 | (setq f 'list) | |
707 | @result{} list | |
708 | @end group | |
709 | @group | |
710 | (apply f 'x 'y 'z) | |
711 | @error{} Wrong type argument: listp, z | |
712 | @end group | |
713 | @group | |
714 | (apply '+ 1 2 '(3 4)) | |
715 | @result{} 10 | |
716 | @end group | |
717 | @group | |
718 | (apply '+ '(1 2 3 4)) | |
719 | @result{} 10 | |
720 | @end group | |
721 | ||
722 | @group | |
723 | (apply 'append '((a b c) nil (x y z) nil)) | |
724 | @result{} (a b c x y z) | |
725 | @end group | |
726 | @end example | |
727 | ||
728 | For an interesting example of using @code{apply}, see @ref{Definition | |
729 | of mapcar}. | |
730 | @end defun | |
731 | ||
80f85d7c EZ |
732 | @cindex partial application of functions |
733 | @cindex currying | |
a18a6d49 | 734 | Sometimes it is useful to fix some of the function's arguments at |
80f85d7c EZ |
735 | certain values, and leave the rest of arguments for when the function |
736 | is actually called. The act of fixing some of the function's | |
737 | arguments is called @dfn{partial application} of the function@footnote{ | |
738 | This is related to, but different from @dfn{currying}, which | |
739 | transforms a function that takes multiple arguments in such a way that | |
740 | it can be called as a chain of functions, each one with a single | |
741 | argument.}. | |
742 | The result is a new function that accepts the rest of | |
743 | arguments and calls the original function with all the arguments | |
a18a6d49 EZ |
744 | combined. |
745 | ||
746 | Here's how to do partial application in Emacs Lisp: | |
80f85d7c EZ |
747 | |
748 | @defun apply-partially func &rest args | |
749 | This function returns a new function which, when called, will call | |
750 | @var{func} with the list of arguments composed from @var{args} and | |
751 | additional arguments specified at the time of the call. If @var{func} | |
752 | accepts @var{n} arguments, then a call to @code{apply-partially} with | |
753 | @w{@code{@var{m} < @var{n}}} arguments will produce a new function of | |
754 | @w{@code{@var{n} - @var{m}}} arguments. | |
755 | ||
834b5485 EZ |
756 | Here's how we could define the built-in function @code{1+}, if it |
757 | didn't exist, using @code{apply-partially} and @code{+}, another | |
758 | built-in function: | |
80f85d7c EZ |
759 | |
760 | @example | |
80f85d7c | 761 | @group |
834b5485 EZ |
762 | (defalias '1+ (apply-partially '+ 1) |
763 | "Increment argument by one.") | |
764 | @end group | |
765 | @group | |
766 | (1+ 10) | |
80f85d7c EZ |
767 | @result{} 11 |
768 | @end group | |
769 | @end example | |
770 | @end defun | |
771 | ||
b8d4c8d0 GM |
772 | @cindex functionals |
773 | It is common for Lisp functions to accept functions as arguments or | |
774 | find them in data structures (especially in hook variables and property | |
775 | lists) and call them using @code{funcall} or @code{apply}. Functions | |
776 | that accept function arguments are often called @dfn{functionals}. | |
777 | ||
778 | Sometimes, when you call a functional, it is useful to supply a no-op | |
779 | function as the argument. Here are two different kinds of no-op | |
780 | function: | |
781 | ||
782 | @defun identity arg | |
783 | This function returns @var{arg} and has no side effects. | |
784 | @end defun | |
785 | ||
786 | @defun ignore &rest args | |
787 | This function ignores any arguments and returns @code{nil}. | |
788 | @end defun | |
789 | ||
735cc5ca CY |
790 | Some functions are user-visible @dfn{commands}, which can be called |
791 | interactively (usually by a key sequence). It is possible to invoke | |
792 | such a command exactly as though it was called interactively, by using | |
793 | the @code{call-interactively} function. @xref{Interactive Call}. | |
413c488d | 794 | |
b8d4c8d0 GM |
795 | @node Mapping Functions |
796 | @section Mapping Functions | |
797 | @cindex mapping functions | |
798 | ||
799 | A @dfn{mapping function} applies a given function (@emph{not} a | |
800 | special form or macro) to each element of a list or other collection. | |
735cc5ca CY |
801 | Emacs Lisp has several such functions; this section describes |
802 | @code{mapcar}, @code{mapc}, and @code{mapconcat}, which map over a | |
803 | list. @xref{Definition of mapatoms}, for the function @code{mapatoms} | |
804 | which maps over the symbols in an obarray. @xref{Definition of | |
805 | maphash}, for the function @code{maphash} which maps over key/value | |
806 | associations in a hash table. | |
b8d4c8d0 GM |
807 | |
808 | These mapping functions do not allow char-tables because a char-table | |
809 | is a sparse array whose nominal range of indices is very large. To map | |
810 | over a char-table in a way that deals properly with its sparse nature, | |
811 | use the function @code{map-char-table} (@pxref{Char-Tables}). | |
812 | ||
813 | @defun mapcar function sequence | |
814 | @anchor{Definition of mapcar} | |
815 | @code{mapcar} applies @var{function} to each element of @var{sequence} | |
816 | in turn, and returns a list of the results. | |
817 | ||
818 | The argument @var{sequence} can be any kind of sequence except a | |
819 | char-table; that is, a list, a vector, a bool-vector, or a string. The | |
820 | result is always a list. The length of the result is the same as the | |
821 | length of @var{sequence}. For example: | |
822 | ||
ddff3351 | 823 | @example |
b8d4c8d0 GM |
824 | @group |
825 | (mapcar 'car '((a b) (c d) (e f))) | |
826 | @result{} (a c e) | |
827 | (mapcar '1+ [1 2 3]) | |
828 | @result{} (2 3 4) | |
3e99b825 | 829 | (mapcar 'string "abc") |
b8d4c8d0 GM |
830 | @result{} ("a" "b" "c") |
831 | @end group | |
832 | ||
833 | @group | |
834 | ;; @r{Call each function in @code{my-hooks}.} | |
835 | (mapcar 'funcall my-hooks) | |
836 | @end group | |
837 | ||
838 | @group | |
839 | (defun mapcar* (function &rest args) | |
840 | "Apply FUNCTION to successive cars of all ARGS. | |
841 | Return the list of results." | |
842 | ;; @r{If no list is exhausted,} | |
843 | (if (not (memq nil args)) | |
844 | ;; @r{apply function to @sc{car}s.} | |
845 | (cons (apply function (mapcar 'car args)) | |
846 | (apply 'mapcar* function | |
847 | ;; @r{Recurse for rest of elements.} | |
848 | (mapcar 'cdr args))))) | |
849 | @end group | |
850 | ||
851 | @group | |
852 | (mapcar* 'cons '(a b c) '(1 2 3 4)) | |
853 | @result{} ((a . 1) (b . 2) (c . 3)) | |
854 | @end group | |
ddff3351 | 855 | @end example |
b8d4c8d0 GM |
856 | @end defun |
857 | ||
858 | @defun mapc function sequence | |
859 | @code{mapc} is like @code{mapcar} except that @var{function} is used for | |
860 | side-effects only---the values it returns are ignored, not collected | |
861 | into a list. @code{mapc} always returns @var{sequence}. | |
862 | @end defun | |
863 | ||
864 | @defun mapconcat function sequence separator | |
865 | @code{mapconcat} applies @var{function} to each element of | |
866 | @var{sequence}: the results, which must be strings, are concatenated. | |
867 | Between each pair of result strings, @code{mapconcat} inserts the string | |
868 | @var{separator}. Usually @var{separator} contains a space or comma or | |
869 | other suitable punctuation. | |
870 | ||
871 | The argument @var{function} must be a function that can take one | |
872 | argument and return a string. The argument @var{sequence} can be any | |
873 | kind of sequence except a char-table; that is, a list, a vector, a | |
874 | bool-vector, or a string. | |
875 | ||
ddff3351 | 876 | @example |
b8d4c8d0 GM |
877 | @group |
878 | (mapconcat 'symbol-name | |
879 | '(The cat in the hat) | |
880 | " ") | |
881 | @result{} "The cat in the hat" | |
882 | @end group | |
883 | ||
884 | @group | |
885 | (mapconcat (function (lambda (x) (format "%c" (1+ x)))) | |
886 | "HAL-8000" | |
887 | "") | |
888 | @result{} "IBM.9111" | |
889 | @end group | |
ddff3351 | 890 | @end example |
b8d4c8d0 GM |
891 | @end defun |
892 | ||
893 | @node Anonymous Functions | |
894 | @section Anonymous Functions | |
895 | @cindex anonymous function | |
896 | ||
735cc5ca CY |
897 | Although functions are usually defined with @code{defun} and given |
898 | names at the same time, it is sometimes convenient to use an explicit | |
899 | lambda expression---an @dfn{anonymous function}. Anonymous functions | |
900 | are valid wherever function names are. They are often assigned as | |
901 | variable values, or as arguments to functions; for instance, you might | |
902 | pass one as the @var{function} argument to @code{mapcar}, which | |
903 | applies that function to each element of a list (@pxref{Mapping | |
904 | Functions}). @xref{describe-symbols example}, for a realistic example | |
905 | of this. | |
906 | ||
907 | When defining a lambda expression that is to be used as an anonymous | |
908 | function, you can in principle use any method to construct the list. | |
909 | But typically you should use the @code{lambda} macro, or the | |
910 | @code{function} special form, or the @code{#'} read syntax: | |
911 | ||
912 | @defmac lambda args body... | |
913 | This macro returns an anonymous function with argument list @var{args} | |
914 | and body forms given by @var{body}. In effect, this macro makes | |
915 | @code{lambda} forms ``self-quoting'': evaluating a form whose @sc{car} | |
916 | is @code{lambda} yields the form itself: | |
b8d4c8d0 | 917 | |
735cc5ca CY |
918 | @example |
919 | (lambda (x) (* x x)) | |
920 | @result{} (lambda (x) (* x x)) | |
921 | @end example | |
b8d4c8d0 | 922 | |
735cc5ca CY |
923 | The @code{lambda} form has one other effect: it tells the Emacs |
924 | evaluator and byte-compiler that its argument is a function, by using | |
925 | @code{function} as a subroutine (see below). | |
926 | @end defmac | |
b8d4c8d0 | 927 | |
735cc5ca CY |
928 | @defspec function function-object |
929 | @cindex function quoting | |
930 | This special form returns @var{function-object} without evaluating it. | |
931 | In this, it is similar to @code{quote} (@pxref{Quoting}). But unlike | |
932 | @code{quote}, it also serves as a note to the Emacs evaluator and | |
933 | byte-compiler that @var{function-object} is intended to be used as a | |
934 | function. Assuming @var{function-object} is a valid lambda | |
935 | expression, this has two effects: | |
b8d4c8d0 | 936 | |
735cc5ca CY |
937 | @itemize |
938 | @item | |
939 | When the code is byte-compiled, @var{function-object} is compiled into | |
940 | a byte-code function object (@pxref{Byte Compilation}). | |
b8d4c8d0 | 941 | |
735cc5ca CY |
942 | @item |
943 | When lexical binding is enabled, @var{function-object} is converted | |
944 | into a closure. @xref{Closures}. | |
945 | @end itemize | |
946 | @end defspec | |
b8d4c8d0 | 947 | |
735cc5ca CY |
948 | @cindex @samp{#'} syntax |
949 | The read syntax @code{#'} is a short-hand for using @code{function}. | |
950 | The following forms are all equivalent: | |
b8d4c8d0 | 951 | |
735cc5ca CY |
952 | @example |
953 | (lambda (x) (* x x)) | |
954 | (function (lambda (x) (* x x))) | |
955 | #'(lambda (x) (* x x)) | |
956 | @end example | |
b8d4c8d0 | 957 | |
5d6ab672 CY |
958 | In the following example, we define a @code{change-property} |
959 | function that takes a function as its third argument, followed by a | |
960 | @code{double-property} function that makes use of | |
961 | @code{change-property} by passing it an anonymous function: | |
b8d4c8d0 GM |
962 | |
963 | @example | |
964 | @group | |
965 | (defun change-property (symbol prop function) | |
966 | (let ((value (get symbol prop))) | |
967 | (put symbol prop (funcall function value)))) | |
968 | @end group | |
b8d4c8d0 | 969 | |
b8d4c8d0 GM |
970 | @group |
971 | (defun double-property (symbol prop) | |
5d6ab672 | 972 | (change-property symbol prop (lambda (x) (* 2 x)))) |
b8d4c8d0 GM |
973 | @end group |
974 | @end example | |
975 | ||
b8d4c8d0 | 976 | @noindent |
735cc5ca | 977 | Note that we do not quote the @code{lambda} form. |
b8d4c8d0 | 978 | |
735cc5ca CY |
979 | If you compile the above code, the anonymous function is also |
980 | compiled. This would not happen if, say, you had constructed the | |
981 | anonymous function by quoting it as a list: | |
b8d4c8d0 GM |
982 | |
983 | @example | |
984 | @group | |
985 | (defun double-property (symbol prop) | |
5d6ab672 | 986 | (change-property symbol prop '(lambda (x) (* 2 x)))) |
b8d4c8d0 GM |
987 | @end group |
988 | @end example | |
989 | ||
990 | @noindent | |
735cc5ca CY |
991 | In that case, the anonymous function is kept as a lambda expression in |
992 | the compiled code. The byte-compiler cannot assume this list is a | |
993 | function, even though it looks like one, since it does not know that | |
994 | @code{change-property} intends to use it as a function. | |
b8d4c8d0 GM |
995 | |
996 | @node Function Cells | |
997 | @section Accessing Function Cell Contents | |
998 | ||
999 | The @dfn{function definition} of a symbol is the object stored in the | |
1000 | function cell of the symbol. The functions described here access, test, | |
1001 | and set the function cell of symbols. | |
1002 | ||
1003 | See also the function @code{indirect-function}. @xref{Definition of | |
1004 | indirect-function}. | |
1005 | ||
1006 | @defun symbol-function symbol | |
1007 | @kindex void-function | |
1008 | This returns the object in the function cell of @var{symbol}. If the | |
1009 | symbol's function cell is void, a @code{void-function} error is | |
1010 | signaled. | |
1011 | ||
1012 | This function does not check that the returned object is a legitimate | |
1013 | function. | |
1014 | ||
1015 | @example | |
1016 | @group | |
1017 | (defun bar (n) (+ n 2)) | |
1018 | @result{} bar | |
1019 | @end group | |
1020 | @group | |
1021 | (symbol-function 'bar) | |
1022 | @result{} (lambda (n) (+ n 2)) | |
1023 | @end group | |
1024 | @group | |
1025 | (fset 'baz 'bar) | |
1026 | @result{} bar | |
1027 | @end group | |
1028 | @group | |
1029 | (symbol-function 'baz) | |
1030 | @result{} bar | |
1031 | @end group | |
1032 | @end example | |
1033 | @end defun | |
1034 | ||
1035 | @cindex void function cell | |
1036 | If you have never given a symbol any function definition, we say that | |
1037 | that symbol's function cell is @dfn{void}. In other words, the function | |
1038 | cell does not have any Lisp object in it. If you try to call such a symbol | |
1039 | as a function, it signals a @code{void-function} error. | |
1040 | ||
1041 | Note that void is not the same as @code{nil} or the symbol | |
1042 | @code{void}. The symbols @code{nil} and @code{void} are Lisp objects, | |
1043 | and can be stored into a function cell just as any other object can be | |
1044 | (and they can be valid functions if you define them in turn with | |
1045 | @code{defun}). A void function cell contains no object whatsoever. | |
1046 | ||
1047 | You can test the voidness of a symbol's function definition with | |
1048 | @code{fboundp}. After you have given a symbol a function definition, you | |
1049 | can make it void once more using @code{fmakunbound}. | |
1050 | ||
1051 | @defun fboundp symbol | |
1052 | This function returns @code{t} if the symbol has an object in its | |
1053 | function cell, @code{nil} otherwise. It does not check that the object | |
1054 | is a legitimate function. | |
1055 | @end defun | |
1056 | ||
1057 | @defun fmakunbound symbol | |
1058 | This function makes @var{symbol}'s function cell void, so that a | |
1059 | subsequent attempt to access this cell will cause a | |
1060 | @code{void-function} error. It returns @var{symbol}. (See also | |
1061 | @code{makunbound}, in @ref{Void Variables}.) | |
1062 | ||
1063 | @example | |
1064 | @group | |
1065 | (defun foo (x) x) | |
1066 | @result{} foo | |
1067 | @end group | |
1068 | @group | |
1069 | (foo 1) | |
1070 | @result{}1 | |
1071 | @end group | |
1072 | @group | |
1073 | (fmakunbound 'foo) | |
1074 | @result{} foo | |
1075 | @end group | |
1076 | @group | |
1077 | (foo 1) | |
1078 | @error{} Symbol's function definition is void: foo | |
1079 | @end group | |
1080 | @end example | |
1081 | @end defun | |
1082 | ||
1083 | @defun fset symbol definition | |
1084 | This function stores @var{definition} in the function cell of | |
1085 | @var{symbol}. The result is @var{definition}. Normally | |
1086 | @var{definition} should be a function or the name of a function, but | |
1087 | this is not checked. The argument @var{symbol} is an ordinary evaluated | |
1088 | argument. | |
1089 | ||
735cc5ca CY |
1090 | The primary use of this function is as a subroutine by constructs that |
1091 | define or alter functions, like @code{defadvice} (@pxref{Advising | |
1092 | Functions}). (If @code{defun} were not a primitive, it could be | |
1093 | written as a Lisp macro using @code{fset}.) You can also use it to | |
1094 | give a symbol a function definition that is not a list, e.g.@: a | |
1095 | keyboard macro (@pxref{Keyboard Macros}): | |
b8d4c8d0 | 1096 | |
735cc5ca CY |
1097 | @example |
1098 | ;; @r{Define a named keyboard macro.} | |
1099 | (fset 'kill-two-lines "\^u2\^k") | |
1100 | @result{} "\^u2\^k" | |
1101 | @end example | |
b8d4c8d0 | 1102 | |
735cc5ca CY |
1103 | It you wish to use @code{fset} to make an alternate name for a |
1104 | function, consider using @code{defalias} instead. @xref{Definition of | |
1105 | defalias}. | |
1106 | @end defun | |
b8d4c8d0 | 1107 | |
735cc5ca CY |
1108 | @node Closures |
1109 | @section Closures | |
b8d4c8d0 | 1110 | |
735cc5ca CY |
1111 | As explained in @ref{Variable Scoping}, Emacs can optionally enable |
1112 | lexical binding of variables. When lexical binding is enabled, any | |
1113 | named function that you create (e.g.@: with @code{defun}), as well as | |
1114 | any anonymous function that you create using the @code{lambda} macro | |
1115 | or the @code{function} special form or the @code{#'} syntax | |
1116 | (@pxref{Anonymous Functions}), is automatically converted into a | |
1117 | closure. | |
b8d4c8d0 | 1118 | |
735cc5ca CY |
1119 | A closure is a function that also carries a record of the lexical |
1120 | environment that existed when the function was defined. When it is | |
1121 | invoked, any lexical variable references within its definition use the | |
1122 | retained lexical environment. In all other respects, closures behave | |
1123 | much like ordinary functions; in particular, they can be called in the | |
1124 | same way as ordinary functions. | |
b8d4c8d0 | 1125 | |
735cc5ca | 1126 | @xref{Lexical Binding}, for an example of using a closure. |
b8d4c8d0 | 1127 | |
735cc5ca CY |
1128 | Currently, an Emacs Lisp closure object is represented by a list |
1129 | with the symbol @code{closure} as the first element, a list | |
1130 | representing the lexical environment as the second element, and the | |
1131 | argument list and body forms as the remaining elements: | |
b8d4c8d0 | 1132 | |
735cc5ca CY |
1133 | @example |
1134 | ;; @r{lexical binding is enabled.} | |
1135 | (lambda (x) (* x x)) | |
1136 | @result{} (closure (t) (x) (* x x)) | |
b8d4c8d0 | 1137 | @end example |
b8d4c8d0 | 1138 | |
735cc5ca CY |
1139 | @noindent |
1140 | However, the fact that the internal structure of a closure is | |
1141 | ``exposed'' to the rest of the Lisp world is considered an internal | |
1142 | implementation detail. For this reason, we recommend against directly | |
1143 | examining or altering the structure of closure objects. | |
b8d4c8d0 GM |
1144 | |
1145 | @node Obsolete Functions | |
1146 | @section Declaring Functions Obsolete | |
1147 | ||
1148 | You can use @code{make-obsolete} to declare a function obsolete. This | |
1149 | indicates that the function may be removed at some stage in the future. | |
1150 | ||
1151 | @defun make-obsolete obsolete-name current-name &optional when | |
1152 | This function makes the byte compiler warn that the function | |
1153 | @var{obsolete-name} is obsolete. If @var{current-name} is a symbol, the | |
1154 | warning message says to use @var{current-name} instead of | |
1155 | @var{obsolete-name}. @var{current-name} does not need to be an alias for | |
1156 | @var{obsolete-name}; it can be a different function with similar | |
1157 | functionality. If @var{current-name} is a string, it is the warning | |
1158 | message. | |
1159 | ||
1160 | If provided, @var{when} should be a string indicating when the function | |
1161 | was first made obsolete---for example, a date or a release number. | |
1162 | @end defun | |
1163 | ||
1164 | You can define a function as an alias and declare it obsolete at the | |
eb5ed549 | 1165 | same time using the macro @code{define-obsolete-function-alias}: |
b8d4c8d0 GM |
1166 | |
1167 | @defmac define-obsolete-function-alias obsolete-name current-name &optional when docstring | |
1168 | This macro marks the function @var{obsolete-name} obsolete and also | |
1169 | defines it as an alias for the function @var{current-name}. It is | |
1170 | equivalent to the following: | |
1171 | ||
1172 | @example | |
1173 | (defalias @var{obsolete-name} @var{current-name} @var{docstring}) | |
1174 | (make-obsolete @var{obsolete-name} @var{current-name} @var{when}) | |
1175 | @end example | |
1176 | @end defmac | |
1177 | ||
eb5ed549 CY |
1178 | In addition, you can mark a certain a particular calling convention |
1179 | for a function as obsolete: | |
1180 | ||
1181 | @defun set-advertised-calling-convention function signature | |
1182 | This function specifies the argument list @var{signature} as the | |
1183 | correct way to call @var{function}. This causes the Emacs byte | |
1184 | compiler to issue a warning whenever it comes across an Emacs Lisp | |
1185 | program that calls @var{function} any other way (however, it will | |
1186 | still allow the code to be byte compiled). | |
1187 | ||
1188 | For instance, in old versions of Emacs the @code{sit-for} function | |
1189 | accepted three arguments, like this | |
1190 | ||
ddff3351 | 1191 | @example |
eb5ed549 | 1192 | (sit-for seconds milliseconds nodisp) |
ddff3351 | 1193 | @end example |
eb5ed549 CY |
1194 | |
1195 | However, calling @code{sit-for} this way is considered obsolete | |
1196 | (@pxref{Waiting}). The old calling convention is deprecated like | |
1197 | this: | |
1198 | ||
ddff3351 | 1199 | @example |
eb5ed549 CY |
1200 | (set-advertised-calling-convention |
1201 | 'sit-for '(seconds &optional nodisp)) | |
ddff3351 | 1202 | @end example |
eb5ed549 CY |
1203 | @end defun |
1204 | ||
b8d4c8d0 GM |
1205 | @node Inline Functions |
1206 | @section Inline Functions | |
1207 | @cindex inline functions | |
1208 | ||
735cc5ca CY |
1209 | @defmac defsubst name argument-list body-forms... |
1210 | Define an inline function. The syntax is exactly the same as | |
1211 | @code{defun} (@pxref{Defining Functions}). | |
1212 | @end defmac | |
1213 | ||
1214 | You can define an @dfn{inline function} by using @code{defsubst} | |
1215 | instead of @code{defun}. An inline function works just like an | |
1216 | ordinary function except for one thing: when you byte-compile a call | |
1217 | to the function (@pxref{Byte Compilation}), the function's definition | |
1218 | is expanded into the caller. | |
b8d4c8d0 | 1219 | |
735cc5ca CY |
1220 | Making a function inline often makes its function calls run faster. |
1221 | But it also has disadvantages. For one thing, it reduces flexibility; | |
1222 | if you change the definition of the function, calls already inlined | |
1223 | still use the old definition until you recompile them. | |
b8d4c8d0 | 1224 | |
735cc5ca CY |
1225 | Another disadvantage is that making a large function inline can |
1226 | increase the size of compiled code both in files and in memory. Since | |
1227 | the speed advantage of inline functions is greatest for small | |
1228 | functions, you generally should not make large functions inline. | |
b8d4c8d0 | 1229 | |
735cc5ca | 1230 | Also, inline functions do not behave well with respect to debugging, |
b8d4c8d0 GM |
1231 | tracing, and advising (@pxref{Advising Functions}). Since ease of |
1232 | debugging and the flexibility of redefining functions are important | |
1233 | features of Emacs, you should not make a function inline, even if it's | |
1234 | small, unless its speed is really crucial, and you've timed the code | |
1235 | to verify that using @code{defun} actually has performance problems. | |
1236 | ||
735cc5ca CY |
1237 | It's possible to define a macro to expand into the same code that an |
1238 | inline function would execute (@pxref{Macros}). But the macro would | |
1239 | be limited to direct use in expressions---a macro cannot be called | |
1240 | with @code{apply}, @code{mapcar} and so on. Also, it takes some work | |
1241 | to convert an ordinary function into a macro. To convert it into an | |
1242 | inline function is easy; just replace @code{defun} with | |
1243 | @code{defsubst}. Since each argument of an inline function is | |
1244 | evaluated exactly once, you needn't worry about how many times the | |
1245 | body uses the arguments, as you do for macros. | |
b8d4c8d0 | 1246 | |
735cc5ca CY |
1247 | After an inline function is defined, its inline expansion can be |
1248 | performed later on in the same file, just like macros. | |
b8d4c8d0 | 1249 | |
e31dfb12 GM |
1250 | @node Declaring Functions |
1251 | @section Telling the Compiler that a Function is Defined | |
1252 | @cindex function declaration | |
1253 | @cindex declaring functions | |
c4540067 | 1254 | @findex declare-function |
e31dfb12 | 1255 | |
a0925923 RS |
1256 | Byte-compiling a file often produces warnings about functions that the |
1257 | compiler doesn't know about (@pxref{Compiler Errors}). Sometimes this | |
1258 | indicates a real problem, but usually the functions in question are | |
1259 | defined in other files which would be loaded if that code is run. For | |
1260 | example, byte-compiling @file{fortran.el} used to warn: | |
e31dfb12 | 1261 | |
ddff3351 | 1262 | @example |
e31dfb12 | 1263 | In end of data: |
bfd0fe7b GM |
1264 | fortran.el:2152:1:Warning: the function `gud-find-c-expr' is not known |
1265 | to be defined. | |
ddff3351 | 1266 | @end example |
e31dfb12 | 1267 | |
a0925923 RS |
1268 | In fact, @code{gud-find-c-expr} is only used in the function that |
1269 | Fortran mode uses for the local value of | |
1270 | @code{gud-find-expr-function}, which is a callback from GUD; if it is | |
1271 | called, the GUD functions will be loaded. When you know that such a | |
1272 | warning does not indicate a real problem, it is good to suppress the | |
1273 | warning. That makes new warnings which might mean real problems more | |
1274 | visible. You do that with @code{declare-function}. | |
e31dfb12 GM |
1275 | |
1276 | All you need to do is add a @code{declare-function} statement before the | |
1277 | first use of the function in question: | |
1278 | ||
ddff3351 | 1279 | @example |
e31dfb12 | 1280 | (declare-function gud-find-c-expr "gud.el" nil) |
ddff3351 | 1281 | @end example |
e31dfb12 GM |
1282 | |
1283 | This says that @code{gud-find-c-expr} is defined in @file{gud.el} (the | |
a0925923 RS |
1284 | @samp{.el} can be omitted). The compiler takes for granted that that file |
1285 | really defines the function, and does not check. | |
7a6a1728 | 1286 | |
a0925923 RS |
1287 | The optional third argument specifies the argument list of |
1288 | @code{gud-find-c-expr}. In this case, it takes no arguments | |
1289 | (@code{nil} is different from not specifying a value). In other | |
1290 | cases, this might be something like @code{(file &optional overwrite)}. | |
1291 | You don't have to specify the argument list, but if you do the | |
1292 | byte compiler can check that the calls match the declaration. | |
1293 | ||
8f4b37d8 | 1294 | @defmac declare-function function file &optional arglist fileonly |
a0925923 | 1295 | Tell the byte compiler to assume that @var{function} is defined, with |
b0fbc500 CY |
1296 | arguments @var{arglist}, and that the definition should come from the |
1297 | file @var{file}. @var{fileonly} non-@code{nil} means only check that | |
8f4b37d8 | 1298 | @var{file} exists, not that it actually defines @var{function}. |
a0925923 RS |
1299 | @end defmac |
1300 | ||
1301 | To verify that these functions really are declared where | |
1302 | @code{declare-function} says they are, use @code{check-declare-file} | |
1303 | to check all @code{declare-function} calls in one source file, or use | |
1304 | @code{check-declare-directory} check all the files in and under a | |
1305 | certain directory. | |
1306 | ||
1307 | These commands find the file that ought to contain a function's | |
1308 | definition using @code{locate-library}; if that finds no file, they | |
1309 | expand the definition file name relative to the directory of the file | |
1310 | that contains the @code{declare-function} call. | |
1311 | ||
735cc5ca CY |
1312 | You can also say that a function is a primitive by specifying a file |
1313 | name ending in @samp{.c} or @samp{.m}. This is useful only when you | |
1314 | call a primitive that is defined only on certain systems. Most | |
1315 | primitives are always defined, so they will never give you a warning. | |
e31dfb12 | 1316 | |
c4540067 GM |
1317 | Sometimes a file will optionally use functions from an external package. |
1318 | If you prefix the filename in the @code{declare-function} statement with | |
1319 | @samp{ext:}, then it will be checked if it is found, otherwise skipped | |
1320 | without error. | |
1321 | ||
8f4b37d8 GM |
1322 | There are some function definitions that @samp{check-declare} does not |
1323 | understand (e.g. @code{defstruct} and some other macros). In such cases, | |
6297397b GM |
1324 | you can pass a non-@code{nil} @var{fileonly} argument to |
1325 | @code{declare-function}, meaning to only check that the file exists, not | |
1326 | that it actually defines the function. Note that to do this without | |
1327 | having to specify an argument list, you should set the @var{arglist} | |
1328 | argument to @code{t} (because @code{nil} means an empty argument list, as | |
1329 | opposed to an unspecified one). | |
8f4b37d8 | 1330 | |
b8d4c8d0 GM |
1331 | @node Function Safety |
1332 | @section Determining whether a Function is Safe to Call | |
1333 | @cindex function safety | |
1334 | @cindex safety of functions | |
1335 | ||
1336 | Some major modes such as SES call functions that are stored in user | |
1337 | files. (@inforef{Top, ,ses}, for more information on SES.) User | |
1338 | files sometimes have poor pedigrees---you can get a spreadsheet from | |
1339 | someone you've just met, or you can get one through email from someone | |
1340 | you've never met. So it is risky to call a function whose source code | |
1341 | is stored in a user file until you have determined that it is safe. | |
1342 | ||
1343 | @defun unsafep form &optional unsafep-vars | |
1344 | Returns @code{nil} if @var{form} is a @dfn{safe} Lisp expression, or | |
1345 | returns a list that describes why it might be unsafe. The argument | |
1346 | @var{unsafep-vars} is a list of symbols known to have temporary | |
1347 | bindings at this point; it is mainly used for internal recursive | |
1348 | calls. The current buffer is an implicit argument, which provides a | |
1349 | list of buffer-local bindings. | |
1350 | @end defun | |
1351 | ||
1352 | Being quick and simple, @code{unsafep} does a very light analysis and | |
1353 | rejects many Lisp expressions that are actually safe. There are no | |
1354 | known cases where @code{unsafep} returns @code{nil} for an unsafe | |
1355 | expression. However, a ``safe'' Lisp expression can return a string | |
1356 | with a @code{display} property, containing an associated Lisp | |
1357 | expression to be executed after the string is inserted into a buffer. | |
1358 | This associated expression can be a virus. In order to be safe, you | |
1359 | must delete properties from all strings calculated by user code before | |
1360 | inserting them into buffers. | |
1361 | ||
1362 | @ignore | |
1363 | What is a safe Lisp expression? Basically, it's an expression that | |
1364 | calls only built-in functions with no side effects (or only innocuous | |
1365 | ones). Innocuous side effects include displaying messages and | |
1366 | altering non-risky buffer-local variables (but not global variables). | |
1367 | ||
1368 | @table @dfn | |
1369 | @item Safe expression | |
1370 | @itemize | |
1371 | @item | |
1372 | An atom or quoted thing. | |
1373 | @item | |
1374 | A call to a safe function (see below), if all its arguments are | |
1375 | safe expressions. | |
1376 | @item | |
1377 | One of the special forms @code{and}, @code{catch}, @code{cond}, | |
1378 | @code{if}, @code{or}, @code{prog1}, @code{prog2}, @code{progn}, | |
1379 | @code{while}, and @code{unwind-protect}], if all its arguments are | |
1380 | safe. | |
1381 | @item | |
1382 | A form that creates temporary bindings (@code{condition-case}, | |
1383 | @code{dolist}, @code{dotimes}, @code{lambda}, @code{let}, or | |
1384 | @code{let*}), if all args are safe and the symbols to be bound are not | |
1385 | explicitly risky (see @pxref{File Local Variables}). | |
1386 | @item | |
1387 | An assignment using @code{add-to-list}, @code{setq}, @code{push}, or | |
1388 | @code{pop}, if all args are safe and the symbols to be assigned are | |
1389 | not explicitly risky and they already have temporary or buffer-local | |
1390 | bindings. | |
1391 | @item | |
1392 | One of [apply, mapc, mapcar, mapconcat] if the first argument is a | |
1393 | safe explicit lambda and the other args are safe expressions. | |
1394 | @end itemize | |
1395 | ||
1396 | @item Safe function | |
1397 | @itemize | |
1398 | @item | |
1399 | A lambda containing safe expressions. | |
1400 | @item | |
1401 | A symbol on the list @code{safe-functions}, so the user says it's safe. | |
1402 | @item | |
1403 | A symbol with a non-@code{nil} @code{side-effect-free} property. | |
1404 | @item | |
27610f35 RS |
1405 | A symbol with a non-@code{nil} @code{safe-function} property. The |
1406 | value @code{t} indicates a function that is safe but has innocuous | |
1407 | side effects. Other values will someday indicate functions with | |
1408 | classes of side effects that are not always safe. | |
b8d4c8d0 GM |
1409 | @end itemize |
1410 | ||
1411 | The @code{side-effect-free} and @code{safe-function} properties are | |
1412 | provided for built-in functions and for low-level functions and macros | |
1413 | defined in @file{subr.el}. You can assign these properties for the | |
1414 | functions you write. | |
1415 | @end table | |
1416 | @end ignore | |
1417 | ||
1418 | @node Related Topics | |
1419 | @section Other Topics Related to Functions | |
1420 | ||
1421 | Here is a table of several functions that do things related to | |
1422 | function calling and function definitions. They are documented | |
1423 | elsewhere, but we provide cross references here. | |
1424 | ||
1425 | @table @code | |
1426 | @item apply | |
1427 | See @ref{Calling Functions}. | |
1428 | ||
1429 | @item autoload | |
1430 | See @ref{Autoload}. | |
1431 | ||
1432 | @item call-interactively | |
1433 | See @ref{Interactive Call}. | |
1434 | ||
39dc0d57 RS |
1435 | @item called-interactively-p |
1436 | See @ref{Distinguish Interactive}. | |
1437 | ||
b8d4c8d0 GM |
1438 | @item commandp |
1439 | See @ref{Interactive Call}. | |
1440 | ||
1441 | @item documentation | |
1442 | See @ref{Accessing Documentation}. | |
1443 | ||
1444 | @item eval | |
1445 | See @ref{Eval}. | |
1446 | ||
1447 | @item funcall | |
1448 | See @ref{Calling Functions}. | |
1449 | ||
1450 | @item function | |
1451 | See @ref{Anonymous Functions}. | |
1452 | ||
1453 | @item ignore | |
1454 | See @ref{Calling Functions}. | |
1455 | ||
1456 | @item indirect-function | |
1457 | See @ref{Function Indirection}. | |
1458 | ||
1459 | @item interactive | |
1460 | See @ref{Using Interactive}. | |
1461 | ||
1462 | @item interactive-p | |
39dc0d57 | 1463 | See @ref{Distinguish Interactive}. |
b8d4c8d0 GM |
1464 | |
1465 | @item mapatoms | |
1466 | See @ref{Creating Symbols}. | |
1467 | ||
1468 | @item mapcar | |
1469 | See @ref{Mapping Functions}. | |
1470 | ||
1471 | @item map-char-table | |
1472 | See @ref{Char-Tables}. | |
1473 | ||
1474 | @item mapconcat | |
1475 | See @ref{Mapping Functions}. | |
1476 | ||
1477 | @item undefined | |
1478 | See @ref{Functions for Key Lookup}. | |
1479 | @end table |