Mercurial > hg > xemacs-beta
annotate man/lispref/functions.texi @ 5090:0ca81354c4c7
Further frame-geometry cleanups
-------------------- ChangeLog entries follow: --------------------
man/ChangeLog addition:
2010-03-03 Ben Wing <ben@xemacs.org>
* internals/internals.texi (Intro to Window and Frame Geometry):
* internals/internals.texi (The Paned Area):
* internals/internals.texi (The Displayable Area):
Update to make note of e.g. the fact that the bottom gutter is
actually above the minibuffer.
src/ChangeLog addition:
2010-03-03 Ben Wing <ben@xemacs.org>
* emacs.c:
* emacs.c (assert_equal_failed):
* lisp.h:
* lisp.h (assert_equal):
New fun assert_equal, asserting that two values == each other, and
printing out both values upon failure.
* frame-gtk.c (gtk_initialize_frame_size):
* frame-impl.h:
* frame-impl.h (FRAME_TOP_INTERNAL_BORDER_START):
* frame-impl.h (FRAME_BOTTOM_INTERNAL_BORDER_START):
* frame-impl.h (FRAME_LEFT_INTERNAL_BORDER_START):
* frame-impl.h (FRAME_PANED_TOP_EDGE):
* frame-impl.h (FRAME_NONPANED_SIZE):
* frame-x.c (x_initialize_frame_size):
* frame.c:
* gutter.c (get_gutter_coords):
* gutter.c (calculate_gutter_size):
* gutter.h:
* gutter.h (WINDOW_REAL_TOP_GUTTER_BOUNDS):
* gutter.h (FRAME_TOP_GUTTER_BOUNDS):
* input-method-xlib.c:
* input-method-xlib.c (XIM_SetGeometry):
* redisplay-output.c (clear_left_border):
* redisplay-output.c (clear_right_border):
* redisplay-output.c (redisplay_output_pixmap):
* redisplay-output.c (redisplay_clear_region):
* redisplay-output.c (redisplay_clear_top_of_window):
* redisplay-output.c (redisplay_clear_to_window_end):
* redisplay-xlike-inc.c (XLIKE_clear_frame):
* redisplay.c:
* redisplay.c (UPDATE_CACHE_RETURN):
* redisplay.c (pixel_to_glyph_translation):
* toolbar.c (update_frame_toolbars_geometry):
* window.c (Fwindow_pixel_edges):
Get rid of some redundant macros. Consistently use the
FRAME_TOP_*_START, FRAME_RIGHT_*_END, etc. format. Rename
FRAME_*_BORDER_* to FRAME_*_INTERNAL_BORDER_*. Comment out
FRAME_BOTTOM_* for gutters and the paned area due to the
uncertainty over where the paned area actually begins. (Eventually
we should probably move the gutters outside the minibuffer so that
the paned area is contiguous.) Use FRAME_PANED_* more often in the
code to make things clearer.
Update the diagram to show that the bottom gutter is inside the
minibuffer (!) and that there are "junk boxes" when you have left
and/or right gutters (dead boxes that are mistakenly left uncleared,
unlike the corresponding scrollbar dead boxes). Update the text
appropriately to cover the bottom gutter position, etc.
Rewrite gutter-geometry code to use the FRAME_*_GUTTER_* in place of
equivalent expressions referencing other frame elements, to make the
code more portable in case we move around the gutter location.
Cleanup FRAME_*_GUTTER_BOUNDS() in gutter.h.
Add some #### GEOM! comments where I think code is incorrect --
typically, it wasn't fixed up properly when the gutter was added.
Some cosmetic changes.
| author | Ben Wing <ben@xemacs.org> |
|---|---|
| date | Wed, 03 Mar 2010 05:07:47 -0600 |
| parents | 755ae5b97edb |
| children | 99f8ebc082d9 |
| rev | line source |
|---|---|
| 428 | 1 @c -*-texinfo-*- |
| 2 @c This is part of the XEmacs Lisp Reference Manual. | |
| 444 | 3 @c Copyright (C) 1990, 1991, 1992, 1993, 1994 Free Software Foundation, Inc. |
| 428 | 4 @c See the file lispref.texi for copying conditions. |
| 5 @setfilename ../../info/functions.info | |
| 2492 | 6 @node Functions and Commands, Macros, Variables, Top |
| 7 @chapter Functions and Commands | |
| 428 | 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. | |
| 444 | 20 * Anonymous Functions:: Lambda expressions are functions with no names. |
| 428 | 21 * Function Cells:: Accessing or setting the function definition |
| 22 of a symbol. | |
| 23 * Inline Functions:: Defining functions that the compiler will open code. | |
| 24 * Related Topics:: Cross-references to specific Lisp primitives | |
| 25 that have a special bearing on how functions work. | |
| 26 @end menu | |
| 27 | |
| 28 @node What Is a Function | |
| 29 @section What Is a Function? | |
| 30 | |
| 31 In a general sense, a function is a rule for carrying on a computation | |
| 32 given several values called @dfn{arguments}. The result of the | |
| 33 computation is called the value of the function. The computation can | |
| 34 also have side effects: lasting changes in the values of variables or | |
| 35 the contents of data structures. | |
| 36 | |
| 37 Here are important terms for functions in XEmacs Lisp and for other | |
| 38 function-like objects. | |
| 39 | |
| 40 @table @dfn | |
| 41 @item function | |
| 42 @cindex function | |
| 43 In XEmacs Lisp, a @dfn{function} is anything that can be applied to | |
| 44 arguments in a Lisp program. In some cases, we use it more | |
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45 specifically to mean a function written in Lisp. Special operators and |
| 428 | 46 macros are not functions. |
| 47 | |
| 2492 | 48 @item command |
| 49 @cindex command | |
| 50 | |
| 51 A @dfn{command} is a possible definition for a key sequence---we count | |
| 52 mouse events and menu accesses as key sequences for this purpose. More | |
| 53 formally, within XEmacs lisp, a command is something that | |
| 54 @code{command-execute} can invoke. | |
| 55 | |
| 56 Some functions are commands; a function written in Lisp is a command if | |
| 57 it contains an interactive declaration. A trivial interactive | |
| 58 declaration is a line @code{(interactive)} immediately after the | |
| 59 documentation string. For more complex examples, with prompting and | |
| 60 completion, see @xref{Defining Commands}. Such a function can be called | |
| 61 from Lisp expressions like other functions; in this case, the fact that | |
| 62 the function is a command makes no difference. | |
| 63 | |
| 64 Keyboard macros (strings and vectors) are commands also, even though | |
| 65 they are not functions. A symbol is a command if its function | |
| 66 definition is a command; such symbols can be invoked with @kbd{M-x}. | |
| 67 The symbol is a function as well if the definition is a function. | |
| 68 | |
| 69 In the case where you want to call a command in reaction to a | |
| 70 user-generated event, you'll need to bind it to that event. For how to | |
| 71 do this, see @xref{Key Binding Commands}. | |
| 72 @xref{Command Overview}. | |
| 73 | |
| 74 @item keystroke command | |
| 75 @cindex keystroke command | |
| 76 A @dfn{keystroke command} is a command that is bound to a key sequence | |
| 77 (typically one to three keystrokes). The distinction is made here | |
| 78 merely to avoid confusion with the meaning of ``command'' in non-Emacs | |
| 79 editors; for Lisp programs, the distinction is normally unimportant. | |
| 80 | |
| 428 | 81 @item primitive |
| 82 @cindex primitive | |
| 83 @cindex subr | |
| 84 @cindex built-in function | |
| 85 A @dfn{primitive} is a function callable from Lisp that is written in C, | |
| 86 such as @code{car} or @code{append}. These functions are also called | |
|
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87 @dfn{built-in} functions or @dfn{subrs}. (Special operators are also |
| 428 | 88 considered primitives.) |
| 89 | |
| 90 Usually the reason that a function is a primitives is because it is | |
| 91 fundamental, because it provides a low-level interface to operating | |
| 92 system services, or because it needs to run fast. Primitives can be | |
| 93 modified or added only by changing the C sources and recompiling the | |
| 94 editor. See @ref{Writing Lisp Primitives,,, internals, XEmacs | |
| 95 Internals Manual}. | |
| 96 | |
| 97 @item lambda expression | |
| 98 A @dfn{lambda expression} is a function written in Lisp. | |
| 99 These are described in the following section. | |
| 100 @ifinfo | |
| 101 @xref{Lambda Expressions}. | |
| 102 @end ifinfo | |
| 103 | |
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104 @item special operator |
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105 A @dfn{special operator} is a primitive that is like a function but does not |
| 428 | 106 evaluate all of its arguments in the usual way. It may evaluate only |
| 107 some of the arguments, or may evaluate them in an unusual order, or | |
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108 several times. Many special operators are described in @ref{Control |
| 428 | 109 Structures}. |
| 110 | |
| 111 @item macro | |
| 112 @cindex macro | |
| 113 A @dfn{macro} is a construct defined in Lisp by the programmer. It | |
| 114 differs from a function in that it translates a Lisp expression that you | |
| 115 write into an equivalent expression to be evaluated instead of the | |
| 116 original expression. Macros enable Lisp programmers to do the sorts of | |
|
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117 things that special operators can do. @xref{Macros}, for how to define and |
| 428 | 118 use macros. |
| 119 | |
| 120 @item compiled function | |
| 121 A @dfn{compiled function} is a function that has been compiled by the | |
| 122 byte compiler. @xref{Compiled-Function Type}. | |
| 123 @end table | |
| 124 | |
| 125 @defun subrp object | |
| 126 This function returns @code{t} if @var{object} is a built-in function | |
| 127 (i.e., a Lisp primitive). | |
| 128 | |
| 129 @example | |
| 130 @group | |
| 131 (subrp 'message) ; @r{@code{message} is a symbol,} | |
| 132 @result{} nil ; @r{not a subr object.} | |
| 133 @end group | |
| 134 @group | |
| 135 (subrp (symbol-function 'message)) | |
| 136 @result{} t | |
| 137 @end group | |
| 138 @end example | |
| 139 @end defun | |
| 140 | |
| 141 @defun compiled-function-p object | |
| 142 This function returns @code{t} if @var{object} is a compiled | |
| 143 function. For example: | |
| 144 | |
| 145 @example | |
| 146 @group | |
| 147 (compiled-function-p (symbol-function 'next-line)) | |
| 148 @result{} t | |
| 149 @end group | |
| 150 @end example | |
| 151 @end defun | |
| 152 | |
| 153 @node Lambda Expressions | |
| 154 @section Lambda Expressions | |
| 155 @cindex lambda expression | |
| 156 | |
| 157 A function written in Lisp is a list that looks like this: | |
| 158 | |
| 159 @example | |
| 160 (lambda (@var{arg-variables}@dots{}) | |
| 161 @r{[}@var{documentation-string}@r{]} | |
| 162 @r{[}@var{interactive-declaration}@r{]} | |
| 163 @var{body-forms}@dots{}) | |
| 164 @end example | |
| 165 | |
| 166 @noindent | |
| 167 Such a list is called a @dfn{lambda expression}. In XEmacs Lisp, it | |
| 168 actually is valid as an expression---it evaluates to itself. In some | |
| 169 other Lisp dialects, a lambda expression is not a valid expression at | |
| 170 all. In either case, its main use is not to be evaluated as an | |
| 171 expression, but to be called as a function. | |
| 172 | |
| 173 @menu | |
| 174 * Lambda Components:: The parts of a lambda expression. | |
| 175 * Simple Lambda:: A simple example. | |
| 176 * Argument List:: Details and special features of argument lists. | |
| 177 * Function Documentation:: How to put documentation in a function. | |
| 178 @end menu | |
| 179 | |
| 180 @node Lambda Components | |
| 181 @subsection Components of a Lambda Expression | |
| 182 | |
| 183 @ifinfo | |
| 184 | |
| 185 A function written in Lisp (a ``lambda expression'') is a list that | |
| 186 looks like this: | |
| 187 | |
| 188 @example | |
| 189 (lambda (@var{arg-variables}@dots{}) | |
| 190 [@var{documentation-string}] | |
| 191 [@var{interactive-declaration}] | |
| 192 @var{body-forms}@dots{}) | |
| 193 @end example | |
| 194 @end ifinfo | |
| 195 | |
| 196 @cindex lambda list | |
| 197 The first element of a lambda expression is always the symbol | |
| 198 @code{lambda}. This indicates that the list represents a function. The | |
| 199 reason functions are defined to start with @code{lambda} is so that | |
| 200 other lists, intended for other uses, will not accidentally be valid as | |
| 201 functions. | |
| 202 | |
| 203 The second element is a list of symbols--the argument variable names. | |
| 204 This is called the @dfn{lambda list}. When a Lisp function is called, | |
| 205 the argument values are matched up against the variables in the lambda | |
| 206 list, which are given local bindings with the values provided. | |
| 207 @xref{Local Variables}. | |
| 208 | |
| 209 The documentation string is a Lisp string object placed within the | |
| 210 function definition to describe the function for the XEmacs help | |
| 211 facilities. @xref{Function Documentation}. | |
| 212 | |
| 213 The interactive declaration is a list of the form @code{(interactive | |
| 214 @var{code-string})}. This declares how to provide arguments if the | |
| 215 function is used interactively. Functions with this declaration are called | |
| 216 @dfn{commands}; they can be called using @kbd{M-x} or bound to a key. | |
| 217 Functions not intended to be called in this way should not have interactive | |
| 218 declarations. @xref{Defining Commands}, for how to write an interactive | |
| 219 declaration. | |
| 220 | |
| 221 @cindex body of function | |
| 222 The rest of the elements are the @dfn{body} of the function: the Lisp | |
| 223 code to do the work of the function (or, as a Lisp programmer would say, | |
| 224 ``a list of Lisp forms to evaluate''). The value returned by the | |
| 225 function is the value returned by the last element of the body. | |
| 226 | |
| 227 @node Simple Lambda | |
| 228 @subsection A Simple Lambda-Expression Example | |
| 229 | |
| 230 Consider for example the following function: | |
| 231 | |
| 232 @example | |
| 233 (lambda (a b c) (+ a b c)) | |
| 234 @end example | |
| 235 | |
| 236 @noindent | |
| 237 We can call this function by writing it as the @sc{car} of an | |
| 238 expression, like this: | |
| 239 | |
| 240 @example | |
| 241 @group | |
| 242 ((lambda (a b c) (+ a b c)) | |
| 243 1 2 3) | |
| 244 @end group | |
| 245 @end example | |
| 246 | |
| 247 @noindent | |
| 248 This call evaluates the body of the lambda expression with the variable | |
| 249 @code{a} bound to 1, @code{b} bound to 2, and @code{c} bound to 3. | |
| 250 Evaluation of the body adds these three numbers, producing the result 6; | |
| 251 therefore, this call to the function returns the value 6. | |
| 252 | |
| 253 Note that the arguments can be the results of other function calls, as in | |
| 254 this example: | |
| 255 | |
| 256 @example | |
| 257 @group | |
| 258 ((lambda (a b c) (+ a b c)) | |
| 259 1 (* 2 3) (- 5 4)) | |
| 260 @end group | |
| 261 @end example | |
| 262 | |
| 263 @noindent | |
| 264 This evaluates the arguments @code{1}, @code{(* 2 3)}, and @code{(- 5 | |
| 265 4)} from left to right. Then it applies the lambda expression to the | |
| 266 argument values 1, 6 and 1 to produce the value 8. | |
| 267 | |
| 268 It is not often useful to write a lambda expression as the @sc{car} of | |
| 269 a form in this way. You can get the same result, of making local | |
|
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parents:
2492
diff
changeset
|
270 variables and giving them values, using the special operator @code{let} |
| 428 | 271 (@pxref{Local Variables}). And @code{let} is clearer and easier to use. |
| 272 In practice, lambda expressions are either stored as the function | |
| 273 definitions of symbols, to produce named functions, or passed as | |
| 274 arguments to other functions (@pxref{Anonymous Functions}). | |
| 275 | |
| 276 However, calls to explicit lambda expressions were very useful in the | |
|
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Aidan Kehoe <kehoea@parhasard.net>
parents:
2492
diff
changeset
|
277 old days of Lisp, before the special operator @code{let} was invented. At |
| 428 | 278 that time, they were the only way to bind and initialize local |
| 279 variables. | |
| 280 | |
| 281 @node Argument List | |
| 282 @subsection Advanced Features of Argument Lists | |
| 283 @kindex wrong-number-of-arguments | |
| 284 @cindex argument binding | |
| 285 @cindex binding arguments | |
| 286 | |
| 287 Our simple sample function, @code{(lambda (a b c) (+ a b c))}, | |
| 288 specifies three argument variables, so it must be called with three | |
| 289 arguments: if you try to call it with only two arguments or four | |
| 290 arguments, you get a @code{wrong-number-of-arguments} error. | |
| 291 | |
| 292 It is often convenient to write a function that allows certain | |
| 293 arguments to be omitted. For example, the function @code{substring} | |
| 294 accepts three arguments---a string, the start index and the end | |
| 295 index---but the third argument defaults to the @var{length} of the | |
| 296 string if you omit it. It is also convenient for certain functions to | |
| 297 accept an indefinite number of arguments, as the functions @code{list} | |
| 298 and @code{+} do. | |
| 299 | |
| 300 @cindex optional arguments | |
| 301 @cindex rest arguments | |
| 302 @kindex &optional | |
| 303 @kindex &rest | |
| 304 To specify optional arguments that may be omitted when a function | |
| 305 is called, simply include the keyword @code{&optional} before the optional | |
| 306 arguments. To specify a list of zero or more extra arguments, include the | |
| 307 keyword @code{&rest} before one final argument. | |
| 308 | |
| 309 Thus, the complete syntax for an argument list is as follows: | |
| 310 | |
| 311 @example | |
| 312 @group | |
| 313 (@var{required-vars}@dots{} | |
| 314 @r{[}&optional @var{optional-vars}@dots{}@r{]} | |
| 315 @r{[}&rest @var{rest-var}@r{]}) | |
| 316 @end group | |
| 317 @end example | |
| 318 | |
| 319 @noindent | |
| 320 The square brackets indicate that the @code{&optional} and @code{&rest} | |
| 321 clauses, and the variables that follow them, are optional. | |
| 322 | |
| 323 A call to the function requires one actual argument for each of the | |
| 324 @var{required-vars}. There may be actual arguments for zero or more of | |
| 325 the @var{optional-vars}, and there cannot be any actual arguments beyond | |
| 326 that unless the lambda list uses @code{&rest}. In that case, there may | |
| 327 be any number of extra actual arguments. | |
| 328 | |
| 329 If actual arguments for the optional and rest variables are omitted, | |
| 330 then they always default to @code{nil}. There is no way for the | |
| 331 function to distinguish between an explicit argument of @code{nil} and | |
| 332 an omitted argument. However, the body of the function is free to | |
| 333 consider @code{nil} an abbreviation for some other meaningful value. | |
| 334 This is what @code{substring} does; @code{nil} as the third argument to | |
| 335 @code{substring} means to use the length of the string supplied. | |
| 336 | |
| 337 @cindex CL note---default optional arg | |
| 338 @quotation | |
| 339 @b{Common Lisp note:} Common Lisp allows the function to specify what | |
| 340 default value to use when an optional argument is omitted; XEmacs Lisp | |
| 341 always uses @code{nil}. | |
| 342 @end quotation | |
| 343 | |
| 344 For example, an argument list that looks like this: | |
| 345 | |
| 346 @example | |
| 347 (a b &optional c d &rest e) | |
| 348 @end example | |
| 349 | |
| 350 @noindent | |
| 351 binds @code{a} and @code{b} to the first two actual arguments, which are | |
| 352 required. If one or two more arguments are provided, @code{c} and | |
| 353 @code{d} are bound to them respectively; any arguments after the first | |
| 354 four are collected into a list and @code{e} is bound to that list. If | |
| 355 there are only two arguments, @code{c} is @code{nil}; if two or three | |
| 356 arguments, @code{d} is @code{nil}; if four arguments or fewer, @code{e} | |
| 357 is @code{nil}. | |
| 358 | |
| 359 There is no way to have required arguments following optional | |
| 360 ones---it would not make sense. To see why this must be so, suppose | |
| 361 that @code{c} in the example were optional and @code{d} were required. | |
| 362 Suppose three actual arguments are given; which variable would the third | |
| 363 argument be for? Similarly, it makes no sense to have any more | |
| 364 arguments (either required or optional) after a @code{&rest} argument. | |
| 365 | |
| 366 Here are some examples of argument lists and proper calls: | |
| 367 | |
| 368 @smallexample | |
| 369 ((lambda (n) (1+ n)) ; @r{One required:} | |
| 370 1) ; @r{requires exactly one argument.} | |
| 371 @result{} 2 | |
| 372 ((lambda (n &optional n1) ; @r{One required and one optional:} | |
| 373 (if n1 (+ n n1) (1+ n))) ; @r{1 or 2 arguments.} | |
| 374 1 2) | |
| 375 @result{} 3 | |
| 376 ((lambda (n &rest ns) ; @r{One required and one rest:} | |
| 377 (+ n (apply '+ ns))) ; @r{1 or more arguments.} | |
| 378 1 2 3 4 5) | |
| 379 @result{} 15 | |
| 380 @end smallexample | |
| 381 | |
| 382 @node Function Documentation | |
| 383 @subsection Documentation Strings of Functions | |
| 384 @cindex documentation of function | |
| 385 | |
| 386 A lambda expression may optionally have a @dfn{documentation string} just | |
| 387 after the lambda list. This string does not affect execution of the | |
| 388 function; it is a kind of comment, but a systematized comment which | |
| 389 actually appears inside the Lisp world and can be used by the XEmacs help | |
| 390 facilities. @xref{Documentation}, for how the @var{documentation-string} is | |
| 391 accessed. | |
| 392 | |
| 393 It is a good idea to provide documentation strings for all the | |
| 394 functions in your program, even those that are only called from within | |
| 395 your program. Documentation strings are like comments, except that they | |
| 396 are easier to access. | |
| 397 | |
| 398 The first line of the documentation string should stand on its own, | |
| 399 because @code{apropos} displays just this first line. It should consist | |
| 400 of one or two complete sentences that summarize the function's purpose. | |
| 401 | |
| 402 The start of the documentation string is usually indented in the source file, | |
| 403 but since these spaces come before the starting double-quote, they are not part of | |
| 404 the string. Some people make a practice of indenting any additional | |
| 405 lines of the string so that the text lines up in the program source. | |
| 406 @emph{This is a mistake.} The indentation of the following lines is | |
| 407 inside the string; what looks nice in the source code will look ugly | |
| 408 when displayed by the help commands. | |
| 409 | |
| 410 You may wonder how the documentation string could be optional, since | |
| 411 there are required components of the function that follow it (the body). | |
| 412 Since evaluation of a string returns that string, without any side effects, | |
| 413 it has no effect if it is not the last form in the body. Thus, in | |
| 414 practice, there is no confusion between the first form of the body and the | |
| 415 documentation string; if the only body form is a string then it serves both | |
| 416 as the return value and as the documentation. | |
| 417 | |
| 418 @node Function Names | |
| 419 @section Naming a Function | |
| 420 @cindex function definition | |
| 421 @cindex named function | |
| 422 @cindex function name | |
| 423 | |
| 424 In most computer languages, every function has a name; the idea of a | |
| 425 function without a name is nonsensical. In Lisp, a function in the | |
| 426 strictest sense has no name. It is simply a list whose first element is | |
| 427 @code{lambda}, or a primitive subr-object. | |
| 428 | |
| 429 However, a symbol can serve as the name of a function. This happens | |
| 430 when you put the function in the symbol's @dfn{function cell} | |
| 431 (@pxref{Symbol Components}). Then the symbol itself becomes a valid, | |
| 432 callable function, equivalent to the list or subr-object that its | |
| 433 function cell refers to. The contents of the function cell are also | |
| 434 called the symbol's @dfn{function definition}. The procedure of using a | |
| 435 symbol's function definition in place of the symbol is called | |
| 436 @dfn{symbol function indirection}; see @ref{Function Indirection}. | |
| 437 | |
| 438 In practice, nearly all functions are given names in this way and | |
| 439 referred to through their names. For example, the symbol @code{car} works | |
| 440 as a function and does what it does because the primitive subr-object | |
| 441 @code{#<subr car>} is stored in its function cell. | |
| 442 | |
| 443 We give functions names because it is convenient to refer to them by | |
| 444 their names in Lisp expressions. For primitive subr-objects such as | |
| 445 @code{#<subr car>}, names are the only way you can refer to them: there | |
| 446 is no read syntax for such objects. For functions written in Lisp, the | |
| 447 name is more convenient to use in a call than an explicit lambda | |
| 448 expression. Also, a function with a name can refer to itself---it can | |
| 449 be recursive. Writing the function's name in its own definition is much | |
| 450 more convenient than making the function definition point to itself | |
| 451 (something that is not impossible but that has various disadvantages in | |
| 452 practice). | |
| 453 | |
| 454 We often identify functions with the symbols used to name them. For | |
| 455 example, we often speak of ``the function @code{car}'', not | |
| 456 distinguishing between the symbol @code{car} and the primitive | |
| 457 subr-object that is its function definition. For most purposes, there | |
| 458 is no need to distinguish. | |
| 459 | |
| 460 Even so, keep in mind that a function need not have a unique name. While | |
| 461 a given function object @emph{usually} appears in the function cell of only | |
| 462 one symbol, this is just a matter of convenience. It is easy to store | |
| 463 it in several symbols using @code{fset}; then each of the symbols is | |
| 464 equally well a name for the same function. | |
| 465 | |
| 466 A symbol used as a function name may also be used as a variable; | |
| 467 these two uses of a symbol are independent and do not conflict. | |
| 468 | |
| 469 @node Defining Functions | |
| 470 @section Defining Functions | |
| 471 @cindex defining a function | |
| 472 | |
| 473 We usually give a name to a function when it is first created. This | |
| 474 is called @dfn{defining a function}, and it is done with the | |
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475 @code{defun} special operator. |
| 428 | 476 |
| 477 @defspec defun name argument-list body-forms | |
| 478 @code{defun} is the usual way to define new Lisp functions. It | |
| 479 defines the symbol @var{name} as a function that looks like this: | |
| 480 | |
| 481 @example | |
| 482 (lambda @var{argument-list} . @var{body-forms}) | |
| 483 @end example | |
| 484 | |
| 485 @code{defun} stores this lambda expression in the function cell of | |
| 486 @var{name}. It returns the value @var{name}, but usually we ignore this | |
| 487 value. | |
| 488 | |
| 489 As described previously (@pxref{Lambda Expressions}), | |
| 490 @var{argument-list} is a list of argument names and may include the | |
| 491 keywords @code{&optional} and @code{&rest}. Also, the first two forms | |
| 492 in @var{body-forms} may be a documentation string and an interactive | |
| 493 declaration. | |
| 494 | |
| 495 There is no conflict if the same symbol @var{name} is also used as a | |
| 496 variable, since the symbol's value cell is independent of the function | |
| 497 cell. @xref{Symbol Components}. | |
| 498 | |
| 499 Here are some examples: | |
| 500 | |
| 501 @example | |
| 502 @group | |
| 503 (defun foo () 5) | |
| 504 @result{} foo | |
| 505 @end group | |
| 506 @group | |
| 507 (foo) | |
| 508 @result{} 5 | |
| 509 @end group | |
| 510 | |
| 511 @group | |
| 512 (defun bar (a &optional b &rest c) | |
| 513 (list a b c)) | |
| 514 @result{} bar | |
| 515 @end group | |
| 516 @group | |
| 517 (bar 1 2 3 4 5) | |
| 518 @result{} (1 2 (3 4 5)) | |
| 519 @end group | |
| 520 @group | |
| 521 (bar 1) | |
| 522 @result{} (1 nil nil) | |
| 523 @end group | |
| 524 @group | |
| 525 (bar) | |
| 526 @error{} Wrong number of arguments. | |
| 527 @end group | |
| 528 | |
| 529 @group | |
| 530 (defun capitalize-backwards () | |
| 531 "Upcase the last letter of a word." | |
| 532 (interactive) | |
| 533 (backward-word 1) | |
| 534 (forward-word 1) | |
| 535 (backward-char 1) | |
| 536 (capitalize-word 1)) | |
| 537 @result{} capitalize-backwards | |
| 538 @end group | |
| 539 @end example | |
| 540 | |
| 541 Be careful not to redefine existing functions unintentionally. | |
| 542 @code{defun} redefines even primitive functions such as @code{car} | |
| 543 without any hesitation or notification. Redefining a function already | |
| 544 defined is often done deliberately, and there is no way to distinguish | |
| 545 deliberate redefinition from unintentional redefinition. | |
| 546 @end defspec | |
| 547 | |
| 548 @defun define-function name definition | |
| 549 @defunx defalias name definition | |
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550 These equivalent primitives define the symbol @var{name} as a |
| 428 | 551 function, with definition @var{definition} (which can be any valid Lisp |
| 552 function). | |
| 553 | |
| 554 The proper place to use @code{define-function} or @code{defalias} is | |
| 555 where a specific function name is being defined---especially where that | |
| 556 name appears explicitly in the source file being loaded. This is | |
| 557 because @code{define-function} and @code{defalias} record which file | |
| 558 defined the function, just like @code{defun}. | |
| 559 (@pxref{Unloading}). | |
| 560 | |
| 561 By contrast, in programs that manipulate function definitions for other | |
| 562 purposes, it is better to use @code{fset}, which does not keep such | |
| 563 records. | |
| 564 @end defun | |
| 565 | |
| 566 See also @code{defsubst}, which defines a function like @code{defun} | |
| 567 and tells the Lisp compiler to open-code it. @xref{Inline Functions}. | |
| 568 | |
| 569 @node Calling Functions | |
| 570 @section Calling Functions | |
| 571 @cindex function invocation | |
| 572 @cindex calling a function | |
| 573 | |
| 574 Defining functions is only half the battle. Functions don't do | |
| 575 anything until you @dfn{call} them, i.e., tell them to run. Calling a | |
| 576 function is also known as @dfn{invocation}. | |
| 577 | |
| 578 The most common way of invoking a function is by evaluating a list. | |
| 579 For example, evaluating the list @code{(concat "a" "b")} calls the | |
| 580 function @code{concat} with arguments @code{"a"} and @code{"b"}. | |
| 581 @xref{Evaluation}, for a description of evaluation. | |
| 582 | |
| 583 When you write a list as an expression in your program, the function | |
| 584 name is part of the program. This means that you choose which function | |
| 585 to call, and how many arguments to give it, when you write the program. | |
| 586 Usually that's just what you want. Occasionally you need to decide at | |
| 587 run time which function to call. To do that, use the functions | |
| 588 @code{funcall} and @code{apply}. | |
| 589 | |
| 590 @defun funcall function &rest arguments | |
| 591 @code{funcall} calls @var{function} with @var{arguments}, and returns | |
| 592 whatever @var{function} returns. | |
| 593 | |
| 594 Since @code{funcall} is a function, all of its arguments, including | |
| 595 @var{function}, are evaluated before @code{funcall} is called. This | |
| 596 means that you can use any expression to obtain the function to be | |
| 597 called. It also means that @code{funcall} does not see the expressions | |
| 598 you write for the @var{arguments}, only their values. These values are | |
| 599 @emph{not} evaluated a second time in the act of calling @var{function}; | |
| 600 @code{funcall} enters the normal procedure for calling a function at the | |
| 601 place where the arguments have already been evaluated. | |
| 602 | |
| 603 The argument @var{function} must be either a Lisp function or a | |
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604 primitive function. Special operators and macros are not allowed, because |
| 428 | 605 they make sense only when given the ``unevaluated'' argument |
| 606 expressions. @code{funcall} cannot provide these because, as we saw | |
| 607 above, it never knows them in the first place. | |
| 608 | |
| 609 @example | |
| 610 @group | |
| 611 (setq f 'list) | |
| 612 @result{} list | |
| 613 @end group | |
| 614 @group | |
| 615 (funcall f 'x 'y 'z) | |
| 616 @result{} (x y z) | |
| 617 @end group | |
| 618 @group | |
| 619 (funcall f 'x 'y '(z)) | |
| 620 @result{} (x y (z)) | |
| 621 @end group | |
| 622 @group | |
| 623 (funcall 'and t nil) | |
| 624 @error{} Invalid function: #<subr and> | |
| 625 @end group | |
| 626 @end example | |
| 627 | |
| 628 Compare these example with the examples of @code{apply}. | |
| 629 @end defun | |
| 630 | |
| 631 @defun apply function &rest arguments | |
| 632 @code{apply} calls @var{function} with @var{arguments}, just like | |
| 633 @code{funcall} but with one difference: the last of @var{arguments} is a | |
| 634 list of arguments to give to @var{function}, rather than a single | |
| 635 argument. We also say that @code{apply} @dfn{spreads} this list so that | |
| 636 each individual element becomes an argument. | |
| 637 | |
| 638 @code{apply} returns the result of calling @var{function}. As with | |
| 639 @code{funcall}, @var{function} must either be a Lisp function or a | |
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640 primitive function; special operators and macros do not make sense in |
| 428 | 641 @code{apply}. |
| 642 | |
| 643 @example | |
| 644 @group | |
| 645 (setq f 'list) | |
| 646 @result{} list | |
| 647 @end group | |
| 648 @group | |
| 649 (apply f 'x 'y 'z) | |
| 650 @error{} Wrong type argument: listp, z | |
| 651 @end group | |
| 652 @group | |
| 653 (apply '+ 1 2 '(3 4)) | |
| 654 @result{} 10 | |
| 655 @end group | |
| 656 @group | |
| 657 (apply '+ '(1 2 3 4)) | |
| 658 @result{} 10 | |
| 659 @end group | |
| 660 | |
| 661 @group | |
| 662 (apply 'append '((a b c) nil (x y z) nil)) | |
| 663 @result{} (a b c x y z) | |
| 664 @end group | |
| 665 @end example | |
| 666 | |
| 667 For an interesting example of using @code{apply}, see the description of | |
| 668 @code{mapcar}, in @ref{Mapping Functions}. | |
| 669 @end defun | |
| 670 | |
| 671 @cindex functionals | |
| 672 It is common for Lisp functions to accept functions as arguments or | |
| 673 find them in data structures (especially in hook variables and property | |
| 674 lists) and call them using @code{funcall} or @code{apply}. Functions | |
| 675 that accept function arguments are often called @dfn{functionals}. | |
| 676 | |
| 677 Sometimes, when you call a functional, it is useful to supply a no-op | |
| 678 function as the argument. Here are two different kinds of no-op | |
| 679 function: | |
| 680 | |
| 681 @defun identity arg | |
| 682 This function returns @var{arg} and has no side effects. | |
| 683 @end defun | |
| 684 | |
| 444 | 685 @deffn Command ignore &rest args |
| 428 | 686 This function ignores any arguments and returns @code{nil}. |
| 444 | 687 @end deffn |
| 428 | 688 |
| 689 @node Mapping Functions | |
| 690 @section Mapping Functions | |
| 691 @cindex mapping functions | |
| 692 | |
| 693 A @dfn{mapping function} applies a given function to each element of a | |
| 434 | 694 list or other collection. XEmacs Lisp has several such functions; |
| 428 | 695 @code{mapcar} and @code{mapconcat}, which scan a list, are described |
| 434 | 696 here. @xref{Creating Symbols}, for the function @code{mapatoms} which |
| 697 maps over the symbols in an obarray. | |
| 698 | |
| 699 Mapping functions should never modify the sequence being mapped over. | |
| 700 The results are unpredictable. | |
| 428 | 701 |
| 702 @defun mapcar function sequence | |
| 703 @code{mapcar} applies @var{function} to each element of @var{sequence} | |
| 704 in turn, and returns a list of the results. | |
| 705 | |
| 434 | 706 The argument @var{sequence} can be any kind of sequence; that is, a |
| 707 list, a vector, a bit vector, or a string. The result is always a list. | |
| 708 The length of the result is the same as the length of @var{sequence}. | |
| 428 | 709 |
| 710 @smallexample | |
| 711 @group | |
| 712 @exdent @r{For example:} | |
| 713 | |
| 714 (mapcar 'car '((a b) (c d) (e f))) | |
| 715 @result{} (a c e) | |
| 716 (mapcar '1+ [1 2 3]) | |
| 717 @result{} (2 3 4) | |
| 718 (mapcar 'char-to-string "abc") | |
| 719 @result{} ("a" "b" "c") | |
| 720 @end group | |
| 721 | |
| 722 @group | |
| 723 ;; @r{Call each function in @code{my-hooks}.} | |
| 724 (mapcar 'funcall my-hooks) | |
| 725 @end group | |
| 726 | |
| 727 @group | |
| 728 (defun mapcar* (f &rest args) | |
| 729 "Apply FUNCTION to successive cars of all ARGS. | |
| 730 Return the list of results." | |
| 731 ;; @r{If no list is exhausted,} | |
| 444 | 732 (if (not (memq 'nil args)) |
| 428 | 733 ;; @r{apply function to @sc{car}s.} |
| 444 | 734 (cons (apply f (mapcar 'car args)) |
| 735 (apply 'mapcar* f | |
| 428 | 736 ;; @r{Recurse for rest of elements.} |
| 737 (mapcar 'cdr args))))) | |
| 738 @end group | |
| 739 | |
| 740 @group | |
| 741 (mapcar* 'cons '(a b c) '(1 2 3 4)) | |
| 742 @result{} ((a . 1) (b . 2) (c . 3)) | |
| 743 @end group | |
| 744 @end smallexample | |
| 745 @end defun | |
| 746 | |
| 747 @defun mapconcat function sequence separator | |
| 748 @code{mapconcat} applies @var{function} to each element of | |
| 749 @var{sequence}: the results, which must be strings, are concatenated. | |
| 750 Between each pair of result strings, @code{mapconcat} inserts the string | |
| 751 @var{separator}. Usually @var{separator} contains a space or comma or | |
| 752 other suitable punctuation. | |
| 753 | |
| 754 The argument @var{function} must be a function that can take one | |
| 434 | 755 argument and return a string. The argument @var{sequence} can be any |
| 756 kind of sequence; that is, a list, a vector, a bit vector, or a string. | |
| 444 | 757 |
| 428 | 758 @smallexample |
| 759 @group | |
| 760 (mapconcat 'symbol-name | |
| 761 '(The cat in the hat) | |
| 762 " ") | |
| 763 @result{} "The cat in the hat" | |
| 764 @end group | |
| 765 | |
| 766 @group | |
| 767 (mapconcat (function (lambda (x) (format "%c" (1+ x)))) | |
| 768 "HAL-8000" | |
| 769 "") | |
| 770 @result{} "IBM.9111" | |
| 771 @end group | |
| 772 @end smallexample | |
| 773 @end defun | |
| 774 | |
| 775 @node Anonymous Functions | |
| 776 @section Anonymous Functions | |
| 777 @cindex anonymous function | |
| 778 | |
| 779 In Lisp, a function is a list that starts with @code{lambda}, a | |
| 780 byte-code function compiled from such a list, or alternatively a | |
| 781 primitive subr-object; names are ``extra''. Although usually functions | |
| 782 are defined with @code{defun} and given names at the same time, it is | |
| 783 occasionally more concise to use an explicit lambda expression---an | |
| 784 anonymous function. Such a list is valid wherever a function name is. | |
| 785 | |
| 786 Any method of creating such a list makes a valid function. Even this: | |
| 787 | |
| 788 @smallexample | |
| 789 @group | |
| 790 (setq silly (append '(lambda (x)) (list (list '+ (* 3 4) 'x)))) | |
| 791 @result{} (lambda (x) (+ 12 x)) | |
| 792 @end group | |
| 793 @end smallexample | |
| 794 | |
| 795 @noindent | |
| 796 This computes a list that looks like @code{(lambda (x) (+ 12 x))} and | |
| 797 makes it the value (@emph{not} the function definition!) of | |
| 798 @code{silly}. | |
| 799 | |
| 800 Here is how we might call this function: | |
| 801 | |
| 802 @example | |
| 803 @group | |
| 804 (funcall silly 1) | |
| 805 @result{} 13 | |
| 806 @end group | |
| 807 @end example | |
| 808 | |
| 809 @noindent | |
| 810 (It does @emph{not} work to write @code{(silly 1)}, because this function | |
| 811 is not the @emph{function definition} of @code{silly}. We have not given | |
| 812 @code{silly} any function definition, just a value as a variable.) | |
| 813 | |
| 814 Most of the time, anonymous functions are constants that appear in | |
| 815 your program. For example, you might want to pass one as an argument | |
| 816 to the function @code{mapcar}, which applies any given function to each | |
| 817 element of a list. Here we pass an anonymous function that multiplies | |
| 818 a number by two: | |
| 819 | |
| 820 @example | |
| 821 @group | |
| 822 (defun double-each (list) | |
| 823 (mapcar '(lambda (x) (* 2 x)) list)) | |
| 824 @result{} double-each | |
| 825 @end group | |
| 826 @group | |
| 827 (double-each '(2 11)) | |
| 828 @result{} (4 22) | |
| 829 @end group | |
| 830 @end example | |
| 831 | |
| 832 @noindent | |
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833 In such cases, we usually use the special operator @code{function} instead |
| 428 | 834 of simple quotation to quote the anonymous function. |
| 835 | |
| 836 @defspec function function-object | |
| 837 @cindex function quoting | |
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838 This special operator returns @var{function-object} without evaluating it. |
| 428 | 839 In this, it is equivalent to @code{quote}. However, it serves as a |
| 840 note to the XEmacs Lisp compiler that @var{function-object} is intended | |
| 841 to be used only as a function, and therefore can safely be compiled. | |
| 842 Contrast this with @code{quote}, in @ref{Quoting}. | |
| 843 @end defspec | |
| 844 | |
| 845 Using @code{function} instead of @code{quote} makes a difference | |
| 846 inside a function or macro that you are going to compile. For example: | |
| 847 | |
| 848 @example | |
| 849 @group | |
| 850 (defun double-each (list) | |
| 851 (mapcar (function (lambda (x) (* 2 x))) list)) | |
| 852 @result{} double-each | |
| 853 @end group | |
| 854 @group | |
| 855 (double-each '(2 11)) | |
| 856 @result{} (4 22) | |
| 857 @end group | |
| 858 @end example | |
| 859 | |
| 860 @noindent | |
| 861 If this definition of @code{double-each} is compiled, the anonymous | |
| 862 function is compiled as well. By contrast, in the previous definition | |
| 863 where ordinary @code{quote} is used, the argument passed to | |
| 864 @code{mapcar} is the precise list shown: | |
| 865 | |
| 866 @example | |
| 867 (lambda (x) (* x 2)) | |
| 868 @end example | |
| 869 | |
| 870 @noindent | |
| 871 The Lisp compiler cannot assume this list is a function, even though it | |
| 872 looks like one, since it does not know what @code{mapcar} does with the | |
| 873 list. Perhaps @code{mapcar} will check that the @sc{car} of the third | |
| 874 element is the symbol @code{*}! The advantage of @code{function} is | |
| 875 that it tells the compiler to go ahead and compile the constant | |
| 876 function. | |
| 877 | |
| 878 We sometimes write @code{function} instead of @code{quote} when | |
| 879 quoting the name of a function, but this usage is just a sort of | |
| 880 comment. | |
| 881 | |
| 882 @example | |
| 883 (function @var{symbol}) @equiv{} (quote @var{symbol}) @equiv{} '@var{symbol} | |
| 884 @end example | |
| 885 | |
| 886 See @code{documentation} in @ref{Accessing Documentation}, for a | |
| 887 realistic example using @code{function} and an anonymous function. | |
| 888 | |
| 889 @node Function Cells | |
| 890 @section Accessing Function Cell Contents | |
| 891 | |
| 892 The @dfn{function definition} of a symbol is the object stored in the | |
| 893 function cell of the symbol. The functions described here access, test, | |
| 894 and set the function cell of symbols. | |
| 895 | |
| 896 See also the function @code{indirect-function} in @ref{Function | |
| 897 Indirection}. | |
| 898 | |
| 899 @defun symbol-function symbol | |
| 900 @kindex void-function | |
| 901 This returns the object in the function cell of @var{symbol}. If the | |
| 902 symbol's function cell is void, a @code{void-function} error is | |
| 903 signaled. | |
| 904 | |
| 905 This function does not check that the returned object is a legitimate | |
| 906 function. | |
| 907 | |
| 908 @example | |
| 909 @group | |
| 910 (defun bar (n) (+ n 2)) | |
| 911 @result{} bar | |
| 912 @end group | |
| 913 @group | |
| 914 (symbol-function 'bar) | |
| 915 @result{} (lambda (n) (+ n 2)) | |
| 916 @end group | |
| 917 @group | |
| 918 (fset 'baz 'bar) | |
| 919 @result{} bar | |
| 920 @end group | |
| 921 @group | |
| 922 (symbol-function 'baz) | |
| 923 @result{} bar | |
| 924 @end group | |
| 925 @end example | |
| 926 @end defun | |
| 927 | |
| 928 @cindex void function cell | |
| 929 If you have never given a symbol any function definition, we say that | |
| 930 that symbol's function cell is @dfn{void}. In other words, the function | |
| 931 cell does not have any Lisp object in it. If you try to call such a symbol | |
| 932 as a function, it signals a @code{void-function} error. | |
| 933 | |
| 934 Note that void is not the same as @code{nil} or the symbol | |
| 935 @code{void}. The symbols @code{nil} and @code{void} are Lisp objects, | |
| 936 and can be stored into a function cell just as any other object can be | |
| 937 (and they can be valid functions if you define them in turn with | |
| 938 @code{defun}). A void function cell contains no object whatsoever. | |
| 939 | |
| 940 You can test the voidness of a symbol's function definition with | |
| 941 @code{fboundp}. After you have given a symbol a function definition, you | |
| 942 can make it void once more using @code{fmakunbound}. | |
| 943 | |
| 944 @defun fboundp symbol | |
| 444 | 945 This function returns @code{t} if @var{symbol} has an object in its |
| 428 | 946 function cell, @code{nil} otherwise. It does not check that the object |
| 947 is a legitimate function. | |
| 948 @end defun | |
| 949 | |
| 950 @defun fmakunbound symbol | |
| 951 This function makes @var{symbol}'s function cell void, so that a | |
| 952 subsequent attempt to access this cell will cause a @code{void-function} | |
| 953 error. (See also @code{makunbound}, in @ref{Local Variables}.) | |
| 954 | |
| 955 @example | |
| 956 @group | |
| 957 (defun foo (x) x) | |
| 958 @result{} x | |
| 959 @end group | |
| 960 @group | |
| 961 (foo 1) | |
| 962 @result{}1 | |
| 963 @end group | |
| 964 @group | |
| 965 (fmakunbound 'foo) | |
| 966 @result{} x | |
| 967 @end group | |
| 968 @group | |
| 969 (foo 1) | |
| 970 @error{} Symbol's function definition is void: foo | |
| 971 @end group | |
| 972 @end example | |
| 973 @end defun | |
| 974 | |
| 975 @defun fset symbol object | |
| 976 This function stores @var{object} in the function cell of @var{symbol}. | |
| 977 The result is @var{object}. Normally @var{object} should be a function | |
| 978 or the name of a function, but this is not checked. | |
| 979 | |
| 980 There are three normal uses of this function: | |
| 981 | |
| 982 @itemize @bullet | |
| 983 @item | |
| 984 Copying one symbol's function definition to another. (In other words, | |
| 985 making an alternate name for a function.) | |
| 986 | |
| 987 @item | |
| 988 Giving a symbol a function definition that is not a list and therefore | |
| 989 cannot be made with @code{defun}. For example, you can use @code{fset} | |
| 444 | 990 to give a symbol @var{symbol1} a function definition which is another symbol |
| 991 @var{symbol2}; then @var{symbol1} serves as an alias for whatever definition | |
| 992 @var{symbol2} presently has. | |
| 428 | 993 |
| 994 @item | |
| 995 In constructs for defining or altering functions. If @code{defun} | |
| 996 were not a primitive, it could be written in Lisp (as a macro) using | |
| 997 @code{fset}. | |
| 998 @end itemize | |
| 999 | |
| 1000 Here are examples of the first two uses: | |
| 1001 | |
| 1002 @example | |
| 1003 @group | |
| 1004 ;; @r{Give @code{first} the same definition @code{car} has.} | |
| 1005 (fset 'first (symbol-function 'car)) | |
| 1006 @result{} #<subr car> | |
| 1007 @end group | |
| 1008 @group | |
| 1009 (first '(1 2 3)) | |
| 1010 @result{} 1 | |
| 1011 @end group | |
| 1012 | |
| 1013 @group | |
| 1014 ;; @r{Make the symbol @code{car} the function definition of @code{xfirst}.} | |
| 1015 (fset 'xfirst 'car) | |
| 1016 @result{} car | |
| 1017 @end group | |
| 1018 @group | |
| 1019 (xfirst '(1 2 3)) | |
| 1020 @result{} 1 | |
| 1021 @end group | |
| 1022 @group | |
| 1023 (symbol-function 'xfirst) | |
| 1024 @result{} car | |
| 1025 @end group | |
| 1026 @group | |
| 1027 (symbol-function (symbol-function 'xfirst)) | |
| 1028 @result{} #<subr car> | |
| 1029 @end group | |
| 1030 | |
| 1031 @group | |
| 1032 ;; @r{Define a named keyboard macro.} | |
| 1033 (fset 'kill-two-lines "\^u2\^k") | |
| 1034 @result{} "\^u2\^k" | |
| 1035 @end group | |
| 1036 @end example | |
| 1037 | |
| 1038 See also the related functions @code{define-function} and | |
| 1039 @code{defalias}, in @ref{Defining Functions}. | |
| 1040 @end defun | |
| 1041 | |
| 1042 When writing a function that extends a previously defined function, | |
| 1043 the following idiom is sometimes used: | |
| 1044 | |
| 1045 @example | |
| 1046 (fset 'old-foo (symbol-function 'foo)) | |
| 1047 (defun foo () | |
| 1048 "Just like old-foo, except more so." | |
| 1049 @group | |
| 1050 (old-foo) | |
| 1051 (more-so)) | |
| 1052 @end group | |
| 1053 @end example | |
| 1054 | |
| 1055 @noindent | |
| 1056 This does not work properly if @code{foo} has been defined to autoload. | |
| 1057 In such a case, when @code{foo} calls @code{old-foo}, Lisp attempts | |
| 1058 to define @code{old-foo} by loading a file. Since this presumably | |
| 1059 defines @code{foo} rather than @code{old-foo}, it does not produce the | |
| 1060 proper results. The only way to avoid this problem is to make sure the | |
| 1061 file is loaded before moving aside the old definition of @code{foo}. | |
| 1062 | |
| 1063 But it is unmodular and unclean, in any case, for a Lisp file to | |
| 1064 redefine a function defined elsewhere. | |
| 1065 | |
| 1066 @node Inline Functions | |
| 1067 @section Inline Functions | |
| 1068 @cindex inline functions | |
| 1069 | |
| 1070 @findex defsubst | |
| 1071 You can define an @dfn{inline function} by using @code{defsubst} instead | |
| 1072 of @code{defun}. An inline function works just like an ordinary | |
| 1073 function except for one thing: when you compile a call to the function, | |
| 1074 the function's definition is open-coded into the caller. | |
| 1075 | |
| 1076 Making a function inline makes explicit calls run faster. But it also | |
| 1077 has disadvantages. For one thing, it reduces flexibility; if you change | |
| 1078 the definition of the function, calls already inlined still use the old | |
| 1079 definition until you recompile them. Since the flexibility of | |
| 1080 redefining functions is an important feature of XEmacs, you should not | |
| 1081 make a function inline unless its speed is really crucial. | |
| 1082 | |
| 1083 Another disadvantage is that making a large function inline can increase | |
| 1084 the size of compiled code both in files and in memory. Since the speed | |
| 1085 advantage of inline functions is greatest for small functions, you | |
| 1086 generally should not make large functions inline. | |
| 1087 | |
| 1088 It's possible to define a macro to expand into the same code that an | |
| 1089 inline function would execute. But the macro would have a limitation: | |
| 1090 you can use it only explicitly---a macro cannot be called with | |
| 1091 @code{apply}, @code{mapcar} and so on. Also, it takes some work to | |
| 1092 convert an ordinary function into a macro. (@xref{Macros}.) To convert | |
| 1093 it into an inline function is very easy; simply replace @code{defun} | |
| 1094 with @code{defsubst}. Since each argument of an inline function is | |
| 1095 evaluated exactly once, you needn't worry about how many times the | |
| 1096 body uses the arguments, as you do for macros. (@xref{Argument | |
| 1097 Evaluation}.) | |
| 1098 | |
| 1099 Inline functions can be used and open-coded later on in the same file, | |
| 1100 following the definition, just like macros. | |
| 1101 | |
| 1102 @c Emacs versions prior to 19 did not have inline functions. | |
| 1103 | |
| 1104 @node Related Topics | |
| 1105 @section Other Topics Related to Functions | |
| 1106 | |
| 1107 Here is a table of several functions that do things related to | |
| 1108 function calling and function definitions. They are documented | |
| 1109 elsewhere, but we provide cross references here. | |
| 1110 | |
| 1111 @table @code | |
| 1112 @item apply | |
| 1113 See @ref{Calling Functions}. | |
| 1114 | |
| 1115 @item autoload | |
| 1116 See @ref{Autoload}. | |
| 1117 | |
| 1118 @item call-interactively | |
| 1119 See @ref{Interactive Call}. | |
| 1120 | |
| 1121 @item commandp | |
| 1122 See @ref{Interactive Call}. | |
| 1123 | |
| 1124 @item documentation | |
| 1125 See @ref{Accessing Documentation}. | |
| 1126 | |
| 1127 @item eval | |
| 1128 See @ref{Eval}. | |
| 1129 | |
| 1130 @item funcall | |
| 1131 See @ref{Calling Functions}. | |
| 1132 | |
| 1133 @item ignore | |
| 1134 See @ref{Calling Functions}. | |
| 1135 | |
| 1136 @item indirect-function | |
| 1137 See @ref{Function Indirection}. | |
| 1138 | |
| 1139 @item interactive | |
| 1140 See @ref{Using Interactive}. | |
| 1141 | |
| 1142 @item interactive-p | |
| 1143 See @ref{Interactive Call}. | |
| 1144 | |
| 1145 @item mapatoms | |
| 1146 See @ref{Creating Symbols}. | |
| 1147 | |
| 1148 @item mapcar | |
| 1149 See @ref{Mapping Functions}. | |
| 1150 | |
| 1151 @item mapconcat | |
| 1152 See @ref{Mapping Functions}. | |
| 1153 | |
| 1154 @item undefined | |
| 1155 See @ref{Key Lookup}. | |
| 1156 @end table | |
| 1157 |