Mercurial > hg > xemacs-beta
annotate man/lispref/variables.texi @ 4623:a9f83990e6bf
Fix a byte compiler bug with characters above ?\xFF.
lisp/ChangeLog addition:
2009-02-22 Aidan Kehoe <kehoea@parhasard.net>
* bytecomp.el (byte-compile-force-escape-quoted): New variable,
used to force `byte-compile-insert-header' to treat the output as
having characters above ?\xFF.
(byte-compile-from-buffer):
If the compiled output contains characters above ?\xFF, and
byte-compile-dynamic-docstrings or byte-compile-dynamic is non-nil
(or we're using an inappropriate coding system) recompile the
file, turning off the dynamic features and using a more
appropriate header.
(byte-compile-insert-header): Pay attention to
byte-compile-force-escape-quoted.
tests/ChangeLog addition:
2009-02-22 Aidan Kehoe <kehoea@parhasard.net>
* automated/mule-tests.el:
Use more realistic tests for the escape-quoted mule encoding
checks; update a comment, change a Known-Bug-Expect-Failure to a
normal test now that we've addressed an old bug.
| author | Aidan Kehoe <kehoea@parhasard.net> |
|---|---|
| date | Sun, 22 Feb 2009 19:57:28 +0000 |
| parents | 57db42ba54fb |
| children | 755ae5b97edb |
| 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/variables.info | |
| 2492 | 6 @node Variables, Functions and Commands, Control Structures, Top |
| 428 | 7 @chapter Variables |
| 8 @cindex variable | |
| 9 | |
| 10 A @dfn{variable} is a name used in a program to stand for a value. | |
| 11 Nearly all programming languages have variables of some sort. In the | |
| 12 text of a Lisp program, variables are written using the syntax for | |
| 13 symbols. | |
| 14 | |
| 15 In Lisp, unlike most programming languages, programs are represented | |
| 16 primarily as Lisp objects and only secondarily as text. The Lisp | |
| 17 objects used for variables are symbols: the symbol name is the variable | |
| 18 name, and the variable's value is stored in the value cell of the | |
| 19 symbol. The use of a symbol as a variable is independent of its use as | |
| 20 a function name. @xref{Symbol Components}. | |
| 21 | |
| 22 The Lisp objects that constitute a Lisp program determine the textual | |
| 23 form of the program---it is simply the read syntax for those Lisp | |
| 24 objects. This is why, for example, a variable in a textual Lisp program | |
| 25 is written using the read syntax for the symbol that represents the | |
| 26 variable. | |
| 27 | |
| 28 @menu | |
| 29 * Global Variables:: Variable values that exist permanently, everywhere. | |
| 30 * Constant Variables:: Certain "variables" have values that never change. | |
| 31 * Local Variables:: Variable values that exist only temporarily. | |
| 32 * Void Variables:: Symbols that lack values. | |
| 33 * Defining Variables:: A definition says a symbol is used as a variable. | |
| 34 * Accessing Variables:: Examining values of variables whose names | |
| 35 are known only at run time. | |
| 36 * Setting Variables:: Storing new values in variables. | |
| 37 * Variable Scoping:: How Lisp chooses among local and global values. | |
| 38 * Buffer-Local Variables:: Variable values in effect only in one buffer. | |
| 39 * Variable Aliases:: Making one variable point to another. | |
| 40 @end menu | |
| 41 | |
| 42 @node Global Variables | |
| 43 @section Global Variables | |
| 44 @cindex global variable | |
| 45 | |
| 46 The simplest way to use a variable is @dfn{globally}. This means that | |
| 47 the variable has just one value at a time, and this value is in effect | |
| 48 (at least for the moment) throughout the Lisp system. The value remains | |
| 49 in effect until you specify a new one. When a new value replaces the | |
| 50 old one, no trace of the old value remains in the variable. | |
| 51 | |
| 52 You specify a value for a symbol with @code{setq}. For example, | |
| 53 | |
| 54 @example | |
| 55 (setq x '(a b)) | |
| 56 @end example | |
| 57 | |
| 58 @noindent | |
| 59 gives the variable @code{x} the value @code{(a b)}. Note that | |
| 60 @code{setq} does not evaluate its first argument, the name of the | |
| 61 variable, but it does evaluate the second argument, the new value. | |
| 62 | |
| 63 Once the variable has a value, you can refer to it by using the symbol | |
| 64 by itself as an expression. Thus, | |
| 65 | |
| 66 @example | |
| 67 @group | |
| 68 x @result{} (a b) | |
| 69 @end group | |
| 70 @end example | |
| 71 | |
| 72 @noindent | |
| 73 assuming the @code{setq} form shown above has already been executed. | |
| 74 | |
| 75 If you do another @code{setq}, the new value replaces the old one: | |
| 76 | |
| 77 @example | |
| 78 @group | |
| 79 x | |
| 80 @result{} (a b) | |
| 81 @end group | |
| 82 @group | |
| 83 (setq x 4) | |
| 84 @result{} 4 | |
| 85 @end group | |
| 86 @group | |
| 87 x | |
| 88 @result{} 4 | |
| 89 @end group | |
| 90 @end example | |
| 91 | |
| 92 @node Constant Variables | |
| 93 @section Variables That Never Change | |
| 94 @vindex nil | |
| 95 @vindex t | |
| 96 @kindex setting-constant | |
| 97 | |
| 98 In XEmacs Lisp, some symbols always evaluate to themselves: the two | |
| 99 special symbols @code{nil} and @code{t}, as well as @dfn{keyword | |
| 100 symbols}, that is, symbols whose name begins with the character | |
| 101 @samp{@code{:}}. These symbols cannot be rebound, nor can their value | |
| 102 cells be changed. An attempt to change the value of @code{nil} or | |
| 103 @code{t} signals a @code{setting-constant} error. | |
| 104 | |
| 105 @example | |
| 106 @group | |
| 107 nil @equiv{} 'nil | |
| 108 @result{} nil | |
| 109 @end group | |
| 110 @group | |
| 111 (setq nil 500) | |
| 112 @error{} Attempt to set constant symbol: nil | |
| 113 @end group | |
| 114 @end example | |
| 115 | |
| 116 @node Local Variables | |
| 117 @section Local Variables | |
| 118 @cindex binding local variables | |
| 119 @cindex local variables | |
| 120 @cindex local binding | |
| 121 @cindex global binding | |
| 122 | |
| 123 Global variables have values that last until explicitly superseded | |
| 124 with new values. Sometimes it is useful to create variable values that | |
| 125 exist temporarily---only while within a certain part of the program. | |
| 126 These values are called @dfn{local}, and the variables so used are | |
| 127 called @dfn{local variables}. | |
| 128 | |
| 129 For example, when a function is called, its argument variables receive | |
| 130 new local values that last until the function exits. The @code{let} | |
| 131 special form explicitly establishes new local values for specified | |
| 132 variables; these last until exit from the @code{let} form. | |
| 133 | |
| 134 @cindex shadowing of variables | |
| 135 Establishing a local value saves away the previous value (or lack of | |
| 136 one) of the variable. When the life span of the local value is over, | |
| 137 the previous value is restored. In the mean time, we say that the | |
| 138 previous value is @dfn{shadowed} and @dfn{not visible}. Both global and | |
| 139 local values may be shadowed (@pxref{Scope}). | |
| 140 | |
| 141 If you set a variable (such as with @code{setq}) while it is local, | |
| 142 this replaces the local value; it does not alter the global value, or | |
| 143 previous local values that are shadowed. To model this behavior, we | |
| 144 speak of a @dfn{local binding} of the variable as well as a local value. | |
| 145 | |
| 146 The local binding is a conceptual place that holds a local value. | |
| 147 Entry to a function, or a special form such as @code{let}, creates the | |
| 148 local binding; exit from the function or from the @code{let} removes the | |
| 149 local binding. As long as the local binding lasts, the variable's value | |
| 150 is stored within it. Use of @code{setq} or @code{set} while there is a | |
| 151 local binding stores a different value into the local binding; it does | |
| 152 not create a new binding. | |
| 153 | |
| 154 We also speak of the @dfn{global binding}, which is where | |
| 155 (conceptually) the global value is kept. | |
| 156 | |
| 157 @cindex current binding | |
| 158 A variable can have more than one local binding at a time (for | |
| 159 example, if there are nested @code{let} forms that bind it). In such a | |
| 160 case, the most recently created local binding that still exists is the | |
| 161 @dfn{current binding} of the variable. (This is called @dfn{dynamic | |
| 162 scoping}; see @ref{Variable Scoping}.) If there are no local bindings, | |
| 163 the variable's global binding is its current binding. We also call the | |
| 164 current binding the @dfn{most-local existing binding}, for emphasis. | |
| 165 Ordinary evaluation of a symbol always returns the value of its current | |
| 166 binding. | |
| 167 | |
| 168 The special forms @code{let} and @code{let*} exist to create | |
| 169 local bindings. | |
| 170 | |
| 171 @defspec let (bindings@dots{}) forms@dots{} | |
| 172 This special form binds variables according to @var{bindings} and then | |
| 173 evaluates all of the @var{forms} in textual order. The @code{let}-form | |
| 174 returns the value of the last form in @var{forms}. | |
| 175 | |
| 176 Each of the @var{bindings} is either @w{(i) a} symbol, in which case | |
| 177 that symbol is bound to @code{nil}; or @w{(ii) a} list of the form | |
| 178 @code{(@var{symbol} @var{value-form})}, in which case @var{symbol} is | |
| 179 bound to the result of evaluating @var{value-form}. If @var{value-form} | |
| 180 is omitted, @code{nil} is used. | |
| 181 | |
| 182 All of the @var{value-form}s in @var{bindings} are evaluated in the | |
| 183 order they appear and @emph{before} any of the symbols are bound. Here | |
| 184 is an example of this: @code{Z} is bound to the old value of @code{Y}, | |
| 185 which is 2, not the new value, 1. | |
| 186 | |
| 187 @example | |
| 188 @group | |
| 189 (setq Y 2) | |
| 190 @result{} 2 | |
| 191 @end group | |
| 192 @group | |
| 444 | 193 (let ((Y 1) |
| 428 | 194 (Z Y)) |
| 195 (list Y Z)) | |
| 196 @result{} (1 2) | |
| 197 @end group | |
| 198 @end example | |
| 199 @end defspec | |
| 200 | |
| 201 @defspec let* (bindings@dots{}) forms@dots{} | |
| 202 This special form is like @code{let}, but it binds each variable right | |
| 203 after computing its local value, before computing the local value for | |
| 204 the next variable. Therefore, an expression in @var{bindings} can | |
| 205 reasonably refer to the preceding symbols bound in this @code{let*} | |
| 206 form. Compare the following example with the example above for | |
| 207 @code{let}. | |
| 208 | |
| 209 @example | |
| 210 @group | |
| 211 (setq Y 2) | |
| 212 @result{} 2 | |
| 213 @end group | |
| 214 @group | |
| 215 (let* ((Y 1) | |
| 216 (Z Y)) ; @r{Use the just-established value of @code{Y}.} | |
| 217 (list Y Z)) | |
| 218 @result{} (1 1) | |
| 219 @end group | |
| 220 @end example | |
| 221 @end defspec | |
| 222 | |
| 223 Here is a complete list of the other facilities that create local | |
| 224 bindings: | |
| 225 | |
| 226 @itemize @bullet | |
| 227 @item | |
| 2492 | 228 Function calls (@pxref{Functions and Commands}). |
| 428 | 229 |
| 230 @item | |
| 231 Macro calls (@pxref{Macros}). | |
| 232 | |
| 233 @item | |
| 234 @code{condition-case} (@pxref{Errors}). | |
| 235 @end itemize | |
| 236 | |
| 237 Variables can also have buffer-local bindings (@pxref{Buffer-Local | |
| 238 Variables}). These kinds of bindings work somewhat like ordinary local | |
| 239 bindings, but they are localized depending on ``where'' you are in | |
| 240 Emacs, rather than localized in time. | |
| 241 | |
| 242 @defvar max-specpdl-size | |
| 243 @cindex variable limit error | |
| 244 @cindex evaluation error | |
| 245 @cindex infinite recursion | |
| 246 This variable defines the limit on the total number of local variable | |
| 247 bindings and @code{unwind-protect} cleanups (@pxref{Nonlocal Exits}) | |
| 248 that are allowed before signaling an error (with data @code{"Variable | |
| 249 binding depth exceeds max-specpdl-size"}). | |
| 250 | |
| 251 This limit, with the associated error when it is exceeded, is one way | |
| 252 that Lisp avoids infinite recursion on an ill-defined function. | |
| 253 | |
| 458 | 254 The default value is 3000. |
| 428 | 255 |
| 256 @code{max-lisp-eval-depth} provides another limit on depth of nesting. | |
| 257 @xref{Eval}. | |
| 258 @end defvar | |
| 259 | |
| 260 @node Void Variables | |
| 261 @section When a Variable is ``Void'' | |
| 262 @kindex void-variable | |
| 263 @cindex void variable | |
| 264 | |
| 265 If you have never given a symbol any value as a global variable, we | |
| 266 say that that symbol's global value is @dfn{void}. In other words, the | |
| 267 symbol's value cell does not have any Lisp object in it. If you try to | |
| 268 evaluate the symbol, you get a @code{void-variable} error rather than | |
| 269 a value. | |
| 270 | |
| 271 Note that a value of @code{nil} is not the same as void. The symbol | |
| 272 @code{nil} is a Lisp object and can be the value of a variable just as any | |
| 273 other object can be; but it is @emph{a value}. A void variable does not | |
| 274 have any value. | |
| 275 | |
| 276 After you have given a variable a value, you can make it void once more | |
| 277 using @code{makunbound}. | |
| 278 | |
| 279 @defun makunbound symbol | |
| 280 This function makes the current binding of @var{symbol} void. | |
| 281 Subsequent attempts to use this symbol's value as a variable will signal | |
| 282 the error @code{void-variable}, unless or until you set it again. | |
| 283 | |
| 284 @code{makunbound} returns @var{symbol}. | |
| 285 | |
| 286 @example | |
| 287 @group | |
| 288 (makunbound 'x) ; @r{Make the global value} | |
| 289 ; @r{of @code{x} void.} | |
| 290 @result{} x | |
| 291 @end group | |
| 292 @group | |
| 293 x | |
| 294 @error{} Symbol's value as variable is void: x | |
| 295 @end group | |
| 296 @end example | |
| 297 | |
| 298 If @var{symbol} is locally bound, @code{makunbound} affects the most | |
| 299 local existing binding. This is the only way a symbol can have a void | |
| 300 local binding, since all the constructs that create local bindings | |
| 301 create them with values. In this case, the voidness lasts at most as | |
| 302 long as the binding does; when the binding is removed due to exit from | |
| 303 the construct that made it, the previous or global binding is reexposed | |
| 304 as usual, and the variable is no longer void unless the newly reexposed | |
| 305 binding was void all along. | |
| 306 | |
| 307 @smallexample | |
| 308 @group | |
| 309 (setq x 1) ; @r{Put a value in the global binding.} | |
| 310 @result{} 1 | |
| 311 (let ((x 2)) ; @r{Locally bind it.} | |
| 312 (makunbound 'x) ; @r{Void the local binding.} | |
| 313 x) | |
| 314 @error{} Symbol's value as variable is void: x | |
| 315 @end group | |
| 316 @group | |
| 317 x ; @r{The global binding is unchanged.} | |
| 318 @result{} 1 | |
| 319 | |
| 320 (let ((x 2)) ; @r{Locally bind it.} | |
| 321 (let ((x 3)) ; @r{And again.} | |
| 322 (makunbound 'x) ; @r{Void the innermost-local binding.} | |
| 323 x)) ; @r{And refer: it's void.} | |
| 324 @error{} Symbol's value as variable is void: x | |
| 325 @end group | |
| 326 | |
| 327 @group | |
| 328 (let ((x 2)) | |
| 329 (let ((x 3)) | |
| 330 (makunbound 'x)) ; @r{Void inner binding, then remove it.} | |
| 331 x) ; @r{Now outer @code{let} binding is visible.} | |
| 332 @result{} 2 | |
| 333 @end group | |
| 334 @end smallexample | |
| 335 @end defun | |
| 336 | |
| 337 A variable that has been made void with @code{makunbound} is | |
| 338 indistinguishable from one that has never received a value and has | |
| 339 always been void. | |
| 340 | |
| 341 You can use the function @code{boundp} to test whether a variable is | |
| 342 currently void. | |
| 343 | |
| 344 @defun boundp variable | |
| 345 @code{boundp} returns @code{t} if @var{variable} (a symbol) is not void; | |
| 346 more precisely, if its current binding is not void. It returns | |
| 347 @code{nil} otherwise. | |
| 348 | |
| 349 @smallexample | |
| 350 @group | |
| 351 (boundp 'abracadabra) ; @r{Starts out void.} | |
| 352 @result{} nil | |
| 353 @end group | |
| 354 @group | |
| 355 (let ((abracadabra 5)) ; @r{Locally bind it.} | |
| 356 (boundp 'abracadabra)) | |
| 357 @result{} t | |
| 358 @end group | |
| 359 @group | |
| 360 (boundp 'abracadabra) ; @r{Still globally void.} | |
| 361 @result{} nil | |
| 362 @end group | |
| 363 @group | |
| 364 (setq abracadabra 5) ; @r{Make it globally nonvoid.} | |
| 365 @result{} 5 | |
| 366 @end group | |
| 367 @group | |
| 368 (boundp 'abracadabra) | |
| 369 @result{} t | |
| 370 @end group | |
| 371 @end smallexample | |
| 372 @end defun | |
| 373 | |
| 374 @node Defining Variables | |
| 375 @section Defining Global Variables | |
| 376 @cindex variable definition | |
| 377 | |
| 378 You may announce your intention to use a symbol as a global variable | |
| 379 with a @dfn{variable definition}: a special form, either @code{defconst} | |
| 380 or @code{defvar}. | |
| 381 | |
| 382 In XEmacs Lisp, definitions serve three purposes. First, they inform | |
| 383 people who read the code that certain symbols are @emph{intended} to be | |
| 384 used a certain way (as variables). Second, they inform the Lisp system | |
| 385 of these things, supplying a value and documentation. Third, they | |
| 386 provide information to utilities such as @code{etags} and | |
| 387 @code{make-docfile}, which create data bases of the functions and | |
| 388 variables in a program. | |
| 389 | |
| 390 The difference between @code{defconst} and @code{defvar} is primarily | |
| 391 a matter of intent, serving to inform human readers of whether programs | |
| 392 will change the variable. XEmacs Lisp does not restrict the ways in | |
| 393 which a variable can be used based on @code{defconst} or @code{defvar} | |
| 394 declarations. However, it does make a difference for initialization: | |
| 395 @code{defconst} unconditionally initializes the variable, while | |
| 396 @code{defvar} initializes it only if it is void. | |
| 397 | |
| 398 One would expect user option variables to be defined with | |
| 399 @code{defconst}, since programs do not change them. Unfortunately, this | |
| 400 has bad results if the definition is in a library that is not preloaded: | |
| 401 @code{defconst} would override any prior value when the library is | |
| 402 loaded. Users would like to be able to set user options in their init | |
| 403 files, and override the default values given in the definitions. For | |
| 404 this reason, user options must be defined with @code{defvar}. | |
| 405 | |
| 406 @defspec defvar symbol [value [doc-string]] | |
| 407 This special form defines @var{symbol} as a value and initializes it. | |
| 408 The definition informs a person reading your code that @var{symbol} is | |
| 409 used as a variable that programs are likely to set or change. It is | |
| 410 also used for all user option variables except in the preloaded parts of | |
| 411 XEmacs. Note that @var{symbol} is not evaluated; the symbol to be | |
| 412 defined must appear explicitly in the @code{defvar}. | |
| 413 | |
| 414 If @var{symbol} already has a value (i.e., it is not void), @var{value} | |
| 415 is not even evaluated, and @var{symbol}'s value remains unchanged. If | |
| 416 @var{symbol} is void and @var{value} is specified, @code{defvar} | |
| 417 evaluates it and sets @var{symbol} to the result. (If @var{value} is | |
| 418 omitted, the value of @var{symbol} is not changed in any case.) | |
| 419 | |
| 420 When you evaluate a top-level @code{defvar} form with @kbd{C-M-x} in | |
| 421 Emacs Lisp mode (@code{eval-defun}), a special feature of | |
| 422 @code{eval-defun} evaluates it as a @code{defconst}. The purpose of | |
| 423 this is to make sure the variable's value is reinitialized, when you ask | |
| 424 for it specifically. | |
| 425 | |
| 426 If @var{symbol} has a buffer-local binding in the current buffer, | |
| 427 @code{defvar} sets the default value, not the local value. | |
| 428 @xref{Buffer-Local Variables}. | |
| 429 | |
| 430 If the @var{doc-string} argument appears, it specifies the documentation | |
| 431 for the variable. (This opportunity to specify documentation is one of | |
| 432 the main benefits of defining the variable.) The documentation is | |
| 433 stored in the symbol's @code{variable-documentation} property. The | |
| 434 XEmacs help functions (@pxref{Documentation}) look for this property. | |
| 435 | |
| 436 If the first character of @var{doc-string} is @samp{*}, it means that | |
| 437 this variable is considered a user option. This lets users set the | |
| 438 variable conveniently using the commands @code{set-variable} and | |
| 439 @code{edit-options}. | |
| 440 | |
| 441 For example, this form defines @code{foo} but does not set its value: | |
| 442 | |
| 443 @example | |
| 444 @group | |
| 445 (defvar foo) | |
| 446 @result{} foo | |
| 447 @end group | |
| 448 @end example | |
| 449 | |
| 450 The following example sets the value of @code{bar} to @code{23}, and | |
| 451 gives it a documentation string: | |
| 452 | |
| 453 @example | |
| 454 @group | |
| 455 (defvar bar 23 | |
| 456 "The normal weight of a bar.") | |
| 457 @result{} bar | |
| 458 @end group | |
| 459 @end example | |
| 460 | |
| 461 The following form changes the documentation string for @code{bar}, | |
| 462 making it a user option, but does not change the value, since @code{bar} | |
| 463 already has a value. (The addition @code{(1+ 23)} is not even | |
| 464 performed.) | |
| 465 | |
| 466 @example | |
| 467 @group | |
| 468 (defvar bar (1+ 23) | |
| 469 "*The normal weight of a bar.") | |
| 470 @result{} bar | |
| 471 @end group | |
| 472 @group | |
| 473 bar | |
| 474 @result{} 23 | |
| 475 @end group | |
| 476 @end example | |
| 477 | |
| 478 Here is an equivalent expression for the @code{defvar} special form: | |
| 479 | |
| 480 @example | |
| 481 @group | |
| 482 (defvar @var{symbol} @var{value} @var{doc-string}) | |
| 483 @equiv{} | |
| 484 (progn | |
| 485 (if (not (boundp '@var{symbol})) | |
| 486 (setq @var{symbol} @var{value})) | |
| 487 (put '@var{symbol} 'variable-documentation '@var{doc-string}) | |
| 488 '@var{symbol}) | |
| 489 @end group | |
| 490 @end example | |
| 491 | |
| 492 The @code{defvar} form returns @var{symbol}, but it is normally used | |
| 493 at top level in a file where its value does not matter. | |
| 494 @end defspec | |
| 495 | |
| 496 @defspec defconst symbol [value [doc-string]] | |
| 497 This special form defines @var{symbol} as a value and initializes it. | |
| 498 It informs a person reading your code that @var{symbol} has a global | |
| 499 value, established here, that will not normally be changed or locally | |
| 500 bound by the execution of the program. The user, however, may be | |
| 501 welcome to change it. Note that @var{symbol} is not evaluated; the | |
| 502 symbol to be defined must appear explicitly in the @code{defconst}. | |
| 503 | |
| 504 @code{defconst} always evaluates @var{value} and sets the global value | |
| 505 of @var{symbol} to the result, provided @var{value} is given. If | |
| 506 @var{symbol} has a buffer-local binding in the current buffer, | |
| 507 @code{defconst} sets the default value, not the local value. | |
| 508 | |
| 509 @strong{Please note:} Don't use @code{defconst} for user option | |
| 510 variables in libraries that are not standardly preloaded. The user | |
| 511 should be able to specify a value for such a variable in the | |
| 512 @file{.emacs} file, so that it will be in effect if and when the library | |
| 513 is loaded later. | |
| 514 | |
| 515 Here, @code{pi} is a constant that presumably ought not to be changed | |
| 516 by anyone (attempts by the Indiana State Legislature notwithstanding). | |
| 517 As the second form illustrates, however, this is only advisory. | |
| 518 | |
| 519 @example | |
| 520 @group | |
| 521 (defconst pi 3.1415 "Pi to five places.") | |
| 522 @result{} pi | |
| 523 @end group | |
| 524 @group | |
| 525 (setq pi 3) | |
| 526 @result{} pi | |
| 527 @end group | |
| 528 @group | |
| 529 pi | |
| 530 @result{} 3 | |
| 531 @end group | |
| 532 @end example | |
| 533 @end defspec | |
| 534 | |
| 535 @defun user-variable-p variable | |
| 536 @cindex user option | |
| 537 This function returns @code{t} if @var{variable} is a user option---a | |
| 538 variable intended to be set by the user for customization---and | |
| 539 @code{nil} otherwise. (Variables other than user options exist for the | |
| 540 internal purposes of Lisp programs, and users need not know about them.) | |
| 541 | |
| 542 User option variables are distinguished from other variables by the | |
| 543 first character of the @code{variable-documentation} property. If the | |
| 544 property exists and is a string, and its first character is @samp{*}, | |
| 545 then the variable is a user option. | |
| 546 @end defun | |
| 547 | |
| 548 If a user option variable has a @code{variable-interactive} property, | |
| 549 the @code{set-variable} command uses that value to control reading the | |
| 550 new value for the variable. The property's value is used as if it were | |
| 551 the argument to @code{interactive}. | |
| 552 | |
| 553 @strong{Warning:} If the @code{defconst} and @code{defvar} special | |
| 554 forms are used while the variable has a local binding, they set the | |
| 555 local binding's value; the global binding is not changed. This is not | |
| 556 what we really want. To prevent it, use these special forms at top | |
| 557 level in a file, where normally no local binding is in effect, and make | |
| 558 sure to load the file before making a local binding for the variable. | |
| 559 | |
| 560 @node Accessing Variables | |
| 561 @section Accessing Variable Values | |
| 562 | |
| 563 The usual way to reference a variable is to write the symbol which | |
| 564 names it (@pxref{Symbol Forms}). This requires you to specify the | |
| 565 variable name when you write the program. Usually that is exactly what | |
| 566 you want to do. Occasionally you need to choose at run time which | |
| 567 variable to reference; then you can use @code{symbol-value}. | |
| 568 | |
| 569 @defun symbol-value symbol | |
| 570 This function returns the value of @var{symbol}. This is the value in | |
| 571 the innermost local binding of the symbol, or its global value if it | |
| 572 has no local bindings. | |
| 573 | |
| 574 @example | |
| 575 @group | |
| 576 (setq abracadabra 5) | |
| 577 @result{} 5 | |
| 578 @end group | |
| 579 @group | |
| 580 (setq foo 9) | |
| 581 @result{} 9 | |
| 582 @end group | |
| 583 | |
| 584 @group | |
| 585 ;; @r{Here the symbol @code{abracadabra}} | |
| 586 ;; @r{is the symbol whose value is examined.} | |
| 587 (let ((abracadabra 'foo)) | |
| 588 (symbol-value 'abracadabra)) | |
| 589 @result{} foo | |
| 590 @end group | |
| 591 | |
| 592 @group | |
| 593 ;; @r{Here the value of @code{abracadabra},} | |
| 594 ;; @r{which is @code{foo},} | |
| 595 ;; @r{is the symbol whose value is examined.} | |
| 596 (let ((abracadabra 'foo)) | |
| 597 (symbol-value abracadabra)) | |
| 598 @result{} 9 | |
| 599 @end group | |
| 600 | |
| 601 @group | |
| 602 (symbol-value 'abracadabra) | |
| 603 @result{} 5 | |
| 604 @end group | |
| 605 @end example | |
| 606 | |
| 607 A @code{void-variable} error is signaled if @var{symbol} has neither a | |
| 608 local binding nor a global value. | |
| 609 @end defun | |
| 610 | |
| 611 @node Setting Variables | |
| 612 @section How to Alter a Variable Value | |
| 613 | |
| 614 The usual way to change the value of a variable is with the special | |
| 615 form @code{setq}. When you need to compute the choice of variable at | |
| 616 run time, use the function @code{set}. | |
| 617 | |
| 618 @defspec setq [symbol form]@dots{} | |
| 619 This special form is the most common method of changing a variable's | |
| 620 value. Each @var{symbol} is given a new value, which is the result of | |
| 621 evaluating the corresponding @var{form}. The most-local existing | |
| 622 binding of the symbol is changed. | |
| 623 | |
| 624 @code{setq} does not evaluate @var{symbol}; it sets the symbol that you | |
| 625 write. We say that this argument is @dfn{automatically quoted}. The | |
| 626 @samp{q} in @code{setq} stands for ``quoted.'' | |
| 627 | |
| 628 The value of the @code{setq} form is the value of the last @var{form}. | |
| 629 | |
| 630 @example | |
| 631 @group | |
| 632 (setq x (1+ 2)) | |
| 633 @result{} 3 | |
| 634 @end group | |
| 635 x ; @r{@code{x} now has a global value.} | |
| 636 @result{} 3 | |
| 637 @group | |
| 444 | 638 (let ((x 5)) |
| 428 | 639 (setq x 6) ; @r{The local binding of @code{x} is set.} |
| 640 x) | |
| 641 @result{} 6 | |
| 642 @end group | |
| 643 x ; @r{The global value is unchanged.} | |
| 644 @result{} 3 | |
| 645 @end example | |
| 646 | |
| 647 Note that the first @var{form} is evaluated, then the first | |
| 648 @var{symbol} is set, then the second @var{form} is evaluated, then the | |
| 649 second @var{symbol} is set, and so on: | |
| 650 | |
| 651 @example | |
| 652 @group | |
| 653 (setq x 10 ; @r{Notice that @code{x} is set before} | |
| 654 y (1+ x)) ; @r{the value of @code{y} is computed.} | |
| 444 | 655 @result{} 11 |
| 428 | 656 @end group |
| 657 @end example | |
| 658 @end defspec | |
| 659 | |
| 660 @defun set symbol value | |
| 661 This function sets @var{symbol}'s value to @var{value}, then returns | |
| 662 @var{value}. Since @code{set} is a function, the expression written for | |
| 663 @var{symbol} is evaluated to obtain the symbol to set. | |
| 664 | |
| 665 The most-local existing binding of the variable is the binding that is | |
| 666 set; shadowed bindings are not affected. | |
| 667 | |
| 668 @example | |
| 669 @group | |
| 670 (set one 1) | |
| 671 @error{} Symbol's value as variable is void: one | |
| 672 @end group | |
| 673 @group | |
| 674 (set 'one 1) | |
| 675 @result{} 1 | |
| 676 @end group | |
| 677 @group | |
| 678 (set 'two 'one) | |
| 679 @result{} one | |
| 680 @end group | |
| 681 @group | |
| 682 (set two 2) ; @r{@code{two} evaluates to symbol @code{one}.} | |
| 683 @result{} 2 | |
| 684 @end group | |
| 685 @group | |
| 686 one ; @r{So it is @code{one} that was set.} | |
| 687 @result{} 2 | |
| 688 (let ((one 1)) ; @r{This binding of @code{one} is set,} | |
| 689 (set 'one 3) ; @r{not the global value.} | |
| 690 one) | |
| 691 @result{} 3 | |
| 692 @end group | |
| 693 @group | |
| 694 one | |
| 695 @result{} 2 | |
| 696 @end group | |
| 697 @end example | |
| 698 | |
| 699 If @var{symbol} is not actually a symbol, a @code{wrong-type-argument} | |
| 700 error is signaled. | |
| 701 | |
| 702 @example | |
| 703 (set '(x y) 'z) | |
| 704 @error{} Wrong type argument: symbolp, (x y) | |
| 705 @end example | |
| 706 | |
| 707 Logically speaking, @code{set} is a more fundamental primitive than | |
| 708 @code{setq}. Any use of @code{setq} can be trivially rewritten to use | |
| 709 @code{set}; @code{setq} could even be defined as a macro, given the | |
| 710 availability of @code{set}. However, @code{set} itself is rarely used; | |
| 711 beginners hardly need to know about it. It is useful only for choosing | |
| 712 at run time which variable to set. For example, the command | |
| 713 @code{set-variable}, which reads a variable name from the user and then | |
| 714 sets the variable, needs to use @code{set}. | |
| 715 | |
| 716 @cindex CL note---@code{set} local | |
| 717 @quotation | |
| 718 @b{Common Lisp note:} In Common Lisp, @code{set} always changes the | |
| 719 symbol's special value, ignoring any lexical bindings. In XEmacs Lisp, | |
| 720 all variables and all bindings are (in effect) special, so @code{set} | |
| 721 always affects the most local existing binding. | |
| 722 @end quotation | |
| 723 @end defun | |
| 724 | |
| 725 One other function for setting a variable is designed to add | |
| 726 an element to a list if it is not already present in the list. | |
| 727 | |
| 1024 | 728 @defun add-to-list symbol element &optional append |
| 428 | 729 This function sets the variable @var{symbol} by consing @var{element} |
| 730 onto the old value, if @var{element} is not already a member of that | |
| 731 value. It returns the resulting list, whether updated or not. The | |
| 732 value of @var{symbol} had better be a list already before the call. | |
| 733 | |
| 1024 | 734 If the optional argument @var{append} is non-@code{nil}, @var{element} |
| 735 is added at the end of the list. | |
| 736 | |
| 428 | 737 The argument @var{symbol} is not implicitly quoted; @code{add-to-list} |
| 738 is an ordinary function, like @code{set} and unlike @code{setq}. Quote | |
| 739 the argument yourself if that is what you want. | |
| 740 | |
| 741 Here's a scenario showing how to use @code{add-to-list}: | |
| 742 | |
| 743 @example | |
| 744 (setq foo '(a b)) | |
| 745 @result{} (a b) | |
| 746 | |
| 747 (add-to-list 'foo 'c) ;; @r{Add @code{c}.} | |
| 748 @result{} (c a b) | |
| 749 | |
| 750 (add-to-list 'foo 'b) ;; @r{No effect.} | |
| 751 @result{} (c a b) | |
| 752 | |
| 753 foo ;; @r{@code{foo} was changed.} | |
| 754 @result{} (c a b) | |
| 755 @end example | |
| 756 @end defun | |
| 757 | |
| 758 An equivalent expression for @code{(add-to-list '@var{var} | |
| 759 @var{value})} is this: | |
| 760 | |
| 761 @example | |
| 762 (or (member @var{value} @var{var}) | |
| 763 (setq @var{var} (cons @var{value} @var{var}))) | |
| 764 @end example | |
| 765 | |
| 766 @node Variable Scoping | |
| 767 @section Scoping Rules for Variable Bindings | |
| 768 | |
| 769 A given symbol @code{foo} may have several local variable bindings, | |
| 770 established at different places in the Lisp program, as well as a global | |
| 771 binding. The most recently established binding takes precedence over | |
| 772 the others. | |
| 773 | |
| 774 @cindex scope | |
| 775 @cindex extent | |
| 776 @cindex dynamic scoping | |
| 777 Local bindings in XEmacs Lisp have @dfn{indefinite scope} and | |
| 778 @dfn{dynamic extent}. @dfn{Scope} refers to @emph{where} textually in | |
| 779 the source code the binding can be accessed. Indefinite scope means | |
| 780 that any part of the program can potentially access the variable | |
| 781 binding. @dfn{Extent} refers to @emph{when}, as the program is | |
| 782 executing, the binding exists. Dynamic extent means that the binding | |
| 783 lasts as long as the activation of the construct that established it. | |
| 784 | |
| 785 The combination of dynamic extent and indefinite scope is called | |
| 786 @dfn{dynamic scoping}. By contrast, most programming languages use | |
| 787 @dfn{lexical scoping}, in which references to a local variable must be | |
| 788 located textually within the function or block that binds the variable. | |
| 789 | |
| 790 @cindex CL note---special variables | |
| 791 @quotation | |
| 792 @b{Common Lisp note:} Variables declared ``special'' in Common Lisp | |
| 793 are dynamically scoped, like variables in XEmacs Lisp. | |
| 794 @end quotation | |
| 795 | |
| 796 @menu | |
| 797 * Scope:: Scope means where in the program a value is visible. | |
| 798 Comparison with other languages. | |
| 799 * Extent:: Extent means how long in time a value exists. | |
| 800 * Impl of Scope:: Two ways to implement dynamic scoping. | |
| 801 * Using Scoping:: How to use dynamic scoping carefully and avoid problems. | |
| 802 @end menu | |
| 803 | |
| 804 @node Scope | |
| 805 @subsection Scope | |
| 806 | |
| 807 XEmacs Lisp uses @dfn{indefinite scope} for local variable bindings. | |
| 808 This means that any function anywhere in the program text might access a | |
| 809 given binding of a variable. Consider the following function | |
| 810 definitions: | |
| 811 | |
| 812 @example | |
| 813 @group | |
| 814 (defun binder (x) ; @r{@code{x} is bound in @code{binder}.} | |
| 815 (foo 5)) ; @r{@code{foo} is some other function.} | |
| 816 @end group | |
| 817 | |
| 818 @group | |
| 819 (defun user () ; @r{@code{x} is used in @code{user}.} | |
| 820 (list x)) | |
| 821 @end group | |
| 822 @end example | |
| 823 | |
| 824 In a lexically scoped language, the binding of @code{x} in | |
| 825 @code{binder} would never be accessible in @code{user}, because | |
| 826 @code{user} is not textually contained within the function | |
| 827 @code{binder}. However, in dynamically scoped XEmacs Lisp, @code{user} | |
| 828 may or may not refer to the binding of @code{x} established in | |
| 829 @code{binder}, depending on circumstances: | |
| 830 | |
| 831 @itemize @bullet | |
| 832 @item | |
| 833 If we call @code{user} directly without calling @code{binder} at all, | |
| 834 then whatever binding of @code{x} is found, it cannot come from | |
| 835 @code{binder}. | |
| 836 | |
| 837 @item | |
| 838 If we define @code{foo} as follows and call @code{binder}, then the | |
| 839 binding made in @code{binder} will be seen in @code{user}: | |
| 840 | |
| 841 @example | |
| 842 @group | |
| 843 (defun foo (lose) | |
| 844 (user)) | |
| 845 @end group | |
| 846 @end example | |
| 847 | |
| 848 @item | |
| 849 If we define @code{foo} as follows and call @code{binder}, then the | |
| 850 binding made in @code{binder} @emph{will not} be seen in @code{user}: | |
| 851 | |
| 852 @example | |
| 853 (defun foo (x) | |
| 854 (user)) | |
| 855 @end example | |
| 856 | |
| 857 @noindent | |
| 858 Here, when @code{foo} is called by @code{binder}, it binds @code{x}. | |
| 859 (The binding in @code{foo} is said to @dfn{shadow} the one made in | |
| 860 @code{binder}.) Therefore, @code{user} will access the @code{x} bound | |
| 861 by @code{foo} instead of the one bound by @code{binder}. | |
| 862 @end itemize | |
| 863 | |
| 864 @node Extent | |
| 865 @subsection Extent | |
| 866 | |
| 867 @dfn{Extent} refers to the time during program execution that a | |
| 868 variable name is valid. In XEmacs Lisp, a variable is valid only while | |
| 869 the form that bound it is executing. This is called @dfn{dynamic | |
| 870 extent}. ``Local'' or ``automatic'' variables in most languages, | |
| 871 including C and Pascal, have dynamic extent. | |
| 872 | |
| 873 One alternative to dynamic extent is @dfn{indefinite extent}. This | |
| 874 means that a variable binding can live on past the exit from the form | |
| 875 that made the binding. Common Lisp and Scheme, for example, support | |
| 876 this, but XEmacs Lisp does not. | |
| 877 | |
| 878 To illustrate this, the function below, @code{make-add}, returns a | |
| 879 function that purports to add @var{n} to its own argument @var{m}. | |
| 880 This would work in Common Lisp, but it does not work as intended in | |
| 881 XEmacs Lisp, because after the call to @code{make-add} exits, the | |
| 882 variable @code{n} is no longer bound to the actual argument 2. | |
| 883 | |
| 884 @example | |
| 885 (defun make-add (n) | |
| 886 (function (lambda (m) (+ n m)))) ; @r{Return a function.} | |
| 887 @result{} make-add | |
| 888 (fset 'add2 (make-add 2)) ; @r{Define function @code{add2}} | |
| 889 ; @r{with @code{(make-add 2)}.} | |
| 890 @result{} (lambda (m) (+ n m)) | |
| 891 (add2 4) ; @r{Try to add 2 to 4.} | |
| 892 @error{} Symbol's value as variable is void: n | |
| 893 @end example | |
| 894 | |
| 895 @cindex closures not available | |
| 896 Some Lisp dialects have ``closures'', objects that are like functions | |
| 4039 | 897 but record additional variable bindings. Closures are available in |
|
4485
57db42ba54fb
Correct a misspelling and add a space, lispref/variables.texi
Aidan Kehoe <kehoea@parhasard.net>
parents:
4039
diff
changeset
|
898 XEmacs Lisp using the @code{lexical-let} and @code{lexical-let*} macros, |
| 4039 | 899 which are autoloaded from @file{cl-macs}. @xref{(cl)Lexical Bindings}. |
| 900 Note that function arguments cannot be closed around using these | |
| 901 macros, and that any lambda expressions returned will not be | |
| 902 byte-compiled. | |
| 428 | 903 |
| 904 @node Impl of Scope | |
| 905 @subsection Implementation of Dynamic Scoping | |
| 906 @cindex deep binding | |
| 907 | |
| 908 A simple sample implementation (which is not how XEmacs Lisp actually | |
| 909 works) may help you understand dynamic binding. This technique is | |
| 910 called @dfn{deep binding} and was used in early Lisp systems. | |
| 911 | |
| 912 Suppose there is a stack of bindings: variable-value pairs. At entry | |
| 913 to a function or to a @code{let} form, we can push bindings on the stack | |
| 914 for the arguments or local variables created there. We can pop those | |
| 915 bindings from the stack at exit from the binding construct. | |
| 916 | |
| 917 We can find the value of a variable by searching the stack from top to | |
| 918 bottom for a binding for that variable; the value from that binding is | |
| 919 the value of the variable. To set the variable, we search for the | |
| 920 current binding, then store the new value into that binding. | |
| 921 | |
| 922 As you can see, a function's bindings remain in effect as long as it | |
| 923 continues execution, even during its calls to other functions. That is | |
| 924 why we say the extent of the binding is dynamic. And any other function | |
| 925 can refer to the bindings, if it uses the same variables while the | |
| 926 bindings are in effect. That is why we say the scope is indefinite. | |
| 927 | |
| 928 @cindex shallow binding | |
| 929 The actual implementation of variable scoping in XEmacs Lisp uses a | |
| 930 technique called @dfn{shallow binding}. Each variable has a standard | |
| 931 place in which its current value is always found---the value cell of the | |
| 932 symbol. | |
| 933 | |
| 934 In shallow binding, setting the variable works by storing a value in | |
| 935 the value cell. Creating a new binding works by pushing the old value | |
| 936 (belonging to a previous binding) on a stack, and storing the local value | |
| 937 in the value cell. Eliminating a binding works by popping the old value | |
| 938 off the stack, into the value cell. | |
| 939 | |
| 940 We use shallow binding because it has the same results as deep | |
| 941 binding, but runs faster, since there is never a need to search for a | |
| 942 binding. | |
| 943 | |
| 944 @node Using Scoping | |
| 945 @subsection Proper Use of Dynamic Scoping | |
| 946 | |
| 947 Binding a variable in one function and using it in another is a | |
| 948 powerful technique, but if used without restraint, it can make programs | |
| 949 hard to understand. There are two clean ways to use this technique: | |
| 950 | |
| 951 @itemize @bullet | |
| 952 @item | |
| 953 Use or bind the variable only in a few related functions, written close | |
| 954 together in one file. Such a variable is used for communication within | |
| 955 one program. | |
| 956 | |
| 957 You should write comments to inform other programmers that they can see | |
| 958 all uses of the variable before them, and to advise them not to add uses | |
| 959 elsewhere. | |
| 960 | |
| 961 @item | |
| 962 Give the variable a well-defined, documented meaning, and make all | |
| 963 appropriate functions refer to it (but not bind it or set it) wherever | |
| 964 that meaning is relevant. For example, the variable | |
| 965 @code{case-fold-search} is defined as ``non-@code{nil} means ignore case | |
| 966 when searching''; various search and replace functions refer to it | |
| 967 directly or through their subroutines, but do not bind or set it. | |
| 968 | |
| 969 Then you can bind the variable in other programs, knowing reliably what | |
| 970 the effect will be. | |
| 971 @end itemize | |
| 972 | |
| 973 In either case, you should define the variable with @code{defvar}. | |
| 974 This helps other people understand your program by telling them to look | |
| 975 for inter-function usage. It also avoids a warning from the byte | |
| 976 compiler. Choose the variable's name to avoid name conflicts---don't | |
| 977 use short names like @code{x}. | |
| 978 | |
| 979 @node Buffer-Local Variables | |
| 980 @section Buffer-Local Variables | |
| 981 @cindex variables, buffer-local | |
| 982 @cindex buffer-local variables | |
| 983 | |
| 984 Global and local variable bindings are found in most programming | |
| 985 languages in one form or another. XEmacs also supports another, unusual | |
| 986 kind of variable binding: @dfn{buffer-local} bindings, which apply only | |
| 987 to one buffer. XEmacs Lisp is meant for programming editing commands, | |
| 988 and having different values for a variable in different buffers is an | |
| 989 important customization method. | |
| 990 | |
| 991 @menu | |
| 992 * Intro to Buffer-Local:: Introduction and concepts. | |
| 993 * Creating Buffer-Local:: Creating and destroying buffer-local bindings. | |
| 994 * Default Value:: The default value is seen in buffers | |
| 995 that don't have their own local values. | |
| 996 @end menu | |
| 997 | |
| 998 @node Intro to Buffer-Local | |
| 999 @subsection Introduction to Buffer-Local Variables | |
| 1000 | |
| 1001 A buffer-local variable has a buffer-local binding associated with a | |
| 1002 particular buffer. The binding is in effect when that buffer is | |
| 1003 current; otherwise, it is not in effect. If you set the variable while | |
| 1004 a buffer-local binding is in effect, the new value goes in that binding, | |
| 1005 so the global binding is unchanged; this means that the change is | |
| 1006 visible in that buffer alone. | |
| 1007 | |
| 1008 A variable may have buffer-local bindings in some buffers but not in | |
| 1009 others. The global binding is shared by all the buffers that don't have | |
| 1010 their own bindings. Thus, if you set the variable in a buffer that does | |
| 1011 not have a buffer-local binding for it, the new value is visible in all | |
| 1012 buffers except those with buffer-local bindings. (Here we are assuming | |
| 1013 that there are no @code{let}-style local bindings to complicate the issue.) | |
| 1014 | |
| 1015 The most common use of buffer-local bindings is for major modes to change | |
| 1016 variables that control the behavior of commands. For example, C mode and | |
| 1017 Lisp mode both set the variable @code{paragraph-start} to specify that only | |
| 1018 blank lines separate paragraphs. They do this by making the variable | |
| 1019 buffer-local in the buffer that is being put into C mode or Lisp mode, and | |
| 1020 then setting it to the new value for that mode. | |
| 1021 | |
| 1022 The usual way to make a buffer-local binding is with | |
| 1023 @code{make-local-variable}, which is what major mode commands use. This | |
| 1024 affects just the current buffer; all other buffers (including those yet to | |
| 1025 be created) continue to share the global value. | |
| 1026 | |
| 1027 @cindex automatically buffer-local | |
| 1028 A more powerful operation is to mark the variable as | |
| 1029 @dfn{automatically buffer-local} by calling | |
| 1030 @code{make-variable-buffer-local}. You can think of this as making the | |
| 1031 variable local in all buffers, even those yet to be created. More | |
| 1032 precisely, the effect is that setting the variable automatically makes | |
| 1033 the variable local to the current buffer if it is not already so. All | |
| 1034 buffers start out by sharing the global value of the variable as usual, | |
| 1035 but any @code{setq} creates a buffer-local binding for the current | |
| 1036 buffer. The new value is stored in the buffer-local binding, leaving | |
| 1037 the (default) global binding untouched. The global value can no longer | |
| 1038 be changed with @code{setq}; you need to use @code{setq-default} to do | |
| 1039 that. | |
| 1040 | |
| 1041 @ignore | |
| 1042 Section about not changing buffers during let bindings. Mly fixed | |
| 1043 this for XEmacs. | |
| 1044 @end ignore | |
| 1045 Local variables in a file you edit are also represented by | |
| 1046 buffer-local bindings for the buffer that holds the file within XEmacs. | |
| 1047 @xref{Auto Major Mode}. | |
| 1048 | |
| 1049 @node Creating Buffer-Local | |
| 1050 @subsection Creating and Deleting Buffer-Local Bindings | |
| 1051 | |
| 3510 | 1052 Besides the functions mentioned here, buffer-local variables are also |
| 1053 created when file-local variables are set. @xref{Auto Major Mode} | |
| 1054 . There is no way (outside of `eval' lines, which are normally disabled) | |
| 1055 to set the global value of a variable from the file local variable | |
| 1056 specifications. | |
| 1057 | |
| 428 | 1058 @deffn Command make-local-variable variable |
| 1059 This function creates a buffer-local binding in the current buffer for | |
| 1060 @var{variable} (a symbol). Other buffers are not affected. The value | |
| 1061 returned is @var{variable}. | |
| 1062 | |
| 1063 @c Emacs 19 feature | |
| 1064 The buffer-local value of @var{variable} starts out as the same value | |
| 1065 @var{variable} previously had. If @var{variable} was void, it remains | |
| 1066 void. | |
| 1067 | |
| 1068 @example | |
| 1069 @group | |
| 1070 ;; @r{In buffer @samp{b1}:} | |
| 1071 (setq foo 5) ; @r{Affects all buffers.} | |
| 1072 @result{} 5 | |
| 1073 @end group | |
| 1074 @group | |
| 1075 (make-local-variable 'foo) ; @r{Now it is local in @samp{b1}.} | |
| 1076 @result{} foo | |
| 1077 @end group | |
| 1078 @group | |
| 1079 foo ; @r{That did not change} | |
| 1080 @result{} 5 ; @r{the value.} | |
| 1081 @end group | |
| 1082 @group | |
| 1083 (setq foo 6) ; @r{Change the value} | |
| 1084 @result{} 6 ; @r{in @samp{b1}.} | |
| 1085 @end group | |
| 1086 @group | |
| 1087 foo | |
| 1088 @result{} 6 | |
| 1089 @end group | |
| 1090 | |
| 1091 @group | |
| 1092 ;; @r{In buffer @samp{b2}, the value hasn't changed.} | |
| 1093 (save-excursion | |
| 1094 (set-buffer "b2") | |
| 1095 foo) | |
| 1096 @result{} 5 | |
| 1097 @end group | |
| 1098 @end example | |
| 1099 | |
| 1100 Making a variable buffer-local within a @code{let}-binding for that | |
| 1101 variable does not work. This is because @code{let} does not distinguish | |
| 1102 between different kinds of bindings; it knows only which variable the | |
| 1103 binding was made for. | |
| 1104 | |
| 1105 @strong{Please note:} do not use @code{make-local-variable} for a hook | |
| 1106 variable. Instead, use @code{make-local-hook}. @xref{Hooks}. | |
| 1107 @end deffn | |
| 1108 | |
| 1109 @deffn Command make-variable-buffer-local variable | |
| 1110 This function marks @var{variable} (a symbol) automatically | |
| 1111 buffer-local, so that any subsequent attempt to set it will make it | |
| 1112 local to the current buffer at the time. | |
| 1113 | |
| 1114 The value returned is @var{variable}. | |
| 1115 @end deffn | |
| 1116 | |
| 444 | 1117 @defun local-variable-p variable buffer &optional after-set |
| 428 | 1118 This returns @code{t} if @var{variable} is buffer-local in buffer |
| 444 | 1119 @var{buffer}; else @code{nil}. |
| 1120 | |
| 1121 If optional third arg @var{after-set} is non-@code{nil}, return @code{t} | |
| 1122 if @var{symbol} would be buffer-local after it is set, regardless of | |
| 1123 whether it is so presently. | |
| 1124 | |
| 1125 A @code{nil} value for @var{buffer} is @emph{not} the same as | |
| 1126 @code{(current-buffer)}, but means "no buffer". Specifically: | |
| 1127 | |
| 1128 If @var{buffer} is @code{nil} and @var{after-set} is @code{nil}, a | |
| 1129 return value of @code{t} indicates that the variable is one of the | |
| 1130 special built-in variables that is always buffer-local. (This includes | |
| 1131 @code{buffer-file-name}, @code{buffer-read-only}, | |
| 1132 @code{buffer-undo-list}, and others.) | |
| 1133 | |
| 1134 If @var{buffer} is @code{nil} and @var{after-set} is @code{t}, a return | |
| 1135 value of @code{t} indicates that the variable has had | |
| 1136 @code{make-variable-buffer-local} applied to it. | |
| 428 | 1137 @end defun |
| 1138 | |
| 1139 @defun buffer-local-variables &optional buffer | |
| 1140 This function returns a list describing the buffer-local variables in | |
| 1141 buffer @var{buffer}. It returns an association list (@pxref{Association | |
| 1142 Lists}) in which each association contains one buffer-local variable and | |
| 1143 its value. When a buffer-local variable is void in @var{buffer}, then | |
| 1144 it appears directly in the resulting list. If @var{buffer} is omitted, | |
| 1145 the current buffer is used. | |
| 1146 | |
| 1147 @example | |
| 1148 @group | |
| 1149 (make-local-variable 'foobar) | |
| 1150 (makunbound 'foobar) | |
| 1151 (make-local-variable 'bind-me) | |
| 1152 (setq bind-me 69) | |
| 1153 @end group | |
| 1154 (setq lcl (buffer-local-variables)) | |
| 1155 ;; @r{First, built-in variables local in all buffers:} | |
| 1156 @result{} ((mark-active . nil) | |
| 1157 (buffer-undo-list nil) | |
| 1158 (mode-name . "Fundamental") | |
| 1159 @dots{} | |
| 1160 @group | |
| 444 | 1161 ;; @r{Next, non-built-in local variables.} |
| 428 | 1162 ;; @r{This one is local and void:} |
| 1163 foobar | |
| 1164 ;; @r{This one is local and nonvoid:} | |
| 1165 (bind-me . 69)) | |
| 1166 @end group | |
| 1167 @end example | |
| 1168 | |
| 1169 Note that storing new values into the @sc{cdr}s of cons cells in this | |
| 1170 list does @emph{not} change the local values of the variables. | |
| 1171 @end defun | |
| 1172 | |
| 1173 @deffn Command kill-local-variable variable | |
| 1174 This function deletes the buffer-local binding (if any) for | |
| 1175 @var{variable} (a symbol) in the current buffer. As a result, the | |
| 1176 global (default) binding of @var{variable} becomes visible in this | |
| 1177 buffer. Usually this results in a change in the value of | |
| 1178 @var{variable}, since the global value is usually different from the | |
| 1179 buffer-local value just eliminated. | |
| 1180 | |
| 1181 If you kill the local binding of a variable that automatically becomes | |
| 1182 local when set, this makes the global value visible in the current | |
| 1183 buffer. However, if you set the variable again, that will once again | |
| 1184 create a local binding for it. | |
| 1185 | |
| 1186 @code{kill-local-variable} returns @var{variable}. | |
| 1187 | |
| 1188 This function is a command because it is sometimes useful to kill one | |
| 1189 buffer-local variable interactively, just as it is useful to create | |
| 1190 buffer-local variables interactively. | |
| 1191 @end deffn | |
| 1192 | |
| 1193 @defun kill-all-local-variables | |
| 1194 This function eliminates all the buffer-local variable bindings of the | |
| 1195 current buffer except for variables marked as ``permanent''. As a | |
| 1196 result, the buffer will see the default values of most variables. | |
| 1197 | |
| 1198 This function also resets certain other information pertaining to the | |
| 1199 buffer: it sets the local keymap to @code{nil}, the syntax table to the | |
| 1200 value of @code{standard-syntax-table}, and the abbrev table to the value | |
| 1201 of @code{fundamental-mode-abbrev-table}. | |
| 1202 | |
| 1203 Every major mode command begins by calling this function, which has the | |
| 1204 effect of switching to Fundamental mode and erasing most of the effects | |
| 1205 of the previous major mode. To ensure that this does its job, the | |
| 1206 variables that major modes set should not be marked permanent. | |
| 1207 | |
| 1208 @code{kill-all-local-variables} returns @code{nil}. | |
| 1209 @end defun | |
| 1210 | |
| 1211 @c Emacs 19 feature | |
| 1212 @cindex permanent local variable | |
| 1213 A local variable is @dfn{permanent} if the variable name (a symbol) has a | |
| 1214 @code{permanent-local} property that is non-@code{nil}. Permanent | |
| 1215 locals are appropriate for data pertaining to where the file came from | |
| 1216 or how to save it, rather than with how to edit the contents. | |
| 1217 | |
| 1218 @node Default Value | |
| 1219 @subsection The Default Value of a Buffer-Local Variable | |
| 1220 @cindex default value | |
| 1221 | |
| 1222 The global value of a variable with buffer-local bindings is also | |
| 1223 called the @dfn{default} value, because it is the value that is in | |
| 1224 effect except when specifically overridden. | |
| 1225 | |
| 1226 The functions @code{default-value} and @code{setq-default} access and | |
| 1227 change a variable's default value regardless of whether the current | |
| 1228 buffer has a buffer-local binding. For example, you could use | |
| 1229 @code{setq-default} to change the default setting of | |
| 1230 @code{paragraph-start} for most buffers; and this would work even when | |
| 1231 you are in a C or Lisp mode buffer that has a buffer-local value for | |
| 1232 this variable. | |
| 1233 | |
| 1234 @c Emacs 19 feature | |
| 1235 The special forms @code{defvar} and @code{defconst} also set the | |
| 1236 default value (if they set the variable at all), rather than any local | |
| 1237 value. | |
| 1238 | |
| 1239 @defun default-value symbol | |
| 1240 This function returns @var{symbol}'s default value. This is the value | |
| 1241 that is seen in buffers that do not have their own values for this | |
| 1242 variable. If @var{symbol} is not buffer-local, this is equivalent to | |
| 1243 @code{symbol-value} (@pxref{Accessing Variables}). | |
| 1244 @end defun | |
| 1245 | |
| 1246 @c Emacs 19 feature | |
| 1247 @defun default-boundp symbol | |
| 1248 The function @code{default-boundp} tells you whether @var{symbol}'s | |
| 1249 default value is nonvoid. If @code{(default-boundp 'foo)} returns | |
| 1250 @code{nil}, then @code{(default-value 'foo)} would get an error. | |
| 1251 | |
| 1252 @code{default-boundp} is to @code{default-value} as @code{boundp} is to | |
| 1253 @code{symbol-value}. | |
| 1254 @end defun | |
| 1255 | |
| 1256 @defspec setq-default symbol value | |
| 1257 This sets the default value of @var{symbol} to @var{value}. It does not | |
| 1258 evaluate @var{symbol}, but does evaluate @var{value}. The value of the | |
| 1259 @code{setq-default} form is @var{value}. | |
| 1260 | |
| 1261 If a @var{symbol} is not buffer-local for the current buffer, and is not | |
| 1262 marked automatically buffer-local, @code{setq-default} has the same | |
| 1263 effect as @code{setq}. If @var{symbol} is buffer-local for the current | |
| 1264 buffer, then this changes the value that other buffers will see (as long | |
| 1265 as they don't have a buffer-local value), but not the value that the | |
| 1266 current buffer sees. | |
| 1267 | |
| 1268 @example | |
| 1269 @group | |
| 1270 ;; @r{In buffer @samp{foo}:} | |
| 1271 (make-local-variable 'local) | |
| 1272 @result{} local | |
| 1273 @end group | |
| 1274 @group | |
| 1275 (setq local 'value-in-foo) | |
| 1276 @result{} value-in-foo | |
| 1277 @end group | |
| 1278 @group | |
| 1279 (setq-default local 'new-default) | |
| 1280 @result{} new-default | |
| 1281 @end group | |
| 1282 @group | |
| 1283 local | |
| 1284 @result{} value-in-foo | |
| 1285 @end group | |
| 1286 @group | |
| 1287 (default-value 'local) | |
| 1288 @result{} new-default | |
| 1289 @end group | |
| 1290 | |
| 1291 @group | |
| 1292 ;; @r{In (the new) buffer @samp{bar}:} | |
| 1293 local | |
| 1294 @result{} new-default | |
| 1295 @end group | |
| 1296 @group | |
| 1297 (default-value 'local) | |
| 1298 @result{} new-default | |
| 1299 @end group | |
| 1300 @group | |
| 1301 (setq local 'another-default) | |
| 1302 @result{} another-default | |
| 1303 @end group | |
| 1304 @group | |
| 1305 (default-value 'local) | |
| 1306 @result{} another-default | |
| 1307 @end group | |
| 1308 | |
| 1309 @group | |
| 1310 ;; @r{Back in buffer @samp{foo}:} | |
| 1311 local | |
| 1312 @result{} value-in-foo | |
| 1313 (default-value 'local) | |
| 1314 @result{} another-default | |
| 1315 @end group | |
| 1316 @end example | |
| 1317 @end defspec | |
| 1318 | |
| 1319 @defun set-default symbol value | |
| 1320 This function is like @code{setq-default}, except that @var{symbol} is | |
| 1321 evaluated. | |
| 1322 | |
| 1323 @example | |
| 1324 @group | |
| 1325 (set-default (car '(a b c)) 23) | |
| 1326 @result{} 23 | |
| 1327 @end group | |
| 1328 @group | |
| 1329 (default-value 'a) | |
| 1330 @result{} 23 | |
| 1331 @end group | |
| 1332 @end example | |
| 1333 @end defun | |
| 1334 | |
| 1335 @node Variable Aliases | |
| 1336 @section Variable Aliases | |
| 1337 @cindex variables, indirect | |
| 1338 @cindex indirect variables | |
| 1339 @cindex variable aliases | |
| 1340 @cindex aliases, for variables | |
| 1341 | |
| 1342 You can define a variable as an @dfn{alias} for another. Any time | |
| 1343 you reference the former variable, the current value of the latter | |
| 1344 is returned. Any time you change the value of the former variable, | |
| 1345 the value of the latter is actually changed. This is useful in | |
| 1346 cases where you want to rename a variable but still make old code | |
| 1347 work (@pxref{Obsoleteness}). | |
| 1348 | |
| 1349 @defun defvaralias variable alias | |
| 1350 This function defines @var{variable} as an alias for @var{alias}. | |
| 1351 Thenceforth, any operations performed on @var{variable} will actually be | |
| 1352 performed on @var{alias}. Both @var{variable} and @var{alias} should be | |
| 1353 symbols. If @var{alias} is @code{nil}, remove any aliases for | |
| 1354 @var{variable}. @var{alias} can itself be aliased, and the chain of | |
| 1355 variable aliases will be followed appropriately. If @var{variable} | |
| 1356 already has a value, this value will be shadowed until the alias is | |
| 1357 removed, at which point it will be restored. Currently @var{variable} | |
| 1358 cannot be a built-in variable, a variable that has a buffer-local value | |
| 1359 in any buffer, or the symbols @code{nil} or @code{t}. | |
| 1360 @end defun | |
| 1361 | |
| 444 | 1362 @defun variable-alias variable &optional follow-past-lisp-magic |
| 428 | 1363 If @var{variable} is aliased to another variable, this function returns |
| 1364 that variable. @var{variable} should be a symbol. If @var{variable} is | |
| 1365 not aliased, this function returns @code{nil}. | |
| 1366 @end defun | |
| 1367 | |
| 444 | 1368 @defun indirect-variable object &optional follow-past-lisp-magic |
| 428 | 1369 This function returns the variable at the end of @var{object}'s |
| 1370 variable-alias chain. If @var{object} is a symbol, follow all variable | |
| 1371 aliases and return the final (non-aliased) symbol. If @var{object} is | |
| 1372 not a symbol, just return it. Signal a | |
| 1373 @code{cyclic-variable-indirection} error if there is a loop in the | |
| 1374 variable chain of symbols. | |
| 1375 @end defun | |
| 1376 | |
| 1377 |