| 1 | @c -*-texinfo-*- |
| 2 | @c This is part of the GNU Emacs Lisp Reference Manual. |
| 3 | @c Copyright (C) 1990-1995, 1998-2014 Free Software Foundation, Inc. |
| 4 | @c See the file elisp.texi for copying conditions. |
| 5 | @node Variables |
| 6 | @chapter Variables |
| 7 | @cindex variable |
| 8 | |
| 9 | A @dfn{variable} is a name used in a program to stand for a value. |
| 10 | In Lisp, each variable is represented by a Lisp symbol |
| 11 | (@pxref{Symbols}). The variable name is simply the symbol's name, and |
| 12 | the variable's value is stored in the symbol's value cell@footnote{To |
| 13 | be precise, under the default @dfn{dynamic scoping} rule, the value |
| 14 | cell always holds the variable's current value, but this is not the |
| 15 | case under the @dfn{lexical scoping} rule. @xref{Variable Scoping}, |
| 16 | for details.}. @xref{Symbol Components}. In Emacs Lisp, the use of a |
| 17 | symbol as a variable is independent of its use as a function name. |
| 18 | |
| 19 | As previously noted in this manual, a Lisp program is represented |
| 20 | primarily by Lisp objects, and only secondarily as text. The textual |
| 21 | form of a Lisp program is given by the read syntax of the Lisp objects |
| 22 | that constitute the program. Hence, the textual form of a variable in |
| 23 | a Lisp program is written using the read syntax for the symbol |
| 24 | representing the variable. |
| 25 | |
| 26 | @menu |
| 27 | * Global Variables:: Variable values that exist permanently, everywhere. |
| 28 | * Constant Variables:: Certain "variables" have values that never change. |
| 29 | * Local Variables:: Variable values that exist only temporarily. |
| 30 | * Void Variables:: Symbols that lack values. |
| 31 | * Defining Variables:: A definition says a symbol is used as a variable. |
| 32 | * Tips for Defining:: Things you should think about when you |
| 33 | define 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 | * File Local Variables:: Handling local variable lists in files. |
| 40 | * Directory Local Variables:: Local variables common to all files in a directory. |
| 41 | * Variable Aliases:: Variables that are aliases for other variables. |
| 42 | * Variables with Restricted Values:: Non-constant variables whose value can |
| 43 | @emph{not} be an arbitrary Lisp object. |
| 44 | * Generalized Variables:: Extending the concept of variables. |
| 45 | @end menu |
| 46 | |
| 47 | @node Global Variables |
| 48 | @section Global Variables |
| 49 | @cindex global variable |
| 50 | |
| 51 | The simplest way to use a variable is @dfn{globally}. This means that |
| 52 | the variable has just one value at a time, and this value is in effect |
| 53 | (at least for the moment) throughout the Lisp system. The value remains |
| 54 | in effect until you specify a new one. When a new value replaces the |
| 55 | old one, no trace of the old value remains in the variable. |
| 56 | |
| 57 | You specify a value for a symbol with @code{setq}. For example, |
| 58 | |
| 59 | @example |
| 60 | (setq x '(a b)) |
| 61 | @end example |
| 62 | |
| 63 | @noindent |
| 64 | gives the variable @code{x} the value @code{(a b)}. Note that |
| 65 | @code{setq} is a special form (@pxref{Special Forms}); it does not |
| 66 | evaluate its first argument, the name of the variable, but it does |
| 67 | evaluate the second argument, the new value. |
| 68 | |
| 69 | Once the variable has a value, you can refer to it by using the |
| 70 | symbol itself as an expression. Thus, |
| 71 | |
| 72 | @example |
| 73 | @group |
| 74 | x @result{} (a b) |
| 75 | @end group |
| 76 | @end example |
| 77 | |
| 78 | @noindent |
| 79 | assuming the @code{setq} form shown above has already been executed. |
| 80 | |
| 81 | If you do set the same variable again, the new value replaces the old |
| 82 | one: |
| 83 | |
| 84 | @example |
| 85 | @group |
| 86 | x |
| 87 | @result{} (a b) |
| 88 | @end group |
| 89 | @group |
| 90 | (setq x 4) |
| 91 | @result{} 4 |
| 92 | @end group |
| 93 | @group |
| 94 | x |
| 95 | @result{} 4 |
| 96 | @end group |
| 97 | @end example |
| 98 | |
| 99 | @node Constant Variables |
| 100 | @section Variables that Never Change |
| 101 | @cindex @code{setting-constant} error |
| 102 | @cindex keyword symbol |
| 103 | @cindex variable with constant value |
| 104 | @cindex constant variables |
| 105 | @cindex symbol that evaluates to itself |
| 106 | @cindex symbol with constant value |
| 107 | |
| 108 | In Emacs Lisp, certain symbols normally evaluate to themselves. These |
| 109 | include @code{nil} and @code{t}, as well as any symbol whose name starts |
| 110 | with @samp{:} (these are called @dfn{keywords}). These symbols cannot |
| 111 | be rebound, nor can their values be changed. Any attempt to set or bind |
| 112 | @code{nil} or @code{t} signals a @code{setting-constant} error. The |
| 113 | same is true for a keyword (a symbol whose name starts with @samp{:}), |
| 114 | if it is interned in the standard obarray, except that setting such a |
| 115 | symbol to itself is not an error. |
| 116 | |
| 117 | @example |
| 118 | @group |
| 119 | nil @equiv{} 'nil |
| 120 | @result{} nil |
| 121 | @end group |
| 122 | @group |
| 123 | (setq nil 500) |
| 124 | @error{} Attempt to set constant symbol: nil |
| 125 | @end group |
| 126 | @end example |
| 127 | |
| 128 | @defun keywordp object |
| 129 | function returns @code{t} if @var{object} is a symbol whose name |
| 130 | starts with @samp{:}, interned in the standard obarray, and returns |
| 131 | @code{nil} otherwise. |
| 132 | @end defun |
| 133 | |
| 134 | These constants are fundamentally different from the ``constants'' |
| 135 | defined using the @code{defconst} special form (@pxref{Defining |
| 136 | Variables}). A @code{defconst} form serves to inform human readers |
| 137 | that you do not intend to change the value of a variable, but Emacs |
| 138 | does not raise an error if you actually change it. |
| 139 | |
| 140 | @node Local Variables |
| 141 | @section Local Variables |
| 142 | @cindex binding local variables |
| 143 | @cindex local variables |
| 144 | @cindex local binding |
| 145 | @cindex global binding |
| 146 | |
| 147 | Global variables have values that last until explicitly superseded |
| 148 | with new values. Sometimes it is useful to give a variable a |
| 149 | @dfn{local value}---a value that takes effect only within a certain |
| 150 | part of a Lisp program. When a variable has a local value, we say |
| 151 | that it is @dfn{locally bound} to that value, and that it is a |
| 152 | @dfn{local variable}. |
| 153 | |
| 154 | For example, when a function is called, its argument variables |
| 155 | receive local values, which are the actual arguments supplied to the |
| 156 | function call; these local bindings take effect within the body of the |
| 157 | function. To take another example, the @code{let} special form |
| 158 | explicitly establishes local bindings for specific variables, which |
| 159 | take effect within the body of the @code{let} form. |
| 160 | |
| 161 | We also speak of the @dfn{global binding}, which is where |
| 162 | (conceptually) the global value is kept. |
| 163 | |
| 164 | @cindex shadowing of variables |
| 165 | Establishing a local binding saves away the variable's previous |
| 166 | value (or lack of one). We say that the previous value is |
| 167 | @dfn{shadowed}. Both global and local values may be shadowed. If a |
| 168 | local binding is in effect, using @code{setq} on the local variable |
| 169 | stores the specified value in the local binding. When that local |
| 170 | binding is no longer in effect, the previously shadowed value (or lack |
| 171 | of one) comes back. |
| 172 | |
| 173 | @cindex current binding |
| 174 | A variable can have more than one local binding at a time (e.g., if |
| 175 | there are nested @code{let} forms that bind the variable). The |
| 176 | @dfn{current binding} is the local binding that is actually in effect. |
| 177 | It determines the value returned by evaluating the variable symbol, |
| 178 | and it is the binding acted on by @code{setq}. |
| 179 | |
| 180 | For most purposes, you can think of the current binding as the |
| 181 | ``innermost'' local binding, or the global binding if there is no |
| 182 | local binding. To be more precise, a rule called the @dfn{scoping |
| 183 | rule} determines where in a program a local binding takes effect. The |
| 184 | default scoping rule in Emacs Lisp is called @dfn{dynamic scoping}, |
| 185 | which simply states that the current binding at any given point in the |
| 186 | execution of a program is the most recently-created binding for that |
| 187 | variable that still exists. For details about dynamic scoping, and an |
| 188 | alternative scoping rule called @dfn{lexical scoping}, @xref{Variable |
| 189 | Scoping}. |
| 190 | |
| 191 | The special forms @code{let} and @code{let*} exist to create local |
| 192 | bindings: |
| 193 | |
| 194 | @defspec let (bindings@dots{}) forms@dots{} |
| 195 | This special form sets up local bindings for a certain set of |
| 196 | variables, as specified by @var{bindings}, and then evaluates all of |
| 197 | the @var{forms} in textual order. Its return value is the value of |
| 198 | the last form in @var{forms}. |
| 199 | |
| 200 | Each of the @var{bindings} is either @w{(i) a} symbol, in which case |
| 201 | that symbol is locally bound to @code{nil}; or @w{(ii) a} list of the |
| 202 | form @code{(@var{symbol} @var{value-form})}, in which case |
| 203 | @var{symbol} is locally bound to the result of evaluating |
| 204 | @var{value-form}. If @var{value-form} is omitted, @code{nil} is used. |
| 205 | |
| 206 | All of the @var{value-form}s in @var{bindings} are evaluated in the |
| 207 | order they appear and @emph{before} binding any of the symbols to them. |
| 208 | Here is an example of this: @code{z} is bound to the old value of |
| 209 | @code{y}, which is 2, not the new value of @code{y}, which is 1. |
| 210 | |
| 211 | @example |
| 212 | @group |
| 213 | (setq y 2) |
| 214 | @result{} 2 |
| 215 | @end group |
| 216 | |
| 217 | @group |
| 218 | (let ((y 1) |
| 219 | (z y)) |
| 220 | (list y z)) |
| 221 | @result{} (1 2) |
| 222 | @end group |
| 223 | @end example |
| 224 | @end defspec |
| 225 | |
| 226 | @defspec let* (bindings@dots{}) forms@dots{} |
| 227 | This special form is like @code{let}, but it binds each variable right |
| 228 | after computing its local value, before computing the local value for |
| 229 | the next variable. Therefore, an expression in @var{bindings} can |
| 230 | refer to the preceding symbols bound in this @code{let*} form. |
| 231 | Compare the following example with the example above for @code{let}. |
| 232 | |
| 233 | @example |
| 234 | @group |
| 235 | (setq y 2) |
| 236 | @result{} 2 |
| 237 | @end group |
| 238 | |
| 239 | @group |
| 240 | (let* ((y 1) |
| 241 | (z y)) ; @r{Use the just-established value of @code{y}.} |
| 242 | (list y z)) |
| 243 | @result{} (1 1) |
| 244 | @end group |
| 245 | @end example |
| 246 | @end defspec |
| 247 | |
| 248 | Here is a complete list of the other facilities that create local |
| 249 | bindings: |
| 250 | |
| 251 | @itemize @bullet |
| 252 | @item |
| 253 | Function calls (@pxref{Functions}). |
| 254 | |
| 255 | @item |
| 256 | Macro calls (@pxref{Macros}). |
| 257 | |
| 258 | @item |
| 259 | @code{condition-case} (@pxref{Errors}). |
| 260 | @end itemize |
| 261 | |
| 262 | Variables can also have buffer-local bindings (@pxref{Buffer-Local |
| 263 | Variables}); a few variables have terminal-local bindings |
| 264 | (@pxref{Multiple Terminals}). These kinds of bindings work somewhat |
| 265 | like ordinary local bindings, but they are localized depending on |
| 266 | ``where'' you are in Emacs. |
| 267 | |
| 268 | @defopt max-specpdl-size |
| 269 | @anchor{Definition of max-specpdl-size} |
| 270 | @cindex variable limit error |
| 271 | @cindex evaluation error |
| 272 | @cindex infinite recursion |
| 273 | This variable defines the limit on the total number of local variable |
| 274 | bindings and @code{unwind-protect} cleanups (see @ref{Cleanups,, |
| 275 | Cleaning Up from Nonlocal Exits}) that are allowed before Emacs |
| 276 | signals an error (with data @code{"Variable binding depth exceeds |
| 277 | max-specpdl-size"}). |
| 278 | |
| 279 | This limit, with the associated error when it is exceeded, is one way |
| 280 | that Lisp avoids infinite recursion on an ill-defined function. |
| 281 | @code{max-lisp-eval-depth} provides another limit on depth of nesting. |
| 282 | @xref{Definition of max-lisp-eval-depth,, Eval}. |
| 283 | |
| 284 | The default value is 1300. Entry to the Lisp debugger increases the |
| 285 | value, if there is little room left, to make sure the debugger itself |
| 286 | has room to execute. |
| 287 | @end defopt |
| 288 | |
| 289 | @node Void Variables |
| 290 | @section When a Variable is ``Void'' |
| 291 | @cindex @code{void-variable} error |
| 292 | @cindex void variable |
| 293 | |
| 294 | We say that a variable is void if its symbol has an unassigned value |
| 295 | cell (@pxref{Symbol Components}). |
| 296 | |
| 297 | Under Emacs Lisp's default dynamic scoping rule (@pxref{Variable |
| 298 | Scoping}), the value cell stores the variable's current (local or |
| 299 | global) value. Note that an unassigned value cell is @emph{not} the |
| 300 | same as having @code{nil} in the value cell. The symbol @code{nil} is |
| 301 | a Lisp object and can be the value of a variable, just as any other |
| 302 | object can be; but it is still a value. If a variable is void, trying |
| 303 | to evaluate the variable signals a @code{void-variable} error, instead |
| 304 | of returning a value. |
| 305 | |
| 306 | Under the optional lexical scoping rule, the value cell only holds |
| 307 | the variable's global value---the value outside of any lexical binding |
| 308 | construct. When a variable is lexically bound, the local value is |
| 309 | determined by the lexical environment; hence, variables can have local |
| 310 | values even if their symbols' value cells are unassigned. |
| 311 | |
| 312 | @defun makunbound symbol |
| 313 | This function empties out the value cell of @var{symbol}, making the |
| 314 | variable void. It returns @var{symbol}. |
| 315 | |
| 316 | If @var{symbol} has a dynamic local binding, @code{makunbound} voids |
| 317 | the current binding, and this voidness lasts only as long as the local |
| 318 | binding is in effect. Afterwards, the previously shadowed local or |
| 319 | global binding is reexposed; then the variable will no longer be void, |
| 320 | unless the reexposed binding is void too. |
| 321 | |
| 322 | Here are some examples (assuming dynamic binding is in effect): |
| 323 | |
| 324 | @smallexample |
| 325 | @group |
| 326 | (setq x 1) ; @r{Put a value in the global binding.} |
| 327 | @result{} 1 |
| 328 | (let ((x 2)) ; @r{Locally bind it.} |
| 329 | (makunbound 'x) ; @r{Void the local binding.} |
| 330 | x) |
| 331 | @error{} Symbol's value as variable is void: x |
| 332 | @end group |
| 333 | @group |
| 334 | x ; @r{The global binding is unchanged.} |
| 335 | @result{} 1 |
| 336 | |
| 337 | (let ((x 2)) ; @r{Locally bind it.} |
| 338 | (let ((x 3)) ; @r{And again.} |
| 339 | (makunbound 'x) ; @r{Void the innermost-local binding.} |
| 340 | x)) ; @r{And refer: it's void.} |
| 341 | @error{} Symbol's value as variable is void: x |
| 342 | @end group |
| 343 | |
| 344 | @group |
| 345 | (let ((x 2)) |
| 346 | (let ((x 3)) |
| 347 | (makunbound 'x)) ; @r{Void inner binding, then remove it.} |
| 348 | x) ; @r{Now outer @code{let} binding is visible.} |
| 349 | @result{} 2 |
| 350 | @end group |
| 351 | @end smallexample |
| 352 | @end defun |
| 353 | |
| 354 | @defun boundp variable |
| 355 | This function returns @code{t} if @var{variable} (a symbol) is not |
| 356 | void, and @code{nil} if it is void. |
| 357 | |
| 358 | Here are some examples (assuming dynamic binding is in effect): |
| 359 | |
| 360 | @smallexample |
| 361 | @group |
| 362 | (boundp 'abracadabra) ; @r{Starts out void.} |
| 363 | @result{} nil |
| 364 | @end group |
| 365 | @group |
| 366 | (let ((abracadabra 5)) ; @r{Locally bind it.} |
| 367 | (boundp 'abracadabra)) |
| 368 | @result{} t |
| 369 | @end group |
| 370 | @group |
| 371 | (boundp 'abracadabra) ; @r{Still globally void.} |
| 372 | @result{} nil |
| 373 | @end group |
| 374 | @group |
| 375 | (setq abracadabra 5) ; @r{Make it globally nonvoid.} |
| 376 | @result{} 5 |
| 377 | @end group |
| 378 | @group |
| 379 | (boundp 'abracadabra) |
| 380 | @result{} t |
| 381 | @end group |
| 382 | @end smallexample |
| 383 | @end defun |
| 384 | |
| 385 | @node Defining Variables |
| 386 | @section Defining Global Variables |
| 387 | @cindex variable definition |
| 388 | |
| 389 | A @dfn{variable definition} is a construct that announces your |
| 390 | intention to use a symbol as a global variable. It uses the special |
| 391 | forms @code{defvar} or @code{defconst}, which are documented below. |
| 392 | |
| 393 | A variable definition serves three purposes. First, it informs |
| 394 | people who read the code that the symbol is @emph{intended} to be used |
| 395 | a certain way (as a variable). Second, it informs the Lisp system of |
| 396 | this, optionally supplying an initial value and a documentation |
| 397 | string. Third, it provides information to programming tools such as |
| 398 | @command{etags}, allowing them to find where the variable was defined. |
| 399 | |
| 400 | The difference between @code{defconst} and @code{defvar} is mainly a |
| 401 | matter of intent, serving to inform human readers of whether the value |
| 402 | should ever change. Emacs Lisp does not actually prevent you from |
| 403 | changing the value of a variable defined with @code{defconst}. One |
| 404 | notable difference between the two forms is that @code{defconst} |
| 405 | unconditionally initializes the variable, whereas @code{defvar} |
| 406 | initializes it only if it is originally void. |
| 407 | |
| 408 | To define a customizable variable, you should use @code{defcustom} |
| 409 | (which calls @code{defvar} as a subroutine). @xref{Variable |
| 410 | Definitions}. |
| 411 | |
| 412 | @defspec defvar symbol [value [doc-string]] |
| 413 | This special form defines @var{symbol} as a variable. Note that |
| 414 | @var{symbol} is not evaluated; the symbol to be defined should appear |
| 415 | explicitly in the @code{defvar} form. The variable is marked as |
| 416 | @dfn{special}, meaning that it should always be dynamically bound |
| 417 | (@pxref{Variable Scoping}). |
| 418 | |
| 419 | If @var{value} is specified, and @var{symbol} is void (i.e., it has no |
| 420 | dynamically bound value; @pxref{Void Variables}), then @var{value} is |
| 421 | evaluated and @var{symbol} is set to the result. But if @var{symbol} |
| 422 | is not void, @var{value} is not evaluated, and @var{symbol}'s value is |
| 423 | left unchanged. If @var{value} is omitted, the value of @var{symbol} |
| 424 | is not changed in any case. |
| 425 | |
| 426 | If @var{symbol} has a buffer-local binding in the current buffer, |
| 427 | @code{defvar} acts on the default value, which is buffer-independent, |
| 428 | rather than the buffer-local binding. It sets the default value if |
| 429 | the default value is void. @xref{Buffer-Local Variables}. |
| 430 | |
| 431 | If @var{symbol} is already lexically bound (e.g., if the @code{defvar} |
| 432 | form occurs in a @code{let} form with lexical binding enabled), then |
| 433 | @code{defvar} sets the dynamic value. The lexical binding remains in |
| 434 | effect until its binding construct exits. @xref{Variable Scoping}. |
| 435 | |
| 436 | When you evaluate a top-level @code{defvar} form with @kbd{C-M-x} in |
| 437 | Emacs Lisp mode (@code{eval-defun}), a special feature of |
| 438 | @code{eval-defun} arranges to set the variable unconditionally, without |
| 439 | testing whether its value is void. |
| 440 | |
| 441 | If the @var{doc-string} argument is supplied, it specifies the |
| 442 | documentation string for the variable (stored in the symbol's |
| 443 | @code{variable-documentation} property). @xref{Documentation}. |
| 444 | |
| 445 | Here are some examples. This form defines @code{foo} but does not |
| 446 | initialize it: |
| 447 | |
| 448 | @example |
| 449 | @group |
| 450 | (defvar foo) |
| 451 | @result{} foo |
| 452 | @end group |
| 453 | @end example |
| 454 | |
| 455 | This example initializes the value of @code{bar} to @code{23}, and gives |
| 456 | it a documentation string: |
| 457 | |
| 458 | @example |
| 459 | @group |
| 460 | (defvar bar 23 |
| 461 | "The normal weight of a bar.") |
| 462 | @result{} bar |
| 463 | @end group |
| 464 | @end example |
| 465 | |
| 466 | The @code{defvar} form returns @var{symbol}, but it is normally used |
| 467 | at top level in a file where its value does not matter. |
| 468 | @end defspec |
| 469 | |
| 470 | @cindex constant variables |
| 471 | @defspec defconst symbol value [doc-string] |
| 472 | This special form defines @var{symbol} as a value and initializes it. |
| 473 | It informs a person reading your code that @var{symbol} has a standard |
| 474 | global value, established here, that should not be changed by the user |
| 475 | or by other programs. Note that @var{symbol} is not evaluated; the |
| 476 | symbol to be defined must appear explicitly in the @code{defconst}. |
| 477 | |
| 478 | The @code{defconst} form, like @code{defvar}, marks the variable as |
| 479 | @dfn{special}, meaning that it should always be dynamically bound |
| 480 | (@pxref{Variable Scoping}). In addition, it marks the variable as |
| 481 | risky (@pxref{File Local Variables}). |
| 482 | |
| 483 | @code{defconst} always evaluates @var{value}, and sets the value of |
| 484 | @var{symbol} to the result. If @var{symbol} does have a buffer-local |
| 485 | binding in the current buffer, @code{defconst} sets the default value, |
| 486 | not the buffer-local value. (But you should not be making |
| 487 | buffer-local bindings for a symbol that is defined with |
| 488 | @code{defconst}.) |
| 489 | |
| 490 | An example of the use of @code{defconst} is Emacs's definition of |
| 491 | @code{float-pi}---the mathematical constant @math{pi}, which ought not |
| 492 | to be changed by anyone (attempts by the Indiana State Legislature |
| 493 | notwithstanding). As the second form illustrates, however, |
| 494 | @code{defconst} is only advisory. |
| 495 | |
| 496 | @example |
| 497 | @group |
| 498 | (defconst float-pi 3.141592653589793 "The value of Pi.") |
| 499 | @result{} float-pi |
| 500 | @end group |
| 501 | @group |
| 502 | (setq float-pi 3) |
| 503 | @result{} float-pi |
| 504 | @end group |
| 505 | @group |
| 506 | float-pi |
| 507 | @result{} 3 |
| 508 | @end group |
| 509 | @end example |
| 510 | @end defspec |
| 511 | |
| 512 | @strong{Warning:} If you use a @code{defconst} or @code{defvar} |
| 513 | special form while the variable has a local binding (made with |
| 514 | @code{let}, or a function argument), it sets the local binding rather |
| 515 | than the global binding. This is not what you usually want. To |
| 516 | prevent this, use these special forms at top level in a file, where |
| 517 | normally no local binding is in effect, and make sure to load the file |
| 518 | before making a local binding for the variable. |
| 519 | |
| 520 | @node Tips for Defining |
| 521 | @section Tips for Defining Variables Robustly |
| 522 | |
| 523 | When you define a variable whose value is a function, or a list of |
| 524 | functions, use a name that ends in @samp{-function} or |
| 525 | @samp{-functions}, respectively. |
| 526 | |
| 527 | There are several other variable name conventions; |
| 528 | here is a complete list: |
| 529 | |
| 530 | @table @samp |
| 531 | @item @dots{}-hook |
| 532 | The variable is a normal hook (@pxref{Hooks}). |
| 533 | |
| 534 | @item @dots{}-function |
| 535 | The value is a function. |
| 536 | |
| 537 | @item @dots{}-functions |
| 538 | The value is a list of functions. |
| 539 | |
| 540 | @item @dots{}-form |
| 541 | The value is a form (an expression). |
| 542 | |
| 543 | @item @dots{}-forms |
| 544 | The value is a list of forms (expressions). |
| 545 | |
| 546 | @item @dots{}-predicate |
| 547 | The value is a predicate---a function of one argument that returns |
| 548 | non-@code{nil} for ``good'' arguments and @code{nil} for ``bad'' |
| 549 | arguments. |
| 550 | |
| 551 | @item @dots{}-flag |
| 552 | The value is significant only as to whether it is @code{nil} or not. |
| 553 | Since such variables often end up acquiring more values over time, |
| 554 | this convention is not strongly recommended. |
| 555 | |
| 556 | @item @dots{}-program |
| 557 | The value is a program name. |
| 558 | |
| 559 | @item @dots{}-command |
| 560 | The value is a whole shell command. |
| 561 | |
| 562 | @item @dots{}-switches |
| 563 | The value specifies options for a command. |
| 564 | @end table |
| 565 | |
| 566 | When you define a variable, always consider whether you should mark |
| 567 | it as ``safe'' or ``risky''; see @ref{File Local Variables}. |
| 568 | |
| 569 | When defining and initializing a variable that holds a complicated |
| 570 | value (such as a keymap with bindings in it), it's best to put the |
| 571 | entire computation of the value into the @code{defvar}, like this: |
| 572 | |
| 573 | @example |
| 574 | (defvar my-mode-map |
| 575 | (let ((map (make-sparse-keymap))) |
| 576 | (define-key map "\C-c\C-a" 'my-command) |
| 577 | @dots{} |
| 578 | map) |
| 579 | @var{docstring}) |
| 580 | @end example |
| 581 | |
| 582 | @noindent |
| 583 | This method has several benefits. First, if the user quits while |
| 584 | loading the file, the variable is either still uninitialized or |
| 585 | initialized properly, never in-between. If it is still uninitialized, |
| 586 | reloading the file will initialize it properly. Second, reloading the |
| 587 | file once the variable is initialized will not alter it; that is |
| 588 | important if the user has run hooks to alter part of the contents |
| 589 | (such as, to rebind keys). Third, evaluating the @code{defvar} form |
| 590 | with @kbd{C-M-x} will reinitialize the map completely. |
| 591 | |
| 592 | Putting so much code in the @code{defvar} form has one disadvantage: |
| 593 | it puts the documentation string far away from the line which names the |
| 594 | variable. Here's a safe way to avoid that: |
| 595 | |
| 596 | @example |
| 597 | (defvar my-mode-map nil |
| 598 | @var{docstring}) |
| 599 | (unless my-mode-map |
| 600 | (let ((map (make-sparse-keymap))) |
| 601 | (define-key map "\C-c\C-a" 'my-command) |
| 602 | @dots{} |
| 603 | (setq my-mode-map map))) |
| 604 | @end example |
| 605 | |
| 606 | @noindent |
| 607 | This has all the same advantages as putting the initialization inside |
| 608 | the @code{defvar}, except that you must type @kbd{C-M-x} twice, once on |
| 609 | each form, if you do want to reinitialize the variable. |
| 610 | |
| 611 | @node Accessing Variables |
| 612 | @section Accessing Variable Values |
| 613 | |
| 614 | The usual way to reference a variable is to write the symbol which |
| 615 | names it. @xref{Symbol Forms}. |
| 616 | |
| 617 | Occasionally, you may want to reference a variable which is only |
| 618 | determined at run time. In that case, you cannot specify the variable |
| 619 | name in the text of the program. You can use the @code{symbol-value} |
| 620 | function to extract the value. |
| 621 | |
| 622 | @defun symbol-value symbol |
| 623 | This function returns the value stored in @var{symbol}'s value cell. |
| 624 | This is where the variable's current (dynamic) value is stored. If |
| 625 | the variable has no local binding, this is simply its global value. |
| 626 | If the variable is void, a @code{void-variable} error is signaled. |
| 627 | |
| 628 | If the variable is lexically bound, the value reported by |
| 629 | @code{symbol-value} is not necessarily the same as the variable's |
| 630 | lexical value, which is determined by the lexical environment rather |
| 631 | than the symbol's value cell. @xref{Variable Scoping}. |
| 632 | |
| 633 | @example |
| 634 | @group |
| 635 | (setq abracadabra 5) |
| 636 | @result{} 5 |
| 637 | @end group |
| 638 | @group |
| 639 | (setq foo 9) |
| 640 | @result{} 9 |
| 641 | @end group |
| 642 | |
| 643 | @group |
| 644 | ;; @r{Here the symbol @code{abracadabra}} |
| 645 | ;; @r{is the symbol whose value is examined.} |
| 646 | (let ((abracadabra 'foo)) |
| 647 | (symbol-value 'abracadabra)) |
| 648 | @result{} foo |
| 649 | @end group |
| 650 | |
| 651 | @group |
| 652 | ;; @r{Here, the value of @code{abracadabra},} |
| 653 | ;; @r{which is @code{foo},} |
| 654 | ;; @r{is the symbol whose value is examined.} |
| 655 | (let ((abracadabra 'foo)) |
| 656 | (symbol-value abracadabra)) |
| 657 | @result{} 9 |
| 658 | @end group |
| 659 | |
| 660 | @group |
| 661 | (symbol-value 'abracadabra) |
| 662 | @result{} 5 |
| 663 | @end group |
| 664 | @end example |
| 665 | @end defun |
| 666 | |
| 667 | @node Setting Variables |
| 668 | @section Setting Variable Values |
| 669 | |
| 670 | The usual way to change the value of a variable is with the special |
| 671 | form @code{setq}. When you need to compute the choice of variable at |
| 672 | run time, use the function @code{set}. |
| 673 | |
| 674 | @defspec setq [symbol form]@dots{} |
| 675 | This special form is the most common method of changing a variable's |
| 676 | value. Each @var{symbol} is given a new value, which is the result of |
| 677 | evaluating the corresponding @var{form}. The current binding of the |
| 678 | symbol is changed. |
| 679 | |
| 680 | @code{setq} does not evaluate @var{symbol}; it sets the symbol that you |
| 681 | write. We say that this argument is @dfn{automatically quoted}. The |
| 682 | @samp{q} in @code{setq} stands for ``quoted''. |
| 683 | |
| 684 | The value of the @code{setq} form is the value of the last @var{form}. |
| 685 | |
| 686 | @example |
| 687 | @group |
| 688 | (setq x (1+ 2)) |
| 689 | @result{} 3 |
| 690 | @end group |
| 691 | x ; @r{@code{x} now has a global value.} |
| 692 | @result{} 3 |
| 693 | @group |
| 694 | (let ((x 5)) |
| 695 | (setq x 6) ; @r{The local binding of @code{x} is set.} |
| 696 | x) |
| 697 | @result{} 6 |
| 698 | @end group |
| 699 | x ; @r{The global value is unchanged.} |
| 700 | @result{} 3 |
| 701 | @end example |
| 702 | |
| 703 | Note that the first @var{form} is evaluated, then the first |
| 704 | @var{symbol} is set, then the second @var{form} is evaluated, then the |
| 705 | second @var{symbol} is set, and so on: |
| 706 | |
| 707 | @example |
| 708 | @group |
| 709 | (setq x 10 ; @r{Notice that @code{x} is set before} |
| 710 | y (1+ x)) ; @r{the value of @code{y} is computed.} |
| 711 | @result{} 11 |
| 712 | @end group |
| 713 | @end example |
| 714 | @end defspec |
| 715 | |
| 716 | @defun set symbol value |
| 717 | This function puts @var{value} in the value cell of @var{symbol}. |
| 718 | Since it is a function rather than a special form, the expression |
| 719 | written for @var{symbol} is evaluated to obtain the symbol to set. |
| 720 | The return value is @var{value}. |
| 721 | |
| 722 | When dynamic variable binding is in effect (the default), @code{set} |
| 723 | has the same effect as @code{setq}, apart from the fact that |
| 724 | @code{set} evaluates its @var{symbol} argument whereas @code{setq} |
| 725 | does not. But when a variable is lexically bound, @code{set} affects |
| 726 | its @emph{dynamic} value, whereas @code{setq} affects its current |
| 727 | (lexical) value. @xref{Variable Scoping}. |
| 728 | |
| 729 | @example |
| 730 | @group |
| 731 | (set one 1) |
| 732 | @error{} Symbol's value as variable is void: one |
| 733 | @end group |
| 734 | @group |
| 735 | (set 'one 1) |
| 736 | @result{} 1 |
| 737 | @end group |
| 738 | @group |
| 739 | (set 'two 'one) |
| 740 | @result{} one |
| 741 | @end group |
| 742 | @group |
| 743 | (set two 2) ; @r{@code{two} evaluates to symbol @code{one}.} |
| 744 | @result{} 2 |
| 745 | @end group |
| 746 | @group |
| 747 | one ; @r{So it is @code{one} that was set.} |
| 748 | @result{} 2 |
| 749 | (let ((one 1)) ; @r{This binding of @code{one} is set,} |
| 750 | (set 'one 3) ; @r{not the global value.} |
| 751 | one) |
| 752 | @result{} 3 |
| 753 | @end group |
| 754 | @group |
| 755 | one |
| 756 | @result{} 2 |
| 757 | @end group |
| 758 | @end example |
| 759 | |
| 760 | If @var{symbol} is not actually a symbol, a @code{wrong-type-argument} |
| 761 | error is signaled. |
| 762 | |
| 763 | @example |
| 764 | (set '(x y) 'z) |
| 765 | @error{} Wrong type argument: symbolp, (x y) |
| 766 | @end example |
| 767 | @end defun |
| 768 | |
| 769 | @node Variable Scoping |
| 770 | @section Scoping Rules for Variable Bindings |
| 771 | @cindex scoping rule |
| 772 | |
| 773 | When you create a local binding for a variable, that binding takes |
| 774 | effect only within a limited portion of the program (@pxref{Local |
| 775 | Variables}). This section describes exactly what this means. |
| 776 | |
| 777 | @cindex scope |
| 778 | @cindex extent |
| 779 | Each local binding has a certain @dfn{scope} and @dfn{extent}. |
| 780 | @dfn{Scope} refers to @emph{where} in the textual source code the |
| 781 | binding can be accessed. @dfn{Extent} refers to @emph{when}, as the |
| 782 | program is executing, the binding exists. |
| 783 | |
| 784 | @cindex dynamic binding |
| 785 | @cindex dynamic scope |
| 786 | @cindex dynamic extent |
| 787 | By default, the local bindings that Emacs creates are @dfn{dynamic |
| 788 | bindings}. Such a binding has @dfn{dynamic scope}, meaning that any |
| 789 | part of the program can potentially access the variable binding. It |
| 790 | also has @dfn{dynamic extent}, meaning that the binding lasts only |
| 791 | while the binding construct (such as the body of a @code{let} form) is |
| 792 | being executed. |
| 793 | |
| 794 | @cindex lexical binding |
| 795 | @cindex lexical scope |
| 796 | @cindex indefinite extent |
| 797 | Emacs can optionally create @dfn{lexical bindings}. A lexical |
| 798 | binding has @dfn{lexical scope}, meaning that any reference to the |
| 799 | variable must be located textually within the binding |
| 800 | construct@footnote{With some exceptions; for instance, a lexical |
| 801 | binding can also be accessed from the Lisp debugger.}. It also has |
| 802 | @dfn{indefinite extent}, meaning that under some circumstances the |
| 803 | binding can live on even after the binding construct has finished |
| 804 | executing, by means of special objects called @dfn{closures}. |
| 805 | |
| 806 | The following subsections describe dynamic binding and lexical |
| 807 | binding in greater detail, and how to enable lexical binding in Emacs |
| 808 | Lisp programs. |
| 809 | |
| 810 | @menu |
| 811 | * Dynamic Binding:: The default for binding local variables in Emacs. |
| 812 | * Dynamic Binding Tips:: Avoiding problems with dynamic binding. |
| 813 | * Lexical Binding:: A different type of local variable binding. |
| 814 | * Using Lexical Binding:: How to enable lexical binding. |
| 815 | @end menu |
| 816 | |
| 817 | @node Dynamic Binding |
| 818 | @subsection Dynamic Binding |
| 819 | |
| 820 | By default, the local variable bindings made by Emacs are dynamic |
| 821 | bindings. When a variable is dynamically bound, its current binding |
| 822 | at any point in the execution of the Lisp program is simply the most |
| 823 | recently-created dynamic local binding for that symbol, or the global |
| 824 | binding if there is no such local binding. |
| 825 | |
| 826 | Dynamic bindings have dynamic scope and extent, as shown by the |
| 827 | following example: |
| 828 | |
| 829 | @example |
| 830 | @group |
| 831 | (defvar x -99) ; @r{@code{x} receives an initial value of @minus{}99.} |
| 832 | |
| 833 | (defun getx () |
| 834 | x) ; @r{@code{x} is used ``free'' in this function.} |
| 835 | |
| 836 | (let ((x 1)) ; @r{@code{x} is dynamically bound.} |
| 837 | (getx)) |
| 838 | @result{} 1 |
| 839 | |
| 840 | ;; @r{After the @code{let} form finishes, @code{x} reverts to its} |
| 841 | ;; @r{previous value, which is @minus{}99.} |
| 842 | |
| 843 | (getx) |
| 844 | @result{} -99 |
| 845 | @end group |
| 846 | @end example |
| 847 | |
| 848 | @noindent |
| 849 | The function @code{getx} refers to @code{x}. This is a ``free'' |
| 850 | reference, in the sense that there is no binding for @code{x} within |
| 851 | that @code{defun} construct itself. When we call @code{getx} from |
| 852 | within a @code{let} form in which @code{x} is (dynamically) bound, it |
| 853 | retrieves the local value (i.e., 1). But when we call @code{getx} |
| 854 | outside the @code{let} form, it retrieves the global value (i.e., |
| 855 | @minus{}99). |
| 856 | |
| 857 | Here is another example, which illustrates setting a dynamically |
| 858 | bound variable using @code{setq}: |
| 859 | |
| 860 | @example |
| 861 | @group |
| 862 | (defvar x -99) ; @r{@code{x} receives an initial value of @minus{}99.} |
| 863 | |
| 864 | (defun addx () |
| 865 | (setq x (1+ x))) ; @r{Add 1 to @code{x} and return its new value.} |
| 866 | |
| 867 | (let ((x 1)) |
| 868 | (addx) |
| 869 | (addx)) |
| 870 | @result{} 3 ; @r{The two @code{addx} calls add to @code{x} twice.} |
| 871 | |
| 872 | ;; @r{After the @code{let} form finishes, @code{x} reverts to its} |
| 873 | ;; @r{previous value, which is @minus{}99.} |
| 874 | |
| 875 | (addx) |
| 876 | @result{} -98 |
| 877 | @end group |
| 878 | @end example |
| 879 | |
| 880 | Dynamic binding is implemented in Emacs Lisp in a simple way. Each |
| 881 | symbol has a value cell, which specifies its current dynamic value (or |
| 882 | absence of value). @xref{Symbol Components}. When a symbol is given |
| 883 | a dynamic local binding, Emacs records the contents of the value cell |
| 884 | (or absence thereof) in a stack, and stores the new local value in the |
| 885 | value cell. When the binding construct finishes executing, Emacs pops |
| 886 | the old value off the stack, and puts it in the value cell. |
| 887 | |
| 888 | @node Dynamic Binding Tips |
| 889 | @subsection Proper Use of Dynamic Binding |
| 890 | |
| 891 | Dynamic binding is a powerful feature, as it allows programs to |
| 892 | refer to variables that are not defined within their local textual |
| 893 | scope. However, if used without restraint, this can also make |
| 894 | programs hard to understand. There are two clean ways to use this |
| 895 | technique: |
| 896 | |
| 897 | @itemize @bullet |
| 898 | @item |
| 899 | If a variable has no global definition, use it as a local variable |
| 900 | only within a binding construct, such as the body of the @code{let} |
| 901 | form where the variable was bound. If this convention is followed |
| 902 | consistently throughout a program, the value of the variable will not |
| 903 | affect, nor be affected by, any uses of the same variable symbol |
| 904 | elsewhere in the program. |
| 905 | |
| 906 | @item |
| 907 | Otherwise, define the variable with @code{defvar}, @code{defconst}, or |
| 908 | @code{defcustom}. @xref{Defining Variables}. Usually, the definition |
| 909 | should be at top-level in an Emacs Lisp file. As far as possible, it |
| 910 | should include a documentation string which explains the meaning and |
| 911 | purpose of the variable. You should also choose the variable's name |
| 912 | to avoid name conflicts (@pxref{Coding Conventions}). |
| 913 | |
| 914 | Then you can bind the variable anywhere in a program, knowing reliably |
| 915 | what the effect will be. Wherever you encounter the variable, it will |
| 916 | be easy to refer back to the definition, e.g., via the @kbd{C-h v} |
| 917 | command (provided the variable definition has been loaded into Emacs). |
| 918 | @xref{Name Help,,, emacs, The GNU Emacs Manual}. |
| 919 | |
| 920 | For example, it is common to use local bindings for customizable |
| 921 | variables like @code{case-fold-search}: |
| 922 | |
| 923 | @example |
| 924 | @group |
| 925 | (defun search-for-abc () |
| 926 | "Search for the string \"abc\", ignoring case differences." |
| 927 | (let ((case-fold-search nil)) |
| 928 | (re-search-forward "abc"))) |
| 929 | @end group |
| 930 | @end example |
| 931 | @end itemize |
| 932 | |
| 933 | @node Lexical Binding |
| 934 | @subsection Lexical Binding |
| 935 | |
| 936 | Lexical binding was introduced to Emacs, as an optional feature, in |
| 937 | version 24.1. We expect its importance to increase in the future. |
| 938 | Lexical binding opens up many more opportunities for optimization, so |
| 939 | programs using it are likely to run faster in future Emacs versions. |
| 940 | Lexical binding is also more compatible with concurrency, which we |
| 941 | want to add to Emacs in the future. |
| 942 | |
| 943 | A lexically-bound variable has @dfn{lexical scope}, meaning that any |
| 944 | reference to the variable must be located textually within the binding |
| 945 | construct. Here is an example |
| 946 | @iftex |
| 947 | (see the next subsection, for how to actually enable lexical binding): |
| 948 | @end iftex |
| 949 | @ifnottex |
| 950 | (@pxref{Using Lexical Binding}, for how to actually enable lexical binding): |
| 951 | @end ifnottex |
| 952 | |
| 953 | @example |
| 954 | @group |
| 955 | (let ((x 1)) ; @r{@code{x} is lexically bound.} |
| 956 | (+ x 3)) |
| 957 | @result{} 4 |
| 958 | |
| 959 | (defun getx () |
| 960 | x) ; @r{@code{x} is used ``free'' in this function.} |
| 961 | |
| 962 | (let ((x 1)) ; @r{@code{x} is lexically bound.} |
| 963 | (getx)) |
| 964 | @error{} Symbol's value as variable is void: x |
| 965 | @end group |
| 966 | @end example |
| 967 | |
| 968 | @noindent |
| 969 | Here, the variable @code{x} has no global value. When it is lexically |
| 970 | bound within a @code{let} form, it can be used in the textual confines |
| 971 | of that @code{let} form. But it can @emph{not} be used from within a |
| 972 | @code{getx} function called from the @code{let} form, since the |
| 973 | function definition of @code{getx} occurs outside the @code{let} form |
| 974 | itself. |
| 975 | |
| 976 | @cindex lexical environment |
| 977 | Here is how lexical binding works. Each binding construct defines a |
| 978 | @dfn{lexical environment}, specifying the symbols that are bound |
| 979 | within the construct and their local values. When the Lisp evaluator |
| 980 | wants the current value of a variable, it looks first in the lexical |
| 981 | environment; if the variable is not specified in there, it looks in |
| 982 | the symbol's value cell, where the dynamic value is stored. |
| 983 | |
| 984 | (Internally, the lexical environment is an alist of symbol-value |
| 985 | pairs, with the final element in the alist being the symbol @code{t} |
| 986 | rather than a cons cell. Such an alist can be passed as the second |
| 987 | argument to the @code{eval} function, in order to specify a lexical |
| 988 | environment in which to evaluate a form. @xref{Eval}. Most Emacs |
| 989 | Lisp programs, however, should not interact directly with lexical |
| 990 | environments in this way; only specialized programs like debuggers.) |
| 991 | |
| 992 | @cindex closures, example of using |
| 993 | Lexical bindings have indefinite extent. Even after a binding |
| 994 | construct has finished executing, its lexical environment can be |
| 995 | ``kept around'' in Lisp objects called @dfn{closures}. A closure is |
| 996 | created when you define a named or anonymous function with lexical |
| 997 | binding enabled. @xref{Closures}, for details. |
| 998 | |
| 999 | When a closure is called as a function, any lexical variable |
| 1000 | references within its definition use the retained lexical environment. |
| 1001 | Here is an example: |
| 1002 | |
| 1003 | @example |
| 1004 | (defvar my-ticker nil) ; @r{We will use this dynamically bound} |
| 1005 | ; @r{variable to store a closure.} |
| 1006 | |
| 1007 | (let ((x 0)) ; @r{@code{x} is lexically bound.} |
| 1008 | (setq my-ticker (lambda () |
| 1009 | (setq x (1+ x))))) |
| 1010 | @result{} (closure ((x . 0) t) () |
| 1011 | (setq x (1+ x))) |
| 1012 | |
| 1013 | (funcall my-ticker) |
| 1014 | @result{} 1 |
| 1015 | |
| 1016 | (funcall my-ticker) |
| 1017 | @result{} 2 |
| 1018 | |
| 1019 | (funcall my-ticker) |
| 1020 | @result{} 3 |
| 1021 | |
| 1022 | x ; @r{Note that @code{x} has no global value.} |
| 1023 | @error{} Symbol's value as variable is void: x |
| 1024 | @end example |
| 1025 | |
| 1026 | @noindent |
| 1027 | The @code{let} binding defines a lexical environment in which the |
| 1028 | variable @code{x} is locally bound to 0. Within this binding |
| 1029 | construct, we define a lambda expression which increments @code{x} by |
| 1030 | one and returns the incremented value. This lambda expression is |
| 1031 | automatically turned into a closure, in which the lexical environment |
| 1032 | lives on even after the @code{let} binding construct has exited. Each |
| 1033 | time we evaluate the closure, it increments @code{x}, using the |
| 1034 | binding of @code{x} in that lexical environment. |
| 1035 | |
| 1036 | Note that functions like @code{symbol-value}, @code{boundp}, and |
| 1037 | @code{set} only retrieve or modify a variable's dynamic binding |
| 1038 | (i.e., the contents of its symbol's value cell). Also, the code in |
| 1039 | the body of a @code{defun} or @code{defmacro} cannot refer to |
| 1040 | surrounding lexical variables. |
| 1041 | |
| 1042 | @node Using Lexical Binding |
| 1043 | @subsection Using Lexical Binding |
| 1044 | |
| 1045 | When loading an Emacs Lisp file or evaluating a Lisp buffer, lexical |
| 1046 | binding is enabled if the buffer-local variable @code{lexical-binding} |
| 1047 | is non-@code{nil}: |
| 1048 | |
| 1049 | @defvar lexical-binding |
| 1050 | If this buffer-local variable is non-@code{nil}, Emacs Lisp files and |
| 1051 | buffers are evaluated using lexical binding instead of dynamic |
| 1052 | binding. (However, special variables are still dynamically bound; see |
| 1053 | below.) If @code{nil}, dynamic binding is used for all local |
| 1054 | variables. This variable is typically set for a whole Emacs Lisp |
| 1055 | file, as a file local variable (@pxref{File Local Variables}). |
| 1056 | Note that unlike other such variables, this one must be set in the |
| 1057 | first line of a file. |
| 1058 | @end defvar |
| 1059 | |
| 1060 | @noindent |
| 1061 | When evaluating Emacs Lisp code directly using an @code{eval} call, |
| 1062 | lexical binding is enabled if the @var{lexical} argument to |
| 1063 | @code{eval} is non-@code{nil}. @xref{Eval}. |
| 1064 | |
| 1065 | @cindex special variables |
| 1066 | Even when lexical binding is enabled, certain variables will |
| 1067 | continue to be dynamically bound. These are called @dfn{special |
| 1068 | variables}. Every variable that has been defined with @code{defvar}, |
| 1069 | @code{defcustom} or @code{defconst} is a special variable |
| 1070 | (@pxref{Defining Variables}). All other variables are subject to |
| 1071 | lexical binding. |
| 1072 | |
| 1073 | @defun special-variable-p symbol |
| 1074 | This function returns non-@code{nil} if @var{symbol} is a special |
| 1075 | variable (i.e., it has a @code{defvar}, @code{defcustom}, or |
| 1076 | @code{defconst} variable definition). Otherwise, the return value is |
| 1077 | @code{nil}. |
| 1078 | @end defun |
| 1079 | |
| 1080 | The use of a special variable as a formal argument in a function is |
| 1081 | discouraged. Doing so gives rise to unspecified behavior when lexical |
| 1082 | binding mode is enabled (it may use lexical binding sometimes, and |
| 1083 | dynamic binding other times). |
| 1084 | |
| 1085 | Converting an Emacs Lisp program to lexical binding is easy. First, |
| 1086 | add a file-local variable setting of @code{lexical-binding} to |
| 1087 | @code{t} in the header line of the Emacs Lisp source file (@pxref{File |
| 1088 | Local Variables}). Second, check that every variable in the program |
| 1089 | which needs to be dynamically bound has a variable definition, so that |
| 1090 | it is not inadvertently bound lexically. |
| 1091 | |
| 1092 | @cindex free variable |
| 1093 | @cindex unused lexical variable |
| 1094 | A simple way to find out which variables need a variable definition |
| 1095 | is to byte-compile the source file. @xref{Byte Compilation}. If a |
| 1096 | non-special variable is used outside of a @code{let} form, the |
| 1097 | byte-compiler will warn about reference or assignment to a ``free |
| 1098 | variable''. If a non-special variable is bound but not used within a |
| 1099 | @code{let} form, the byte-compiler will warn about an ``unused lexical |
| 1100 | variable''. The byte-compiler will also issue a warning if you use a |
| 1101 | special variable as a function argument. |
| 1102 | |
| 1103 | (To silence byte-compiler warnings about unused variables, just use |
| 1104 | a variable name that start with an underscore. The byte-compiler |
| 1105 | interprets this as an indication that this is a variable known not to |
| 1106 | be used.) |
| 1107 | |
| 1108 | @node Buffer-Local Variables |
| 1109 | @section Buffer-Local Variables |
| 1110 | @cindex variable, buffer-local |
| 1111 | @cindex buffer-local variables |
| 1112 | |
| 1113 | Global and local variable bindings are found in most programming |
| 1114 | languages in one form or another. Emacs, however, also supports |
| 1115 | additional, unusual kinds of variable binding, such as |
| 1116 | @dfn{buffer-local} bindings, which apply only in one buffer. Having |
| 1117 | different values for a variable in different buffers is an important |
| 1118 | customization method. (Variables can also have bindings that are |
| 1119 | local to each terminal. @xref{Multiple Terminals}.) |
| 1120 | |
| 1121 | @menu |
| 1122 | * Intro to Buffer-Local:: Introduction and concepts. |
| 1123 | * Creating Buffer-Local:: Creating and destroying buffer-local bindings. |
| 1124 | * Default Value:: The default value is seen in buffers |
| 1125 | that don't have their own buffer-local values. |
| 1126 | @end menu |
| 1127 | |
| 1128 | @node Intro to Buffer-Local |
| 1129 | @subsection Introduction to Buffer-Local Variables |
| 1130 | |
| 1131 | A buffer-local variable has a buffer-local binding associated with a |
| 1132 | particular buffer. The binding is in effect when that buffer is |
| 1133 | current; otherwise, it is not in effect. If you set the variable while |
| 1134 | a buffer-local binding is in effect, the new value goes in that binding, |
| 1135 | so its other bindings are unchanged. This means that the change is |
| 1136 | visible only in the buffer where you made it. |
| 1137 | |
| 1138 | The variable's ordinary binding, which is not associated with any |
| 1139 | specific buffer, is called the @dfn{default binding}. In most cases, |
| 1140 | this is the global binding. |
| 1141 | |
| 1142 | A variable can have buffer-local bindings in some buffers but not in |
| 1143 | other buffers. The default binding is shared by all the buffers that |
| 1144 | don't have their own bindings for the variable. (This includes all |
| 1145 | newly-created buffers.) If you set the variable in a buffer that does |
| 1146 | not have a buffer-local binding for it, this sets the default binding, |
| 1147 | so the new value is visible in all the buffers that see the default |
| 1148 | binding. |
| 1149 | |
| 1150 | The most common use of buffer-local bindings is for major modes to change |
| 1151 | variables that control the behavior of commands. For example, C mode and |
| 1152 | Lisp mode both set the variable @code{paragraph-start} to specify that only |
| 1153 | blank lines separate paragraphs. They do this by making the variable |
| 1154 | buffer-local in the buffer that is being put into C mode or Lisp mode, and |
| 1155 | then setting it to the new value for that mode. @xref{Major Modes}. |
| 1156 | |
| 1157 | The usual way to make a buffer-local binding is with |
| 1158 | @code{make-local-variable}, which is what major mode commands typically |
| 1159 | use. This affects just the current buffer; all other buffers (including |
| 1160 | those yet to be created) will continue to share the default value unless |
| 1161 | they are explicitly given their own buffer-local bindings. |
| 1162 | |
| 1163 | @cindex automatically buffer-local |
| 1164 | A more powerful operation is to mark the variable as |
| 1165 | @dfn{automatically buffer-local} by calling |
| 1166 | @code{make-variable-buffer-local}. You can think of this as making the |
| 1167 | variable local in all buffers, even those yet to be created. More |
| 1168 | precisely, the effect is that setting the variable automatically makes |
| 1169 | the variable local to the current buffer if it is not already so. All |
| 1170 | buffers start out by sharing the default value of the variable as usual, |
| 1171 | but setting the variable creates a buffer-local binding for the current |
| 1172 | buffer. The new value is stored in the buffer-local binding, leaving |
| 1173 | the default binding untouched. This means that the default value cannot |
| 1174 | be changed with @code{setq} in any buffer; the only way to change it is |
| 1175 | with @code{setq-default}. |
| 1176 | |
| 1177 | @strong{Warning:} When a variable has buffer-local |
| 1178 | bindings in one or more buffers, @code{let} rebinds the binding that's |
| 1179 | currently in effect. For instance, if the current buffer has a |
| 1180 | buffer-local value, @code{let} temporarily rebinds that. If no |
| 1181 | buffer-local bindings are in effect, @code{let} rebinds |
| 1182 | the default value. If inside the @code{let} you then change to a |
| 1183 | different current buffer in which a different binding is in effect, |
| 1184 | you won't see the @code{let} binding any more. And if you exit the |
| 1185 | @code{let} while still in the other buffer, you won't see the |
| 1186 | unbinding occur (though it will occur properly). Here is an example |
| 1187 | to illustrate: |
| 1188 | |
| 1189 | @example |
| 1190 | @group |
| 1191 | (setq foo 'g) |
| 1192 | (set-buffer "a") |
| 1193 | (make-local-variable 'foo) |
| 1194 | @end group |
| 1195 | (setq foo 'a) |
| 1196 | (let ((foo 'temp)) |
| 1197 | ;; foo @result{} 'temp ; @r{let binding in buffer @samp{a}} |
| 1198 | (set-buffer "b") |
| 1199 | ;; foo @result{} 'g ; @r{the global value since foo is not local in @samp{b}} |
| 1200 | @var{body}@dots{}) |
| 1201 | @group |
| 1202 | foo @result{} 'g ; @r{exiting restored the local value in buffer @samp{a},} |
| 1203 | ; @r{but we don't see that in buffer @samp{b}} |
| 1204 | @end group |
| 1205 | @group |
| 1206 | (set-buffer "a") ; @r{verify the local value was restored} |
| 1207 | foo @result{} 'a |
| 1208 | @end group |
| 1209 | @end example |
| 1210 | |
| 1211 | @noindent |
| 1212 | Note that references to @code{foo} in @var{body} access the |
| 1213 | buffer-local binding of buffer @samp{b}. |
| 1214 | |
| 1215 | When a file specifies local variable values, these become buffer-local |
| 1216 | values when you visit the file. @xref{File Variables,,, emacs, The |
| 1217 | GNU Emacs Manual}. |
| 1218 | |
| 1219 | A buffer-local variable cannot be made terminal-local |
| 1220 | (@pxref{Multiple Terminals}). |
| 1221 | |
| 1222 | @node Creating Buffer-Local |
| 1223 | @subsection Creating and Deleting Buffer-Local Bindings |
| 1224 | |
| 1225 | @deffn Command make-local-variable variable |
| 1226 | This function creates a buffer-local binding in the current buffer for |
| 1227 | @var{variable} (a symbol). Other buffers are not affected. The value |
| 1228 | returned is @var{variable}. |
| 1229 | |
| 1230 | The buffer-local value of @var{variable} starts out as the same value |
| 1231 | @var{variable} previously had. If @var{variable} was void, it remains |
| 1232 | void. |
| 1233 | |
| 1234 | @example |
| 1235 | @group |
| 1236 | ;; @r{In buffer @samp{b1}:} |
| 1237 | (setq foo 5) ; @r{Affects all buffers.} |
| 1238 | @result{} 5 |
| 1239 | @end group |
| 1240 | @group |
| 1241 | (make-local-variable 'foo) ; @r{Now it is local in @samp{b1}.} |
| 1242 | @result{} foo |
| 1243 | @end group |
| 1244 | @group |
| 1245 | foo ; @r{That did not change} |
| 1246 | @result{} 5 ; @r{the value.} |
| 1247 | @end group |
| 1248 | @group |
| 1249 | (setq foo 6) ; @r{Change the value} |
| 1250 | @result{} 6 ; @r{in @samp{b1}.} |
| 1251 | @end group |
| 1252 | @group |
| 1253 | foo |
| 1254 | @result{} 6 |
| 1255 | @end group |
| 1256 | |
| 1257 | @group |
| 1258 | ;; @r{In buffer @samp{b2}, the value hasn't changed.} |
| 1259 | (with-current-buffer "b2" |
| 1260 | foo) |
| 1261 | @result{} 5 |
| 1262 | @end group |
| 1263 | @end example |
| 1264 | |
| 1265 | Making a variable buffer-local within a @code{let}-binding for that |
| 1266 | variable does not work reliably, unless the buffer in which you do this |
| 1267 | is not current either on entry to or exit from the @code{let}. This is |
| 1268 | because @code{let} does not distinguish between different kinds of |
| 1269 | bindings; it knows only which variable the binding was made for. |
| 1270 | |
| 1271 | If the variable is terminal-local (@pxref{Multiple Terminals}), this |
| 1272 | function signals an error. Such variables cannot have buffer-local |
| 1273 | bindings as well. |
| 1274 | |
| 1275 | @strong{Warning:} do not use @code{make-local-variable} for a hook |
| 1276 | variable. The hook variables are automatically made buffer-local as |
| 1277 | needed if you use the @var{local} argument to @code{add-hook} or |
| 1278 | @code{remove-hook}. |
| 1279 | @end deffn |
| 1280 | |
| 1281 | @defmac setq-local variable value |
| 1282 | This macro creates a buffer-local binding in the current buffer for |
| 1283 | @var{variable}, and gives it the buffer-local value @var{value}. It |
| 1284 | is equivalent to calling @code{make-local-variable} followed by |
| 1285 | @code{setq}. @var{variable} should be an unquoted symbol. |
| 1286 | @end defmac |
| 1287 | |
| 1288 | @deffn Command make-variable-buffer-local variable |
| 1289 | This function marks @var{variable} (a symbol) automatically |
| 1290 | buffer-local, so that any subsequent attempt to set it will make it |
| 1291 | local to the current buffer at the time. Unlike |
| 1292 | @code{make-local-variable}, with which it is often confused, this |
| 1293 | cannot be undone, and affects the behavior of the variable in all |
| 1294 | buffers. |
| 1295 | |
| 1296 | A peculiar wrinkle of this feature is that binding the variable (with |
| 1297 | @code{let} or other binding constructs) does not create a buffer-local |
| 1298 | binding for it. Only setting the variable (with @code{set} or |
| 1299 | @code{setq}), while the variable does not have a @code{let}-style |
| 1300 | binding that was made in the current buffer, does so. |
| 1301 | |
| 1302 | If @var{variable} does not have a default value, then calling this |
| 1303 | command will give it a default value of @code{nil}. If @var{variable} |
| 1304 | already has a default value, that value remains unchanged. |
| 1305 | Subsequently calling @code{makunbound} on @var{variable} will result |
| 1306 | in a void buffer-local value and leave the default value unaffected. |
| 1307 | |
| 1308 | The value returned is @var{variable}. |
| 1309 | |
| 1310 | @strong{Warning:} Don't assume that you should use |
| 1311 | @code{make-variable-buffer-local} for user-option variables, simply |
| 1312 | because users @emph{might} want to customize them differently in |
| 1313 | different buffers. Users can make any variable local, when they wish |
| 1314 | to. It is better to leave the choice to them. |
| 1315 | |
| 1316 | The time to use @code{make-variable-buffer-local} is when it is crucial |
| 1317 | that no two buffers ever share the same binding. For example, when a |
| 1318 | variable is used for internal purposes in a Lisp program which depends |
| 1319 | on having separate values in separate buffers, then using |
| 1320 | @code{make-variable-buffer-local} can be the best solution. |
| 1321 | @end deffn |
| 1322 | |
| 1323 | @defmac defvar-local variable value &optional docstring |
| 1324 | This macro defines @var{variable} as a variable with initial value |
| 1325 | @var{value} and @var{docstring}, and marks it as automatically |
| 1326 | buffer-local. It is equivalent to calling @code{defvar} followed by |
| 1327 | @code{make-variable-buffer-local}. @var{variable} should be an |
| 1328 | unquoted symbol. |
| 1329 | @end defmac |
| 1330 | |
| 1331 | @defun local-variable-p variable &optional buffer |
| 1332 | This returns @code{t} if @var{variable} is buffer-local in buffer |
| 1333 | @var{buffer} (which defaults to the current buffer); otherwise, |
| 1334 | @code{nil}. |
| 1335 | @end defun |
| 1336 | |
| 1337 | @defun local-variable-if-set-p variable &optional buffer |
| 1338 | This returns @code{t} if @var{variable} either has a buffer-local |
| 1339 | value in buffer @var{buffer}, or is automatically buffer-local. |
| 1340 | Otherwise, it returns @code{nil}. If omitted or @code{nil}, |
| 1341 | @var{buffer} defaults to the current buffer. |
| 1342 | @end defun |
| 1343 | |
| 1344 | @defun buffer-local-value variable buffer |
| 1345 | This function returns the buffer-local binding of @var{variable} (a |
| 1346 | symbol) in buffer @var{buffer}. If @var{variable} does not have a |
| 1347 | buffer-local binding in buffer @var{buffer}, it returns the default |
| 1348 | value (@pxref{Default Value}) of @var{variable} instead. |
| 1349 | @end defun |
| 1350 | |
| 1351 | @defun buffer-local-variables &optional buffer |
| 1352 | This function returns a list describing the buffer-local variables in |
| 1353 | buffer @var{buffer}. (If @var{buffer} is omitted, the current buffer |
| 1354 | is used.) Normally, each list element has the form |
| 1355 | @w{@code{(@var{sym} . @var{val})}}, where @var{sym} is a buffer-local |
| 1356 | variable (a symbol) and @var{val} is its buffer-local value. But when |
| 1357 | a variable's buffer-local binding in @var{buffer} is void, its list |
| 1358 | element is just @var{sym}. |
| 1359 | |
| 1360 | @example |
| 1361 | @group |
| 1362 | (make-local-variable 'foobar) |
| 1363 | (makunbound 'foobar) |
| 1364 | (make-local-variable 'bind-me) |
| 1365 | (setq bind-me 69) |
| 1366 | @end group |
| 1367 | (setq lcl (buffer-local-variables)) |
| 1368 | ;; @r{First, built-in variables local in all buffers:} |
| 1369 | @result{} ((mark-active . nil) |
| 1370 | (buffer-undo-list . nil) |
| 1371 | (mode-name . "Fundamental") |
| 1372 | @dots{} |
| 1373 | @group |
| 1374 | ;; @r{Next, non-built-in buffer-local variables.} |
| 1375 | ;; @r{This one is buffer-local and void:} |
| 1376 | foobar |
| 1377 | ;; @r{This one is buffer-local and nonvoid:} |
| 1378 | (bind-me . 69)) |
| 1379 | @end group |
| 1380 | @end example |
| 1381 | |
| 1382 | Note that storing new values into the @sc{cdr}s of cons cells in this |
| 1383 | list does @emph{not} change the buffer-local values of the variables. |
| 1384 | @end defun |
| 1385 | |
| 1386 | @deffn Command kill-local-variable variable |
| 1387 | This function deletes the buffer-local binding (if any) for |
| 1388 | @var{variable} (a symbol) in the current buffer. As a result, the |
| 1389 | default binding of @var{variable} becomes visible in this buffer. This |
| 1390 | typically results in a change in the value of @var{variable}, since the |
| 1391 | default value is usually different from the buffer-local value just |
| 1392 | eliminated. |
| 1393 | |
| 1394 | If you kill the buffer-local binding of a variable that automatically |
| 1395 | becomes buffer-local when set, this makes the default value visible in |
| 1396 | the current buffer. However, if you set the variable again, that will |
| 1397 | once again create a buffer-local binding for it. |
| 1398 | |
| 1399 | @code{kill-local-variable} returns @var{variable}. |
| 1400 | |
| 1401 | This function is a command because it is sometimes useful to kill one |
| 1402 | buffer-local variable interactively, just as it is useful to create |
| 1403 | buffer-local variables interactively. |
| 1404 | @end deffn |
| 1405 | |
| 1406 | @defun kill-all-local-variables |
| 1407 | This function eliminates all the buffer-local variable bindings of the |
| 1408 | current buffer except for variables marked as ``permanent'' and local |
| 1409 | hook functions that have a non-@code{nil} @code{permanent-local-hook} |
| 1410 | property (@pxref{Setting Hooks}). As a result, the buffer will see |
| 1411 | the default values of most variables. |
| 1412 | |
| 1413 | This function also resets certain other information pertaining to the |
| 1414 | buffer: it sets the local keymap to @code{nil}, the syntax table to the |
| 1415 | value of @code{(standard-syntax-table)}, the case table to |
| 1416 | @code{(standard-case-table)}, and the abbrev table to the value of |
| 1417 | @code{fundamental-mode-abbrev-table}. |
| 1418 | |
| 1419 | The very first thing this function does is run the normal hook |
| 1420 | @code{change-major-mode-hook} (see below). |
| 1421 | |
| 1422 | Every major mode command begins by calling this function, which has the |
| 1423 | effect of switching to Fundamental mode and erasing most of the effects |
| 1424 | of the previous major mode. To ensure that this does its job, the |
| 1425 | variables that major modes set should not be marked permanent. |
| 1426 | |
| 1427 | @code{kill-all-local-variables} returns @code{nil}. |
| 1428 | @end defun |
| 1429 | |
| 1430 | @defvar change-major-mode-hook |
| 1431 | The function @code{kill-all-local-variables} runs this normal hook |
| 1432 | before it does anything else. This gives major modes a way to arrange |
| 1433 | for something special to be done if the user switches to a different |
| 1434 | major mode. It is also useful for buffer-specific minor modes |
| 1435 | that should be forgotten if the user changes the major mode. |
| 1436 | |
| 1437 | For best results, make this variable buffer-local, so that it will |
| 1438 | disappear after doing its job and will not interfere with the |
| 1439 | subsequent major mode. @xref{Hooks}. |
| 1440 | @end defvar |
| 1441 | |
| 1442 | @cindex permanent local variable |
| 1443 | A buffer-local variable is @dfn{permanent} if the variable name (a |
| 1444 | symbol) has a @code{permanent-local} property that is non-@code{nil}. |
| 1445 | Such variables are unaffected by @code{kill-all-local-variables}, and |
| 1446 | their local bindings are therefore not cleared by changing major modes. |
| 1447 | Permanent locals are appropriate for data pertaining to where the file |
| 1448 | came from or how to save it, rather than with how to edit the contents. |
| 1449 | |
| 1450 | @node Default Value |
| 1451 | @subsection The Default Value of a Buffer-Local Variable |
| 1452 | @cindex default value |
| 1453 | |
| 1454 | The global value of a variable with buffer-local bindings is also |
| 1455 | called the @dfn{default} value, because it is the value that is in |
| 1456 | effect whenever neither the current buffer nor the selected frame has |
| 1457 | its own binding for the variable. |
| 1458 | |
| 1459 | The functions @code{default-value} and @code{setq-default} access and |
| 1460 | change a variable's default value regardless of whether the current |
| 1461 | buffer has a buffer-local binding. For example, you could use |
| 1462 | @code{setq-default} to change the default setting of |
| 1463 | @code{paragraph-start} for most buffers; and this would work even when |
| 1464 | you are in a C or Lisp mode buffer that has a buffer-local value for |
| 1465 | this variable. |
| 1466 | |
| 1467 | @c Emacs 19 feature |
| 1468 | The special forms @code{defvar} and @code{defconst} also set the |
| 1469 | default value (if they set the variable at all), rather than any |
| 1470 | buffer-local value. |
| 1471 | |
| 1472 | @defun default-value symbol |
| 1473 | This function returns @var{symbol}'s default value. This is the value |
| 1474 | that is seen in buffers and frames that do not have their own values for |
| 1475 | this variable. If @var{symbol} is not buffer-local, this is equivalent |
| 1476 | to @code{symbol-value} (@pxref{Accessing Variables}). |
| 1477 | @end defun |
| 1478 | |
| 1479 | @c Emacs 19 feature |
| 1480 | @defun default-boundp symbol |
| 1481 | The function @code{default-boundp} tells you whether @var{symbol}'s |
| 1482 | default value is nonvoid. If @code{(default-boundp 'foo)} returns |
| 1483 | @code{nil}, then @code{(default-value 'foo)} would get an error. |
| 1484 | |
| 1485 | @code{default-boundp} is to @code{default-value} as @code{boundp} is to |
| 1486 | @code{symbol-value}. |
| 1487 | @end defun |
| 1488 | |
| 1489 | @defspec setq-default [symbol form]@dots{} |
| 1490 | This special form gives each @var{symbol} a new default value, which is |
| 1491 | the result of evaluating the corresponding @var{form}. It does not |
| 1492 | evaluate @var{symbol}, but does evaluate @var{form}. The value of the |
| 1493 | @code{setq-default} form is the value of the last @var{form}. |
| 1494 | |
| 1495 | If a @var{symbol} is not buffer-local for the current buffer, and is not |
| 1496 | marked automatically buffer-local, @code{setq-default} has the same |
| 1497 | effect as @code{setq}. If @var{symbol} is buffer-local for the current |
| 1498 | buffer, then this changes the value that other buffers will see (as long |
| 1499 | as they don't have a buffer-local value), but not the value that the |
| 1500 | current buffer sees. |
| 1501 | |
| 1502 | @example |
| 1503 | @group |
| 1504 | ;; @r{In buffer @samp{foo}:} |
| 1505 | (make-local-variable 'buffer-local) |
| 1506 | @result{} buffer-local |
| 1507 | @end group |
| 1508 | @group |
| 1509 | (setq buffer-local 'value-in-foo) |
| 1510 | @result{} value-in-foo |
| 1511 | @end group |
| 1512 | @group |
| 1513 | (setq-default buffer-local 'new-default) |
| 1514 | @result{} new-default |
| 1515 | @end group |
| 1516 | @group |
| 1517 | buffer-local |
| 1518 | @result{} value-in-foo |
| 1519 | @end group |
| 1520 | @group |
| 1521 | (default-value 'buffer-local) |
| 1522 | @result{} new-default |
| 1523 | @end group |
| 1524 | |
| 1525 | @group |
| 1526 | ;; @r{In (the new) buffer @samp{bar}:} |
| 1527 | buffer-local |
| 1528 | @result{} new-default |
| 1529 | @end group |
| 1530 | @group |
| 1531 | (default-value 'buffer-local) |
| 1532 | @result{} new-default |
| 1533 | @end group |
| 1534 | @group |
| 1535 | (setq buffer-local 'another-default) |
| 1536 | @result{} another-default |
| 1537 | @end group |
| 1538 | @group |
| 1539 | (default-value 'buffer-local) |
| 1540 | @result{} another-default |
| 1541 | @end group |
| 1542 | |
| 1543 | @group |
| 1544 | ;; @r{Back in buffer @samp{foo}:} |
| 1545 | buffer-local |
| 1546 | @result{} value-in-foo |
| 1547 | (default-value 'buffer-local) |
| 1548 | @result{} another-default |
| 1549 | @end group |
| 1550 | @end example |
| 1551 | @end defspec |
| 1552 | |
| 1553 | @defun set-default symbol value |
| 1554 | This function is like @code{setq-default}, except that @var{symbol} is |
| 1555 | an ordinary evaluated argument. |
| 1556 | |
| 1557 | @example |
| 1558 | @group |
| 1559 | (set-default (car '(a b c)) 23) |
| 1560 | @result{} 23 |
| 1561 | @end group |
| 1562 | @group |
| 1563 | (default-value 'a) |
| 1564 | @result{} 23 |
| 1565 | @end group |
| 1566 | @end example |
| 1567 | @end defun |
| 1568 | |
| 1569 | @node File Local Variables |
| 1570 | @section File Local Variables |
| 1571 | @cindex file local variables |
| 1572 | |
| 1573 | A file can specify local variable values; Emacs uses these to create |
| 1574 | buffer-local bindings for those variables in the buffer visiting that |
| 1575 | file. @xref{File Variables, , Local Variables in Files, emacs, The |
| 1576 | GNU Emacs Manual}, for basic information about file-local variables. |
| 1577 | This section describes the functions and variables that affect how |
| 1578 | file-local variables are processed. |
| 1579 | |
| 1580 | If a file-local variable could specify an arbitrary function or Lisp |
| 1581 | expression that would be called later, visiting a file could take over |
| 1582 | your Emacs. Emacs protects against this by automatically setting only |
| 1583 | those file-local variables whose specified values are known to be |
| 1584 | safe. Other file-local variables are set only if the user agrees. |
| 1585 | |
| 1586 | For additional safety, @code{read-circle} is temporarily bound to |
| 1587 | @code{nil} when Emacs reads file-local variables (@pxref{Input |
| 1588 | Functions}). This prevents the Lisp reader from recognizing circular |
| 1589 | and shared Lisp structures (@pxref{Circular Objects}). |
| 1590 | |
| 1591 | @defopt enable-local-variables |
| 1592 | This variable controls whether to process file-local variables. |
| 1593 | The possible values are: |
| 1594 | |
| 1595 | @table @asis |
| 1596 | @item @code{t} (the default) |
| 1597 | Set the safe variables, and query (once) about any unsafe variables. |
| 1598 | @item @code{:safe} |
| 1599 | Set only the safe variables and do not query. |
| 1600 | @item @code{:all} |
| 1601 | Set all the variables and do not query. |
| 1602 | @item @code{nil} |
| 1603 | Don't set any variables. |
| 1604 | @item anything else |
| 1605 | Query (once) about all the variables. |
| 1606 | @end table |
| 1607 | @end defopt |
| 1608 | |
| 1609 | @defvar inhibit-local-variables-regexps |
| 1610 | This is a list of regular expressions. If a file has a name |
| 1611 | matching an element of this list, then it is not scanned for |
| 1612 | any form of file-local variable. For examples of why you might want |
| 1613 | to use this, @pxref{Auto Major Mode}. |
| 1614 | @end defvar |
| 1615 | |
| 1616 | @defun hack-local-variables &optional mode-only |
| 1617 | This function parses, and binds or evaluates as appropriate, any local |
| 1618 | variables specified by the contents of the current buffer. The variable |
| 1619 | @code{enable-local-variables} has its effect here. However, this |
| 1620 | function does not look for the @samp{mode:} local variable in the |
| 1621 | @w{@samp{-*-}} line. @code{set-auto-mode} does that, also taking |
| 1622 | @code{enable-local-variables} into account (@pxref{Auto Major Mode}). |
| 1623 | |
| 1624 | This function works by walking the alist stored in |
| 1625 | @code{file-local-variables-alist} and applying each local variable in |
| 1626 | turn. It calls @code{before-hack-local-variables-hook} and |
| 1627 | @code{hack-local-variables-hook} before and after applying the |
| 1628 | variables, respectively. It only calls the before-hook if the alist |
| 1629 | is non-@code{nil}; it always calls the other hook. This |
| 1630 | function ignores a @samp{mode} element if it specifies the same major |
| 1631 | mode as the buffer already has. |
| 1632 | |
| 1633 | If the optional argument @var{mode-only} is non-@code{nil}, then all |
| 1634 | this function does is return a symbol specifying the major mode, |
| 1635 | if the @w{@samp{-*-}} line or the local variables list specifies one, |
| 1636 | and @code{nil} otherwise. It does not set the mode nor any other |
| 1637 | file-local variable. |
| 1638 | @end defun |
| 1639 | |
| 1640 | @defvar file-local-variables-alist |
| 1641 | This buffer-local variable holds the alist of file-local variable |
| 1642 | settings. Each element of the alist is of the form |
| 1643 | @w{@code{(@var{var} . @var{value})}}, where @var{var} is a symbol of |
| 1644 | the local variable and @var{value} is its value. When Emacs visits a |
| 1645 | file, it first collects all the file-local variables into this alist, |
| 1646 | and then the @code{hack-local-variables} function applies them one by |
| 1647 | one. |
| 1648 | @end defvar |
| 1649 | |
| 1650 | @defvar before-hack-local-variables-hook |
| 1651 | Emacs calls this hook immediately before applying file-local variables |
| 1652 | stored in @code{file-local-variables-alist}. |
| 1653 | @end defvar |
| 1654 | |
| 1655 | @defvar hack-local-variables-hook |
| 1656 | Emacs calls this hook immediately after it finishes applying |
| 1657 | file-local variables stored in @code{file-local-variables-alist}. |
| 1658 | @end defvar |
| 1659 | |
| 1660 | @cindex safe local variable |
| 1661 | You can specify safe values for a variable with a |
| 1662 | @code{safe-local-variable} property. The property has to be a |
| 1663 | function of one argument; any value is safe if the function returns |
| 1664 | non-@code{nil} given that value. Many commonly-encountered file |
| 1665 | variables have @code{safe-local-variable} properties; these include |
| 1666 | @code{fill-column}, @code{fill-prefix}, and @code{indent-tabs-mode}. |
| 1667 | For boolean-valued variables that are safe, use @code{booleanp} as the |
| 1668 | property value. |
| 1669 | |
| 1670 | When defining a user option using @code{defcustom}, you can set its |
| 1671 | @code{safe-local-variable} property by adding the arguments |
| 1672 | @code{:safe @var{function}} to @code{defcustom} (@pxref{Variable |
| 1673 | Definitions}). |
| 1674 | |
| 1675 | @defopt safe-local-variable-values |
| 1676 | This variable provides another way to mark some variable values as |
| 1677 | safe. It is a list of cons cells @code{(@var{var} . @var{val})}, |
| 1678 | where @var{var} is a variable name and @var{val} is a value which is |
| 1679 | safe for that variable. |
| 1680 | |
| 1681 | When Emacs asks the user whether or not to obey a set of file-local |
| 1682 | variable specifications, the user can choose to mark them as safe. |
| 1683 | Doing so adds those variable/value pairs to |
| 1684 | @code{safe-local-variable-values}, and saves it to the user's custom |
| 1685 | file. |
| 1686 | @end defopt |
| 1687 | |
| 1688 | @defun safe-local-variable-p sym val |
| 1689 | This function returns non-@code{nil} if it is safe to give @var{sym} |
| 1690 | the value @var{val}, based on the above criteria. |
| 1691 | @end defun |
| 1692 | |
| 1693 | @c @cindex risky local variable Duplicates risky-local-variable |
| 1694 | Some variables are considered @dfn{risky}. If a variable is risky, |
| 1695 | it is never entered automatically into |
| 1696 | @code{safe-local-variable-values}; Emacs always queries before setting |
| 1697 | a risky variable, unless the user explicitly allows a value by |
| 1698 | customizing @code{safe-local-variable-values} directly. |
| 1699 | |
| 1700 | Any variable whose name has a non-@code{nil} |
| 1701 | @code{risky-local-variable} property is considered risky. When you |
| 1702 | define a user option using @code{defcustom}, you can set its |
| 1703 | @code{risky-local-variable} property by adding the arguments |
| 1704 | @code{:risky @var{value}} to @code{defcustom} (@pxref{Variable |
| 1705 | Definitions}). In addition, any variable whose name ends in any of |
| 1706 | @samp{-command}, @samp{-frame-alist}, @samp{-function}, |
| 1707 | @samp{-functions}, @samp{-hook}, @samp{-hooks}, @samp{-form}, |
| 1708 | @samp{-forms}, @samp{-map}, @samp{-map-alist}, @samp{-mode-alist}, |
| 1709 | @samp{-program}, or @samp{-predicate} is automatically considered |
| 1710 | risky. The variables @samp{font-lock-keywords}, |
| 1711 | @samp{font-lock-keywords} followed by a digit, and |
| 1712 | @samp{font-lock-syntactic-keywords} are also considered risky. |
| 1713 | |
| 1714 | @defun risky-local-variable-p sym |
| 1715 | This function returns non-@code{nil} if @var{sym} is a risky variable, |
| 1716 | based on the above criteria. |
| 1717 | @end defun |
| 1718 | |
| 1719 | @defvar ignored-local-variables |
| 1720 | This variable holds a list of variables that should not be given local |
| 1721 | values by files. Any value specified for one of these variables is |
| 1722 | completely ignored. |
| 1723 | @end defvar |
| 1724 | |
| 1725 | The @samp{Eval:} ``variable'' is also a potential loophole, so Emacs |
| 1726 | normally asks for confirmation before handling it. |
| 1727 | |
| 1728 | @defopt enable-local-eval |
| 1729 | This variable controls processing of @samp{Eval:} in @samp{-*-} lines |
| 1730 | or local variables |
| 1731 | lists in files being visited. A value of @code{t} means process them |
| 1732 | unconditionally; @code{nil} means ignore them; anything else means ask |
| 1733 | the user what to do for each file. The default value is @code{maybe}. |
| 1734 | @end defopt |
| 1735 | |
| 1736 | @defopt safe-local-eval-forms |
| 1737 | This variable holds a list of expressions that are safe to |
| 1738 | evaluate when found in the @samp{Eval:} ``variable'' in a file |
| 1739 | local variables list. |
| 1740 | @end defopt |
| 1741 | |
| 1742 | If the expression is a function call and the function has a |
| 1743 | @code{safe-local-eval-function} property, the property value |
| 1744 | determines whether the expression is safe to evaluate. The property |
| 1745 | value can be a predicate to call to test the expression, a list of |
| 1746 | such predicates (it's safe if any predicate succeeds), or @code{t} |
| 1747 | (always safe provided the arguments are constant). |
| 1748 | |
| 1749 | Text properties are also potential loopholes, since their values |
| 1750 | could include functions to call. So Emacs discards all text |
| 1751 | properties from string values specified for file-local variables. |
| 1752 | |
| 1753 | @node Directory Local Variables |
| 1754 | @section Directory Local Variables |
| 1755 | @cindex directory local variables |
| 1756 | |
| 1757 | A directory can specify local variable values common to all files in |
| 1758 | that directory; Emacs uses these to create buffer-local bindings for |
| 1759 | those variables in buffers visiting any file in that directory. This |
| 1760 | is useful when the files in the directory belong to some @dfn{project} |
| 1761 | and therefore share the same local variables. |
| 1762 | |
| 1763 | There are two different methods for specifying directory local |
| 1764 | variables: by putting them in a special file, or by defining a |
| 1765 | @dfn{project class} for that directory. |
| 1766 | |
| 1767 | @defvr Constant dir-locals-file |
| 1768 | This constant is the name of the file where Emacs expects to find the |
| 1769 | directory-local variables. The name of the file is |
| 1770 | @file{.dir-locals.el}@footnote{ |
| 1771 | The MS-DOS version of Emacs uses @file{_dir-locals.el} instead, due to |
| 1772 | limitations of the DOS filesystems. |
| 1773 | }. A file by that name in a directory causes Emacs to apply its |
| 1774 | settings to any file in that directory or any of its subdirectories |
| 1775 | (optionally, you can exclude subdirectories; see below). |
| 1776 | If some of the subdirectories have their own @file{.dir-locals.el} |
| 1777 | files, Emacs uses the settings from the deepest file it finds starting |
| 1778 | from the file's directory and moving up the directory tree. The file |
| 1779 | specifies local variables as a specially formatted list; see |
| 1780 | @ref{Directory Variables, , Per-directory Local Variables, emacs, The |
| 1781 | GNU Emacs Manual}, for more details. |
| 1782 | @end defvr |
| 1783 | |
| 1784 | @defun hack-dir-local-variables |
| 1785 | This function reads the @code{.dir-locals.el} file and stores the |
| 1786 | directory-local variables in @code{file-local-variables-alist} that is |
| 1787 | local to the buffer visiting any file in the directory, without |
| 1788 | applying them. It also stores the directory-local settings in |
| 1789 | @code{dir-locals-class-alist}, where it defines a special class for |
| 1790 | the directory in which @file{.dir-locals.el} file was found. This |
| 1791 | function works by calling @code{dir-locals-set-class-variables} and |
| 1792 | @code{dir-locals-set-directory-class}, described below. |
| 1793 | @end defun |
| 1794 | |
| 1795 | @defun hack-dir-local-variables-non-file-buffer |
| 1796 | This function looks for directory-local variables, and immediately |
| 1797 | applies them in the current buffer. It is intended to be called in |
| 1798 | the mode commands for non-file buffers, such as Dired buffers, to let |
| 1799 | them obey directory-local variable settings. For non-file buffers, |
| 1800 | Emacs looks for directory-local variables in @code{default-directory} |
| 1801 | and its parent directories. |
| 1802 | @end defun |
| 1803 | |
| 1804 | @defun dir-locals-set-class-variables class variables |
| 1805 | This function defines a set of variable settings for the named |
| 1806 | @var{class}, which is a symbol. You can later assign the class to one |
| 1807 | or more directories, and Emacs will apply those variable settings to |
| 1808 | all files in those directories. The list in @var{variables} can be of |
| 1809 | one of the two forms: @code{(@var{major-mode} . @var{alist})} or |
| 1810 | @code{(@var{directory} . @var{list})}. With the first form, if the |
| 1811 | file's buffer turns on a mode that is derived from @var{major-mode}, |
| 1812 | then the all the variables in the associated @var{alist} are applied; |
| 1813 | @var{alist} should be of the form @code{(@var{name} . @var{value})}. |
| 1814 | A special value @code{nil} for @var{major-mode} means the settings are |
| 1815 | applicable to any mode. In @var{alist}, you can use a special |
| 1816 | @var{name}: @code{subdirs}. If the associated value is |
| 1817 | @code{nil}, the alist is only applied to files in the relevant |
| 1818 | directory, not to those in any subdirectories. |
| 1819 | |
| 1820 | With the second form of @var{variables}, if @var{directory} is the |
| 1821 | initial substring of the file's directory, then @var{list} is applied |
| 1822 | recursively by following the above rules; @var{list} should be of one |
| 1823 | of the two forms accepted by this function in @var{variables}. |
| 1824 | @end defun |
| 1825 | |
| 1826 | @defun dir-locals-set-directory-class directory class &optional mtime |
| 1827 | This function assigns @var{class} to all the files in @code{directory} |
| 1828 | and its subdirectories. Thereafter, all the variable settings |
| 1829 | specified for @var{class} will be applied to any visited file in |
| 1830 | @var{directory} and its children. @var{class} must have been already |
| 1831 | defined by @code{dir-locals-set-class-variables}. |
| 1832 | |
| 1833 | Emacs uses this function internally when it loads directory variables |
| 1834 | from a @code{.dir-locals.el} file. In that case, the optional |
| 1835 | argument @var{mtime} holds the file modification time (as returned by |
| 1836 | @code{file-attributes}). Emacs uses this time to check stored |
| 1837 | local variables are still valid. If you are assigning a class |
| 1838 | directly, not via a file, this argument should be @code{nil}. |
| 1839 | @end defun |
| 1840 | |
| 1841 | @defvar dir-locals-class-alist |
| 1842 | This alist holds the class symbols and the associated variable |
| 1843 | settings. It is updated by @code{dir-locals-set-class-variables}. |
| 1844 | @end defvar |
| 1845 | |
| 1846 | @defvar dir-locals-directory-cache |
| 1847 | This alist holds directory names, their assigned class names, and |
| 1848 | modification times of the associated directory local variables file |
| 1849 | (if there is one). The function @code{dir-locals-set-directory-class} |
| 1850 | updates this list. |
| 1851 | @end defvar |
| 1852 | |
| 1853 | @defvar enable-dir-local-variables |
| 1854 | If @code{nil}, directory-local variables are ignored. This variable |
| 1855 | may be useful for modes that want to ignore directory-locals while |
| 1856 | still respecting file-local variables (@pxref{File Local Variables}). |
| 1857 | @end defvar |
| 1858 | |
| 1859 | @node Variable Aliases |
| 1860 | @section Variable Aliases |
| 1861 | @cindex variable aliases |
| 1862 | @cindex alias, for variables |
| 1863 | |
| 1864 | It is sometimes useful to make two variables synonyms, so that both |
| 1865 | variables always have the same value, and changing either one also |
| 1866 | changes the other. Whenever you change the name of a |
| 1867 | variable---either because you realize its old name was not well |
| 1868 | chosen, or because its meaning has partly changed---it can be useful |
| 1869 | to keep the old name as an @emph{alias} of the new one for |
| 1870 | compatibility. You can do this with @code{defvaralias}. |
| 1871 | |
| 1872 | @defun defvaralias new-alias base-variable &optional docstring |
| 1873 | This function defines the symbol @var{new-alias} as a variable alias |
| 1874 | for symbol @var{base-variable}. This means that retrieving the value |
| 1875 | of @var{new-alias} returns the value of @var{base-variable}, and |
| 1876 | changing the value of @var{new-alias} changes the value of |
| 1877 | @var{base-variable}. The two aliased variable names always share the |
| 1878 | same value and the same bindings. |
| 1879 | |
| 1880 | If the @var{docstring} argument is non-@code{nil}, it specifies the |
| 1881 | documentation for @var{new-alias}; otherwise, the alias gets the same |
| 1882 | documentation as @var{base-variable} has, if any, unless |
| 1883 | @var{base-variable} is itself an alias, in which case @var{new-alias} gets |
| 1884 | the documentation of the variable at the end of the chain of aliases. |
| 1885 | |
| 1886 | This function returns @var{base-variable}. |
| 1887 | @end defun |
| 1888 | |
| 1889 | Variable aliases are convenient for replacing an old name for a |
| 1890 | variable with a new name. @code{make-obsolete-variable} declares that |
| 1891 | the old name is obsolete and therefore that it may be removed at some |
| 1892 | stage in the future. |
| 1893 | |
| 1894 | @defun make-obsolete-variable obsolete-name current-name when &optional access-type |
| 1895 | This function makes the byte compiler warn that the variable |
| 1896 | @var{obsolete-name} is obsolete. If @var{current-name} is a symbol, |
| 1897 | it is the variable's new name; then the warning message says to use |
| 1898 | @var{current-name} instead of @var{obsolete-name}. If |
| 1899 | @var{current-name} is a string, this is the message and there is no |
| 1900 | replacement variable. @var{when} should be a string indicating when |
| 1901 | the variable was first made obsolete (usually a version number |
| 1902 | string). |
| 1903 | |
| 1904 | The optional argument @var{access-type}, if non-@code{nil}, should |
| 1905 | should specify the kind of access that will trigger obsolescence |
| 1906 | warnings; it can be either @code{get} or @code{set}. |
| 1907 | @end defun |
| 1908 | |
| 1909 | You can make two variables synonyms and declare one obsolete at the |
| 1910 | same time using the macro @code{define-obsolete-variable-alias}. |
| 1911 | |
| 1912 | @defmac define-obsolete-variable-alias obsolete-name current-name &optional when docstring |
| 1913 | This macro marks the variable @var{obsolete-name} as obsolete and also |
| 1914 | makes it an alias for the variable @var{current-name}. It is |
| 1915 | equivalent to the following: |
| 1916 | |
| 1917 | @example |
| 1918 | (defvaralias @var{obsolete-name} @var{current-name} @var{docstring}) |
| 1919 | (make-obsolete-variable @var{obsolete-name} @var{current-name} @var{when}) |
| 1920 | @end example |
| 1921 | @end defmac |
| 1922 | |
| 1923 | @defun indirect-variable variable |
| 1924 | This function returns the variable at the end of the chain of aliases |
| 1925 | of @var{variable}. If @var{variable} is not a symbol, or if @var{variable} is |
| 1926 | not defined as an alias, the function returns @var{variable}. |
| 1927 | |
| 1928 | This function signals a @code{cyclic-variable-indirection} error if |
| 1929 | there is a loop in the chain of symbols. |
| 1930 | @end defun |
| 1931 | |
| 1932 | @example |
| 1933 | (defvaralias 'foo 'bar) |
| 1934 | (indirect-variable 'foo) |
| 1935 | @result{} bar |
| 1936 | (indirect-variable 'bar) |
| 1937 | @result{} bar |
| 1938 | (setq bar 2) |
| 1939 | bar |
| 1940 | @result{} 2 |
| 1941 | @group |
| 1942 | foo |
| 1943 | @result{} 2 |
| 1944 | @end group |
| 1945 | (setq foo 0) |
| 1946 | bar |
| 1947 | @result{} 0 |
| 1948 | foo |
| 1949 | @result{} 0 |
| 1950 | @end example |
| 1951 | |
| 1952 | @node Variables with Restricted Values |
| 1953 | @section Variables with Restricted Values |
| 1954 | |
| 1955 | Ordinary Lisp variables can be assigned any value that is a valid |
| 1956 | Lisp object. However, certain Lisp variables are not defined in Lisp, |
| 1957 | but in C@. Most of these variables are defined in the C code using |
| 1958 | @code{DEFVAR_LISP}. Like variables defined in Lisp, these can take on |
| 1959 | any value. However, some variables are defined using |
| 1960 | @code{DEFVAR_INT} or @code{DEFVAR_BOOL}. @xref{Defining Lisp |
| 1961 | variables in C,, Writing Emacs Primitives}, in particular the |
| 1962 | description of functions of the type @code{syms_of_@var{filename}}, |
| 1963 | for a brief discussion of the C implementation. |
| 1964 | |
| 1965 | Variables of type @code{DEFVAR_BOOL} can only take on the values |
| 1966 | @code{nil} or @code{t}. Attempting to assign them any other value |
| 1967 | will set them to @code{t}: |
| 1968 | |
| 1969 | @example |
| 1970 | (let ((display-hourglass 5)) |
| 1971 | display-hourglass) |
| 1972 | @result{} t |
| 1973 | @end example |
| 1974 | |
| 1975 | @defvar byte-boolean-vars |
| 1976 | This variable holds a list of all variables of type @code{DEFVAR_BOOL}. |
| 1977 | @end defvar |
| 1978 | |
| 1979 | Variables of type @code{DEFVAR_INT} can take on only integer values. |
| 1980 | Attempting to assign them any other value will result in an error: |
| 1981 | |
| 1982 | @example |
| 1983 | (setq undo-limit 1000.0) |
| 1984 | @error{} Wrong type argument: integerp, 1000.0 |
| 1985 | @end example |
| 1986 | |
| 1987 | @node Generalized Variables |
| 1988 | @section Generalized Variables |
| 1989 | |
| 1990 | A @dfn{generalized variable} or @dfn{place form} is one of the many places |
| 1991 | in Lisp memory where values can be stored. The simplest place form is |
| 1992 | a regular Lisp variable. But the @sc{car}s and @sc{cdr}s of lists, elements |
| 1993 | of arrays, properties of symbols, and many other locations are also |
| 1994 | places where Lisp values are stored. |
| 1995 | |
| 1996 | Generalized variables are analogous to ``lvalues'' in the C |
| 1997 | language, where @samp{x = a[i]} gets an element from an array |
| 1998 | and @samp{a[i] = x} stores an element using the same notation. |
| 1999 | Just as certain forms like @code{a[i]} can be lvalues in C, there |
| 2000 | is a set of forms that can be generalized variables in Lisp. |
| 2001 | |
| 2002 | @menu |
| 2003 | * Setting Generalized Variables:: The @code{setf} macro. |
| 2004 | * Adding Generalized Variables:: Defining new @code{setf} forms. |
| 2005 | @end menu |
| 2006 | |
| 2007 | @node Setting Generalized Variables |
| 2008 | @subsection The @code{setf} Macro |
| 2009 | |
| 2010 | The @code{setf} macro is the most basic way to operate on generalized |
| 2011 | variables. The @code{setf} form is like @code{setq}, except that it |
| 2012 | accepts arbitrary place forms on the left side rather than just |
| 2013 | symbols. For example, @code{(setf (car a) b)} sets the car of |
| 2014 | @code{a} to @code{b}, doing the same operation as @code{(setcar a b)}, |
| 2015 | but without having to remember two separate functions for setting and |
| 2016 | accessing every type of place. |
| 2017 | |
| 2018 | @defmac setf [place form]@dots{} |
| 2019 | This macro evaluates @var{form} and stores it in @var{place}, which |
| 2020 | must be a valid generalized variable form. If there are several |
| 2021 | @var{place} and @var{form} pairs, the assignments are done sequentially |
| 2022 | just as with @code{setq}. @code{setf} returns the value of the last |
| 2023 | @var{form}. |
| 2024 | @end defmac |
| 2025 | |
| 2026 | The following Lisp forms will work as generalized variables, and |
| 2027 | so may appear in the @var{place} argument of @code{setf}: |
| 2028 | |
| 2029 | @itemize |
| 2030 | @item |
| 2031 | A symbol naming a variable. In other words, @code{(setf x y)} is |
| 2032 | exactly equivalent to @code{(setq x y)}, and @code{setq} itself is |
| 2033 | strictly speaking redundant given that @code{setf} exists. Many |
| 2034 | programmers continue to prefer @code{setq} for setting simple |
| 2035 | variables, though, purely for stylistic or historical reasons. |
| 2036 | The macro @code{(setf x y)} actually expands to @code{(setq x y)}, |
| 2037 | so there is no performance penalty for using it in compiled code. |
| 2038 | |
| 2039 | @item |
| 2040 | A call to any of the following standard Lisp functions: |
| 2041 | |
| 2042 | @smallexample |
| 2043 | aref cddr symbol-function |
| 2044 | car elt symbol-plist |
| 2045 | caar get symbol-value |
| 2046 | cadr gethash |
| 2047 | cdr nth |
| 2048 | cdar nthcdr |
| 2049 | @end smallexample |
| 2050 | |
| 2051 | @item |
| 2052 | A call to any of the following Emacs-specific functions: |
| 2053 | |
| 2054 | @smallexample |
| 2055 | default-value process-get |
| 2056 | frame-parameter process-sentinel |
| 2057 | terminal-parameter window-buffer |
| 2058 | keymap-parent window-display-table |
| 2059 | match-data window-dedicated-p |
| 2060 | overlay-get window-hscroll |
| 2061 | overlay-start window-parameter |
| 2062 | overlay-end window-point |
| 2063 | process-buffer window-start |
| 2064 | process-filter |
| 2065 | @end smallexample |
| 2066 | @end itemize |
| 2067 | |
| 2068 | @noindent |
| 2069 | @code{setf} signals an error if you pass a @var{place} form that it |
| 2070 | does not know how to handle. |
| 2071 | |
| 2072 | @c And for cl-lib's cl-getf. |
| 2073 | Note that for @code{nthcdr}, the list argument of the function must |
| 2074 | itself be a valid @var{place} form. For example, @code{(setf (nthcdr |
| 2075 | 0 foo) 7)} will set @code{foo} itself to 7. |
| 2076 | @c The use of @code{nthcdr} as a @var{place} form is an extension |
| 2077 | @c to standard Common Lisp. |
| 2078 | |
| 2079 | @c FIXME I don't think is a particularly good way to do it, |
| 2080 | @c but these macros are introduced before generalized variables are. |
| 2081 | The macros @code{push} (@pxref{List Variables}) and @code{pop} |
| 2082 | (@pxref{List Elements}) can manipulate generalized variables, |
| 2083 | not just lists. @code{(pop @var{place})} removes and returns the first |
| 2084 | element of the list stored in @var{place}. It is analogous to |
| 2085 | @code{(prog1 (car @var{place}) (setf @var{place} (cdr @var{place})))}, |
| 2086 | except that it takes care to evaluate all subforms only once. |
| 2087 | @code{(push @var{x} @var{place})} inserts @var{x} at the front of |
| 2088 | the list stored in @var{place}. It is analogous to @code{(setf |
| 2089 | @var{place} (cons @var{x} @var{place}))}, except for evaluation of the |
| 2090 | subforms. Note that @code{push} and @code{pop} on an @code{nthcdr} |
| 2091 | place can be used to insert or delete at any position in a list. |
| 2092 | |
| 2093 | The @file{cl-lib} library defines various extensions for generalized |
| 2094 | variables, including additional @code{setf} places. |
| 2095 | @xref{Generalized Variables,,, cl, Common Lisp Extensions}. |
| 2096 | |
| 2097 | |
| 2098 | @node Adding Generalized Variables |
| 2099 | @subsection Defining new @code{setf} forms |
| 2100 | |
| 2101 | This section describes how to define new forms that @code{setf} can |
| 2102 | operate on. |
| 2103 | |
| 2104 | @defmac gv-define-simple-setter name setter &optional fix-return |
| 2105 | This macro enables you to easily define @code{setf} methods for simple |
| 2106 | cases. @var{name} is the name of a function, macro, or special form. |
| 2107 | You can use this macro whenever @var{name} has a directly |
| 2108 | corresponding @var{setter} function that updates it, e.g., |
| 2109 | @code{(gv-define-simple-setter car setcar)}. |
| 2110 | |
| 2111 | This macro translates a call of the form |
| 2112 | |
| 2113 | @example |
| 2114 | (setf (@var{name} @var{args}@dots{}) @var{value}) |
| 2115 | @end example |
| 2116 | |
| 2117 | into |
| 2118 | @example |
| 2119 | (@var{setter} @var{args}@dots{} @var{value}) |
| 2120 | @end example |
| 2121 | |
| 2122 | @noindent |
| 2123 | Such a @code{setf} call is documented to return @var{value}. This is |
| 2124 | no problem with, e.g., @code{car} and @code{setcar}, because |
| 2125 | @code{setcar} returns the value that it set. If your @var{setter} |
| 2126 | function does not return @var{value}, use a non-@code{nil} value for |
| 2127 | the @var{fix-return} argument of @code{gv-define-simple-setter}. This |
| 2128 | expands into something equivalent to |
| 2129 | @example |
| 2130 | (let ((temp @var{value})) |
| 2131 | (@var{setter} @var{args}@dots{} temp) |
| 2132 | temp) |
| 2133 | @end example |
| 2134 | so ensuring that it returns the correct result. |
| 2135 | @end defmac |
| 2136 | |
| 2137 | |
| 2138 | @defmac gv-define-setter name arglist &rest body |
| 2139 | This macro allows for more complex @code{setf} expansions than the |
| 2140 | previous form. You may need to use this form, for example, if there |
| 2141 | is no simple setter function to call, or if there is one but it |
| 2142 | requires different arguments to the place form. |
| 2143 | |
| 2144 | This macro expands the form |
| 2145 | @code{(setf (@var{name} @var{args}@dots{}) @var{value})} by |
| 2146 | first binding the @code{setf} argument forms |
| 2147 | @code{(@var{value} @var{args}@dots{})} according to @var{arglist}, |
| 2148 | and then executing @var{body}. @var{body} should return a Lisp |
| 2149 | form that does the assignment, and finally returns the value that was |
| 2150 | set. An example of using this macro is: |
| 2151 | |
| 2152 | @example |
| 2153 | (gv-define-setter caar (val x) `(setcar (car ,x) ,val)) |
| 2154 | @end example |
| 2155 | @end defmac |
| 2156 | |
| 2157 | @findex gv-define-expander |
| 2158 | @findex gv-letplace |
| 2159 | @c FIXME? Not sure what or how much to say about these. |
| 2160 | @c See cl.texi for an example of using gv-letplace. |
| 2161 | For more control over the expansion, see the macro @code{gv-define-expander}. |
| 2162 | The macro @code{gv-letplace} can be useful in defining macros that |
| 2163 | perform similarly to @code{setf}; for example, the @code{incf} macro |
| 2164 | of Common Lisp. Consult the source file @file{gv.el} for more details. |
| 2165 | |
| 2166 | @cindex CL note---no @code{setf} functions |
| 2167 | @quotation |
| 2168 | @b{Common Lisp note:} Common Lisp defines another way to specify the |
| 2169 | @code{setf} behavior of a function, namely ``@code{setf} functions'', |
| 2170 | whose names are lists @code{(setf @var{name})} rather than symbols. |
| 2171 | For example, @code{(defun (setf foo) @dots{})} defines the function |
| 2172 | that is used when @code{setf} is applied to @code{foo}. Emacs does |
| 2173 | not support this. It is a compile-time error to use @code{setf} on a |
| 2174 | form that has not already had an appropriate expansion defined. In |
| 2175 | Common Lisp, this is not an error since the function @code{(setf |
| 2176 | @var{func})} might be defined later. |
| 2177 | @end quotation |