| 1 | @c -*-texinfo-*- |
| 2 | @c This is part of the GNU Emacs Lisp Reference Manual. |
| 3 | @c Copyright (C) 1990-1995, 1998-2011 Free Software Foundation, Inc. |
| 4 | @c See the file elisp.texi for copying conditions. |
| 5 | @setfilename ../../info/variables |
| 6 | @node Variables, Functions, Control Structures, Top |
| 7 | @chapter Variables |
| 8 | @cindex variable |
| 9 | |
| 10 | A @dfn{variable} is a name used in a program to stand for a value. |
| 11 | Nearly all programming languages have variables of some sort. In the |
| 12 | text of a Lisp program, variables are written using the syntax for |
| 13 | symbols. |
| 14 | |
| 15 | In Lisp, unlike most programming languages, programs are represented |
| 16 | primarily as Lisp objects and only secondarily as text. The Lisp |
| 17 | objects used for variables are symbols: the symbol name is the |
| 18 | variable name, and the variable's value is stored in the value cell of |
| 19 | the symbol. The use of a symbol as a variable is independent of its |
| 20 | use as a function name. @xref{Symbol Components}. |
| 21 | |
| 22 | The textual form of a Lisp program is given by the read syntax of |
| 23 | the Lisp objects that constitute the program. Hence, a variable in a |
| 24 | textual Lisp program is written using the read syntax for the symbol |
| 25 | representing the variable. |
| 26 | |
| 27 | @menu |
| 28 | * Global Variables:: Variable values that exist permanently, everywhere. |
| 29 | * Constant Variables:: Certain "variables" have values that never change. |
| 30 | * Local Variables:: Variable values that exist only temporarily. |
| 31 | * Void Variables:: Symbols that lack values. |
| 32 | * Defining Variables:: A definition says a symbol is used as a variable. |
| 33 | * Tips for Defining:: Things you should think about when you |
| 34 | define a variable. |
| 35 | * Accessing Variables:: Examining values of variables whose names |
| 36 | are known only at run time. |
| 37 | * Setting Variables:: Storing new values in variables. |
| 38 | * Variable Scoping:: How Lisp chooses among local and global values. |
| 39 | * Buffer-Local Variables:: Variable values in effect only in one buffer. |
| 40 | * File Local Variables:: Handling local variable lists in files. |
| 41 | * Directory Local Variables:: Local variables common to all files in a directory. |
| 42 | * Frame-Local Variables:: Frame-local bindings for variables. |
| 43 | * Variable Aliases:: Variables that are aliases for other variables. |
| 44 | * Variables with Restricted Values:: Non-constant variables whose value can |
| 45 | @emph{not} be an arbitrary Lisp object. |
| 46 | @end menu |
| 47 | |
| 48 | @node Global Variables |
| 49 | @section Global Variables |
| 50 | @cindex global variable |
| 51 | |
| 52 | The simplest way to use a variable is @dfn{globally}. This means that |
| 53 | the variable has just one value at a time, and this value is in effect |
| 54 | (at least for the moment) throughout the Lisp system. The value remains |
| 55 | in effect until you specify a new one. When a new value replaces the |
| 56 | old one, no trace of the old value remains in the variable. |
| 57 | |
| 58 | You specify a value for a symbol with @code{setq}. For example, |
| 59 | |
| 60 | @example |
| 61 | (setq x '(a b)) |
| 62 | @end example |
| 63 | |
| 64 | @noindent |
| 65 | gives the variable @code{x} the value @code{(a b)}. Note that |
| 66 | @code{setq} is a special form (@pxref{Special Forms}); it does not |
| 67 | evaluate its first argument, the name of the variable, but it does |
| 68 | evaluate the second argument, the new value. |
| 69 | |
| 70 | Once the variable has a value, you can refer to it by using the |
| 71 | symbol itself as an expression. Thus, |
| 72 | |
| 73 | @example |
| 74 | @group |
| 75 | x @result{} (a b) |
| 76 | @end group |
| 77 | @end example |
| 78 | |
| 79 | @noindent |
| 80 | assuming the @code{setq} form shown above has already been executed. |
| 81 | |
| 82 | If you do set the same variable again, the new value replaces the old |
| 83 | one: |
| 84 | |
| 85 | @example |
| 86 | @group |
| 87 | x |
| 88 | @result{} (a b) |
| 89 | @end group |
| 90 | @group |
| 91 | (setq x 4) |
| 92 | @result{} 4 |
| 93 | @end group |
| 94 | @group |
| 95 | x |
| 96 | @result{} 4 |
| 97 | @end group |
| 98 | @end example |
| 99 | |
| 100 | @node Constant Variables |
| 101 | @section Variables that Never Change |
| 102 | @kindex setting-constant |
| 103 | @cindex keyword symbol |
| 104 | @cindex variable with constant value |
| 105 | @cindex constant variables |
| 106 | @cindex symbol that evaluates to itself |
| 107 | @cindex symbol with constant value |
| 108 | |
| 109 | In Emacs Lisp, certain symbols normally evaluate to themselves. These |
| 110 | include @code{nil} and @code{t}, as well as any symbol whose name starts |
| 111 | with @samp{:} (these are called @dfn{keywords}). These symbols cannot |
| 112 | be rebound, nor can their values be changed. Any attempt to set or bind |
| 113 | @code{nil} or @code{t} signals a @code{setting-constant} error. The |
| 114 | same is true for a keyword (a symbol whose name starts with @samp{:}), |
| 115 | if it is interned in the standard obarray, except that setting such a |
| 116 | symbol to itself is not an error. |
| 117 | |
| 118 | @example |
| 119 | @group |
| 120 | nil @equiv{} 'nil |
| 121 | @result{} nil |
| 122 | @end group |
| 123 | @group |
| 124 | (setq nil 500) |
| 125 | @error{} Attempt to set constant symbol: nil |
| 126 | @end group |
| 127 | @end example |
| 128 | |
| 129 | @defun keywordp object |
| 130 | function returns @code{t} if @var{object} is a symbol whose name |
| 131 | starts with @samp{:}, interned in the standard obarray, and returns |
| 132 | @code{nil} otherwise. |
| 133 | @end defun |
| 134 | |
| 135 | These constants are fundamentally different from the ``constants'' |
| 136 | defined using the @code{defconst} special form (@pxref{Defining |
| 137 | Variables}). A @code{defconst} form serves to inform human readers |
| 138 | that you do not intend to change the value of a variable, but Emacs |
| 139 | does not raise an error if you actually change it. |
| 140 | |
| 141 | @node Local Variables |
| 142 | @section Local Variables |
| 143 | @cindex binding local variables |
| 144 | @cindex local variables |
| 145 | @cindex local binding |
| 146 | @cindex global binding |
| 147 | |
| 148 | Global variables have values that last until explicitly superseded |
| 149 | with new values. Sometimes it is useful to create variable values that |
| 150 | exist temporarily---only until a certain part of the program finishes. |
| 151 | These values are called @dfn{local}, and the variables so used are |
| 152 | called @dfn{local variables}. |
| 153 | |
| 154 | For example, when a function is called, its argument variables receive |
| 155 | new local values that last until the function exits. The @code{let} |
| 156 | special form explicitly establishes new local values for specified |
| 157 | variables; these last until exit from the @code{let} form. |
| 158 | |
| 159 | @cindex shadowing of variables |
| 160 | Establishing a local value saves away the variable's previous value |
| 161 | (or lack of one). We say that the previous value is @dfn{shadowed} |
| 162 | and @dfn{not visible}. Both global and local values may be shadowed |
| 163 | (@pxref{Scope}). After the life span of the local value is over, the |
| 164 | previous value (or lack of one) is restored. |
| 165 | |
| 166 | If you set a variable (such as with @code{setq}) while it is local, |
| 167 | this replaces the local value; it does not alter the global value, or |
| 168 | previous local values, that are shadowed. To model this behavior, we |
| 169 | speak of a @dfn{local binding} of the variable as well as a local value. |
| 170 | |
| 171 | The local binding is a conceptual place that holds a local value. |
| 172 | Entering a function, or a special form such as @code{let}, creates the |
| 173 | local binding; exiting the function or the @code{let} removes the |
| 174 | local binding. While the local binding lasts, the variable's value is |
| 175 | stored within it. Using @code{setq} or @code{set} while there is a |
| 176 | local binding stores a different value into the local binding; it does |
| 177 | not create a new binding. |
| 178 | |
| 179 | We also speak of the @dfn{global binding}, which is where |
| 180 | (conceptually) the global value is kept. |
| 181 | |
| 182 | @cindex current binding |
| 183 | A variable can have more than one local binding at a time (for |
| 184 | example, if there are nested @code{let} forms that bind it). In such a |
| 185 | case, the most recently created local binding that still exists is the |
| 186 | @dfn{current binding} of the variable. (This rule is called |
| 187 | @dfn{dynamic scoping}; see @ref{Variable Scoping}.) If there are no |
| 188 | local bindings, the variable's global binding is its current binding. |
| 189 | We sometimes call the current binding the @dfn{most-local existing |
| 190 | binding}, for emphasis. Ordinary evaluation of a symbol always returns |
| 191 | the value of its current binding. |
| 192 | |
| 193 | The special forms @code{let} and @code{let*} exist to create |
| 194 | local bindings. |
| 195 | |
| 196 | @defspec let (bindings@dots{}) forms@dots{} |
| 197 | This special form binds variables according to @var{bindings} and then |
| 198 | evaluates all of the @var{forms} in textual order. The @code{let}-form |
| 199 | returns the value of the last form in @var{forms}. |
| 200 | |
| 201 | Each of the @var{bindings} is either @w{(i) a} symbol, in which case |
| 202 | that symbol is bound to @code{nil}; or @w{(ii) a} list of the form |
| 203 | @code{(@var{symbol} @var{value-form})}, in which case @var{symbol} is |
| 204 | bound to the result of evaluating @var{value-form}. If @var{value-form} |
| 205 | is omitted, @code{nil} is used. |
| 206 | |
| 207 | All of the @var{value-form}s in @var{bindings} are evaluated in the |
| 208 | order they appear and @emph{before} binding any of the symbols to them. |
| 209 | Here is an example of this: @code{z} is bound to the old value of |
| 210 | @code{y}, which is 2, not the new value of @code{y}, which is 1. |
| 211 | |
| 212 | @example |
| 213 | @group |
| 214 | (setq y 2) |
| 215 | @result{} 2 |
| 216 | @end group |
| 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 | reasonably refer to the preceding symbols bound in this @code{let*} |
| 231 | form. Compare the following example with the example above for |
| 232 | @code{let}. |
| 233 | |
| 234 | @example |
| 235 | @group |
| 236 | (setq y 2) |
| 237 | @result{} 2 |
| 238 | @end group |
| 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, rather than localized in time. |
| 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 1000. 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 | @kindex void-variable |
| 292 | @cindex void variable |
| 293 | |
| 294 | If you have never given a symbol any value as a global variable, we |
| 295 | say that that symbol's global value is @dfn{void}. In other words, the |
| 296 | symbol's value cell does not have any Lisp object in it. If you try to |
| 297 | evaluate the symbol, you get a @code{void-variable} error rather than |
| 298 | a value. |
| 299 | |
| 300 | Note that a value of @code{nil} is not the same as void. The symbol |
| 301 | @code{nil} is a Lisp object and can be the value of a variable just as any |
| 302 | other object can be; but it is @emph{a value}. A void variable does not |
| 303 | have any value. |
| 304 | |
| 305 | After you have given a variable a value, you can make it void once more |
| 306 | using @code{makunbound}. |
| 307 | |
| 308 | @defun makunbound symbol |
| 309 | This function makes the current variable binding of @var{symbol} void. |
| 310 | Subsequent attempts to use this symbol's value as a variable will signal |
| 311 | the error @code{void-variable}, unless and until you set it again. |
| 312 | |
| 313 | @code{makunbound} returns @var{symbol}. |
| 314 | |
| 315 | @example |
| 316 | @group |
| 317 | (makunbound 'x) ; @r{Make the global value of @code{x} void.} |
| 318 | @result{} x |
| 319 | @end group |
| 320 | @group |
| 321 | x |
| 322 | @error{} Symbol's value as variable is void: x |
| 323 | @end group |
| 324 | @end example |
| 325 | |
| 326 | If @var{symbol} is locally bound, @code{makunbound} affects the most |
| 327 | local existing binding. This is the only way a symbol can have a void |
| 328 | local binding, since all the constructs that create local bindings |
| 329 | create them with values. In this case, the voidness lasts at most as |
| 330 | long as the binding does; when the binding is removed due to exit from |
| 331 | the construct that made it, the previous local or global binding is |
| 332 | reexposed as usual, and the variable is no longer void unless the newly |
| 333 | reexposed binding was void all along. |
| 334 | |
| 335 | @smallexample |
| 336 | @group |
| 337 | (setq x 1) ; @r{Put a value in the global binding.} |
| 338 | @result{} 1 |
| 339 | (let ((x 2)) ; @r{Locally bind it.} |
| 340 | (makunbound 'x) ; @r{Void the local binding.} |
| 341 | x) |
| 342 | @error{} Symbol's value as variable is void: x |
| 343 | @end group |
| 344 | @group |
| 345 | x ; @r{The global binding is unchanged.} |
| 346 | @result{} 1 |
| 347 | |
| 348 | (let ((x 2)) ; @r{Locally bind it.} |
| 349 | (let ((x 3)) ; @r{And again.} |
| 350 | (makunbound 'x) ; @r{Void the innermost-local binding.} |
| 351 | x)) ; @r{And refer: it's void.} |
| 352 | @error{} Symbol's value as variable is void: x |
| 353 | @end group |
| 354 | |
| 355 | @group |
| 356 | (let ((x 2)) |
| 357 | (let ((x 3)) |
| 358 | (makunbound 'x)) ; @r{Void inner binding, then remove it.} |
| 359 | x) ; @r{Now outer @code{let} binding is visible.} |
| 360 | @result{} 2 |
| 361 | @end group |
| 362 | @end smallexample |
| 363 | @end defun |
| 364 | |
| 365 | A variable that has been made void with @code{makunbound} is |
| 366 | indistinguishable from one that has never received a value and has |
| 367 | always been void. |
| 368 | |
| 369 | You can use the function @code{boundp} to test whether a variable is |
| 370 | currently void. |
| 371 | |
| 372 | @defun boundp variable |
| 373 | @code{boundp} returns @code{t} if @var{variable} (a symbol) is not void; |
| 374 | more precisely, if its current binding is not void. It returns |
| 375 | @code{nil} otherwise. |
| 376 | |
| 377 | @smallexample |
| 378 | @group |
| 379 | (boundp 'abracadabra) ; @r{Starts out void.} |
| 380 | @result{} nil |
| 381 | @end group |
| 382 | @group |
| 383 | (let ((abracadabra 5)) ; @r{Locally bind it.} |
| 384 | (boundp 'abracadabra)) |
| 385 | @result{} t |
| 386 | @end group |
| 387 | @group |
| 388 | (boundp 'abracadabra) ; @r{Still globally void.} |
| 389 | @result{} nil |
| 390 | @end group |
| 391 | @group |
| 392 | (setq abracadabra 5) ; @r{Make it globally nonvoid.} |
| 393 | @result{} 5 |
| 394 | @end group |
| 395 | @group |
| 396 | (boundp 'abracadabra) |
| 397 | @result{} t |
| 398 | @end group |
| 399 | @end smallexample |
| 400 | @end defun |
| 401 | |
| 402 | @node Defining Variables |
| 403 | @section Defining Global Variables |
| 404 | @cindex variable definition |
| 405 | |
| 406 | You may announce your intention to use a symbol as a global variable |
| 407 | with a @dfn{variable definition}: a special form, either @code{defconst} |
| 408 | or @code{defvar}. |
| 409 | |
| 410 | In Emacs Lisp, definitions serve three purposes. First, they inform |
| 411 | people who read the code that certain symbols are @emph{intended} to be |
| 412 | used a certain way (as variables). Second, they inform the Lisp system |
| 413 | of these things, supplying a value and documentation. Third, they |
| 414 | provide information to utilities such as @code{etags} and |
| 415 | @code{make-docfile}, which create data bases of the functions and |
| 416 | variables in a program. |
| 417 | |
| 418 | The difference between @code{defconst} and @code{defvar} is primarily |
| 419 | a matter of intent, serving to inform human readers of whether the value |
| 420 | should ever change. Emacs Lisp does not restrict the ways in which a |
| 421 | variable can be used based on @code{defconst} or @code{defvar} |
| 422 | declarations. However, it does make a difference for initialization: |
| 423 | @code{defconst} unconditionally initializes the variable, while |
| 424 | @code{defvar} initializes it only if it is void. |
| 425 | |
| 426 | @ignore |
| 427 | One would expect user option variables to be defined with |
| 428 | @code{defconst}, since programs do not change them. Unfortunately, this |
| 429 | has bad results if the definition is in a library that is not preloaded: |
| 430 | @code{defconst} would override any prior value when the library is |
| 431 | loaded. Users would like to be able to set user options in their init |
| 432 | files, and override the default values given in the definitions. For |
| 433 | this reason, user options must be defined with @code{defvar}. |
| 434 | @end ignore |
| 435 | |
| 436 | @defspec defvar symbol [value [doc-string]] |
| 437 | This special form defines @var{symbol} as a variable and can also |
| 438 | initialize and document it. The definition informs a person reading |
| 439 | your code that @var{symbol} is used as a variable that might be set or |
| 440 | changed. Note that @var{symbol} is not evaluated; the symbol to be |
| 441 | defined must appear explicitly in the @code{defvar}. |
| 442 | |
| 443 | If @var{symbol} is void and @var{value} is specified, @code{defvar} |
| 444 | evaluates it and sets @var{symbol} to the result. But if @var{symbol} |
| 445 | already has a value (i.e., it is not void), @var{value} is not even |
| 446 | evaluated, and @var{symbol}'s value remains unchanged. If @var{value} |
| 447 | is omitted, the value of @var{symbol} is not changed in any case. |
| 448 | |
| 449 | If @var{symbol} has a buffer-local binding in the current buffer, |
| 450 | @code{defvar} operates on the default value, which is buffer-independent, |
| 451 | not the current (buffer-local) binding. It sets the default value if |
| 452 | the default value is void. @xref{Buffer-Local Variables}. |
| 453 | |
| 454 | When you evaluate a top-level @code{defvar} form with @kbd{C-M-x} in |
| 455 | Emacs Lisp mode (@code{eval-defun}), a special feature of |
| 456 | @code{eval-defun} arranges to set the variable unconditionally, without |
| 457 | testing whether its value is void. |
| 458 | |
| 459 | If the @var{doc-string} argument appears, it specifies the documentation |
| 460 | for the variable. (This opportunity to specify documentation is one of |
| 461 | the main benefits of defining the variable.) The documentation is |
| 462 | stored in the symbol's @code{variable-documentation} property. The |
| 463 | Emacs help functions (@pxref{Documentation}) look for this property. |
| 464 | |
| 465 | If the documentation string begins with the character @samp{*}, Emacs |
| 466 | allows users to set it interactively using the @code{set-variable} |
| 467 | command. However, you should nearly always use @code{defcustom} |
| 468 | instead of @code{defvar} to define such variables, so that users can |
| 469 | use @kbd{M-x customize} and related commands to set them. In that |
| 470 | case, it is not necessary to begin the documentation string with |
| 471 | @samp{*}. @xref{Customization}. |
| 472 | |
| 473 | Here are some examples. This form defines @code{foo} but does not |
| 474 | initialize it: |
| 475 | |
| 476 | @example |
| 477 | @group |
| 478 | (defvar foo) |
| 479 | @result{} foo |
| 480 | @end group |
| 481 | @end example |
| 482 | |
| 483 | This example initializes the value of @code{bar} to @code{23}, and gives |
| 484 | it a documentation string: |
| 485 | |
| 486 | @example |
| 487 | @group |
| 488 | (defvar bar 23 |
| 489 | "The normal weight of a bar.") |
| 490 | @result{} bar |
| 491 | @end group |
| 492 | @end example |
| 493 | |
| 494 | The following form changes the documentation string for @code{bar}, |
| 495 | making it a user option, but does not change the value, since @code{bar} |
| 496 | already has a value. (The addition @code{(1+ nil)} would get an error |
| 497 | if it were evaluated, but since it is not evaluated, there is no error.) |
| 498 | |
| 499 | @example |
| 500 | @group |
| 501 | (defvar bar (1+ nil) |
| 502 | "*The normal weight of a bar.") |
| 503 | @result{} bar |
| 504 | @end group |
| 505 | @group |
| 506 | bar |
| 507 | @result{} 23 |
| 508 | @end group |
| 509 | @end example |
| 510 | |
| 511 | Here is an equivalent expression for the @code{defvar} special form: |
| 512 | |
| 513 | @example |
| 514 | @group |
| 515 | (defvar @var{symbol} @var{value} @var{doc-string}) |
| 516 | @equiv{} |
| 517 | (progn |
| 518 | (if (not (boundp '@var{symbol})) |
| 519 | (setq @var{symbol} @var{value})) |
| 520 | (if '@var{doc-string} |
| 521 | (put '@var{symbol} 'variable-documentation '@var{doc-string})) |
| 522 | '@var{symbol}) |
| 523 | @end group |
| 524 | @end example |
| 525 | |
| 526 | The @code{defvar} form returns @var{symbol}, but it is normally used |
| 527 | at top level in a file where its value does not matter. |
| 528 | @end defspec |
| 529 | |
| 530 | @cindex constant variables |
| 531 | @defspec defconst symbol value [doc-string] |
| 532 | This special form defines @var{symbol} as a value and initializes it. |
| 533 | It informs a person reading your code that @var{symbol} has a standard |
| 534 | global value, established here, that should not be changed by the user |
| 535 | or by other programs. Note that @var{symbol} is not evaluated; the |
| 536 | symbol to be defined must appear explicitly in the @code{defconst}. |
| 537 | |
| 538 | @code{defconst} always evaluates @var{value}, and sets the value of |
| 539 | @var{symbol} to the result. If @var{symbol} does have a buffer-local |
| 540 | binding in the current buffer, @code{defconst} sets the default value, |
| 541 | not the buffer-local value. (But you should not be making |
| 542 | buffer-local bindings for a symbol that is defined with |
| 543 | @code{defconst}.) |
| 544 | |
| 545 | An example of the use of @code{defconst} is Emacs' definition of |
| 546 | @code{float-pi}---the mathematical constant @math{pi}, which ought not |
| 547 | to be changed by anyone (attempts by the Indiana State Legislature |
| 548 | notwithstanding). As the second form illustrates, however, |
| 549 | @code{defconst} is only advisory. |
| 550 | |
| 551 | @example |
| 552 | @group |
| 553 | (defconst float-pi 3.141592653589793 "The value of Pi.") |
| 554 | @result{} float-pi |
| 555 | @end group |
| 556 | @group |
| 557 | (setq float-pi 3) |
| 558 | @result{} float-pi |
| 559 | @end group |
| 560 | @group |
| 561 | float-pi |
| 562 | @result{} 3 |
| 563 | @end group |
| 564 | @end example |
| 565 | @end defspec |
| 566 | |
| 567 | @defun user-variable-p variable |
| 568 | @cindex user option |
| 569 | This function returns @code{t} if @var{variable} is a user option---a |
| 570 | variable intended to be set by the user for customization---and |
| 571 | @code{nil} otherwise. (Variables other than user options exist for the |
| 572 | internal purposes of Lisp programs, and users need not know about them.) |
| 573 | |
| 574 | User option variables are distinguished from other variables either |
| 575 | though being declared using @code{defcustom}@footnote{They may also be |
| 576 | declared equivalently in @file{cus-start.el}.} or by the first character |
| 577 | of their @code{variable-documentation} property. If the property exists |
| 578 | and is a string, and its first character is @samp{*}, then the variable |
| 579 | is a user option. Aliases of user options are also user options. |
| 580 | @end defun |
| 581 | |
| 582 | @kindex variable-interactive |
| 583 | If a user option variable has a @code{variable-interactive} property, |
| 584 | the @code{set-variable} command uses that value to control reading the |
| 585 | new value for the variable. The property's value is used as if it were |
| 586 | specified in @code{interactive} (@pxref{Using Interactive}). However, |
| 587 | this feature is largely obsoleted by @code{defcustom} |
| 588 | (@pxref{Customization}). |
| 589 | |
| 590 | @strong{Warning:} If the @code{defconst} and @code{defvar} special |
| 591 | forms are used while the variable has a local binding (made with |
| 592 | @code{let}, or a function argument), they set the local-binding's |
| 593 | value; the top-level binding is not changed. This is not what you |
| 594 | usually want. To prevent it, use these special forms at top level in |
| 595 | a file, where normally no local binding is in effect, and make sure to |
| 596 | load the file before making a local binding for the variable. |
| 597 | |
| 598 | @node Tips for Defining |
| 599 | @section Tips for Defining Variables Robustly |
| 600 | |
| 601 | When you define a variable whose value is a function, or a list of |
| 602 | functions, use a name that ends in @samp{-function} or |
| 603 | @samp{-functions}, respectively. |
| 604 | |
| 605 | There are several other variable name conventions; |
| 606 | here is a complete list: |
| 607 | |
| 608 | @table @samp |
| 609 | @item @dots{}-hook |
| 610 | The variable is a normal hook (@pxref{Hooks}). |
| 611 | |
| 612 | @item @dots{}-function |
| 613 | The value is a function. |
| 614 | |
| 615 | @item @dots{}-functions |
| 616 | The value is a list of functions. |
| 617 | |
| 618 | @item @dots{}-form |
| 619 | The value is a form (an expression). |
| 620 | |
| 621 | @item @dots{}-forms |
| 622 | The value is a list of forms (expressions). |
| 623 | |
| 624 | @item @dots{}-predicate |
| 625 | The value is a predicate---a function of one argument that returns |
| 626 | non-@code{nil} for ``good'' arguments and @code{nil} for ``bad'' |
| 627 | arguments. |
| 628 | |
| 629 | @item @dots{}-flag |
| 630 | The value is significant only as to whether it is @code{nil} or not. |
| 631 | Since such variables often end up acquiring more values over time, |
| 632 | this convention is not strongly recommended. |
| 633 | |
| 634 | @item @dots{}-program |
| 635 | The value is a program name. |
| 636 | |
| 637 | @item @dots{}-command |
| 638 | The value is a whole shell command. |
| 639 | |
| 640 | @item @dots{}-switches |
| 641 | The value specifies options for a command. |
| 642 | @end table |
| 643 | |
| 644 | When you define a variable, always consider whether you should mark |
| 645 | it as ``safe'' or ``risky''; see @ref{File Local Variables}. |
| 646 | |
| 647 | When defining and initializing a variable that holds a complicated |
| 648 | value (such as a keymap with bindings in it), it's best to put the |
| 649 | entire computation of the value into the @code{defvar}, like this: |
| 650 | |
| 651 | @example |
| 652 | (defvar my-mode-map |
| 653 | (let ((map (make-sparse-keymap))) |
| 654 | (define-key map "\C-c\C-a" 'my-command) |
| 655 | @dots{} |
| 656 | map) |
| 657 | @var{docstring}) |
| 658 | @end example |
| 659 | |
| 660 | @noindent |
| 661 | This method has several benefits. First, if the user quits while |
| 662 | loading the file, the variable is either still uninitialized or |
| 663 | initialized properly, never in-between. If it is still uninitialized, |
| 664 | reloading the file will initialize it properly. Second, reloading the |
| 665 | file once the variable is initialized will not alter it; that is |
| 666 | important if the user has run hooks to alter part of the contents (such |
| 667 | as, to rebind keys). Third, evaluating the @code{defvar} form with |
| 668 | @kbd{C-M-x} @emph{will} reinitialize the map completely. |
| 669 | |
| 670 | Putting so much code in the @code{defvar} form has one disadvantage: |
| 671 | it puts the documentation string far away from the line which names the |
| 672 | variable. Here's a safe way to avoid that: |
| 673 | |
| 674 | @example |
| 675 | (defvar my-mode-map nil |
| 676 | @var{docstring}) |
| 677 | (unless my-mode-map |
| 678 | (let ((map (make-sparse-keymap))) |
| 679 | (define-key map "\C-c\C-a" 'my-command) |
| 680 | @dots{} |
| 681 | (setq my-mode-map map))) |
| 682 | @end example |
| 683 | |
| 684 | @noindent |
| 685 | This has all the same advantages as putting the initialization inside |
| 686 | the @code{defvar}, except that you must type @kbd{C-M-x} twice, once on |
| 687 | each form, if you do want to reinitialize the variable. |
| 688 | |
| 689 | But be careful not to write the code like this: |
| 690 | |
| 691 | @example |
| 692 | (defvar my-mode-map nil |
| 693 | @var{docstring}) |
| 694 | (unless my-mode-map |
| 695 | (setq my-mode-map (make-sparse-keymap)) |
| 696 | (define-key my-mode-map "\C-c\C-a" 'my-command) |
| 697 | @dots{}) |
| 698 | @end example |
| 699 | |
| 700 | @noindent |
| 701 | This code sets the variable, then alters it, but it does so in more than |
| 702 | one step. If the user quits just after the @code{setq}, that leaves the |
| 703 | variable neither correctly initialized nor void nor @code{nil}. Once |
| 704 | that happens, reloading the file will not initialize the variable; it |
| 705 | will remain incomplete. |
| 706 | |
| 707 | @node Accessing Variables |
| 708 | @section Accessing Variable Values |
| 709 | |
| 710 | The usual way to reference a variable is to write the symbol which |
| 711 | names it (@pxref{Symbol Forms}). This requires you to specify the |
| 712 | variable name when you write the program. Usually that is exactly what |
| 713 | you want to do. Occasionally you need to choose at run time which |
| 714 | variable to reference; then you can use @code{symbol-value}. |
| 715 | |
| 716 | @defun symbol-value symbol |
| 717 | This function returns the value of @var{symbol}. This is the value in |
| 718 | the innermost local binding of the symbol, or its global value if it |
| 719 | has no local bindings. |
| 720 | |
| 721 | @example |
| 722 | @group |
| 723 | (setq abracadabra 5) |
| 724 | @result{} 5 |
| 725 | @end group |
| 726 | @group |
| 727 | (setq foo 9) |
| 728 | @result{} 9 |
| 729 | @end group |
| 730 | |
| 731 | @group |
| 732 | ;; @r{Here the symbol @code{abracadabra}} |
| 733 | ;; @r{is the symbol whose value is examined.} |
| 734 | (let ((abracadabra 'foo)) |
| 735 | (symbol-value 'abracadabra)) |
| 736 | @result{} foo |
| 737 | @end group |
| 738 | |
| 739 | @group |
| 740 | ;; @r{Here, the value of @code{abracadabra},} |
| 741 | ;; @r{which is @code{foo},} |
| 742 | ;; @r{is the symbol whose value is examined.} |
| 743 | (let ((abracadabra 'foo)) |
| 744 | (symbol-value abracadabra)) |
| 745 | @result{} 9 |
| 746 | @end group |
| 747 | |
| 748 | @group |
| 749 | (symbol-value 'abracadabra) |
| 750 | @result{} 5 |
| 751 | @end group |
| 752 | @end example |
| 753 | |
| 754 | A @code{void-variable} error is signaled if the current binding of |
| 755 | @var{symbol} is void. |
| 756 | @end defun |
| 757 | |
| 758 | @node Setting Variables |
| 759 | @section How to Alter a Variable Value |
| 760 | |
| 761 | The usual way to change the value of a variable is with the special |
| 762 | form @code{setq}. When you need to compute the choice of variable at |
| 763 | run time, use the function @code{set}. |
| 764 | |
| 765 | @defspec setq [symbol form]@dots{} |
| 766 | This special form is the most common method of changing a variable's |
| 767 | value. Each @var{symbol} is given a new value, which is the result of |
| 768 | evaluating the corresponding @var{form}. The most-local existing |
| 769 | binding of the symbol is changed. |
| 770 | |
| 771 | @code{setq} does not evaluate @var{symbol}; it sets the symbol that you |
| 772 | write. We say that this argument is @dfn{automatically quoted}. The |
| 773 | @samp{q} in @code{setq} stands for ``quoted.'' |
| 774 | |
| 775 | The value of the @code{setq} form is the value of the last @var{form}. |
| 776 | |
| 777 | @example |
| 778 | @group |
| 779 | (setq x (1+ 2)) |
| 780 | @result{} 3 |
| 781 | @end group |
| 782 | x ; @r{@code{x} now has a global value.} |
| 783 | @result{} 3 |
| 784 | @group |
| 785 | (let ((x 5)) |
| 786 | (setq x 6) ; @r{The local binding of @code{x} is set.} |
| 787 | x) |
| 788 | @result{} 6 |
| 789 | @end group |
| 790 | x ; @r{The global value is unchanged.} |
| 791 | @result{} 3 |
| 792 | @end example |
| 793 | |
| 794 | Note that the first @var{form} is evaluated, then the first |
| 795 | @var{symbol} is set, then the second @var{form} is evaluated, then the |
| 796 | second @var{symbol} is set, and so on: |
| 797 | |
| 798 | @example |
| 799 | @group |
| 800 | (setq x 10 ; @r{Notice that @code{x} is set before} |
| 801 | y (1+ x)) ; @r{the value of @code{y} is computed.} |
| 802 | @result{} 11 |
| 803 | @end group |
| 804 | @end example |
| 805 | @end defspec |
| 806 | |
| 807 | @defun set symbol value |
| 808 | This function sets @var{symbol}'s value to @var{value}, then returns |
| 809 | @var{value}. Since @code{set} is a function, the expression written for |
| 810 | @var{symbol} is evaluated to obtain the symbol to set. |
| 811 | |
| 812 | The most-local existing binding of the variable is the binding that is |
| 813 | set; shadowed bindings are not affected. |
| 814 | |
| 815 | @example |
| 816 | @group |
| 817 | (set one 1) |
| 818 | @error{} Symbol's value as variable is void: one |
| 819 | @end group |
| 820 | @group |
| 821 | (set 'one 1) |
| 822 | @result{} 1 |
| 823 | @end group |
| 824 | @group |
| 825 | (set 'two 'one) |
| 826 | @result{} one |
| 827 | @end group |
| 828 | @group |
| 829 | (set two 2) ; @r{@code{two} evaluates to symbol @code{one}.} |
| 830 | @result{} 2 |
| 831 | @end group |
| 832 | @group |
| 833 | one ; @r{So it is @code{one} that was set.} |
| 834 | @result{} 2 |
| 835 | (let ((one 1)) ; @r{This binding of @code{one} is set,} |
| 836 | (set 'one 3) ; @r{not the global value.} |
| 837 | one) |
| 838 | @result{} 3 |
| 839 | @end group |
| 840 | @group |
| 841 | one |
| 842 | @result{} 2 |
| 843 | @end group |
| 844 | @end example |
| 845 | |
| 846 | If @var{symbol} is not actually a symbol, a @code{wrong-type-argument} |
| 847 | error is signaled. |
| 848 | |
| 849 | @example |
| 850 | (set '(x y) 'z) |
| 851 | @error{} Wrong type argument: symbolp, (x y) |
| 852 | @end example |
| 853 | |
| 854 | Logically speaking, @code{set} is a more fundamental primitive than |
| 855 | @code{setq}. Any use of @code{setq} can be trivially rewritten to use |
| 856 | @code{set}; @code{setq} could even be defined as a macro, given the |
| 857 | availability of @code{set}. However, @code{set} itself is rarely used; |
| 858 | beginners hardly need to know about it. It is useful only for choosing |
| 859 | at run time which variable to set. For example, the command |
| 860 | @code{set-variable}, which reads a variable name from the user and then |
| 861 | sets the variable, needs to use @code{set}. |
| 862 | |
| 863 | @cindex CL note---@code{set} local |
| 864 | @quotation |
| 865 | @b{Common Lisp note:} In Common Lisp, @code{set} always changes the |
| 866 | symbol's ``special'' or dynamic value, ignoring any lexical bindings. |
| 867 | In Emacs Lisp, all variables and all bindings are dynamic, so @code{set} |
| 868 | always affects the most local existing binding. |
| 869 | @end quotation |
| 870 | @end defun |
| 871 | |
| 872 | @node Variable Scoping |
| 873 | @section Scoping Rules for Variable Bindings |
| 874 | |
| 875 | A given symbol @code{foo} can have several local variable bindings, |
| 876 | established at different places in the Lisp program, as well as a global |
| 877 | binding. The most recently established binding takes precedence over |
| 878 | the others. |
| 879 | |
| 880 | @cindex scope |
| 881 | @cindex extent |
| 882 | @cindex dynamic scoping |
| 883 | @cindex lexical scoping |
| 884 | Local bindings in Emacs Lisp have @dfn{indefinite scope} and |
| 885 | @dfn{dynamic extent}. @dfn{Scope} refers to @emph{where} textually in |
| 886 | the source code the binding can be accessed. ``Indefinite scope'' means |
| 887 | that any part of the program can potentially access the variable |
| 888 | binding. @dfn{Extent} refers to @emph{when}, as the program is |
| 889 | executing, the binding exists. ``Dynamic extent'' means that the binding |
| 890 | lasts as long as the activation of the construct that established it. |
| 891 | |
| 892 | The combination of dynamic extent and indefinite scope is called |
| 893 | @dfn{dynamic scoping}. By contrast, most programming languages use |
| 894 | @dfn{lexical scoping}, in which references to a local variable must be |
| 895 | located textually within the function or block that binds the variable. |
| 896 | |
| 897 | @cindex CL note---special variables |
| 898 | @quotation |
| 899 | @b{Common Lisp note:} Variables declared ``special'' in Common Lisp are |
| 900 | dynamically scoped, like all variables in Emacs Lisp. |
| 901 | @end quotation |
| 902 | |
| 903 | @menu |
| 904 | * Scope:: Scope means where in the program a value is visible. |
| 905 | Comparison with other languages. |
| 906 | * Extent:: Extent means how long in time a value exists. |
| 907 | * Impl of Scope:: Two ways to implement dynamic scoping. |
| 908 | * Using Scoping:: How to use dynamic scoping carefully and avoid problems. |
| 909 | @end menu |
| 910 | |
| 911 | @node Scope |
| 912 | @subsection Scope |
| 913 | |
| 914 | Emacs Lisp uses @dfn{indefinite scope} for local variable bindings. |
| 915 | This means that any function anywhere in the program text might access a |
| 916 | given binding of a variable. Consider the following function |
| 917 | definitions: |
| 918 | |
| 919 | @example |
| 920 | @group |
| 921 | (defun binder (x) ; @r{@code{x} is bound in @code{binder}.} |
| 922 | (foo 5)) ; @r{@code{foo} is some other function.} |
| 923 | @end group |
| 924 | |
| 925 | @group |
| 926 | (defun user () ; @r{@code{x} is used ``free'' in @code{user}.} |
| 927 | (list x)) |
| 928 | @end group |
| 929 | @end example |
| 930 | |
| 931 | In a lexically scoped language, the binding of @code{x} in |
| 932 | @code{binder} would never be accessible in @code{user}, because |
| 933 | @code{user} is not textually contained within the function |
| 934 | @code{binder}. However, in dynamically-scoped Emacs Lisp, @code{user} |
| 935 | may or may not refer to the binding of @code{x} established in |
| 936 | @code{binder}, depending on the circumstances: |
| 937 | |
| 938 | @itemize @bullet |
| 939 | @item |
| 940 | If we call @code{user} directly without calling @code{binder} at all, |
| 941 | then whatever binding of @code{x} is found, it cannot come from |
| 942 | @code{binder}. |
| 943 | |
| 944 | @item |
| 945 | If we define @code{foo} as follows and then call @code{binder}, then the |
| 946 | binding made in @code{binder} will be seen in @code{user}: |
| 947 | |
| 948 | @example |
| 949 | @group |
| 950 | (defun foo (lose) |
| 951 | (user)) |
| 952 | @end group |
| 953 | @end example |
| 954 | |
| 955 | @item |
| 956 | However, if we define @code{foo} as follows and then call @code{binder}, |
| 957 | then the binding made in @code{binder} @emph{will not} be seen in |
| 958 | @code{user}: |
| 959 | |
| 960 | @example |
| 961 | (defun foo (x) |
| 962 | (user)) |
| 963 | @end example |
| 964 | |
| 965 | @noindent |
| 966 | Here, when @code{foo} is called by @code{binder}, it binds @code{x}. |
| 967 | (The binding in @code{foo} is said to @dfn{shadow} the one made in |
| 968 | @code{binder}.) Therefore, @code{user} will access the @code{x} bound |
| 969 | by @code{foo} instead of the one bound by @code{binder}. |
| 970 | @end itemize |
| 971 | |
| 972 | Emacs Lisp uses dynamic scoping because simple implementations of |
| 973 | lexical scoping are slow. In addition, every Lisp system needs to offer |
| 974 | dynamic scoping at least as an option; if lexical scoping is the norm, |
| 975 | there must be a way to specify dynamic scoping instead for a particular |
| 976 | variable. It might not be a bad thing for Emacs to offer both, but |
| 977 | implementing it with dynamic scoping only was much easier. |
| 978 | |
| 979 | @node Extent |
| 980 | @subsection Extent |
| 981 | |
| 982 | @dfn{Extent} refers to the time during program execution that a |
| 983 | variable name is valid. In Emacs Lisp, a variable is valid only while |
| 984 | the form that bound it is executing. This is called @dfn{dynamic |
| 985 | extent}. ``Local'' or ``automatic'' variables in most languages, |
| 986 | including C and Pascal, have dynamic extent. |
| 987 | |
| 988 | One alternative to dynamic extent is @dfn{indefinite extent}. This |
| 989 | means that a variable binding can live on past the exit from the form |
| 990 | that made the binding. Common Lisp and Scheme, for example, support |
| 991 | this, but Emacs Lisp does not. |
| 992 | |
| 993 | To illustrate this, the function below, @code{make-add}, returns a |
| 994 | function that purports to add @var{n} to its own argument @var{m}. This |
| 995 | would work in Common Lisp, but it does not do the job in Emacs Lisp, |
| 996 | because after the call to @code{make-add} exits, the variable @code{n} |
| 997 | is no longer bound to the actual argument 2. |
| 998 | |
| 999 | @example |
| 1000 | (defun make-add (n) |
| 1001 | (function (lambda (m) (+ n m)))) ; @r{Return a function.} |
| 1002 | @result{} make-add |
| 1003 | (fset 'add2 (make-add 2)) ; @r{Define function @code{add2}} |
| 1004 | ; @r{with @code{(make-add 2)}.} |
| 1005 | @result{} (lambda (m) (+ n m)) |
| 1006 | (add2 4) ; @r{Try to add 2 to 4.} |
| 1007 | @error{} Symbol's value as variable is void: n |
| 1008 | @end example |
| 1009 | |
| 1010 | @cindex closures not available |
| 1011 | Some Lisp dialects have ``closures,'' objects that are like functions |
| 1012 | but record additional variable bindings. Emacs Lisp does not have |
| 1013 | closures. |
| 1014 | |
| 1015 | @node Impl of Scope |
| 1016 | @subsection Implementation of Dynamic Scoping |
| 1017 | @cindex deep binding |
| 1018 | |
| 1019 | A simple sample implementation (which is not how Emacs Lisp actually |
| 1020 | works) may help you understand dynamic binding. This technique is |
| 1021 | called @dfn{deep binding} and was used in early Lisp systems. |
| 1022 | |
| 1023 | Suppose there is a stack of bindings, which are variable-value pairs. |
| 1024 | At entry to a function or to a @code{let} form, we can push bindings |
| 1025 | onto the stack for the arguments or local variables created there. We |
| 1026 | can pop those bindings from the stack at exit from the binding |
| 1027 | construct. |
| 1028 | |
| 1029 | We can find the value of a variable by searching the stack from top to |
| 1030 | bottom for a binding for that variable; the value from that binding is |
| 1031 | the value of the variable. To set the variable, we search for the |
| 1032 | current binding, then store the new value into that binding. |
| 1033 | |
| 1034 | As you can see, a function's bindings remain in effect as long as it |
| 1035 | continues execution, even during its calls to other functions. That is |
| 1036 | why we say the extent of the binding is dynamic. And any other function |
| 1037 | can refer to the bindings, if it uses the same variables while the |
| 1038 | bindings are in effect. That is why we say the scope is indefinite. |
| 1039 | |
| 1040 | @cindex shallow binding |
| 1041 | The actual implementation of variable scoping in GNU Emacs Lisp uses a |
| 1042 | technique called @dfn{shallow binding}. Each variable has a standard |
| 1043 | place in which its current value is always found---the value cell of the |
| 1044 | symbol. |
| 1045 | |
| 1046 | In shallow binding, setting the variable works by storing a value in |
| 1047 | the value cell. Creating a new binding works by pushing the old value |
| 1048 | (belonging to a previous binding) onto a stack, and storing the new |
| 1049 | local value in the value cell. Eliminating a binding works by popping |
| 1050 | the old value off the stack, into the value cell. |
| 1051 | |
| 1052 | We use shallow binding because it has the same results as deep |
| 1053 | binding, but runs faster, since there is never a need to search for a |
| 1054 | binding. |
| 1055 | |
| 1056 | @node Using Scoping |
| 1057 | @subsection Proper Use of Dynamic Scoping |
| 1058 | |
| 1059 | Binding a variable in one function and using it in another is a |
| 1060 | powerful technique, but if used without restraint, it can make programs |
| 1061 | hard to understand. There are two clean ways to use this technique: |
| 1062 | |
| 1063 | @itemize @bullet |
| 1064 | @item |
| 1065 | Use or bind the variable only in a few related functions, written close |
| 1066 | together in one file. Such a variable is used for communication within |
| 1067 | one program. |
| 1068 | |
| 1069 | You should write comments to inform other programmers that they can see |
| 1070 | all uses of the variable before them, and to advise them not to add uses |
| 1071 | elsewhere. |
| 1072 | |
| 1073 | @item |
| 1074 | Give the variable a well-defined, documented meaning, and make all |
| 1075 | appropriate functions refer to it (but not bind it or set it) wherever |
| 1076 | that meaning is relevant. For example, the variable |
| 1077 | @code{case-fold-search} is defined as ``non-@code{nil} means ignore case |
| 1078 | when searching''; various search and replace functions refer to it |
| 1079 | directly or through their subroutines, but do not bind or set it. |
| 1080 | |
| 1081 | Then you can bind the variable in other programs, knowing reliably what |
| 1082 | the effect will be. |
| 1083 | @end itemize |
| 1084 | |
| 1085 | In either case, you should define the variable with @code{defvar}. |
| 1086 | This helps other people understand your program by telling them to look |
| 1087 | for inter-function usage. It also avoids a warning from the byte |
| 1088 | compiler. Choose the variable's name to avoid name conflicts---don't |
| 1089 | use short names like @code{x}. |
| 1090 | |
| 1091 | @node Buffer-Local Variables |
| 1092 | @section Buffer-Local Variables |
| 1093 | @cindex variable, buffer-local |
| 1094 | @cindex buffer-local variables |
| 1095 | |
| 1096 | Global and local variable bindings are found in most programming |
| 1097 | languages in one form or another. Emacs, however, also supports |
| 1098 | additional, unusual kinds of variable binding, such as |
| 1099 | @dfn{buffer-local} bindings, which apply only in one buffer. Having |
| 1100 | different values for a variable in different buffers is an important |
| 1101 | customization method. (Variables can also have bindings that are |
| 1102 | local to each terminal, or to each frame. @xref{Multiple Terminals}, |
| 1103 | and @xref{Frame-Local Variables}.) |
| 1104 | |
| 1105 | @menu |
| 1106 | * Intro to Buffer-Local:: Introduction and concepts. |
| 1107 | * Creating Buffer-Local:: Creating and destroying buffer-local bindings. |
| 1108 | * Default Value:: The default value is seen in buffers |
| 1109 | that don't have their own buffer-local values. |
| 1110 | @end menu |
| 1111 | |
| 1112 | @node Intro to Buffer-Local |
| 1113 | @subsection Introduction to Buffer-Local Variables |
| 1114 | |
| 1115 | A buffer-local variable has a buffer-local binding associated with a |
| 1116 | particular buffer. The binding is in effect when that buffer is |
| 1117 | current; otherwise, it is not in effect. If you set the variable while |
| 1118 | a buffer-local binding is in effect, the new value goes in that binding, |
| 1119 | so its other bindings are unchanged. This means that the change is |
| 1120 | visible only in the buffer where you made it. |
| 1121 | |
| 1122 | The variable's ordinary binding, which is not associated with any |
| 1123 | specific buffer, is called the @dfn{default binding}. In most cases, |
| 1124 | this is the global binding. |
| 1125 | |
| 1126 | A variable can have buffer-local bindings in some buffers but not in |
| 1127 | other buffers. The default binding is shared by all the buffers that |
| 1128 | don't have their own bindings for the variable. (This includes all |
| 1129 | newly-created buffers.) If you set the variable in a buffer that does |
| 1130 | not have a buffer-local binding for it, this sets the default binding, |
| 1131 | so the new value is visible in all the buffers that see the default |
| 1132 | binding. |
| 1133 | |
| 1134 | The most common use of buffer-local bindings is for major modes to change |
| 1135 | variables that control the behavior of commands. For example, C mode and |
| 1136 | Lisp mode both set the variable @code{paragraph-start} to specify that only |
| 1137 | blank lines separate paragraphs. They do this by making the variable |
| 1138 | buffer-local in the buffer that is being put into C mode or Lisp mode, and |
| 1139 | then setting it to the new value for that mode. @xref{Major Modes}. |
| 1140 | |
| 1141 | The usual way to make a buffer-local binding is with |
| 1142 | @code{make-local-variable}, which is what major mode commands typically |
| 1143 | use. This affects just the current buffer; all other buffers (including |
| 1144 | those yet to be created) will continue to share the default value unless |
| 1145 | they are explicitly given their own buffer-local bindings. |
| 1146 | |
| 1147 | @cindex automatically buffer-local |
| 1148 | A more powerful operation is to mark the variable as |
| 1149 | @dfn{automatically buffer-local} by calling |
| 1150 | @code{make-variable-buffer-local}. You can think of this as making the |
| 1151 | variable local in all buffers, even those yet to be created. More |
| 1152 | precisely, the effect is that setting the variable automatically makes |
| 1153 | the variable local to the current buffer if it is not already so. All |
| 1154 | buffers start out by sharing the default value of the variable as usual, |
| 1155 | but setting the variable creates a buffer-local binding for the current |
| 1156 | buffer. The new value is stored in the buffer-local binding, leaving |
| 1157 | the default binding untouched. This means that the default value cannot |
| 1158 | be changed with @code{setq} in any buffer; the only way to change it is |
| 1159 | with @code{setq-default}. |
| 1160 | |
| 1161 | @strong{Warning:} When a variable has buffer-local |
| 1162 | bindings in one or more buffers, @code{let} rebinds the binding that's |
| 1163 | currently in effect. For instance, if the current buffer has a |
| 1164 | buffer-local value, @code{let} temporarily rebinds that. If no |
| 1165 | buffer-local bindings are in effect, @code{let} rebinds |
| 1166 | the default value. If inside the @code{let} you then change to a |
| 1167 | different current buffer in which a different binding is in effect, |
| 1168 | you won't see the @code{let} binding any more. And if you exit the |
| 1169 | @code{let} while still in the other buffer, you won't see the |
| 1170 | unbinding occur (though it will occur properly). Here is an example |
| 1171 | to illustrate: |
| 1172 | |
| 1173 | @example |
| 1174 | @group |
| 1175 | (setq foo 'g) |
| 1176 | (set-buffer "a") |
| 1177 | (make-local-variable 'foo) |
| 1178 | @end group |
| 1179 | (setq foo 'a) |
| 1180 | (let ((foo 'temp)) |
| 1181 | ;; foo @result{} 'temp ; @r{let binding in buffer @samp{a}} |
| 1182 | (set-buffer "b") |
| 1183 | ;; foo @result{} 'g ; @r{the global value since foo is not local in @samp{b}} |
| 1184 | @var{body}@dots{}) |
| 1185 | @group |
| 1186 | foo @result{} 'g ; @r{exiting restored the local value in buffer @samp{a},} |
| 1187 | ; @r{but we don't see that in buffer @samp{b}} |
| 1188 | @end group |
| 1189 | @group |
| 1190 | (set-buffer "a") ; @r{verify the local value was restored} |
| 1191 | foo @result{} 'a |
| 1192 | @end group |
| 1193 | @end example |
| 1194 | |
| 1195 | Note that references to @code{foo} in @var{body} access the |
| 1196 | buffer-local binding of buffer @samp{b}. |
| 1197 | |
| 1198 | When a file specifies local variable values, these become buffer-local |
| 1199 | values when you visit the file. @xref{File Variables,,, emacs, The |
| 1200 | GNU Emacs Manual}. |
| 1201 | |
| 1202 | A buffer-local variable cannot be made frame-local |
| 1203 | (@pxref{Frame-Local Variables}) or terminal-local (@pxref{Multiple |
| 1204 | Terminals}). |
| 1205 | |
| 1206 | @node Creating Buffer-Local |
| 1207 | @subsection Creating and Deleting Buffer-Local Bindings |
| 1208 | |
| 1209 | @deffn Command make-local-variable variable |
| 1210 | This function creates a buffer-local binding in the current buffer for |
| 1211 | @var{variable} (a symbol). Other buffers are not affected. The value |
| 1212 | returned is @var{variable}. |
| 1213 | |
| 1214 | The buffer-local value of @var{variable} starts out as the same value |
| 1215 | @var{variable} previously had. If @var{variable} was void, it remains |
| 1216 | void. |
| 1217 | |
| 1218 | @example |
| 1219 | @group |
| 1220 | ;; @r{In buffer @samp{b1}:} |
| 1221 | (setq foo 5) ; @r{Affects all buffers.} |
| 1222 | @result{} 5 |
| 1223 | @end group |
| 1224 | @group |
| 1225 | (make-local-variable 'foo) ; @r{Now it is local in @samp{b1}.} |
| 1226 | @result{} foo |
| 1227 | @end group |
| 1228 | @group |
| 1229 | foo ; @r{That did not change} |
| 1230 | @result{} 5 ; @r{the value.} |
| 1231 | @end group |
| 1232 | @group |
| 1233 | (setq foo 6) ; @r{Change the value} |
| 1234 | @result{} 6 ; @r{in @samp{b1}.} |
| 1235 | @end group |
| 1236 | @group |
| 1237 | foo |
| 1238 | @result{} 6 |
| 1239 | @end group |
| 1240 | |
| 1241 | @group |
| 1242 | ;; @r{In buffer @samp{b2}, the value hasn't changed.} |
| 1243 | (with-current-buffer "b2" |
| 1244 | foo) |
| 1245 | @result{} 5 |
| 1246 | @end group |
| 1247 | @end example |
| 1248 | |
| 1249 | Making a variable buffer-local within a @code{let}-binding for that |
| 1250 | variable does not work reliably, unless the buffer in which you do this |
| 1251 | is not current either on entry to or exit from the @code{let}. This is |
| 1252 | because @code{let} does not distinguish between different kinds of |
| 1253 | bindings; it knows only which variable the binding was made for. |
| 1254 | |
| 1255 | If the variable is terminal-local (@pxref{Multiple Terminals}), or |
| 1256 | frame-local (@pxref{Frame-Local Variables}), this function signals an |
| 1257 | error. Such variables cannot have buffer-local bindings as well. |
| 1258 | |
| 1259 | @strong{Warning:} do not use @code{make-local-variable} for a hook |
| 1260 | variable. The hook variables are automatically made buffer-local as |
| 1261 | needed if you use the @var{local} argument to @code{add-hook} or |
| 1262 | @code{remove-hook}. |
| 1263 | @end deffn |
| 1264 | |
| 1265 | @deffn Command make-variable-buffer-local variable |
| 1266 | This function marks @var{variable} (a symbol) automatically |
| 1267 | buffer-local, so that any subsequent attempt to set it will make it |
| 1268 | local to the current buffer at the time. |
| 1269 | |
| 1270 | A peculiar wrinkle of this feature is that binding the variable (with |
| 1271 | @code{let} or other binding constructs) does not create a buffer-local |
| 1272 | binding for it. Only setting the variable (with @code{set} or |
| 1273 | @code{setq}), while the variable does not have a @code{let}-style |
| 1274 | binding that was made in the current buffer, does so. |
| 1275 | |
| 1276 | If @var{variable} does not have a default value, then calling this |
| 1277 | command will give it a default value of @code{nil}. If @var{variable} |
| 1278 | already has a default value, that value remains unchanged. |
| 1279 | Subsequently calling @code{makunbound} on @var{variable} will result |
| 1280 | in a void buffer-local value and leave the default value unaffected. |
| 1281 | |
| 1282 | The value returned is @var{variable}. |
| 1283 | |
| 1284 | @strong{Warning:} Don't assume that you should use |
| 1285 | @code{make-variable-buffer-local} for user-option variables, simply |
| 1286 | because users @emph{might} want to customize them differently in |
| 1287 | different buffers. Users can make any variable local, when they wish |
| 1288 | to. It is better to leave the choice to them. |
| 1289 | |
| 1290 | The time to use @code{make-variable-buffer-local} is when it is crucial |
| 1291 | that no two buffers ever share the same binding. For example, when a |
| 1292 | variable is used for internal purposes in a Lisp program which depends |
| 1293 | on having separate values in separate buffers, then using |
| 1294 | @code{make-variable-buffer-local} can be the best solution. |
| 1295 | @end deffn |
| 1296 | |
| 1297 | @defun local-variable-p variable &optional buffer |
| 1298 | This returns @code{t} if @var{variable} is buffer-local in buffer |
| 1299 | @var{buffer} (which defaults to the current buffer); otherwise, |
| 1300 | @code{nil}. |
| 1301 | @end defun |
| 1302 | |
| 1303 | @defun local-variable-if-set-p variable &optional buffer |
| 1304 | This returns @code{t} if @var{variable} will become buffer-local in |
| 1305 | buffer @var{buffer} (which defaults to the current buffer) if it is |
| 1306 | set there. |
| 1307 | @end defun |
| 1308 | |
| 1309 | @defun buffer-local-value variable buffer |
| 1310 | This function returns the buffer-local binding of @var{variable} (a |
| 1311 | symbol) in buffer @var{buffer}. If @var{variable} does not have a |
| 1312 | buffer-local binding in buffer @var{buffer}, it returns the default |
| 1313 | value (@pxref{Default Value}) of @var{variable} instead. |
| 1314 | @end defun |
| 1315 | |
| 1316 | @defun buffer-local-variables &optional buffer |
| 1317 | This function returns a list describing the buffer-local variables in |
| 1318 | buffer @var{buffer}. (If @var{buffer} is omitted, the current buffer is |
| 1319 | used.) It returns an association list (@pxref{Association Lists}) in |
| 1320 | which each element contains one buffer-local variable and its value. |
| 1321 | However, when a variable's buffer-local binding in @var{buffer} is void, |
| 1322 | then the variable appears directly in the resulting list. |
| 1323 | |
| 1324 | @example |
| 1325 | @group |
| 1326 | (make-local-variable 'foobar) |
| 1327 | (makunbound 'foobar) |
| 1328 | (make-local-variable 'bind-me) |
| 1329 | (setq bind-me 69) |
| 1330 | @end group |
| 1331 | (setq lcl (buffer-local-variables)) |
| 1332 | ;; @r{First, built-in variables local in all buffers:} |
| 1333 | @result{} ((mark-active . nil) |
| 1334 | (buffer-undo-list . nil) |
| 1335 | (mode-name . "Fundamental") |
| 1336 | @dots{} |
| 1337 | @group |
| 1338 | ;; @r{Next, non-built-in buffer-local variables.} |
| 1339 | ;; @r{This one is buffer-local and void:} |
| 1340 | foobar |
| 1341 | ;; @r{This one is buffer-local and nonvoid:} |
| 1342 | (bind-me . 69)) |
| 1343 | @end group |
| 1344 | @end example |
| 1345 | |
| 1346 | Note that storing new values into the @sc{cdr}s of cons cells in this |
| 1347 | list does @emph{not} change the buffer-local values of the variables. |
| 1348 | @end defun |
| 1349 | |
| 1350 | @deffn Command kill-local-variable variable |
| 1351 | This function deletes the buffer-local binding (if any) for |
| 1352 | @var{variable} (a symbol) in the current buffer. As a result, the |
| 1353 | default binding of @var{variable} becomes visible in this buffer. This |
| 1354 | typically results in a change in the value of @var{variable}, since the |
| 1355 | default value is usually different from the buffer-local value just |
| 1356 | eliminated. |
| 1357 | |
| 1358 | If you kill the buffer-local binding of a variable that automatically |
| 1359 | becomes buffer-local when set, this makes the default value visible in |
| 1360 | the current buffer. However, if you set the variable again, that will |
| 1361 | once again create a buffer-local binding for it. |
| 1362 | |
| 1363 | @code{kill-local-variable} returns @var{variable}. |
| 1364 | |
| 1365 | This function is a command because it is sometimes useful to kill one |
| 1366 | buffer-local variable interactively, just as it is useful to create |
| 1367 | buffer-local variables interactively. |
| 1368 | @end deffn |
| 1369 | |
| 1370 | @defun kill-all-local-variables |
| 1371 | This function eliminates all the buffer-local variable bindings of the |
| 1372 | current buffer except for variables marked as ``permanent'' and local |
| 1373 | hook functions that have a non-@code{nil} @code{permanent-local-hook} |
| 1374 | property (@pxref{Setting Hooks}). As a result, the buffer will see |
| 1375 | the default values of most variables. |
| 1376 | |
| 1377 | This function also resets certain other information pertaining to the |
| 1378 | buffer: it sets the local keymap to @code{nil}, the syntax table to the |
| 1379 | value of @code{(standard-syntax-table)}, the case table to |
| 1380 | @code{(standard-case-table)}, and the abbrev table to the value of |
| 1381 | @code{fundamental-mode-abbrev-table}. |
| 1382 | |
| 1383 | The very first thing this function does is run the normal hook |
| 1384 | @code{change-major-mode-hook} (see below). |
| 1385 | |
| 1386 | Every major mode command begins by calling this function, which has the |
| 1387 | effect of switching to Fundamental mode and erasing most of the effects |
| 1388 | of the previous major mode. To ensure that this does its job, the |
| 1389 | variables that major modes set should not be marked permanent. |
| 1390 | |
| 1391 | @code{kill-all-local-variables} returns @code{nil}. |
| 1392 | @end defun |
| 1393 | |
| 1394 | @defvar change-major-mode-hook |
| 1395 | The function @code{kill-all-local-variables} runs this normal hook |
| 1396 | before it does anything else. This gives major modes a way to arrange |
| 1397 | for something special to be done if the user switches to a different |
| 1398 | major mode. It is also useful for buffer-specific minor modes |
| 1399 | that should be forgotten if the user changes the major mode. |
| 1400 | |
| 1401 | For best results, make this variable buffer-local, so that it will |
| 1402 | disappear after doing its job and will not interfere with the |
| 1403 | subsequent major mode. @xref{Hooks}. |
| 1404 | @end defvar |
| 1405 | |
| 1406 | @c Emacs 19 feature |
| 1407 | @cindex permanent local variable |
| 1408 | A buffer-local variable is @dfn{permanent} if the variable name (a |
| 1409 | symbol) has a @code{permanent-local} property that is non-@code{nil}. |
| 1410 | Permanent locals are appropriate for data pertaining to where the file |
| 1411 | came from or how to save it, rather than with how to edit the contents. |
| 1412 | |
| 1413 | @node Default Value |
| 1414 | @subsection The Default Value of a Buffer-Local Variable |
| 1415 | @cindex default value |
| 1416 | |
| 1417 | The global value of a variable with buffer-local bindings is also |
| 1418 | called the @dfn{default} value, because it is the value that is in |
| 1419 | effect whenever neither the current buffer nor the selected frame has |
| 1420 | its own binding for the variable. |
| 1421 | |
| 1422 | The functions @code{default-value} and @code{setq-default} access and |
| 1423 | change a variable's default value regardless of whether the current |
| 1424 | buffer has a buffer-local binding. For example, you could use |
| 1425 | @code{setq-default} to change the default setting of |
| 1426 | @code{paragraph-start} for most buffers; and this would work even when |
| 1427 | you are in a C or Lisp mode buffer that has a buffer-local value for |
| 1428 | this variable. |
| 1429 | |
| 1430 | @c Emacs 19 feature |
| 1431 | The special forms @code{defvar} and @code{defconst} also set the |
| 1432 | default value (if they set the variable at all), rather than any |
| 1433 | buffer-local value. |
| 1434 | |
| 1435 | @defun default-value symbol |
| 1436 | This function returns @var{symbol}'s default value. This is the value |
| 1437 | that is seen in buffers and frames that do not have their own values for |
| 1438 | this variable. If @var{symbol} is not buffer-local, this is equivalent |
| 1439 | to @code{symbol-value} (@pxref{Accessing Variables}). |
| 1440 | @end defun |
| 1441 | |
| 1442 | @c Emacs 19 feature |
| 1443 | @defun default-boundp symbol |
| 1444 | The function @code{default-boundp} tells you whether @var{symbol}'s |
| 1445 | default value is nonvoid. If @code{(default-boundp 'foo)} returns |
| 1446 | @code{nil}, then @code{(default-value 'foo)} would get an error. |
| 1447 | |
| 1448 | @code{default-boundp} is to @code{default-value} as @code{boundp} is to |
| 1449 | @code{symbol-value}. |
| 1450 | @end defun |
| 1451 | |
| 1452 | @defspec setq-default [symbol form]@dots{} |
| 1453 | This special form gives each @var{symbol} a new default value, which is |
| 1454 | the result of evaluating the corresponding @var{form}. It does not |
| 1455 | evaluate @var{symbol}, but does evaluate @var{form}. The value of the |
| 1456 | @code{setq-default} form is the value of the last @var{form}. |
| 1457 | |
| 1458 | If a @var{symbol} is not buffer-local for the current buffer, and is not |
| 1459 | marked automatically buffer-local, @code{setq-default} has the same |
| 1460 | effect as @code{setq}. If @var{symbol} is buffer-local for the current |
| 1461 | buffer, then this changes the value that other buffers will see (as long |
| 1462 | as they don't have a buffer-local value), but not the value that the |
| 1463 | current buffer sees. |
| 1464 | |
| 1465 | @example |
| 1466 | @group |
| 1467 | ;; @r{In buffer @samp{foo}:} |
| 1468 | (make-local-variable 'buffer-local) |
| 1469 | @result{} buffer-local |
| 1470 | @end group |
| 1471 | @group |
| 1472 | (setq buffer-local 'value-in-foo) |
| 1473 | @result{} value-in-foo |
| 1474 | @end group |
| 1475 | @group |
| 1476 | (setq-default buffer-local 'new-default) |
| 1477 | @result{} new-default |
| 1478 | @end group |
| 1479 | @group |
| 1480 | buffer-local |
| 1481 | @result{} value-in-foo |
| 1482 | @end group |
| 1483 | @group |
| 1484 | (default-value 'buffer-local) |
| 1485 | @result{} new-default |
| 1486 | @end group |
| 1487 | |
| 1488 | @group |
| 1489 | ;; @r{In (the new) buffer @samp{bar}:} |
| 1490 | buffer-local |
| 1491 | @result{} new-default |
| 1492 | @end group |
| 1493 | @group |
| 1494 | (default-value 'buffer-local) |
| 1495 | @result{} new-default |
| 1496 | @end group |
| 1497 | @group |
| 1498 | (setq buffer-local 'another-default) |
| 1499 | @result{} another-default |
| 1500 | @end group |
| 1501 | @group |
| 1502 | (default-value 'buffer-local) |
| 1503 | @result{} another-default |
| 1504 | @end group |
| 1505 | |
| 1506 | @group |
| 1507 | ;; @r{Back in buffer @samp{foo}:} |
| 1508 | buffer-local |
| 1509 | @result{} value-in-foo |
| 1510 | (default-value 'buffer-local) |
| 1511 | @result{} another-default |
| 1512 | @end group |
| 1513 | @end example |
| 1514 | @end defspec |
| 1515 | |
| 1516 | @defun set-default symbol value |
| 1517 | This function is like @code{setq-default}, except that @var{symbol} is |
| 1518 | an ordinary evaluated argument. |
| 1519 | |
| 1520 | @example |
| 1521 | @group |
| 1522 | (set-default (car '(a b c)) 23) |
| 1523 | @result{} 23 |
| 1524 | @end group |
| 1525 | @group |
| 1526 | (default-value 'a) |
| 1527 | @result{} 23 |
| 1528 | @end group |
| 1529 | @end example |
| 1530 | @end defun |
| 1531 | |
| 1532 | @node File Local Variables |
| 1533 | @section File Local Variables |
| 1534 | @cindex file local variables |
| 1535 | |
| 1536 | A file can specify local variable values; Emacs uses these to create |
| 1537 | buffer-local bindings for those variables in the buffer visiting that |
| 1538 | file. @xref{File variables, , Local Variables in Files, emacs, The |
| 1539 | GNU Emacs Manual}, for basic information about file-local variables. |
| 1540 | This section describes the functions and variables that affect how |
| 1541 | file-local variables are processed. |
| 1542 | |
| 1543 | If a file-local variable could specify an arbitrary function or Lisp |
| 1544 | expression that would be called later, visiting a file could take over |
| 1545 | your Emacs. Emacs protects against this by automatically setting only |
| 1546 | those file-local variables whose specified values are known to be |
| 1547 | safe. Other file-local variables are set only if the user agrees. |
| 1548 | |
| 1549 | For additional safety, @code{read-circle} is temporarily bound to |
| 1550 | @code{nil} when Emacs reads file-local variables (@pxref{Input |
| 1551 | Functions}). This prevents the Lisp reader from recognizing circular |
| 1552 | and shared Lisp structures (@pxref{Circular Objects}). |
| 1553 | |
| 1554 | @defopt enable-local-variables |
| 1555 | This variable controls whether to process file-local variables. |
| 1556 | The possible values are: |
| 1557 | |
| 1558 | @table @asis |
| 1559 | @item @code{t} (the default) |
| 1560 | Set the safe variables, and query (once) about any unsafe variables. |
| 1561 | @item @code{:safe} |
| 1562 | Set only the safe variables and do not query. |
| 1563 | @item @code{:all} |
| 1564 | Set all the variables and do not query. |
| 1565 | @item @code{nil} |
| 1566 | Don't set any variables. |
| 1567 | @item anything else |
| 1568 | Query (once) about all the variables. |
| 1569 | @end table |
| 1570 | @end defopt |
| 1571 | |
| 1572 | @defun hack-local-variables &optional mode-only |
| 1573 | This function parses, and binds or evaluates as appropriate, any local |
| 1574 | variables specified by the contents of the current buffer. The variable |
| 1575 | @code{enable-local-variables} has its effect here. However, this |
| 1576 | function does not look for the @samp{mode:} local variable in the |
| 1577 | @w{@samp{-*-}} line. @code{set-auto-mode} does that, also taking |
| 1578 | @code{enable-local-variables} into account (@pxref{Auto Major Mode}). |
| 1579 | |
| 1580 | This function works by walking the alist stored in |
| 1581 | @code{file-local-variables-alist} and applying each local variable in |
| 1582 | turn. It calls @code{before-hack-local-variables-hook} and |
| 1583 | @code{hack-local-variables-hook} before and after applying the |
| 1584 | variables, respectively. |
| 1585 | |
| 1586 | If the optional argument @var{mode-only} is non-@code{nil}, then all |
| 1587 | this function does is return @code{t} if the @w{@samp{-*-}} line or |
| 1588 | the local variables list specifies a mode and @code{nil} otherwise. |
| 1589 | It does not set the mode nor any other file-local variable. |
| 1590 | @end defun |
| 1591 | |
| 1592 | @defvar file-local-variables-alist |
| 1593 | This buffer-local variable holds the alist of file-local variable |
| 1594 | settings. Each element of the alist is of the form |
| 1595 | @w{@code{(@var{var} . @var{value})}}, where @var{var} is a symbol of |
| 1596 | the local variable and @var{value} is its value. When Emacs visits a |
| 1597 | file, it first collects all the file-local variables into this alist, |
| 1598 | and then the @code{hack-local-variables} function applies them one by |
| 1599 | one. |
| 1600 | @end defvar |
| 1601 | |
| 1602 | @defvar before-hack-local-variables-hook |
| 1603 | Emacs calls this hook immediately before applying file-local variables |
| 1604 | stored in @code{file-local-variables-alist}. |
| 1605 | @end defvar |
| 1606 | |
| 1607 | @defvar hack-local-variables-hook |
| 1608 | Emacs calls this hook immediately after it finishes applying |
| 1609 | file-local variables stored in @code{file-local-variables-alist}. |
| 1610 | @end defvar |
| 1611 | |
| 1612 | @cindex safe local variable |
| 1613 | You can specify safe values for a variable with a |
| 1614 | @code{safe-local-variable} property. The property has to be a |
| 1615 | function of one argument; any value is safe if the function returns |
| 1616 | non-@code{nil} given that value. Many commonly-encountered file |
| 1617 | variables have @code{safe-local-variable} properties; these include |
| 1618 | @code{fill-column}, @code{fill-prefix}, and @code{indent-tabs-mode}. |
| 1619 | For boolean-valued variables that are safe, use @code{booleanp} as the |
| 1620 | property value. Lambda expressions should be quoted so that |
| 1621 | @code{describe-variable} can display the predicate. |
| 1622 | |
| 1623 | @defopt safe-local-variable-values |
| 1624 | This variable provides another way to mark some variable values as |
| 1625 | safe. It is a list of cons cells @code{(@var{var} . @var{val})}, |
| 1626 | where @var{var} is a variable name and @var{val} is a value which is |
| 1627 | safe for that variable. |
| 1628 | |
| 1629 | When Emacs asks the user whether or not to obey a set of file-local |
| 1630 | variable specifications, the user can choose to mark them as safe. |
| 1631 | Doing so adds those variable/value pairs to |
| 1632 | @code{safe-local-variable-values}, and saves it to the user's custom |
| 1633 | file. |
| 1634 | @end defopt |
| 1635 | |
| 1636 | @defun safe-local-variable-p sym val |
| 1637 | This function returns non-@code{nil} if it is safe to give @var{sym} |
| 1638 | the value @var{val}, based on the above criteria. |
| 1639 | @end defun |
| 1640 | |
| 1641 | @c @cindex risky local variable Duplicates risky-local-variable |
| 1642 | Some variables are considered @dfn{risky}. A variable whose name |
| 1643 | ends in any of @samp{-command}, @samp{-frame-alist}, @samp{-function}, |
| 1644 | @samp{-functions}, @samp{-hook}, @samp{-hooks}, @samp{-form}, |
| 1645 | @samp{-forms}, @samp{-map}, @samp{-map-alist}, @samp{-mode-alist}, |
| 1646 | @samp{-program}, or @samp{-predicate} is considered risky. The |
| 1647 | variables @samp{font-lock-keywords}, @samp{font-lock-keywords} |
| 1648 | followed by a digit, and @samp{font-lock-syntactic-keywords} are also |
| 1649 | considered risky. Finally, any variable whose name has a |
| 1650 | non-@code{nil} @code{risky-local-variable} property is considered |
| 1651 | risky. |
| 1652 | |
| 1653 | @defun risky-local-variable-p sym |
| 1654 | This function returns non-@code{nil} if @var{sym} is a risky variable, |
| 1655 | based on the above criteria. |
| 1656 | @end defun |
| 1657 | |
| 1658 | If a variable is risky, it will not be entered automatically into |
| 1659 | @code{safe-local-variable-values} as described above. Therefore, |
| 1660 | Emacs will always query before setting a risky variable, unless the |
| 1661 | user explicitly allows the setting by customizing |
| 1662 | @code{safe-local-variable-values} directly. |
| 1663 | |
| 1664 | @defvar ignored-local-variables |
| 1665 | This variable holds a list of variables that should not be given local |
| 1666 | values by files. Any value specified for one of these variables is |
| 1667 | completely ignored. |
| 1668 | @end defvar |
| 1669 | |
| 1670 | The @samp{Eval:} ``variable'' is also a potential loophole, so Emacs |
| 1671 | normally asks for confirmation before handling it. |
| 1672 | |
| 1673 | @defopt enable-local-eval |
| 1674 | This variable controls processing of @samp{Eval:} in @samp{-*-} lines |
| 1675 | or local variables |
| 1676 | lists in files being visited. A value of @code{t} means process them |
| 1677 | unconditionally; @code{nil} means ignore them; anything else means ask |
| 1678 | the user what to do for each file. The default value is @code{maybe}. |
| 1679 | @end defopt |
| 1680 | |
| 1681 | @defopt safe-local-eval-forms |
| 1682 | This variable holds a list of expressions that are safe to |
| 1683 | evaluate when found in the @samp{Eval:} ``variable'' in a file |
| 1684 | local variables list. |
| 1685 | @end defopt |
| 1686 | |
| 1687 | If the expression is a function call and the function has a |
| 1688 | @code{safe-local-eval-function} property, the property value |
| 1689 | determines whether the expression is safe to evaluate. The property |
| 1690 | value can be a predicate to call to test the expression, a list of |
| 1691 | such predicates (it's safe if any predicate succeeds), or @code{t} |
| 1692 | (always safe provided the arguments are constant). |
| 1693 | |
| 1694 | Text properties are also potential loopholes, since their values |
| 1695 | could include functions to call. So Emacs discards all text |
| 1696 | properties from string values specified for file-local variables. |
| 1697 | |
| 1698 | @node Directory Local Variables |
| 1699 | @section Directory Local Variables |
| 1700 | @cindex directory local variables |
| 1701 | |
| 1702 | A directory can specify local variable values common to all files in |
| 1703 | that directory; Emacs uses these to create buffer-local bindings for |
| 1704 | those variables in buffers visiting any file in that directory. This |
| 1705 | is useful when the files in the directory belong to some @dfn{project} |
| 1706 | and therefore share the same local variables. |
| 1707 | |
| 1708 | There are two different methods for specifying directory local |
| 1709 | variables: by putting them in a special file, or by defining a |
| 1710 | @dfn{project class} for that directory. |
| 1711 | |
| 1712 | @defvr Constant dir-locals-file |
| 1713 | This constant is the name of the file where Emacs expects to find the |
| 1714 | directory-local variables. The name of the file is |
| 1715 | @file{.dir-locals.el}@footnote{ |
| 1716 | The MS-DOS version of Emacs uses @file{_dir-locals.el} instead, due to |
| 1717 | limitations of the DOS filesystems. |
| 1718 | }. A file by that name in a directory causes Emacs to apply its |
| 1719 | settings to any file in that directory or any of its subdirectories. |
| 1720 | If some of the subdirectories have their own @file{.dir-locals.el} |
| 1721 | files, Emacs uses the settings from the deepest file it finds starting |
| 1722 | from the file's directory and moving up the directory tree. The file |
| 1723 | specifies local variables as a specially formatted list; see |
| 1724 | @ref{Directory Variables, , Per-directory Local Variables, emacs, The |
| 1725 | GNU Emacs Manual}, for more details. |
| 1726 | @end defvr |
| 1727 | |
| 1728 | @defun hack-dir-local-variables |
| 1729 | This function reads the @code{.dir-locals.el} file and stores the |
| 1730 | directory-local variables in @code{file-local-variables-alist} that is |
| 1731 | local to the buffer visiting any file in the directory, without |
| 1732 | applying them. It also stores the directory-local settings in |
| 1733 | @code{dir-locals-class-alist}, where it defines a special class for |
| 1734 | the directory in which @file{.dir-locals.el} file was found. This |
| 1735 | function works by calling @code{dir-locals-set-class-variables} and |
| 1736 | @code{dir-locals-set-directory-class}, described below. |
| 1737 | @end defun |
| 1738 | |
| 1739 | @defun dir-locals-set-class-variables class variables |
| 1740 | This function defines a set of variable settings for the named |
| 1741 | @var{class}, which is a symbol. You can later assign the class to one |
| 1742 | or more directories, and Emacs will apply those variable settings to |
| 1743 | all files in those directories. The list in @var{variables} can be of |
| 1744 | one of the two forms: @code{(@var{major-mode} . @var{alist})} or |
| 1745 | @code{(@var{directory} . @var{list})}. With the first form, if the |
| 1746 | file's buffer turns on a mode that is derived from @var{major-mode}, |
| 1747 | then the all the variables in the associated @var{alist} are applied; |
| 1748 | @var{alist} should be of the form @code{(@var{name} . @var{value})}. |
| 1749 | A special value @code{nil} for @var{major-mode} means the settings are |
| 1750 | applicable to any mode. |
| 1751 | |
| 1752 | With the second form of @var{variables}, if @var{directory} is the |
| 1753 | initial substring of the file's directory, then @var{list} is applied |
| 1754 | recursively by following the above rules; @var{list} should be of one |
| 1755 | of the two forms accepted by this function in @var{variables}. |
| 1756 | @end defun |
| 1757 | |
| 1758 | @defun dir-locals-set-directory-class directory class |
| 1759 | This function assigns @var{class} to all the files in @code{directory} |
| 1760 | and its subdirectories. Thereafter, all the variable settings |
| 1761 | specified for @var{class} will be applied to any visited file in |
| 1762 | @var{directory} and its children. @var{class} must have been already |
| 1763 | defined by @code{dir-locals-set-class-variables} |
| 1764 | @end defun |
| 1765 | |
| 1766 | @defvar dir-locals-class-alist |
| 1767 | This alist holds the class symbols and the associated variable |
| 1768 | settings. It is updated by @code{dir-locals-set-class-variables}. |
| 1769 | @end defvar |
| 1770 | |
| 1771 | @defvar dir-locals-directory-cache |
| 1772 | This alist holds directory names, their assigned class names, and |
| 1773 | modification times of the associated directory local variables file. |
| 1774 | It is updated by @code{dir-locals-set-directory-class}. |
| 1775 | @end defvar |
| 1776 | |
| 1777 | @node Frame-Local Variables |
| 1778 | @section Frame-Local Values for Variables |
| 1779 | @cindex frame-local variables |
| 1780 | |
| 1781 | In addition to buffer-local variable bindings (@pxref{Buffer-Local |
| 1782 | Variables}), Emacs supports @dfn{frame-local} bindings. A frame-local |
| 1783 | binding for a variable is in effect in a frame for which it was |
| 1784 | defined. |
| 1785 | |
| 1786 | In practice, frame-local variables have not proven very useful. |
| 1787 | Ordinary frame parameters are generally used instead (@pxref{Frame |
| 1788 | Parameters}). The function @code{make-variable-frame-local}, which |
| 1789 | was used to define frame-local variables, has been deprecated since |
| 1790 | Emacs 22.2. However, you can still define a frame-specific binding |
| 1791 | for a variable @var{var} in frame @var{frame}, by setting the |
| 1792 | @var{var} frame parameter for that frame: |
| 1793 | |
| 1794 | @lisp |
| 1795 | (modify-frame-parameters @var{frame} '((@var{var} . @var{value}))) |
| 1796 | @end lisp |
| 1797 | |
| 1798 | @noindent |
| 1799 | This causes the variable @var{var} to be bound to the specified |
| 1800 | @var{value} in the named @var{frame}. To check the frame-specific |
| 1801 | values of such variables, use @code{frame-parameter}. @xref{Parameter |
| 1802 | Access}. |
| 1803 | |
| 1804 | Note that you cannot have a frame-local binding for a variable that |
| 1805 | has a buffer-local binding. |
| 1806 | |
| 1807 | @node Variable Aliases |
| 1808 | @section Variable Aliases |
| 1809 | @cindex variable aliases |
| 1810 | |
| 1811 | It is sometimes useful to make two variables synonyms, so that both |
| 1812 | variables always have the same value, and changing either one also |
| 1813 | changes the other. Whenever you change the name of a |
| 1814 | variable---either because you realize its old name was not well |
| 1815 | chosen, or because its meaning has partly changed---it can be useful |
| 1816 | to keep the old name as an @emph{alias} of the new one for |
| 1817 | compatibility. You can do this with @code{defvaralias}. |
| 1818 | |
| 1819 | @defun defvaralias new-alias base-variable &optional docstring |
| 1820 | This function defines the symbol @var{new-alias} as a variable alias |
| 1821 | for symbol @var{base-variable}. This means that retrieving the value |
| 1822 | of @var{new-alias} returns the value of @var{base-variable}, and |
| 1823 | changing the value of @var{new-alias} changes the value of |
| 1824 | @var{base-variable}. The two aliased variable names always share the |
| 1825 | same value and the same bindings. |
| 1826 | |
| 1827 | If the @var{docstring} argument is non-@code{nil}, it specifies the |
| 1828 | documentation for @var{new-alias}; otherwise, the alias gets the same |
| 1829 | documentation as @var{base-variable} has, if any, unless |
| 1830 | @var{base-variable} is itself an alias, in which case @var{new-alias} gets |
| 1831 | the documentation of the variable at the end of the chain of aliases. |
| 1832 | |
| 1833 | This function returns @var{base-variable}. |
| 1834 | @end defun |
| 1835 | |
| 1836 | Variable aliases are convenient for replacing an old name for a |
| 1837 | variable with a new name. @code{make-obsolete-variable} declares that |
| 1838 | the old name is obsolete and therefore that it may be removed at some |
| 1839 | stage in the future. |
| 1840 | |
| 1841 | @defun make-obsolete-variable obsolete-name current-name &optional when |
| 1842 | This function makes the byte compiler warn that the variable |
| 1843 | @var{obsolete-name} is obsolete. If @var{current-name} is a symbol, it is |
| 1844 | the variable's new name; then the warning message says to use |
| 1845 | @var{current-name} instead of @var{obsolete-name}. If @var{current-name} |
| 1846 | is a string, this is the message and there is no replacement variable. |
| 1847 | |
| 1848 | If provided, @var{when} should be a string indicating when the |
| 1849 | variable was first made obsolete---for example, a date or a release |
| 1850 | number. |
| 1851 | @end defun |
| 1852 | |
| 1853 | You can make two variables synonyms and declare one obsolete at the |
| 1854 | same time using the macro @code{define-obsolete-variable-alias}. |
| 1855 | |
| 1856 | @defmac define-obsolete-variable-alias obsolete-name current-name &optional when docstring |
| 1857 | This macro marks the variable @var{obsolete-name} as obsolete and also |
| 1858 | makes it an alias for the variable @var{current-name}. It is |
| 1859 | equivalent to the following: |
| 1860 | |
| 1861 | @example |
| 1862 | (defvaralias @var{obsolete-name} @var{current-name} @var{docstring}) |
| 1863 | (make-obsolete-variable @var{obsolete-name} @var{current-name} @var{when}) |
| 1864 | @end example |
| 1865 | @end defmac |
| 1866 | |
| 1867 | @defun indirect-variable variable |
| 1868 | This function returns the variable at the end of the chain of aliases |
| 1869 | of @var{variable}. If @var{variable} is not a symbol, or if @var{variable} is |
| 1870 | not defined as an alias, the function returns @var{variable}. |
| 1871 | |
| 1872 | This function signals a @code{cyclic-variable-indirection} error if |
| 1873 | there is a loop in the chain of symbols. |
| 1874 | @end defun |
| 1875 | |
| 1876 | @example |
| 1877 | (defvaralias 'foo 'bar) |
| 1878 | (indirect-variable 'foo) |
| 1879 | @result{} bar |
| 1880 | (indirect-variable 'bar) |
| 1881 | @result{} bar |
| 1882 | (setq bar 2) |
| 1883 | bar |
| 1884 | @result{} 2 |
| 1885 | @group |
| 1886 | foo |
| 1887 | @result{} 2 |
| 1888 | @end group |
| 1889 | (setq foo 0) |
| 1890 | bar |
| 1891 | @result{} 0 |
| 1892 | foo |
| 1893 | @result{} 0 |
| 1894 | @end example |
| 1895 | |
| 1896 | @node Variables with Restricted Values |
| 1897 | @section Variables with Restricted Values |
| 1898 | |
| 1899 | Ordinary Lisp variables can be assigned any value that is a valid |
| 1900 | Lisp object. However, certain Lisp variables are not defined in Lisp, |
| 1901 | but in C. Most of these variables are defined in the C code using |
| 1902 | @code{DEFVAR_LISP}. Like variables defined in Lisp, these can take on |
| 1903 | any value. However, some variables are defined using |
| 1904 | @code{DEFVAR_INT} or @code{DEFVAR_BOOL}. @xref{Defining Lisp |
| 1905 | variables in C,, Writing Emacs Primitives}, in particular the |
| 1906 | description of functions of the type @code{syms_of_@var{filename}}, |
| 1907 | for a brief discussion of the C implementation. |
| 1908 | |
| 1909 | Variables of type @code{DEFVAR_BOOL} can only take on the values |
| 1910 | @code{nil} or @code{t}. Attempting to assign them any other value |
| 1911 | will set them to @code{t}: |
| 1912 | |
| 1913 | @example |
| 1914 | (let ((display-hourglass 5)) |
| 1915 | display-hourglass) |
| 1916 | @result{} t |
| 1917 | @end example |
| 1918 | |
| 1919 | @defvar byte-boolean-vars |
| 1920 | This variable holds a list of all variables of type @code{DEFVAR_BOOL}. |
| 1921 | @end defvar |
| 1922 | |
| 1923 | Variables of type @code{DEFVAR_INT} can only take on integer values. |
| 1924 | Attempting to assign them any other value will result in an error: |
| 1925 | |
| 1926 | @example |
| 1927 | (setq window-min-height 5.0) |
| 1928 | @error{} Wrong type argument: integerp, 5.0 |
| 1929 | @end example |