variables.texi 72.1 KB
Newer Older
Glenn Morris's avatar
Glenn Morris committed
1 2
@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
3
@c Copyright (C) 1990-1995, 1998-2012 Free Software Foundation, Inc.
Glenn Morris's avatar
Glenn Morris committed
4 5 6 7 8 9
@c See the file elisp.texi for copying conditions.
@node Variables, Functions, Control Structures, Top
@chapter Variables
@cindex variable

  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 12 13 14 15 16 17
(@pxref{Symbols}).  The variable name is simply the symbol's name, and
the variable's value is stored in the symbol's value cell@footnote{To
be precise, under the default @dfn{dynamic binding} rules the value
cell always holds the variable's current value, but this is not the
case under @dfn{lexical binding} rules.  @xref{Variable Scoping}, for
details.}.  @xref{Symbol Components}.  In Emacs Lisp, the use of a
symbol as a variable is independent of its use as a function name.
18 19 20 21 22 23

  As previously noted in this manual, a Lisp program is represented
primarily by Lisp objects, and only secondarily as text.  The textual
form of a Lisp program is given by the read syntax of the Lisp objects
that constitute the program.  Hence, the textual form of a variable in
a Lisp program is written using the read syntax for the symbol
24
representing the variable.
Glenn Morris's avatar
Glenn Morris committed
25 26

@menu
27 28 29 30 31 32
* Global Variables::            Variable values that exist permanently, everywhere.
* Constant Variables::          Certain "variables" have values that never change.
* Local Variables::             Variable values that exist only temporarily.
* Void Variables::              Symbols that lack values.
* Defining Variables::          A definition says a symbol is used as a variable.
* Tips for Defining::           Things you should think about when you
Glenn Morris's avatar
Glenn Morris committed
33
                            define a variable.
34
* Accessing Variables::         Examining values of variables whose names
Glenn Morris's avatar
Glenn Morris committed
35
                            are known only at run time.
36 37 38 39 40 41
* Setting Variables::           Storing new values in variables.
* Variable Scoping::            How Lisp chooses among local and global values.
* Buffer-Local Variables::      Variable values in effect only in one buffer.
* File Local Variables::        Handling local variable lists in files.
* Directory Local Variables::   Local variables common to all files in a directory.
* Variable Aliases::            Variables that are aliases for other variables.
Glenn Morris's avatar
Glenn Morris committed
42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63
* Variables with Restricted Values::  Non-constant variables whose value can
                                        @emph{not} be an arbitrary Lisp object.
@end menu

@node Global Variables
@section Global Variables
@cindex global variable

  The simplest way to use a variable is @dfn{globally}.  This means that
the variable has just one value at a time, and this value is in effect
(at least for the moment) throughout the Lisp system.  The value remains
in effect until you specify a new one.  When a new value replaces the
old one, no trace of the old value remains in the variable.

  You specify a value for a symbol with @code{setq}.  For example,

@example
(setq x '(a b))
@end example

@noindent
gives the variable @code{x} the value @code{(a b)}.  Note that
64 65 66
@code{setq} is a special form (@pxref{Special Forms}); it does not
evaluate its first argument, the name of the variable, but it does
evaluate the second argument, the new value.
Glenn Morris's avatar
Glenn Morris committed
67

68 69
  Once the variable has a value, you can refer to it by using the
symbol itself as an expression.  Thus,
Glenn Morris's avatar
Glenn Morris committed
70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99

@example
@group
x @result{} (a b)
@end group
@end example

@noindent
assuming the @code{setq} form shown above has already been executed.

  If you do set the same variable again, the new value replaces the old
one:

@example
@group
x
     @result{} (a b)
@end group
@group
(setq x 4)
     @result{} 4
@end group
@group
x
     @result{} 4
@end group
@end example

@node Constant Variables
@section Variables that Never Change
100
@cindex @code{setting-constant} error
Glenn Morris's avatar
Glenn Morris committed
101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132
@cindex keyword symbol
@cindex variable with constant value
@cindex constant variables
@cindex symbol that evaluates to itself
@cindex symbol with constant value

  In Emacs Lisp, certain symbols normally evaluate to themselves.  These
include @code{nil} and @code{t}, as well as any symbol whose name starts
with @samp{:} (these are called @dfn{keywords}).  These symbols cannot
be rebound, nor can their values be changed.  Any attempt to set or bind
@code{nil} or @code{t} signals a @code{setting-constant} error.  The
same is true for a keyword (a symbol whose name starts with @samp{:}),
if it is interned in the standard obarray, except that setting such a
symbol to itself is not an error.

@example
@group
nil @equiv{} 'nil
     @result{} nil
@end group
@group
(setq nil 500)
@error{} Attempt to set constant symbol: nil
@end group
@end example

@defun keywordp object
function returns @code{t} if @var{object} is a symbol whose name
starts with @samp{:}, interned in the standard obarray, and returns
@code{nil} otherwise.
@end defun

133 134 135 136 137 138
These constants are fundamentally different from the ``constants''
defined using the @code{defconst} special form (@pxref{Defining
Variables}).  A @code{defconst} form serves to inform human readers
that you do not intend to change the value of a variable, but Emacs
does not raise an error if you actually change it.

Glenn Morris's avatar
Glenn Morris committed
139 140 141 142 143 144 145 146
@node Local Variables
@section Local Variables
@cindex binding local variables
@cindex local variables
@cindex local binding
@cindex global binding

  Global variables have values that last until explicitly superseded
147 148 149
with new values.  Sometimes it is useful to give a variable a
@dfn{local value}---a value that takes effect only within a certain
part of a Lisp program.  When a variable has a local value, we say
150 151
that it is @dfn{locally bound} to that value, and that it is a
@dfn{local variable}.
152 153 154 155 156 157 158

  For example, when a function is called, its argument variables
receive local values, which are the actual arguments supplied to the
function call; these local bindings take effect within the body of the
function.  To take another example, the @code{let} special form
explicitly establishes local bindings for specific variables, which
take effect within the body of the @code{let} form.
Glenn Morris's avatar
Glenn Morris committed
159 160 161 162

  We also speak of the @dfn{global binding}, which is where
(conceptually) the global value is kept.

163 164 165 166 167 168 169 170 171
@cindex shadowing of variables
  Establishing a local binding saves away the variable's previous
value (or lack of one).  We say that the previous value is
@dfn{shadowed}.  Both global and local values may be shadowed.  If a
local binding is in effect, using @code{setq} on the local variable
stores the specified value in the local binding.  When that local
binding is no longer in effect, the previously shadowed value (or lack
of one) comes back.

Glenn Morris's avatar
Glenn Morris committed
172
@cindex current binding
173 174 175 176 177 178 179 180 181 182 183 184
  A variable can have more than one local binding at a time (e.g.@: if
there are nested @code{let} forms that bind the variable).  The
@dfn{current binding} is the local binding that is actually in effect.
It determines the value returned by evaluating the variable symbol,
and it is the binding acted on by @code{setq}.

  For most purposes, you can think of the current binding as the
``innermost'' local binding, or the global binding if there is no
local binding.  To be more precise, a rule called the @dfn{scoping
rule} determines where in a program a local binding takes effect.  The
default scoping rule in Emacs Lisp is called @dfn{dynamic scoping},
which simply states that the current binding at any given point in the
185 186 187 188
execution of a program is the most recently-created binding for that
variable that still exists.  For details about dynamic scoping, and an
alternative scoping rule called @dfn{lexical scoping}, @xref{Variable
Scoping}.
189 190 191

  The special forms @code{let} and @code{let*} exist to create local
bindings:
Glenn Morris's avatar
Glenn Morris committed
192 193

@defspec let (bindings@dots{}) forms@dots{}
194 195 196 197
This special form sets up local bindings for a certain set of
variables, as specified by @var{bindings}, and then evaluates all of
the @var{forms} in textual order.  Its return value is the value of
the last form in @var{forms}.
Glenn Morris's avatar
Glenn Morris committed
198 199

Each of the @var{bindings} is either @w{(i) a} symbol, in which case
200 201 202 203
that symbol is locally bound to @code{nil}; or @w{(ii) a} list of the
form @code{(@var{symbol} @var{value-form})}, in which case
@var{symbol} is locally bound to the result of evaluating
@var{value-form}.  If @var{value-form} is omitted, @code{nil} is used.
Glenn Morris's avatar
Glenn Morris committed
204 205 206 207 208 209 210 211 212 213 214

All of the @var{value-form}s in @var{bindings} are evaluated in the
order they appear and @emph{before} binding any of the symbols to them.
Here is an example of this: @code{z} is bound to the old value of
@code{y}, which is 2, not the new value of @code{y}, which is 1.

@example
@group
(setq y 2)
     @result{} 2
@end group
215

Glenn Morris's avatar
Glenn Morris committed
216 217 218 219 220 221 222 223 224 225 226 227 228
@group
(let ((y 1)
      (z y))
  (list y z))
     @result{} (1 2)
@end group
@end example
@end defspec

@defspec let* (bindings@dots{}) forms@dots{}
This special form is like @code{let}, but it binds each variable right
after computing its local value, before computing the local value for
the next variable.  Therefore, an expression in @var{bindings} can
229 230
refer to the preceding symbols bound in this @code{let*} form.
Compare the following example with the example above for @code{let}.
Glenn Morris's avatar
Glenn Morris committed
231 232 233 234 235 236

@example
@group
(setq y 2)
     @result{} 2
@end group
237

Glenn Morris's avatar
Glenn Morris committed
238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261
@group
(let* ((y 1)
       (z y))    ; @r{Use the just-established value of @code{y}.}
  (list y z))
     @result{} (1 1)
@end group
@end example
@end defspec

  Here is a complete list of the other facilities that create local
bindings:

@itemize @bullet
@item
Function calls (@pxref{Functions}).

@item
Macro calls (@pxref{Macros}).

@item
@code{condition-case} (@pxref{Errors}).
@end itemize

  Variables can also have buffer-local bindings (@pxref{Buffer-Local
262
Variables}); a few variables have terminal-local bindings
263
(@pxref{Multiple Terminals}).  These kinds of bindings work somewhat
264
like ordinary local bindings, but they are localized depending on
265
``where'' you are in Emacs.
Glenn Morris's avatar
Glenn Morris committed
266

267
@defopt max-specpdl-size
Glenn Morris's avatar
Glenn Morris committed
268 269 270 271 272
@anchor{Definition of max-specpdl-size}
@cindex variable limit error
@cindex evaluation error
@cindex infinite recursion
This variable defines the limit on the total number of local variable
273 274 275
bindings and @code{unwind-protect} cleanups (see @ref{Cleanups,,
Cleaning Up from Nonlocal Exits}) that are allowed before Emacs
signals an error (with data @code{"Variable binding depth exceeds
Glenn Morris's avatar
Glenn Morris committed
276 277 278 279 280 281 282
max-specpdl-size"}).

This limit, with the associated error when it is exceeded, is one way
that Lisp avoids infinite recursion on an ill-defined function.
@code{max-lisp-eval-depth} provides another limit on depth of nesting.
@xref{Definition of max-lisp-eval-depth,, Eval}.

283
The default value is 1300.  Entry to the Lisp debugger increases the
Glenn Morris's avatar
Glenn Morris committed
284 285
value, if there is little room left, to make sure the debugger itself
has room to execute.
286
@end defopt
Glenn Morris's avatar
Glenn Morris committed
287 288 289

@node Void Variables
@section When a Variable is ``Void''
290
@cindex @code{void-variable} error
Glenn Morris's avatar
Glenn Morris committed
291 292
@cindex void variable

293 294 295 296 297 298 299 300 301 302 303 304
  We say that a variable is void if its symbol has an unassigned value
cell (@pxref{Symbol Components}).  Under Emacs Lisp's default dynamic
binding rules (@pxref{Variable Scoping}), the value cell stores the
variable's current (local or global) value.  Note that an unassigned
value cell is @emph{not} the same as having @code{nil} in the value
cell.  The symbol @code{nil} is a Lisp object and can be the value of
a variable, just as any other object can be; but it is still a value.
If a variable is void, trying to evaluate the variable signals a
@code{void-variable} error rather than a value.

  Under lexical binding rules, the value cell only holds the
variable's global value, i.e.@: the value outside of any lexical
Paul Eggert's avatar
Paul Eggert committed
305
binding construct.  When a variable is lexically bound, the local value
306 307
is determined by the lexical environment; the variable may have a
local value if its symbol's value cell is unassigned.
Glenn Morris's avatar
Glenn Morris committed
308 309

@defun makunbound symbol
310 311
This function empties out the value cell of @var{symbol}, making the
variable void.  It returns @var{symbol}.
Glenn Morris's avatar
Glenn Morris committed
312

313
If @var{symbol} has a dynamic local binding, @code{makunbound} voids
314 315 316 317
the current binding, and this voidness lasts only as long as the local
binding is in effect.  Afterwards, the previously shadowed local or
global binding is reexposed; then the variable will no longer be void,
unless the reexposed binding is void too.
Glenn Morris's avatar
Glenn Morris committed
318

319
Here are some examples (assuming dynamic binding is in effect):
Glenn Morris's avatar
Glenn Morris committed
320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351

@smallexample
@group
(setq x 1)               ; @r{Put a value in the global binding.}
     @result{} 1
(let ((x 2))             ; @r{Locally bind it.}
  (makunbound 'x)        ; @r{Void the local binding.}
  x)
@error{} Symbol's value as variable is void: x
@end group
@group
x                        ; @r{The global binding is unchanged.}
     @result{} 1

(let ((x 2))             ; @r{Locally bind it.}
  (let ((x 3))           ; @r{And again.}
    (makunbound 'x)      ; @r{Void the innermost-local binding.}
    x))                  ; @r{And refer: it's void.}
@error{} Symbol's value as variable is void: x
@end group

@group
(let ((x 2))
  (let ((x 3))
    (makunbound 'x))     ; @r{Void inner binding, then remove it.}
  x)                     ; @r{Now outer @code{let} binding is visible.}
     @result{} 2
@end group
@end smallexample
@end defun

@defun boundp variable
352 353 354 355
This function returns @code{t} if @var{variable} (a symbol) is not
void, and @code{nil} if it is void.

Here are some examples (assuming dynamic binding is in effect):
Glenn Morris's avatar
Glenn Morris committed
356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385

@smallexample
@group
(boundp 'abracadabra)          ; @r{Starts out void.}
     @result{} nil
@end group
@group
(let ((abracadabra 5))         ; @r{Locally bind it.}
  (boundp 'abracadabra))
     @result{} t
@end group
@group
(boundp 'abracadabra)          ; @r{Still globally void.}
     @result{} nil
@end group
@group
(setq abracadabra 5)           ; @r{Make it globally nonvoid.}
     @result{} 5
@end group
@group
(boundp 'abracadabra)
     @result{} t
@end group
@end smallexample
@end defun

@node Defining Variables
@section Defining Global Variables
@cindex variable definition

386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406
  A @dfn{variable definition} is a construct that announces your
intention to use a symbol as a global variable.  It uses the special
forms @code{defvar} or @code{defconst}, which are documented below.

  A variable definition serves three purposes.  First, it informs
people who read the code that the symbol is @emph{intended} to be used
a certain way (as a variable).  Second, it informs the Lisp system of
this, optionally supplying an initial value and a documentation
string.  Third, it provides information to programming tools such as
@command{etags}, allowing them to find where the variable was defined.

  The difference between @code{defconst} and @code{defvar} is mainly a
matter of intent, serving to inform human readers of whether the value
should ever change.  Emacs Lisp does not actually prevent you from
changing the value of a variable defined with @code{defconst}.  One
notable difference between the two forms is that @code{defconst}
unconditionally initializes the variable, whereas @code{defvar}
initializes it only if it is originally void.

  To define a customizable variable, you should use @code{defcustom}
(which calls @code{defvar} as a subroutine).  @xref{Customization}.
Glenn Morris's avatar
Glenn Morris committed
407 408

@defspec defvar symbol [value [doc-string]]
409 410 411 412 413
This special form defines @var{symbol} as a variable.  Note that
@var{symbol} is not evaluated; the symbol to be defined should appear
explicitly in the @code{defvar} form.  The variable is marked as
@dfn{special}, meaning that it should always be dynamically bound
(@pxref{Variable Scoping}).
Glenn Morris's avatar
Glenn Morris committed
414 415

If @var{symbol} is void and @var{value} is specified, @code{defvar}
416 417 418 419 420
evaluates @var{value} and sets @var{symbol} to the result.  But if
@var{symbol} already has a value (i.e.@: it is not void), @var{value}
is not even evaluated, and @var{symbol}'s value remains unchanged.  If
@var{value} is omitted, the value of @var{symbol} is not changed in
any case.
Glenn Morris's avatar
Glenn Morris committed
421 422 423 424 425 426 427 428 429 430 431

If @var{symbol} has a buffer-local binding in the current buffer,
@code{defvar} operates on the default value, which is buffer-independent,
not the current (buffer-local) binding.  It sets the default value if
the default value is void.  @xref{Buffer-Local Variables}.

When you evaluate a top-level @code{defvar} form with @kbd{C-M-x} in
Emacs Lisp mode (@code{eval-defun}), a special feature of
@code{eval-defun} arranges to set the variable unconditionally, without
testing whether its value is void.

432 433 434
If the @var{doc-string} argument is supplied, it specifies the
documentation string for the variable (stored in the symbol's
@code{variable-documentation} property).  @xref{Documentation}.
Glenn Morris's avatar
Glenn Morris committed
435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460

Here are some examples.  This form defines @code{foo} but does not
initialize it:

@example
@group
(defvar foo)
     @result{} foo
@end group
@end example

This example initializes the value of @code{bar} to @code{23}, and gives
it a documentation string:

@example
@group
(defvar bar 23
  "The normal weight of a bar.")
     @result{} bar
@end group
@end example

The @code{defvar} form returns @var{symbol}, but it is normally used
at top level in a file where its value does not matter.
@end defspec

461
@cindex constant variables
Glenn Morris's avatar
Glenn Morris committed
462 463 464 465 466 467 468
@defspec defconst symbol value [doc-string]
This special form defines @var{symbol} as a value and initializes it.
It informs a person reading your code that @var{symbol} has a standard
global value, established here, that should not be changed by the user
or by other programs.  Note that @var{symbol} is not evaluated; the
symbol to be defined must appear explicitly in the @code{defconst}.

469 470 471 472 473
The @code{defconst} form, like @code{defvar}, marks the variable as
@dfn{special}, meaning that it should always be dynamically bound
(@pxref{Variable Scoping}).  In addition, it marks the variable as
risky (@pxref{File Local Variables}).

Glenn Morris's avatar
Glenn Morris committed
474 475 476 477 478 479 480
@code{defconst} always evaluates @var{value}, and sets the value of
@var{symbol} to the result.  If @var{symbol} does have a buffer-local
binding in the current buffer, @code{defconst} sets the default value,
not the buffer-local value.  (But you should not be making
buffer-local bindings for a symbol that is defined with
@code{defconst}.)

481
An example of the use of @code{defconst} is Emacs's definition of
482 483 484 485
@code{float-pi}---the mathematical constant @math{pi}, which ought not
to be changed by anyone (attempts by the Indiana State Legislature
notwithstanding).  As the second form illustrates, however,
@code{defconst} is only advisory.
Glenn Morris's avatar
Glenn Morris committed
486 487 488

@example
@group
489 490
(defconst float-pi 3.141592653589793 "The value of Pi.")
     @result{} float-pi
Glenn Morris's avatar
Glenn Morris committed
491 492
@end group
@group
493 494
(setq float-pi 3)
     @result{} float-pi
Glenn Morris's avatar
Glenn Morris committed
495 496
@end group
@group
497
float-pi
Glenn Morris's avatar
Glenn Morris committed
498 499 500 501 502
     @result{} 3
@end group
@end example
@end defspec

503 504 505 506 507 508 509
  @strong{Warning:} If you use a @code{defconst} or @code{defvar}
special form while the variable has a local binding (made with
@code{let}, or a function argument), it sets the local binding rather
than the global binding.  This is not what you usually want.  To
prevent this, use these special forms at top level in a file, where
normally no local binding is in effect, and make sure to load the file
before making a local binding for the variable.
Glenn Morris's avatar
Glenn Morris committed
510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543

@node Tips for Defining
@section Tips for Defining Variables Robustly

  When you define a variable whose value is a function, or a list of
functions, use a name that ends in @samp{-function} or
@samp{-functions}, respectively.

  There are several other variable name conventions;
here is a complete list:

@table @samp
@item @dots{}-hook
The variable is a normal hook (@pxref{Hooks}).

@item @dots{}-function
The value is a function.

@item @dots{}-functions
The value is a list of functions.

@item @dots{}-form
The value is a form (an expression).

@item @dots{}-forms
The value is a list of forms (expressions).

@item @dots{}-predicate
The value is a predicate---a function of one argument that returns
non-@code{nil} for ``good'' arguments and @code{nil} for ``bad''
arguments.

@item @dots{}-flag
The value is significant only as to whether it is @code{nil} or not.
544
Since such variables often end up acquiring more values over time,
Stefan Monnier's avatar
Typo.  
Stefan Monnier committed
545
this convention is not strongly recommended.
Glenn Morris's avatar
Glenn Morris committed
546 547 548 549 550 551 552 553 554 555 556 557

@item @dots{}-program
The value is a program name.

@item @dots{}-command
The value is a whole shell command.

@item @dots{}-switches
The value specifies options for a command.
@end table

  When you define a variable, always consider whether you should mark
558
it as ``safe'' or ``risky''; see @ref{File Local Variables}.
Glenn Morris's avatar
Glenn Morris committed
559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578

  When defining and initializing a variable that holds a complicated
value (such as a keymap with bindings in it), it's best to put the
entire computation of the value into the @code{defvar}, like this:

@example
(defvar my-mode-map
  (let ((map (make-sparse-keymap)))
    (define-key map "\C-c\C-a" 'my-command)
    @dots{}
    map)
  @var{docstring})
@end example

@noindent
This method has several benefits.  First, if the user quits while
loading the file, the variable is either still uninitialized or
initialized properly, never in-between.  If it is still uninitialized,
reloading the file will initialize it properly.  Second, reloading the
file once the variable is initialized will not alter it; that is
579 580 581
important if the user has run hooks to alter part of the contents
(such as, to rebind keys).  Third, evaluating the @code{defvar} form
with @kbd{C-M-x} will reinitialize the map completely.
Glenn Morris's avatar
Glenn Morris committed
582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605

  Putting so much code in the @code{defvar} form has one disadvantage:
it puts the documentation string far away from the line which names the
variable.  Here's a safe way to avoid that:

@example
(defvar my-mode-map nil
  @var{docstring})
(unless my-mode-map
  (let ((map (make-sparse-keymap)))
    (define-key map "\C-c\C-a" 'my-command)
    @dots{}
    (setq my-mode-map map)))
@end example

@noindent
This has all the same advantages as putting the initialization inside
the @code{defvar}, except that you must type @kbd{C-M-x} twice, once on
each form, if you do want to reinitialize the variable.

@node Accessing Variables
@section Accessing Variable Values

  The usual way to reference a variable is to write the symbol which
606 607 608 609 610 611
names it.  @xref{Symbol Forms}.

  Occasionally, you may want to reference a variable which is only
determined at run time.  In that case, you cannot specify the variable
name in the text of the program.  You can use the @code{symbol-value}
function to extract the value.
Glenn Morris's avatar
Glenn Morris committed
612 613

@defun symbol-value symbol
614 615 616 617
This function returns the value stored in @var{symbol}'s value cell.
This is where the variable's current (dynamic) value is stored.  If
the variable has no local binding, this is simply its global value.
If the variable is void, a @code{void-variable} error is signaled.
618 619

If the variable is lexically bound, the value reported by
620 621 622
@code{symbol-value} is not necessarily the same as the variable's
lexical value, which is determined by the lexical environment rather
than the symbol's value cell.  @xref{Variable Scoping}.
Glenn Morris's avatar
Glenn Morris committed
623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658

@example
@group
(setq abracadabra 5)
     @result{} 5
@end group
@group
(setq foo 9)
     @result{} 9
@end group

@group
;; @r{Here the symbol @code{abracadabra}}
;;   @r{is the symbol whose value is examined.}
(let ((abracadabra 'foo))
  (symbol-value 'abracadabra))
     @result{} foo
@end group

@group
;; @r{Here, the value of @code{abracadabra},}
;;   @r{which is @code{foo},}
;;   @r{is the symbol whose value is examined.}
(let ((abracadabra 'foo))
  (symbol-value abracadabra))
     @result{} 9
@end group

@group
(symbol-value 'abracadabra)
     @result{} 5
@end group
@end example
@end defun

@node Setting Variables
659
@section Setting Variable Values
Glenn Morris's avatar
Glenn Morris committed
660 661 662 663 664 665 666 667

  The usual way to change the value of a variable is with the special
form @code{setq}.  When you need to compute the choice of variable at
run time, use the function @code{set}.

@defspec setq [symbol form]@dots{}
This special form is the most common method of changing a variable's
value.  Each @var{symbol} is given a new value, which is the result of
668 669
evaluating the corresponding @var{form}.  The current binding of the
symbol is changed.
Glenn Morris's avatar
Glenn Morris committed
670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707

@code{setq} does not evaluate @var{symbol}; it sets the symbol that you
write.  We say that this argument is @dfn{automatically quoted}.  The
@samp{q} in @code{setq} stands for ``quoted.''

The value of the @code{setq} form is the value of the last @var{form}.

@example
@group
(setq x (1+ 2))
     @result{} 3
@end group
x                   ; @r{@code{x} now has a global value.}
     @result{} 3
@group
(let ((x 5))
  (setq x 6)        ; @r{The local binding of @code{x} is set.}
  x)
     @result{} 6
@end group
x                   ; @r{The global value is unchanged.}
     @result{} 3
@end example

Note that the first @var{form} is evaluated, then the first
@var{symbol} is set, then the second @var{form} is evaluated, then the
second @var{symbol} is set, and so on:

@example
@group
(setq x 10          ; @r{Notice that @code{x} is set before}
      y (1+ x))     ;   @r{the value of @code{y} is computed.}
     @result{} 11
@end group
@end example
@end defspec

@defun set symbol value
708 709 710 711 712 713 714 715 716 717 718
This function puts @var{value} in the value cell of @var{symbol}.
Since it is a function rather than a special form, the expression
written for @var{symbol} is evaluated to obtain the symbol to set.
The return value is @var{value}.

When dynamic variable binding is in effect (the default), @code{set}
has the same effect as @code{setq}, apart from the fact that
@code{set} evaluates its @var{symbol} argument whereas @code{setq}
does not.  But when a variable is lexically bound, @code{set} affects
its @emph{dynamic} value, whereas @code{setq} affects its current
(lexical) value.  @xref{Variable Scoping}.
Glenn Morris's avatar
Glenn Morris committed
719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761 762

@example
@group
(set one 1)
@error{} Symbol's value as variable is void: one
@end group
@group
(set 'one 1)
     @result{} 1
@end group
@group
(set 'two 'one)
     @result{} one
@end group
@group
(set two 2)         ; @r{@code{two} evaluates to symbol @code{one}.}
     @result{} 2
@end group
@group
one                 ; @r{So it is @code{one} that was set.}
     @result{} 2
(let ((one 1))      ; @r{This binding of @code{one} is set,}
  (set 'one 3)      ;   @r{not the global value.}
  one)
     @result{} 3
@end group
@group
one
     @result{} 2
@end group
@end example

If @var{symbol} is not actually a symbol, a @code{wrong-type-argument}
error is signaled.

@example
(set '(x y) 'z)
@error{} Wrong type argument: symbolp, (x y)
@end example
@end defun

@node Variable Scoping
@section Scoping Rules for Variable Bindings

763 764 765
  When you create a local binding for a variable, that binding takes
effect only within a limited portion of the program (@pxref{Local
Variables}).  This section describes exactly what this means.
Glenn Morris's avatar
Glenn Morris committed
766 767 768

@cindex scope
@cindex extent
769 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786 787 788 789 790 791 792 793 794 795 796 797
  Each local binding has a certain @dfn{scope} and @dfn{extent}.
@dfn{Scope} refers to @emph{where} in the textual source code the
binding can be accessed.  @dfn{Extent} refers to @emph{when}, as the
program is executing, the binding exists.

@cindex dynamic binding
@cindex indefinite scope
@cindex dynamic extent
  By default, the local bindings that Emacs creates are @dfn{dynamic
bindings}.  Such a binding has @dfn{indefinite scope}, meaning that
any part of the program can potentially access the variable binding.
It also has @dfn{dynamic extent}, meaning that the binding lasts only
while the binding construct (such as the body of a @code{let} form) is
being executed.

@cindex lexical binding
@cindex lexical scope
@cindex indefinite extent
  Emacs can optionally create @dfn{lexical bindings}.  A lexical
binding has @dfn{lexical scope}, meaning that any reference to the
variable must be located textually within the binding construct.  It
also has @dfn{indefinite extent}, meaning that under some
circumstances the binding can live on even after the binding construct
has finished executing, by means of special objects called
@dfn{closures}.

  The following subsections describe dynamic binding and lexical
binding in greater detail, and how to enable lexical binding in Emacs
Lisp programs.
Glenn Morris's avatar
Glenn Morris committed
798 799

@menu
800 801 802 803
* Dynamic Binding::         The default for binding local variables in Emacs.
* Dynamic Binding Tips::    Avoiding problems with dynamic binding.
* Lexical Binding::         A different type of local variable binding.
* Using Lexical Binding::   How to enable lexical binding.
Glenn Morris's avatar
Glenn Morris committed
804 805
@end menu

806 807 808 809 810 811 812 813
@node Dynamic Binding
@subsection Dynamic Binding

  By default, the local variable bindings made by Emacs are dynamic
bindings.  When a variable is dynamically bound, its current binding
at any point in the execution of the Lisp program is simply the most
recently-created dynamic local binding for that symbol, or the global
binding if there is no such local binding.
Glenn Morris's avatar
Glenn Morris committed
814

815 816
  Dynamic bindings have indefinite scope and dynamic extent, as shown
by the following example:
Glenn Morris's avatar
Glenn Morris committed
817 818 819

@example
@group
820
(defvar x -99)  ; @r{@code{x} receives an initial value of -99.}
Glenn Morris's avatar
Glenn Morris committed
821

822 823 824 825 826 827 828 829 830 831 832 833
(defun getx ()
  x)            ; @r{@code{x} is used ``free'' in this function.}

(let ((x 1))    ; @r{@code{x} is dynamically bound.}
  (getx))
     @result{} 1

;; @r{After the @code{let} form finishes, @code{x} reverts to its}
;; @r{previous value, which is -99.}

(getx)
     @result{} -99
Glenn Morris's avatar
Glenn Morris committed
834 835 836
@end group
@end example

837 838 839 840 841 842 843 844
@noindent
The function @code{getx} refers to @code{x}.  This is a ``free''
reference, in the sense that there is no binding for @code{x} within
that @code{defun} construct itself.  When we call @code{getx} from
within a @code{let} form in which @code{x} is (dynamically) bound, it
retrieves the local value of @code{x} (i.e.@: 1).  But when we call
@code{getx} outside the @code{let} form, it retrieves the global value
of @code{x} (i.e.@: -99).
Glenn Morris's avatar
Glenn Morris committed
845

846 847
  Here is another example, which illustrates setting a dynamically
bound variable using @code{setq}:
Glenn Morris's avatar
Glenn Morris committed
848 849 850

@example
@group
851 852 853 854 855 856 857 858 859 860 861 862 863 864 865
(defvar x -99)      ; @r{@code{x} receives an initial value of -99.}

(defun addx ()
  (setq x (1+ x)))  ; @r{Add 1 to @code{x} and return its new value.}

(let ((x 1))
  (addx)
  (addx))
     @result{} 3           ; @r{The two @code{addx} calls add to @code{x} twice.}

;; @r{After the @code{let} form finishes, @code{x} reverts to its}
;; @r{previous value, which is -99.}

(addx)
     @result{} -98
Glenn Morris's avatar
Glenn Morris committed
866 867 868
@end group
@end example

869 870 871 872 873 874 875
  Dynamic binding is implemented in Emacs Lisp in a simple way.  Each
symbol has a value cell, which specifies its current dynamic value (or
absence of value).  @xref{Symbol Components}.  When a symbol is given
a dynamic local binding, Emacs records the contents of the value cell
(or absence thereof) in a stack, and stores the new local value in the
value cell.  When the binding construct finishes executing, Emacs pops
the old value off the stack, and puts it in the value cell.
Glenn Morris's avatar
Glenn Morris committed
876

877 878
@node Dynamic Binding Tips
@subsection Proper Use of Dynamic Binding
Glenn Morris's avatar
Glenn Morris committed
879

880 881 882 883 884
  Dynamic binding is a powerful feature, as it allows programs to
refer to variables that are not defined within their local textual
scope.  However, if used without restraint, this can also make
programs hard to understand.  There are two clean ways to use this
technique:
Glenn Morris's avatar
Glenn Morris committed
885

886 887 888 889 890 891 892 893 894 895 896 897 898 899 900 901 902 903 904 905 906 907 908 909 910 911
@itemize @bullet
@item
If a variable has no global definition, use it as a local variable
only within a binding construct, e.g.@: the body of the @code{let}
form where the variable was bound, or the body of the function for an
argument variable.  If this convention is followed consistently
throughout a program, the value of the variable will not affect, nor
be affected by, any uses of the same variable symbol elsewhere in the
program.

@item
Otherwise, define the variable with @code{defvar}, @code{defconst}, or
@code{defcustom}.  @xref{Defining Variables}.  Usually, the definition
should be at top-level in an Emacs Lisp file.  As far as possible, it
should include a documentation string which explains the meaning and
purpose of the variable.  You should also choose the variable's name
to avoid name conflicts (@pxref{Coding Conventions}).

Then you can bind the variable anywhere in a program, knowing reliably
what the effect will be.  Wherever you encounter the variable, it will
be easy to refer back to the definition, e.g.@: via the @kbd{C-h v}
command (provided the variable definition has been loaded into Emacs).
@xref{Name Help,,, emacs, The GNU Emacs Manual}.

For example, it is common to use local bindings for customizable
variables like @code{case-fold-search}:
Glenn Morris's avatar
Glenn Morris committed
912 913

@example
914 915 916 917 918 919
@group
(defun search-for-abc ()
  "Search for the string \"abc\", ignoring case differences."
  (let ((case-fold-search nil))
    (re-search-forward "abc")))
@end group
Glenn Morris's avatar
Glenn Morris committed
920
@end example
921
@end itemize
Glenn Morris's avatar
Glenn Morris committed
922

923 924
@node Lexical Binding
@subsection Lexical Binding
Glenn Morris's avatar
Glenn Morris committed
925

926 927 928
Optionally, you can create lexical bindings in Emacs Lisp.  A
lexically bound variable has @dfn{lexical scope}, meaning that any
reference to the variable must be located textually within the binding
Glenn Morris's avatar
Glenn Morris committed
929 930
construct.

931 932 933 934 935 936 937
  Here is an example
@iftex
(see the next subsection, for how to actually enable lexical binding):
@end iftex
@ifnottex
(@pxref{Using Lexical Binding}, for how to actually enable lexical binding):
@end ifnottex
Glenn Morris's avatar
Glenn Morris committed
938

939 940 941 942 943
@example
@group
(let ((x 1))    ; @r{@code{x} is lexically bound.}
  (+ x 3))
     @result{} 4
Glenn Morris's avatar
Glenn Morris committed
944

945 946 947
(defun getx ()
  x)            ; @r{@code{x} is used ``free'' in this function.}

948 949 950 951 952
(let ((x 1))    ; @r{@code{x} is lexically bound.}
  (getx))
@error{} Symbol's value as variable is void: x
@end group
@end example
Glenn Morris's avatar
Glenn Morris committed
953

954 955 956 957 958 959 960 961 962 963 964 965 966 967
@noindent
Here, the variable @code{x} has no global value.  When it is lexically
bound within a @code{let} form, it can be used in the textual confines
of that @code{let} form.  But it can @emph{not} be used from within a
@code{getx} function called from the @code{let} form, since the
function definition of @code{getx} occurs outside the @code{let} form
itself.

@cindex lexical environment
  Here is how lexical binding works.  Each binding construct defines a
@dfn{lexical environment}, specifying the symbols that are bound
within the construct and their local values.  When the Lisp evaluator
wants the current value of a variable, it looks first in the lexical
environment; if the variable is not specified in there, it looks in
968
the symbol's value cell, where the dynamic value is stored.
969 970 971 972 973

@cindex closures
  Lexical bindings have indefinite extent.  Even after a binding
construct has finished executing, its lexical environment can be
``kept around'' in Lisp objects called @dfn{closures}.  A closure is
974 975
created when you create a named or anonymous function with lexical
binding enabled.  @xref{Closures}, for details.
976

977 978 979
  When a closure is called as a function, any lexical variable
references within its definition use the retained lexical environment.
Here is an example:
Glenn Morris's avatar
Glenn Morris committed
980

981 982 983
@example
(defvar my-ticker nil)   ; @r{We will use this dynamically bound}
                         ; @r{variable to store a closure.}
Glenn Morris's avatar
Glenn Morris committed
984

985 986 987 988 989
(let ((x 0))             ; @r{@code{x} is lexically bound.}
  (setq my-ticker (lambda ()
                    (setq x (1+ x)))))
    @result{} (closure ((x . 0) t) ()
          (1+ x))
Glenn Morris's avatar
Glenn Morris committed
990

991 992
(funcall my-ticker)
    @result{} 1
Glenn Morris's avatar
Glenn Morris committed
993

994 995
(funcall my-ticker)
    @result{} 2
Glenn Morris's avatar
Glenn Morris committed
996

997 998
(funcall my-ticker)
    @result{} 3
999

1000 1001 1002
x                        ; @r{Note that @code{x} has no global value.}
@error{} Symbol's value as variable is void: x
@end example
1003

1004 1005 1006 1007 1008 1009 1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021 1022 1023 1024 1025 1026 1027 1028 1029 1030 1031 1032
@noindent
The @code{let} binding defines a lexical environment in which the
variable @code{x} is locally bound to 0.  Within this binding
construct, we define a lambda expression which increments @code{x} by
one and returns the incremented value.  This lambda expression is
automatically turned into a closure, in which the lexical environment
lives on even after the @code{let} binding construct has exited.  Each
time we evaluate the closure, it increments @code{x}, using the
binding of @code{x} in that lexical environment.

  Note that functions like @code{symbol-value}, @code{boundp}, and
@code{set} only retrieve or modify a variable's dynamic binding
(i.e.@: the contents of its symbol's value cell).  Also, the code in
the body of a @code{defun} or @code{defmacro} cannot refer to
surrounding lexical variables.

  Currently, lexical binding is not much used within the Emacs
sources.  However, we expect its importance to increase in the future.
Lexical binding opens up a lot more opportunities for optimization, so
Emacs Lisp code that makes use of lexical binding is likely to run
faster in future Emacs versions.  Such code is also much more friendly
to concurrency, which we want to add to Emacs in the near future.

@node Using Lexical Binding
@subsection Using Lexical Binding

  When loading an Emacs Lisp file or evaluating a Lisp buffer, lexical
binding is enabled if the buffer-local variable @code{lexical-binding}
is non-@code{nil}:
1033 1034

@defvar lexical-binding
1035 1036 1037 1038 1039 1040
If this buffer-local variable is non-@code{nil}, Emacs Lisp files and
buffers are evaluated using lexical binding instead of dynamic
binding.  (However, special variables are still dynamically bound; see
below.)  If @code{nil}, dynamic binding is used for all local
variables.  This variable is typically set for a whole Emacs Lisp
file, as a file local variable (@pxref{File Local Variables}).
1041 1042
Note that unlike other such variables, this one must be set in the
first line of a file.
1043 1044
@end defvar

1045 1046 1047 1048 1049 1050 1051 1052 1053 1054 1055 1056 1057
@noindent
When evaluating Emacs Lisp code directly using an @code{eval} call,
lexical binding is enabled if the @var{lexical} argument to
@code{eval} is non-@code{nil}.  @xref{Eval}.

@cindex special variables
  Even when lexical binding is enabled, certain variables will
continue to be dynamically bound.  These are called @dfn{special
variables}.  Every variable that has been defined with @code{defvar},
@code{defcustom} or @code{defconst} is a special variable
(@pxref{Defining Variables}).  All other variables are subject to
lexical binding.

1058
@defun special-variable-p SYMBOL
1059 1060 1061 1062
This function returns non-@code{nil} if @var{symbol} is a special
variable (i.e.@: it has a @code{defvar}, @code{defcustom}, or
@code{defconst} variable definition).  Otherwise, the return value is
@code{nil}.
1063 1064
@end defun

1065 1066 1067 1068 1069 1070 1071 1072 1073 1074 1075 1076 1077 1078 1079 1080 1081 1082 1083 1084 1085 1086 1087 1088
  The use of a special variable as a formal argument in a function is
discouraged.  Doing so gives rise to unspecified behavior when lexical
binding mode is enabled (it may use lexical binding sometimes, and
dynamic binding other times).

  Converting an Emacs Lisp program to lexical binding is pretty easy.
First, add a file-local variable setting of @code{lexical-binding} to
@code{t} in the Emacs Lisp source file.  Second, check that every
variable in the program which needs to be dynamically bound has a
variable definition, so that it is not inadvertently bound lexically.

  A simple way to find out which variables need a variable definition
is to byte-compile the source file.  @xref{Byte Compilation}.  If a
non-special variable is used outside of a @code{let} form, the
byte-compiler will warn about reference or assignment to a ``free
variable''.  If a non-special variable is bound but not used within a
@code{let} form, the byte-compiler will warn about an ``unused lexical
variable''.  The byte-compiler will also issue a warning if you use a
special variable as a function argument.

  (To silence byte-compiler warnings about unused variables, just use
a variable name that start with an underscore.  The byte-compiler
interprets this as an indication that this is a variable known not to
be used.)
1089

Glenn Morris's avatar
Glenn Morris committed
1090 1091 1092 1093 1094 1095
@node Buffer-Local Variables
@section Buffer-Local Variables
@cindex variable, buffer-local
@cindex buffer-local variables

  Global and local variable bindings are found in most programming
1096 1097 1098 1099
languages in one form or another.  Emacs, however, also supports
additional, unusual kinds of variable binding, such as
@dfn{buffer-local} bindings, which apply only in one buffer.  Having
different values for a variable in different buffers is an important
1100
customization method.  (Variables can also have bindings that are
1101
local to each terminal.  @xref{Multiple Terminals}.)
Glenn Morris's avatar
Glenn Morris committed
1102 1103

@menu
1104 1105 1106
* Intro to Buffer-Local::       Introduction and concepts.
* Creating Buffer-Local::       Creating and destroying buffer-local bindings.
* Default Value::               The default value is seen in buffers
Glenn Morris's avatar
Glenn Morris committed
1107 1108 1109 1110 1111 1112 1113 1114 1115 1116 1117 1118 1119 1120 1121 1122 1123 1124 1125 1126 1127
                                 that don't have their own buffer-local values.
@end menu

@node Intro to Buffer-Local
@subsection Introduction to Buffer-Local Variables

  A buffer-local variable has a buffer-local binding associated with a
particular buffer.  The binding is in effect when that buffer is
current; otherwise, it is not in effect.  If you set the variable while
a buffer-local binding is in effect, the new value goes in that binding,
so its other bindings are unchanged.  This means that the change is
visible only in the buffer where you made it.

  The variable's ordinary binding, which is not associated with any
specific buffer, is called the @dfn{default binding}.  In most cases,
this is the global binding.

  A variable can have buffer-local bindings in some buffers but not in
other buffers.  The default binding is shared by all the buffers that
don't have their own bindings for the variable.  (This includes all
newly-created buffers.)  If you set the variable in a buffer that does
1128
not have a buffer-local binding for it, this sets the default binding,
Glenn Morris's avatar
Glenn Morris committed
1129 1130 1131 1132 1133 1134 1135 1136 1137 1138 1139 1140 1141 1142 1143 1144 1145 1146 1147 1148 1149 1150 1151 1152 1153 1154 1155 1156 1157 1158
so the new value is visible in all the buffers that see the default
binding.

  The most common use of buffer-local bindings is for major modes to change
variables that control the behavior of commands.  For example, C mode and
Lisp mode both set the variable @code{paragraph-start} to specify that only
blank lines separate paragraphs.  They do this by making the variable
buffer-local in the buffer that is being put into C mode or Lisp mode, and
then setting it to the new value for that mode.  @xref{Major Modes}.

  The usual way to make a buffer-local binding is with
@code{make-local-variable}, which is what major mode commands typically
use.  This affects just the current buffer; all other buffers (including
those yet to be created) will continue to share the default value unless
they are explicitly given their own buffer-local bindings.

@cindex automatically buffer-local
  A more powerful operation is to mark the variable as
@dfn{automatically buffer-local} by calling
@code{make-variable-buffer-local}.  You can think of this as making the
variable local in all buffers, even those yet to be created.  More
precisely, the effect is that setting the variable automatically makes
the variable local to the current buffer if it is not already so.  All
buffers start out by sharing the default value of the variable as usual,
but setting the variable creates a buffer-local binding for the current
buffer.  The new value is stored in the buffer-local binding, leaving
the default binding untouched.  This means that the default value cannot
be changed with @code{setq} in any buffer; the only way to change it is
with @code{setq-default}.

1159
  @strong{Warning:} When a variable has buffer-local
Glenn Morris's avatar
Glenn Morris committed
1160 1161 1162
bindings in one or more buffers, @code{let} rebinds the binding that's
currently in effect.  For instance, if the current buffer has a
buffer-local value, @code{let} temporarily rebinds that.  If no
1163
buffer-local bindings are in effect, @code{let} rebinds
Glenn Morris's avatar
Glenn Morris committed
1164 1165 1166 1167 1168 1169 1170 1171 1172 1173 1174 1175 1176 1177 1178 1179 1180 1181 1182 1183 1184 1185 1186 1187 1188 1189 1190 1191 1192
the default value.  If inside the @code{let} you then change to a
different current buffer in which a different binding is in effect,
you won't see the @code{let} binding any more.  And if you exit the
@code{let} while still in the other buffer, you won't see the
unbinding occur (though it will occur properly).  Here is an example
to illustrate:

@example
@group
(setq foo 'g)
(set-buffer "a")
(make-local-variable 'foo)
@end group
(setq foo 'a)
(let ((foo 'temp))
  ;; foo @result{} 'temp  ; @r{let binding in buffer @samp{a}}
  (set-buffer "b")
  ;; foo @result{} 'g     ; @r{the global value since foo is not local in @samp{b}}
  @var{body}@dots{})
@group
foo @result{} 'g        ; @r{exiting restored the local value in buffer @samp{a},}
                 ; @r{but we don't see that in buffer @samp{b}}
@end group
@group
(set-buffer "a") ; @r{verify the local value was restored}
foo @result{} 'a
@end group
@end example

1193 1194
@noindent
Note that references to @code{foo} in @var{body} access the
Glenn Morris's avatar
Glenn Morris committed
1195 1196 1197 1198 1199 1200
buffer-local binding of buffer @samp{b}.

  When a file specifies local variable values, these become buffer-local
values when you visit the file.  @xref{File Variables,,, emacs, The
GNU Emacs Manual}.

1201 1202
  A buffer-local variable cannot be made terminal-local
(@pxref{Multiple Terminals}).
1203

Glenn Morris's avatar
Glenn Morris committed
1204 1205 1206 1207 1208 1209 1210 1211 1212 1213 1214 1215 1216 1217 1218 1219 1220 1221 1222 1223 1224 1225 1226 1227 1228 1229 1230 1231 1232 1233 1234 1235 1236 1237 1238 1239 1240
@node Creating Buffer-Local
@subsection Creating and Deleting Buffer-Local Bindings

@deffn Command make-local-variable variable
This function creates a buffer-local binding in the current buffer for
@var{variable} (a symbol).  Other buffers are not affected.  The value
returned is @var{variable}.

The buffer-local value of @var{variable} starts out as the same value
@var{variable} previously had.  If @var{variable} was void, it remains
void.

@example
@group
;; @r{In buffer @samp{b1}:}
(setq foo 5)                ; @r{Affects all buffers.}
     @result{} 5
@end group
@group
(make-local-variable 'foo)  ; @r{Now it is local in @samp{b1}.}
     @result{} foo
@end group
@group
foo                         ; @r{That did not change}
     @result{} 5                   ;   @r{the value.}
@end group
@group
(setq foo 6)                ; @r{Change the value}
     @result{} 6                   ;   @r{in @samp{b1}.}
@end group
@group
foo
     @result{} 6
@end group

@group
;; @r{In buffer @samp{b2}, the value hasn't changed.}
1241
(with-current-buffer "b2"
Glenn Morris's avatar
Glenn Morris committed
1242 1243 1244 1245 1246 1247 1248 1249 1250 1251 1252
  foo)
     @result{} 5
@end group
@end example

Making a variable buffer-local within a @code{let}-binding for that
variable does not work reliably, unless the buffer in which you do this
is not current either on entry to or exit from the @code{let}.  This is
because @code{let} does not distinguish between different kinds of
bindings; it knows only which variable the binding was made for.

1253 1254 1255
If the variable is terminal-local (@pxref{Multiple Terminals}), this
function signals an error.  Such variables cannot have buffer-local
bindings as well.
Glenn Morris's avatar
Glenn Morris committed
1256 1257 1258 1259 1260 1261 1262 1263 1264 1265

@strong{Warning:} do not use @code{make-local-variable} for a hook
variable.  The hook variables are automatically made buffer-local as
needed if you use the @var{local} argument to @code{add-hook} or
@code{remove-hook}.
@end deffn

@deffn Command make-variable-buffer-local variable
This function marks @var{variable} (a symbol) automatically
buffer-local, so that any subsequent attempt to set it will make it
1266 1267 1268 1269
local to the current buffer at the time.  Unlike
@code{make-local-variable}, with which it is often confused, this
cannot be undone, and affects the behavior of the variable in all
buffers.
Glenn Morris's avatar
Glenn Morris committed
1270 1271 1272 1273 1274 1275 1276 1277 1278 1279 1280 1281 1282 1283 1284 1285 1286 1287 1288 1289 1290 1291 1292 1293 1294 1295 1296 1297 1298 1299 1300 1301 1302 1303 1304 1305 1306 1307 1308 1309 1310 1311 1312 1313 1314 1315 1316 1317 1318

A peculiar wrinkle of this feature is that binding the variable (with
@code{let} or other binding constructs) does not create a buffer-local
binding for it.  Only setting the variable (with @code{set} or
@code{setq}), while the variable does not have a @code{let}-style
binding that was made in the current buffer, does so.

If @var{variable} does not have a default value, then calling this
command will give it a default value of @code{nil}.  If @var{variable}
already has a default value, that value remains unchanged.
Subsequently calling @code{makunbound} on @var{variable} will result
in a void buffer-local value and leave the default value unaffected.

The value returned is @var{variable}.

@strong{Warning:} Don't assume that you should use
@code{make-variable-buffer-local} for user-option variables, simply
because users @emph{might} want to customize them differently in
different buffers.  Users can make any variable local, when they wish
to.  It is better to leave the choice to them.

The time to use @code{make-variable-buffer-local} is when it is crucial
that no two buffers ever share the same binding.  For example, when a
variable is used for internal purposes in a Lisp program which depends
on having separate values in separate buffers, then using
@code{make-variable-buffer-local} can be the best solution.
@end deffn

@defun local-variable-p variable &optional buffer
This returns @code{t} if @var{variable} is buffer-local in buffer
@var{buffer} (which defaults to the current buffer); otherwise,
@code{nil}.
@end defun

@defun local-variable-if-set-p variable &optional buffer
This returns @code{t} if @var{variable} will become buffer-local in
buffer @var{buffer} (which defaults to the current buffer) if it is
set there.
@end defun

@defun buffer-local-value variable buffer
This function returns the buffer-local binding of @var{variable} (a
symbol) in buffer @var{buffer}.  If @var{variable} does not have a
buffer-local binding in buffer @var{buffer}, it returns the default
value (@pxref{Default Value}) of @var{variable} instead.
@end defun

@defun buffer-local-variables &optional buffer
This function returns a list describing the buffer-local variables in
1319 1320 1321 1322 1323 1324
buffer @var{buffer}.  (If @var{buffer} is omitted, the current buffer
is used.)  Normally, each list element has the form
@w{@code{(@var{sym} . @var{val})}}, where @var{sym} is a buffer-local
variable (a symbol) and @var{val} is its buffer-local value.  But when
a variable's buffer-local binding in @var{buffer} is void, its list
element is just @var{sym}.
Glenn Morris's avatar
Glenn Morris committed
1325 1326 1327 1328 1329 1330 1331 1332 1333 1334 1335 1336 1337 1338 1339 1340 1341 1342 1343 1344 1345 1346 1347 1348 1349 1350 1351 1352 1353 1354 1355 1356 1357 1358 1359 1360 1361 1362 1363 1364 1365 1366 1367 1368 1369 1370 1371 1372 1373

@example
@group
(make-local-variable 'foobar)
(makunbound 'foobar)
(make-local-variable 'bind-me)
(setq bind-me 69)
@end group
(setq lcl (buffer-local-variables))
    ;; @r{First, built-in variables local in all buffers:}
@result{} ((mark-active . nil)
    (buffer-undo-list . nil)
    (mode-name . "Fundamental")
    @dots{}
@group
    ;; @r{Next, non-built-in buffer-local variables.}
    ;; @r{This one is buffer-local and void:}
    foobar
    ;; @r{This one is buffer-local and nonvoid:}
    (bind-me . 69))
@end group
@end example

Note that storing new values into the @sc{cdr}s of cons cells in this
list does @emph{not} change the buffer-local values of the variables.
@end defun

@deffn Command kill-local-variable variable
This function deletes the buffer-local binding (if any) for
@var{variable} (a symbol) in the current buffer.  As a result, the
default binding of @var{variable} becomes visible in this buffer.  This
typically results in a change in the value of @var{variable}, since the
default value is usually different from the buffer-local value just
eliminated.

If you kill the buffer-local binding of a variable that automatically
becomes buffer-local when set, this makes the default value visible in
the current buffer.  However, if you set the variable again, that will
once again create a buffer-local binding for it.

@code{kill-local-variable} returns @var{variable}.

This function is a command because it is sometimes useful to kill one
buffer-local variable interactively, just as it is useful to create
buffer-local variables interactively.
@end deffn

@defun kill-all-local-variables
This function eliminates all the buffer-local variable bindings of the
1374 1375 1376 1377
current buffer except for variables marked as ``permanent'' and local
hook functions that have a non-@code{nil} @code{permanent-local-hook}
property (@pxref{Setting Hooks}).  As a result, the buffer will see
the default values of most variables.
Glenn Morris's avatar
Glenn Morris committed
1378 1379 1380 1381 1382 1383 1384 1385 1386 1387 1388 1389 1390 1391 1392 1393 1394 1395 1396 1397 1398 1399 1400 1401 1402 1403 1404 1405 1406 1407 1408 1409 1410 1411

This function also resets certain other information pertaining to the
buffer: it sets the local keymap to @code{nil}, the syntax table to the
value of @code{(standard-syntax-table)}, the case table to
@code{(standard-case-table)}, and the abbrev table to the value of
@code{fundamental-mode-abbrev-table}.

The very first thing this function does is run the normal hook
@code{change-major-mode-hook} (see below).

Every major mode command begins by calling this function, which has the
effect of switching to Fundamental mode and erasing most of the effects
of the previous major mode.  To ensure that this does its job, the
variables that major modes set should not be marked permanent.

@code{kill-all-local-variables} returns @code{nil}.
@end defun

@defvar change-major-mode-hook
The function @code{kill-all-local-variables} runs this normal hook
before it does anything else.  This gives major modes a way to arrange
for something special to be done if the user switches to a different
major mode.  It is also useful for buffer-specific minor modes
that should be forgotten if the user changes the major mode.

For best results, make this variable buffer-local, so that it will
disappear after doing its job and will not interfere with the
subsequent major mode.  @xref{Hooks}.
@end defvar

@c Emacs 19 feature
@cindex permanent local variable
A buffer-local variable is @dfn{permanent} if the variable name (a
symbol) has a @code{permanent-local} property that is non-@code{nil}.
1412 1413
Such variables are unaffected by @code{kill-all-local-variables}, and
their local bindings are therefore not cleared by changing major modes.
Glenn Morris's avatar
Glenn Morris committed
1414 1415 1416 1417 1418 1419 1420 1421 1422 1423 1424 1425 1426 1427 1428 1429 1430 1431 1432 1433 1434 1435 1436
Permanent locals are appropriate for data pertaining to where the file
came from or how to save it, rather than with how to edit the contents.

@node Default Value
@subsection The Default Value of a Buffer-Local Variable
@cindex default value

  The global value of a variable with buffer-local bindings is also
called the @dfn{default} value, because it is the value that is in
effect whenever neither the current buffer nor the selected frame has
its own binding for the variable.

  The functions @code{default-value} and @code{setq-default} access and
change a variable's default value regardless of whether the current
buffer has a buffer-local binding.  For example, you could use
@code{setq-default} to change the default setting of
@code{paragraph-start} for most buffers; and this would work even when
you are in a C or Lisp mode buffer that has a buffer-local value for
this variable.

@c Emacs 19 feature
  The special forms @code{defvar} and @code{defconst} also set the
default value (if they set the variable at all), rather than any
1437
buffer-local value.
Glenn Morris's avatar
Glenn Morris committed
1438 1439 1440 1441 1442 1443 1444 1445 1446 1447 1448 1449 1450 1451 1452 1453 1454 1455 1456 1457 1458 1459 1460 1461 1462 1463 1464 1465 1466 1467 1468 1469 1470 1471 1472 1473 1474 1475 1476 1477 1478 1479 1480 1481 1482 1483 1484 1485 1486 1487 1488 1489 1490 1491 1492 1493 1494 1495 1496 1497 1498 1499 1500 1501 1502 1503 1504 1505 1506 1507 1508 1509 1510 1511 1512 1513 1514 1515 1516 1517 1518 1519 1520 1521 1522 1523 1524 1525 1526 1527 1528 1529 1530 1531 1532 1533 1534 1535 1536 1537 1538 1539 1540 1541 1542

@defun default-value symbol
This function returns @var{symbol}'s default value.  This is the value
that is seen in buffers and frames that do not have their own values for
this variable.  If @var{symbol} is not buffer-local, this is equivalent
to @code{symbol-value} (@pxref{Accessing Variables}).
@end defun

@c Emacs 19 feature
@defun default-boundp symbol
The function @code{default-boundp} tells you whether @var{symbol}'s
default value is nonvoid.  If @code{(default-boundp 'foo)} returns
@code{nil}, then @code{(default-value 'foo)} would get an error.

@code{default-boundp} is to @code{default-value} as @code{boundp} is to
@code{symbol-value}.
@end defun

@defspec setq-default [symbol form]@dots{}
This special form gives each @var{symbol} a new default value, which is
the result of evaluating the corresponding @var{form}.  It does not
evaluate @var{symbol}, but does evaluate @var{form}.  The value of the
@code{setq-default} form is the value of the last @var{form}.

If a @var{symbol} is not buffer-local for the current buffer, and is not
marked automatically buffer-local, @code{setq-default} has the same
effect as @code{setq}.  If @var{symbol} is buffer-local for the current
buffer, then this changes the value that other buffers will see (as long
as they don't have a buffer-local value), but not the value that the
current buffer sees.

@example
@group
;; @r{In buffer @samp{foo}:}
(make-local-variable 'buffer-local)
     @result{} buffer-local
@end group
@group
(setq buffer-local 'value-in-foo)
     @result{} value-in-foo
@end group
@group
(setq-default buffer-local 'new-default)
     @result{} new-default
@end group
@group
buffer-local
     @result{} value-in-foo
@end group
@group
(default-value 'buffer-local)
     @result{} new-default
@end group

@group
;; @r{In (the new) buffer @samp{bar}:}
buffer-local
     @result{} new-default
@end group
@group
(default-value 'buffer-local)
     @result{} new-default
@end group
@group
(setq buffer-local 'another-default)
     @result{} another-default
@end group
@group
(default-value 'buffer-local)
     @result{} another-default
@end group

@group
;; @r{Back in buffer @samp{foo}:}
buffer-local
     @result{} value-in-foo
(default-value 'buffer-local)
     @result{} another-default
@end group
@end example
@end defspec

@defun set-default symbol value
This function is like @code{setq-default}, except that @var{symbol} is
an ordinary evaluated argument.

@example
@group
(set-default (car '(a b c)) 23)
     @result{} 23
@end group
@group
(default-value 'a)
     @result{} 23
@end group
@end example
@end defun

@node File Local Variables
@section File Local Variables
@cindex file local variables

  A file can specify local variable values; Emacs uses these to create
buffer-local bindings for those variables in the buffer visiting that
file.  @xref{File variables, , Local Variables in Files, emacs, The
1543 1544 1545 1546 1547 1548 1549 1550 1551
GNU Emacs Manual}, for basic information about file-local variables.
This section describes the functions and variables that affect how
file-local variables are processed.

  If a file-local variable could specify an arbitrary function or Lisp
expression that would be called later, visiting a file could take over
your Emacs.  Emacs protects against this by automatically setting only
those file-local variables whose specified values are known to be
safe.  Other file-local variables are set only if the user agrees.
Glenn Morris's avatar
Glenn Morris committed
1552

1553 1554 1555 1556 1557
  For additional safety, @code{read-circle} is temporarily bound to
@code{nil} when Emacs reads file-local variables (@pxref{Input
Functions}).  This prevents the Lisp reader from recognizing circular
and shared Lisp structures (@pxref{Circular Objects}).

Glenn Morris's avatar
Glenn Morris committed
1558
@defopt enable-local-variables
1559
This variable controls whether to process file-local variables.
Glenn Morris's avatar
Glenn Morris committed
1560 1561 1562 1563 1564 1565 1566 1567 1568 1569 1570 1571 1572 1573 1574 1575
The possible values are:

@table @asis
@item @code{t} (the default)
Set the safe variables, and query (once) about any unsafe variables.
@item @code{:safe}
Set only the safe variables and do not query.
@item @code{:all}
Set all the variables and do not query.
@item @code{nil}
Don't set any variables.
@item anything else
Query (once) about all the variables.
@end table
@end defopt

1576 1577 1578 1579 1580 1581 1582
@defvar inhibit-local-variables-regexps
This is a list of regular expressions.  If a file has a name
matching an element of this list, then it is not scanned for
any form of file-local variable.  For examples of why you might want
to use this, @pxref{Auto Major Mode}.
@end defvar

Glenn Morris's avatar
Glenn Morris committed
1583 1584 1585 1586 1587 1588 1589 1590
@defun hack-local-variables &optional mode-only
This function parses, and binds or evaluates as appropriate, any local
variables specified by the contents of the current buffer.  The variable
@code{enable-local-variables} has its effect here.  However, this
function does not look for the @samp{mode:} local variable in the
@w{@samp{-*-}} line.  @code{set-auto-mode} does that, also taking
@code{enable-local-variables} into account (@pxref{Auto Major Mode}).

1591
This function works by walking the alist stored in
1592
@code{file-local-variables-alist} and applying each local variable in
1593 1594
turn.  It calls @code{before-hack-local-variables-hook} and
@code{hack-local-variables-hook} before and after applying the
1595 1596 1597 1598
variables, respectively.  It only calls the before-hook if the alist
is non-@code{nil}; it always calls the other hook.  This
function ignores a @samp{mode} element if it specifies the same major
mode as the buffer already has.
1599

Glenn Morris's avatar
Glenn Morris committed
1600
If the optional argument @var{mode-only} is non-@code{nil}, then all
1601 1602 1603 1604
this function does is return a symbol specifying the major mode,
if the @w{@samp{-*-}} line or the local variables list specifies one,
and @code{nil} otherwise.  It does not set the mode nor any other
file-local variable.
Glenn Morris's avatar
Glenn Morris committed
1605 1606
@end defun

1607 1608 1609 1610 1611 1612 1613 1614 1615 1616 1617 1618 1619 1620 1621 1622 1623 1624 1625 1626
@defvar file-local-variables-alist
This buffer-local variable holds the alist of file-local variable
settings.  Each element of the alist is of the form
@w{@code{(@var{var} . @var{value})}}, where @var{var} is a symbol of
the local variable and @var{value} is its value.  When Emacs visits a
file, it first collects all the file-local variables into this alist,
and then the @code{hack-local-variables} function applies them one by
one.
@end defvar

@defvar before-hack-local-variables-hook
Emacs calls this hook immediately before applying file-local variables
stored in @code{file-local-variables-alist}.
@end defvar

@defvar hack-local-variables-hook
Emacs calls this hook immediately after it finishes applying
file-local variables stored in @code{file-local-variables-alist}.
@end defvar

Glenn Morris's avatar
Glenn Morris committed
1627 1628
@cindex safe local variable
  You can specify safe values for a variable with a
1629 1630 1631 1632 1633 1634 1635 1636
@code{safe-local-variable} property.  The property has to be a
function of one argument; any value is safe if the function returns
non-@code{nil} given that value.  Many commonly-encountered file
variables have @code{safe-local-variable} properties; these include
@code{fill-column}, @code{fill-prefix}, and @code{indent-tabs-mode}.
For boolean-valued variables that are safe, use @code{booleanp} as the
property value.  Lambda expressions should be quoted so that
@code{describe-variable} can display the predicate.
Glenn Morris's avatar
Glenn Morris committed
1637

1638 1639 1640 1641 1642
  When defining a user option using @code{defcustom}, you can set its
@code{safe-local-variable} property by adding the arguments
@code{:safe @var{function}} to @code{defcustom} (@pxref{Variable
Definitions}).

Glenn Morris's avatar
Glenn Morris committed
1643 1644 1645 1646 1647 1648
@defopt safe-local-variable-values
This variable provides another way to mark some variable values as
safe.  It is a list of cons cells @code{(@var{var} . @var{val})},
where @var{var} is a variable name and @var{val} is a value which is
safe for that variable.

1649
When Emacs asks the user whether or not to obey a set of file-local
Glenn Morris's avatar
Glenn Morris committed
1650 1651 1652 1653 1654 1655 1656 1657 1658 1659