-
-
Notifications
You must be signed in to change notification settings - Fork 39
/
variables.texi
1931 lines (1637 loc) · 70.9 KB
/
variables.texi
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
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
100
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
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
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
352
353
354
355
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
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
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
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
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
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
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
659
660
661
662
663
664
665
666
667
668
669
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
708
709
710
711
712
713
714
715
716
717
718
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
763
764
765
766
767
768
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
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
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
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999, 2000,
@c 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009 Free Software Foundation, Inc.
@c See the file elisp.texi for copying conditions.
@setfilename ../../info/variables
@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.
Nearly all programming languages have variables of some sort. In the
text of a Lisp program, variables are written using the syntax for
symbols.
In Lisp, unlike most programming languages, programs are represented
primarily as Lisp objects and only secondarily as text. The Lisp
objects used for variables are symbols: the symbol name is the
variable name, and the variable's value is stored in the value cell of
the symbol. The use of a symbol as a variable is independent of its
use as a function name. @xref{Symbol Components}.
The textual form of a Lisp program is given by the read syntax of
the Lisp objects that constitute the program. Hence, a variable in a
textual Lisp program is written using the read syntax for the symbol
representing the variable.
@menu
* 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
define a variable.
* Accessing Variables:: Examining values of variables whose names
are known only at run time.
* 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.
* Frame-Local Variables:: Frame-local bindings for variables.
* Variable Aliases:: Variables that are aliases for other variables.
* 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
@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.
Once the variable has a value, you can refer to it by using the
symbol itself as an expression. Thus,
@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
@kindex setting-constant
@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
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.
@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
with new values. Sometimes it is useful to create variable values that
exist temporarily---only until a certain part of the program finishes.
These values are called @dfn{local}, and the variables so used are
called @dfn{local variables}.
For example, when a function is called, its argument variables receive
new local values that last until the function exits. The @code{let}
special form explicitly establishes new local values for specified
variables; these last until exit from the @code{let} form.
@cindex shadowing of variables
Establishing a local value saves away the variable's previous value
(or lack of one). We say that the previous value is @dfn{shadowed}
and @dfn{not visible}. Both global and local values may be shadowed
(@pxref{Scope}). After the life span of the local value is over, the
previous value (or lack of one) is restored.
If you set a variable (such as with @code{setq}) while it is local,
this replaces the local value; it does not alter the global value, or
previous local values, that are shadowed. To model this behavior, we
speak of a @dfn{local binding} of the variable as well as a local value.
The local binding is a conceptual place that holds a local value.
Entering a function, or a special form such as @code{let}, creates the
local binding; exiting the function or the @code{let} removes the
local binding. While the local binding lasts, the variable's value is
stored within it. Using @code{setq} or @code{set} while there is a
local binding stores a different value into the local binding; it does
not create a new binding.
We also speak of the @dfn{global binding}, which is where
(conceptually) the global value is kept.
@cindex current binding
A variable can have more than one local binding at a time (for
example, if there are nested @code{let} forms that bind it). In such a
case, the most recently created local binding that still exists is the
@dfn{current binding} of the variable. (This rule is called
@dfn{dynamic scoping}; see @ref{Variable Scoping}.) If there are no
local bindings, the variable's global binding is its current binding.
We sometimes call the current binding the @dfn{most-local existing
binding}, for emphasis. Ordinary evaluation of a symbol always returns
the value of its current binding.
The special forms @code{let} and @code{let*} exist to create
local bindings.
@defspec let (bindings@dots{}) forms@dots{}
This special form binds variables according to @var{bindings} and then
evaluates all of the @var{forms} in textual order. The @code{let}-form
returns the value of the last form in @var{forms}.
Each of the @var{bindings} is either @w{(i) a} symbol, in which case
that symbol is bound to @code{nil}; or @w{(ii) a} list of the form
@code{(@var{symbol} @var{value-form})}, in which case @var{symbol} is
bound to the result of evaluating @var{value-form}. If @var{value-form}
is omitted, @code{nil} is used.
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
@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
reasonably refer to the preceding symbols bound in this @code{let*}
form. Compare the following example with the example above for
@code{let}.
@example
@group
(setq y 2)
@result{} 2
@end group
@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
Variables}); a few variables have terminal-local bindings
(@pxref{Multiple Terminals}). These kinds of bindings work somewhat
like ordinary local bindings, but they are localized depending on
``where'' you are in Emacs, rather than localized in time.
@defopt max-specpdl-size
@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
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
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}.
The default value is 1000. Entry to the Lisp debugger increases the
value, if there is little room left, to make sure the debugger itself
has room to execute.
@end defopt
@node Void Variables
@section When a Variable is ``Void''
@kindex void-variable
@cindex void variable
If you have never given a symbol any value as a global variable, we
say that that symbol's global value is @dfn{void}. In other words, the
symbol's value cell does not have any Lisp object in it. If you try to
evaluate the symbol, you get a @code{void-variable} error rather than
a value.
Note that a value of @code{nil} is not the same as void. 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 @emph{a value}. A void variable does not
have any value.
After you have given a variable a value, you can make it void once more
using @code{makunbound}.
@defun makunbound symbol
This function makes the current variable binding of @var{symbol} void.
Subsequent attempts to use this symbol's value as a variable will signal
the error @code{void-variable}, unless and until you set it again.
@code{makunbound} returns @var{symbol}.
@example
@group
(makunbound 'x) ; @r{Make the global value of @code{x} void.}
@result{} x
@end group
@group
x
@error{} Symbol's value as variable is void: x
@end group
@end example
If @var{symbol} is locally bound, @code{makunbound} affects the most
local existing binding. This is the only way a symbol can have a void
local binding, since all the constructs that create local bindings
create them with values. In this case, the voidness lasts at most as
long as the binding does; when the binding is removed due to exit from
the construct that made it, the previous local or global binding is
reexposed as usual, and the variable is no longer void unless the newly
reexposed binding was void all along.
@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
A variable that has been made void with @code{makunbound} is
indistinguishable from one that has never received a value and has
always been void.
You can use the function @code{boundp} to test whether a variable is
currently void.
@defun boundp variable
@code{boundp} returns @code{t} if @var{variable} (a symbol) is not void;
more precisely, if its current binding is not void. It returns
@code{nil} otherwise.
@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
You may announce your intention to use a symbol as a global variable
with a @dfn{variable definition}: a special form, either @code{defconst}
or @code{defvar}.
In Emacs Lisp, definitions serve three purposes. First, they inform
people who read the code that certain symbols are @emph{intended} to be
used a certain way (as variables). Second, they inform the Lisp system
of these things, supplying a value and documentation. Third, they
provide information to utilities such as @code{etags} and
@code{make-docfile}, which create data bases of the functions and
variables in a program.
The difference between @code{defconst} and @code{defvar} is primarily
a matter of intent, serving to inform human readers of whether the value
should ever change. Emacs Lisp does not restrict the ways in which a
variable can be used based on @code{defconst} or @code{defvar}
declarations. However, it does make a difference for initialization:
@code{defconst} unconditionally initializes the variable, while
@code{defvar} initializes it only if it is void.
@ignore
One would expect user option variables to be defined with
@code{defconst}, since programs do not change them. Unfortunately, this
has bad results if the definition is in a library that is not preloaded:
@code{defconst} would override any prior value when the library is
loaded. Users would like to be able to set user options in their init
files, and override the default values given in the definitions. For
this reason, user options must be defined with @code{defvar}.
@end ignore
@defspec defvar symbol [value [doc-string]]
This special form defines @var{symbol} as a variable and can also
initialize and document it. The definition informs a person reading
your code that @var{symbol} is used as a variable that might be set or
changed. Note that @var{symbol} is not evaluated; the symbol to be
defined must appear explicitly in the @code{defvar}.
If @var{symbol} is void and @var{value} is specified, @code{defvar}
evaluates it 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.
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.
If the @var{doc-string} argument appears, it specifies the documentation
for the variable. (This opportunity to specify documentation is one of
the main benefits of defining the variable.) The documentation is
stored in the symbol's @code{variable-documentation} property. The
Emacs help functions (@pxref{Documentation}) look for this property.
If the documentation string begins with the character @samp{*}, Emacs
allows users to set it interactively using the @code{set-variable}
command. However, you should nearly always use @code{defcustom}
instead of @code{defvar} to define such variables, so that users can
use @kbd{M-x customize} and related commands to set them. In that
case, it is not necessary to begin the documentation string with
@samp{*}. @xref{Customization}.
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 following form changes the documentation string for @code{bar},
making it a user option, but does not change the value, since @code{bar}
already has a value. (The addition @code{(1+ nil)} would get an error
if it were evaluated, but since it is not evaluated, there is no error.)
@example
@group
(defvar bar (1+ nil)
"*The normal weight of a bar.")
@result{} bar
@end group
@group
bar
@result{} 23
@end group
@end example
Here is an equivalent expression for the @code{defvar} special form:
@example
@group
(defvar @var{symbol} @var{value} @var{doc-string})
@equiv{}
(progn
(if (not (boundp '@var{symbol}))
(setq @var{symbol} @var{value}))
(if '@var{doc-string}
(put '@var{symbol} 'variable-documentation '@var{doc-string}))
'@var{symbol})
@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
@cindex constant variables
@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}.
@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}.)
Here, @code{pi} is a constant that presumably ought not to be changed
by anyone (attempts by the Indiana State Legislature notwithstanding).
As the second form illustrates, however, this is only advisory.
@example
@group
(defconst pi 3.1415 "Pi to five places.")
@result{} pi
@end group
@group
(setq pi 3)
@result{} pi
@end group
@group
pi
@result{} 3
@end group
@end example
@end defspec
@defun user-variable-p variable
@cindex user option
This function returns @code{t} if @var{variable} is a user option---a
variable intended to be set by the user for customization---and
@code{nil} otherwise. (Variables other than user options exist for the
internal purposes of Lisp programs, and users need not know about them.)
User option variables are distinguished from other variables either
though being declared using @code{defcustom}@footnote{They may also be
declared equivalently in @file{cus-start.el}.} or by the first character
of their @code{variable-documentation} property. If the property exists
and is a string, and its first character is @samp{*}, then the variable
is a user option. Aliases of user options are also user options.
@end defun
@kindex variable-interactive
If a user option variable has a @code{variable-interactive} property,
the @code{set-variable} command uses that value to control reading the
new value for the variable. The property's value is used as if it were
specified in @code{interactive} (@pxref{Using Interactive}). However,
this feature is largely obsoleted by @code{defcustom}
(@pxref{Customization}).
@strong{Warning:} If the @code{defconst} and @code{defvar} special
forms are used while the variable has a local binding (made with
@code{let}, or a function argument), they set the local-binding's
value; the top-level binding is not changed. This is not what you
usually want. To prevent it, 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.
@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.
Since such variables often end up acquiring more values over time,
this convention is not strongly recommended.
@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
it as ``safe'' or ``risky''; see @ref{File Local Variables}.
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
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} @emph{will} reinitialize the map completely.
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.
But be careful not to write the code like this:
@example
(defvar my-mode-map nil
@var{docstring})
(unless my-mode-map
(setq my-mode-map (make-sparse-keymap))
(define-key my-mode-map "\C-c\C-a" 'my-command)
@dots{})
@end example
@noindent
This code sets the variable, then alters it, but it does so in more than
one step. If the user quits just after the @code{setq}, that leaves the
variable neither correctly initialized nor void nor @code{nil}. Once
that happens, reloading the file will not initialize the variable; it
will remain incomplete.
@node Accessing Variables
@section Accessing Variable Values
The usual way to reference a variable is to write the symbol which
names it (@pxref{Symbol Forms}). This requires you to specify the
variable name when you write the program. Usually that is exactly what
you want to do. Occasionally you need to choose at run time which
variable to reference; then you can use @code{symbol-value}.
@defun symbol-value symbol
This function returns the value of @var{symbol}. This is the value in
the innermost local binding of the symbol, or its global value if it
has no local bindings.
@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
A @code{void-variable} error is signaled if the current binding of
@var{symbol} is void.
@end defun
@node Setting Variables
@section How to Alter a Variable Value
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
evaluating the corresponding @var{form}. The most-local existing
binding of the symbol is changed.
@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
This function sets @var{symbol}'s value to @var{value}, then returns
@var{value}. Since @code{set} is a function, the expression written for
@var{symbol} is evaluated to obtain the symbol to set.
The most-local existing binding of the variable is the binding that is
set; shadowed bindings are not affected.
@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
Logically speaking, @code{set} is a more fundamental primitive than
@code{setq}. Any use of @code{setq} can be trivially rewritten to use
@code{set}; @code{setq} could even be defined as a macro, given the
availability of @code{set}. However, @code{set} itself is rarely used;
beginners hardly need to know about it. It is useful only for choosing
at run time which variable to set. For example, the command
@code{set-variable}, which reads a variable name from the user and then
sets the variable, needs to use @code{set}.
@cindex CL note---@code{set} local
@quotation
@b{Common Lisp note:} In Common Lisp, @code{set} always changes the
symbol's ``special'' or dynamic value, ignoring any lexical bindings.
In Emacs Lisp, all variables and all bindings are dynamic, so @code{set}
always affects the most local existing binding.
@end quotation
@end defun
@node Variable Scoping
@section Scoping Rules for Variable Bindings
A given symbol @code{foo} can have several local variable bindings,
established at different places in the Lisp program, as well as a global
binding. The most recently established binding takes precedence over
the others.
@cindex scope
@cindex extent
@cindex dynamic scoping
@cindex lexical scoping
Local bindings in Emacs Lisp have @dfn{indefinite scope} and
@dfn{dynamic extent}. @dfn{Scope} refers to @emph{where} textually in
the source code the binding can be accessed. ``Indefinite scope'' means
that any part of the program can potentially access the variable
binding. @dfn{Extent} refers to @emph{when}, as the program is
executing, the binding exists. ``Dynamic extent'' means that the binding
lasts as long as the activation of the construct that established it.
The combination of dynamic extent and indefinite scope is called
@dfn{dynamic scoping}. By contrast, most programming languages use
@dfn{lexical scoping}, in which references to a local variable must be
located textually within the function or block that binds the variable.
@cindex CL note---special variables
@quotation
@b{Common Lisp note:} Variables declared ``special'' in Common Lisp are
dynamically scoped, like all variables in Emacs Lisp.
@end quotation
@menu
* Scope:: Scope means where in the program a value is visible.
Comparison with other languages.
* Extent:: Extent means how long in time a value exists.
* Impl of Scope:: Two ways to implement dynamic scoping.
* Using Scoping:: How to use dynamic scoping carefully and avoid problems.
@end menu
@node Scope
@subsection Scope
Emacs Lisp uses @dfn{indefinite scope} for local variable bindings.
This means that any function anywhere in the program text might access a
given binding of a variable. Consider the following function
definitions:
@example
@group
(defun binder (x) ; @r{@code{x} is bound in @code{binder}.}
(foo 5)) ; @r{@code{foo} is some other function.}
@end group
@group
(defun user () ; @r{@code{x} is used ``free'' in @code{user}.}
(list x))
@end group
@end example
In a lexically scoped language, the binding of @code{x} in
@code{binder} would never be accessible in @code{user}, because
@code{user} is not textually contained within the function
@code{binder}. However, in dynamically-scoped Emacs Lisp, @code{user}
may or may not refer to the binding of @code{x} established in
@code{binder}, depending on the circumstances:
@itemize @bullet
@item
If we call @code{user} directly without calling @code{binder} at all,
then whatever binding of @code{x} is found, it cannot come from
@code{binder}.
@item
If we define @code{foo} as follows and then call @code{binder}, then the
binding made in @code{binder} will be seen in @code{user}:
@example
@group
(defun foo (lose)
(user))
@end group
@end example
@item
However, if we define @code{foo} as follows and then call @code{binder},
then the binding made in @code{binder} @emph{will not} be seen in
@code{user}:
@example
(defun foo (x)
(user))
@end example
@noindent
Here, when @code{foo} is called by @code{binder}, it binds @code{x}.
(The binding in @code{foo} is said to @dfn{shadow} the one made in
@code{binder}.) Therefore, @code{user} will access the @code{x} bound
by @code{foo} instead of the one bound by @code{binder}.
@end itemize
Emacs Lisp uses dynamic scoping because simple implementations of
lexical scoping are slow. In addition, every Lisp system needs to offer
dynamic scoping at least as an option; if lexical scoping is the norm,
there must be a way to specify dynamic scoping instead for a particular
variable. It might not be a bad thing for Emacs to offer both, but
implementing it with dynamic scoping only was much easier.
@node Extent
@subsection Extent
@dfn{Extent} refers to the time during program execution that a
variable name is valid. In Emacs Lisp, a variable is valid only while
the form that bound it is executing. This is called @dfn{dynamic
extent}. ``Local'' or ``automatic'' variables in most languages,
including C and Pascal, have dynamic extent.
One alternative to dynamic extent is @dfn{indefinite extent}. This
means that a variable binding can live on past the exit from the form
that made the binding. Common Lisp and Scheme, for example, support
this, but Emacs Lisp does not.
To illustrate this, the function below, @code{make-add}, returns a
function that purports to add @var{n} to its own argument @var{m}. This
would work in Common Lisp, but it does not do the job in Emacs Lisp,
because after the call to @code{make-add} exits, the variable @code{n}
is no longer bound to the actual argument 2.
@example
(defun make-add (n)
(function (lambda (m) (+ n m)))) ; @r{Return a function.}