summaryrefslogtreecommitdiff
path: root/doc/lispref/customize.texi
blob: c3de589b01f2c517c1f30b70a2e5e5a2d79c3759 (plain)
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
1001
1002
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
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
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
1159
1160
1161
1162
1163
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
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
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
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
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
1319
1320
1321
1322
1323
1324
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
1374
1375
1376
1377
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
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1997-2012 Free Software Foundation, Inc.
@c See the file elisp.texi for copying conditions.
@node Customization
@chapter Customization Settings

@cindex customization item
  Users of Emacs can customize variables and faces without writing
Lisp code, by using the Customize interface.  @xref{Easy
Customization,,, emacs, The GNU Emacs Manual}.  This chapter describes
how to define @dfn{customization items} that users can interact with
through the Customize interface.

  Customization items include customizable variables, which are
defined with the
@ifinfo
@code{defcustom} macro (@pxref{Variable Definitions});
@end ifinfo
@ifnotinfo
@code{defcustom} macro;
@end ifnotinfo
customizable faces, which are defined with @code{defface} (described
separately in @ref{Defining Faces}); and @dfn{customization groups},
defined with
@ifinfo
@code{defgroup} (@pxref{Group Definitions}),
@end ifinfo
@ifnotinfo
@code{defgroup},
@end ifnotinfo
which act as containers for groups of related customization items.

@menu
* Common Keywords::         Common keyword arguments for all kinds of
                             customization declarations.
* Group Definitions::       Writing customization group definitions.
* Variable Definitions::    Declaring user options.
* Customization Types::     Specifying the type of a user option.
* Applying Customizations:: Functions to apply customization settings.
* Custom Themes::           Writing Custom themes.
@end menu

@node Common Keywords
@section Common Item Keywords

@cindex customization keywords
  The customization declarations that we will describe in the next few
sections---@code{defcustom}, @code{defgroup}, etc.---all accept
keyword arguments (@pxref{Constant Variables}) for specifying various
information.  This section describes keywords that apply to all types
of customization declarations.

  All of these keywords, except @code{:tag}, can be used more than once
in a given item.  Each use of the keyword has an independent effect.
The keyword @code{:tag} is an exception because any given item can only
display one name.

@table @code
@item :tag @var{label}
@kindex tag@r{, customization keyword}
Use @var{label}, a string, instead of the item's name, to label the
item in customization menus and buffers.  @strong{Don't use a tag
which is substantially different from the item's real name; that would
cause confusion.}

@kindex group@r{, customization keyword}
@item :group @var{group}
Put this customization item in group @var{group}.  When you use
@code{:group} in a @code{defgroup}, it makes the new group a subgroup of
@var{group}.

If you use this keyword more than once, you can put a single item into
more than one group.  Displaying any of those groups will show this
item.  Please don't overdo this, since the result would be annoying.

@item :link @var{link-data}
@kindex link@r{, customization keyword}
Include an external link after the documentation string for this item.
This is a sentence containing a button that references some
other documentation.

There are several alternatives you can use for @var{link-data}:

@table @code
@item (custom-manual @var{info-node})
Link to an Info node; @var{info-node} is a string which specifies the
node name, as in @code{"(emacs)Top"}.  The link appears as
@samp{[Manual]} in the customization buffer and enters the built-in
Info reader on @var{info-node}.

@item (info-link @var{info-node})
Like @code{custom-manual} except that the link appears
in the customization buffer with the Info node name.

@item (url-link @var{url})
Link to a web page; @var{url} is a string which specifies the
@acronym{URL}.  The link appears in the customization buffer as
@var{url} and invokes the WWW browser specified by
@code{browse-url-browser-function}.

@item (emacs-commentary-link @var{library})
Link to the commentary section of a library; @var{library} is a string
which specifies the library name.  @xref{Library Headers}.

@item (emacs-library-link @var{library})
Link to an Emacs Lisp library file; @var{library} is a string which
specifies the library name.

@item (file-link @var{file})
Link to a file; @var{file} is a string which specifies the name of the
file to visit with @code{find-file} when the user invokes this link.

@item (function-link @var{function})
Link to the documentation of a function; @var{function} is a string
which specifies the name of the function to describe with
@code{describe-function} when the user invokes this link.

@item (variable-link @var{variable})
Link to the documentation of a variable; @var{variable} is a string
which specifies the name of the variable to describe with
@code{describe-variable} when the user invokes this link.

@item (custom-group-link @var{group})
Link to another customization group.  Invoking it creates a new
customization buffer for @var{group}.
@end table

You can specify the text to use in the customization buffer by adding
@code{:tag @var{name}} after the first element of the @var{link-data};
for example, @code{(info-link :tag "foo" "(emacs)Top")} makes a link to
the Emacs manual which appears in the buffer as @samp{foo}.

You can use this keyword more than once, to add multiple links.

@item :load @var{file}
@kindex load@r{, customization keyword}
Load file @var{file} (a string) before displaying this customization
item (@pxref{Loading}).  Loading is done with @code{load}, and only if
the file is not already loaded.

@item :require @var{feature}
@kindex require@r{, customization keyword}
Execute @code{(require '@var{feature})} when your saved customizations
set the value of this item.  @var{feature} should be a symbol.

The most common reason to use @code{:require} is when a variable enables
a feature such as a minor mode, and just setting the variable won't have
any effect unless the code which implements the mode is loaded.

@item :version @var{version}
@kindex version@r{, customization keyword}
This keyword specifies that the item was first introduced in Emacs
version @var{version}, or that its default value was changed in that
version.  The value @var{version} must be a string.

@item :package-version '(@var{package} . @var{version})
@kindex package-version@r{, customization keyword}
This keyword specifies that the item was first introduced in
@var{package} version @var{version}, or that its meaning or default
value was changed in that version.  This keyword takes priority over
@code{:version}.

@var{package} should be the official name of the package, as a symbol
(e.g., @code{MH-E}).  @var{version} should be a string.  If the
package @var{package} is released as part of Emacs, @var{package} and
@var{version} should appear in the value of
@code{customize-package-emacs-version-alist}.
@end table

Packages distributed as part of Emacs that use the
@code{:package-version} keyword must also update the
@code{customize-package-emacs-version-alist} variable.

@defvar customize-package-emacs-version-alist
This alist provides a mapping for the versions of Emacs that are
associated with versions of a package listed in the
@code{:package-version} keyword.  Its elements are:

@example
(@var{package} (@var{pversion} . @var{eversion})@dots{})
@end example

For each @var{package}, which is a symbol, there are one or more
elements that contain a package version @var{pversion} with an
associated Emacs version @var{eversion}.  These versions are strings.
For example, the MH-E package updates this alist with the following:

@c Must be small else too wide.
@c FIXME obviously this is out of date (in the code).
@smallexample
(add-to-list 'customize-package-emacs-version-alist
             '(MH-E ("6.0" . "22.1") ("6.1" . "22.1") ("7.0" . "22.1")
                    ("7.1" . "22.1") ("7.2" . "22.1") ("7.3" . "22.1")
                    ("7.4" . "22.1") ("8.0" . "22.1")))
@end smallexample

The value of @var{package} needs to be unique and it needs to match
the @var{package} value appearing in the @code{:package-version}
keyword.  Since the user might see the value in an error message, a good
choice is the official name of the package, such as MH-E or Gnus.
@end defvar

@node Group Definitions
@section Defining Customization Groups
@cindex define customization group
@cindex customization groups, defining

  Each Emacs Lisp package should have one main customization group
which contains all the options, faces and other groups in the package.
If the package has a small number of options and faces, use just one
group and put everything in it.  When there are more than twenty or so
options and faces, then you should structure them into subgroups, and
put the subgroups under the package's main customization group.  It is
OK to put some of the options and faces in the package's main group
alongside the subgroups.

  The package's main or only group should be a member of one or more of
the standard customization groups.  (To display the full list of them,
use @kbd{M-x customize}.)  Choose one or more of them (but not too
many), and add your group to each of them using the @code{:group}
keyword.

  The way to declare new customization groups is with @code{defgroup}.

@defmac defgroup group members doc [keyword value]@dots{}
Declare @var{group} as a customization group containing @var{members}.
Do not quote the symbol @var{group}.  The argument @var{doc} specifies
the documentation string for the group.

The argument @var{members} is a list specifying an initial set of
customization items to be members of the group.  However, most often
@var{members} is @code{nil}, and you specify the group's members by
using the @code{:group} keyword when defining those members.

If you want to specify group members through @var{members}, each element
should have the form @code{(@var{name} @var{widget})}.  Here @var{name}
is a symbol, and @var{widget} is a widget type for editing that symbol.
Useful widgets are @code{custom-variable} for a variable,
@code{custom-face} for a face, and @code{custom-group} for a group.

When you introduce a new group into Emacs, use the @code{:version}
keyword in the @code{defgroup}; then you need not use it for
the individual members of the group.

In addition to the common keywords (@pxref{Common Keywords}), you can
also use this keyword in @code{defgroup}:

@table @code
@item :prefix @var{prefix}
@kindex prefix@r{, @code{defgroup} keyword}
If the name of an item in the group starts with @var{prefix}, and the
customizable variable @code{custom-unlispify-remove-prefixes} is
non-@code{nil}, the item's tag will omit @var{prefix}.  A group can
have any number of prefixes.
@end table
@end defmac

@defopt custom-unlispify-remove-prefixes
If this variable is non-@code{nil}, the prefixes specified by a
group's @code{:prefix} keyword are omitted from tag names, whenever
the user customizes the group.

The default value is @code{nil}, i.e., the prefix-discarding feature
is disabled.  This is because discarding prefixes often leads to
confusing names for options and faces.
@end defopt

@node Variable Definitions
@section Defining Customization Variables
@cindex define customization options
@cindex customizable variables, how to define
@cindex user options, how to define

  @dfn{Customizable variables}, also called @dfn{user options}, are
global Lisp variables whose values can be set through the Customize
interface.  Unlike other global variables, which are defined with
@code{defvar} (@pxref{Defining Variables}), customizable variables are
defined using the @code{defcustom} macro.  In addition to calling
@code{defvar} as a subroutine, @code{defcustom} states how the
variable should be displayed in the Customize interface, the values it
is allowed to take, etc.

@defmac defcustom option standard doc [keyword value]@dots{}
This macro declares @var{option} as a user option (i.e., a
customizable variable).  You should not quote @var{option}.

The argument @var{standard} is an expression that specifies the
standard value for @var{option}.  Evaluating the @code{defcustom} form
evaluates @var{standard}, but does not necessarily install the
standard value.  If @var{option} already has a default value,
@code{defcustom} does not change it.  If the user has saved a
customization for @var{option}, @code{defcustom} installs the user's
customized value as @var{option}'s default value.  If neither of those
cases applies, @code{defcustom} installs the result of evaluating
@var{standard} as the default value.

The expression @var{standard} can be evaluated at various other times,
too---whenever the customization facility needs to know @var{option}'s
standard value.  So be sure to use an expression which is harmless to
evaluate at any time.

The argument @var{doc} specifies the documentation string for the
variable.

If a @code{defcustom} does not specify any @code{:group}, the last group
defined with @code{defgroup} in the same file will be used.  This way, most
@code{defcustom} do not need an explicit @code{:group}.

When you evaluate a @code{defcustom} 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.  (The same feature applies to @code{defvar}.)
@xref{Defining Variables}.

If you put a @code{defcustom} in a pre-loaded Emacs Lisp file
(@pxref{Building Emacs}), the standard value installed at dump time
might be incorrect, e.g., because another variable that it depends on
has not been assigned the right value yet.  In that case, use
@code{custom-reevaluate-setting}, described below, to re-evaluate the
standard value after Emacs starts up.
@end defmac

  In addition to the keywords listed in @ref{Common Keywords}, this
macro accepts the following keywords:

@table @code
@item :type @var{type}
Use @var{type} as the data type for this option.  It specifies which
values are legitimate, and how to display the value
(@pxref{Customization Types}).

@item :options @var{value-list}
@kindex options@r{, @code{defcustom} keyword}
Specify the list of reasonable values for use in this
option.  The user is not restricted to using only these values, but they
are offered as convenient alternatives.

This is meaningful only for certain types, currently including
@code{hook}, @code{plist} and @code{alist}.  See the definition of the
individual types for a description of how to use @code{:options}.

@item :set @var{setfunction}
@kindex set@r{, @code{defcustom} keyword}
Specify @var{setfunction} as the way to change the value of this
option when using the Customize interface.  The function
@var{setfunction} should take two arguments, a symbol (the option
name) and the new value, and should do whatever is necessary to update
the value properly for this option (which may not mean simply setting
the option as a Lisp variable).  The default for @var{setfunction} is
@code{set-default}.

If you specify this keyword, the variable's documentation string
should describe how to do the same job in hand-written Lisp code.

@item :get @var{getfunction}
@kindex get@r{, @code{defcustom} keyword}
Specify @var{getfunction} as the way to extract the value of this
option.  The function @var{getfunction} should take one argument, a
symbol, and should return whatever customize should use as the
``current value'' for that symbol (which need not be the symbol's Lisp
value).  The default is @code{default-value}.

You have to really understand the workings of Custom to use
@code{:get} correctly.  It is meant for values that are treated in
Custom as variables but are not actually stored in Lisp variables.  It
is almost surely a mistake to specify @var{getfunction} for a value
that really is stored in a Lisp variable.

@item :initialize @var{function}
@kindex initialize@r{, @code{defcustom} keyword}
@var{function} should be a function used to initialize the variable
when the @code{defcustom} is evaluated.  It should take two arguments,
the option name (a symbol) and the value.  Here are some predefined
functions meant for use in this way:

@table @code
@item custom-initialize-set
Use the variable's @code{:set} function to initialize the variable, but
do not reinitialize it if it is already non-void.

@item custom-initialize-default
Like @code{custom-initialize-set}, but use the function
@code{set-default} to set the variable, instead of the variable's
@code{:set} function.  This is the usual choice for a variable whose
@code{:set} function enables or disables a minor mode; with this choice,
defining the variable will not call the minor mode function, but
customizing the variable will do so.

@item custom-initialize-reset
Always use the @code{:set} function to initialize the variable.  If
the variable is already non-void, reset it by calling the @code{:set}
function using the current value (returned by the @code{:get} method).
This is the default @code{:initialize} function.

@item custom-initialize-changed
Use the @code{:set} function to initialize the variable, if it is
already set or has been customized; otherwise, just use
@code{set-default}.

@item custom-initialize-safe-set
@itemx custom-initialize-safe-default
These functions behave like @code{custom-initialize-set}
(@code{custom-initialize-default}, respectively), but catch errors.
If an error occurs during initialization, they set the variable to
@code{nil} using @code{set-default}, and signal no error.

These functions are meant for options defined in pre-loaded files,
where the @var{standard} expression may signal an error because some
required variable or function is not yet defined.  The value normally
gets updated in @file{startup.el}, ignoring the value computed by
@code{defcustom}.  After startup, if one unsets the value and
reevaluates the @code{defcustom}, the @var{standard} expression can be
evaluated without error.
@end table

@item :risky @var{value}
@kindex risky@r{, @code{defcustom} keyword}
Set the variable's @code{risky-local-variable} property to
@var{value} (@pxref{File Local Variables}).

@item :safe @var{function}
@kindex safe@r{, @code{defcustom} keyword}
Set the variable's @code{safe-local-variable} property to
@var{function} (@pxref{File Local Variables}).

@item :set-after @var{variables}
@kindex set-after@r{, @code{defcustom} keyword}
When setting variables according to saved customizations, make sure to
set the variables @var{variables} before this one; i.e., delay
setting this variable until after those others have been handled.  Use
@code{:set-after} if setting this variable won't work properly unless
those other variables already have their intended values.
@end table

  It is useful to specify the @code{:require} keyword for an option
that ``turns on'' a certain feature.  This causes Emacs to load the
feature, if it is not already loaded, whenever the option is set.
@xref{Common Keywords}.  Here is an example, from the library
@file{saveplace.el}:

@example
(defcustom save-place nil
  "Non-nil means automatically save place in each file..."
  :type 'boolean
  :require 'saveplace
  :group 'save-place)
@end example

If a customization item has a type such as @code{hook} or
@code{alist}, which supports @code{:options}, you can add additional
values to the list from outside the @code{defcustom} declaration by
calling @code{custom-add-frequent-value}.  For example, if you define a
function @code{my-lisp-mode-initialization} intended to be called from
@code{emacs-lisp-mode-hook}, you might want to add that to the list of
reasonable values for @code{emacs-lisp-mode-hook}, but not by editing
its definition.  You can do it thus:

@example
(custom-add-frequent-value 'emacs-lisp-mode-hook
   'my-lisp-mode-initialization)
@end example

@defun custom-add-frequent-value symbol value
For the customization option @var{symbol}, add @var{value} to the
list of reasonable values.

The precise effect of adding a value depends on the customization type
of @var{symbol}.
@end defun

Internally, @code{defcustom} uses the symbol property
@code{standard-value} to record the expression for the standard value,
@code{saved-value} to record the value saved by the user with the
customization buffer, and @code{customized-value} to record the value
set by the user with the customization buffer, but not saved.
@xref{Symbol Properties}.  These properties are lists, the car of
which is an expression that evaluates to the value.

@defun custom-reevaluate-setting symbol
This function re-evaluates the standard value of @var{symbol}, which
should be a user option declared via @code{defcustom}.  If the
variable was customized, this function re-evaluates the saved value
instead.  Then it sets the user option to that value (using the
option's @code{:set} property if that is defined).

This is useful for customizable options that are defined before their
value could be computed correctly.  For example, during startup Emacs
calls this function for some user options that were defined in
pre-loaded Emacs Lisp files, but whose initial values depend on
information available only at run-time.
@end defun

@defun custom-variable-p arg
This function returns non-@code{nil} if @var{arg} is a customizable
variable.  A customizable variable is either a variable that has a
@code{standard-value} or @code{custom-autoload} property (usually
meaning it was declared with @code{defcustom}), or an alias for
another customizable variable.
@end defun

@node Customization Types
@section Customization Types

@cindex customization types
  When you define a user option with @code{defcustom}, you must specify
its @dfn{customization type}.  That is a Lisp object which describes (1)
which values are legitimate and (2) how to display the value in the
customization buffer for editing.

@kindex type@r{, @code{defcustom} keyword}
  You specify the customization type in @code{defcustom} with the
@code{:type} keyword.  The argument of @code{:type} is evaluated, but
only once when the @code{defcustom} is executed, so it isn't useful
for the value to vary.  Normally we use a quoted constant.  For
example:

@example
(defcustom diff-command "diff"
  "The command to use to run diff."
  :type '(string)
  :group 'diff)
@end example

  In general, a customization type is a list whose first element is a
symbol, one of the customization type names defined in the following
sections.  After this symbol come a number of arguments, depending on
the symbol.  Between the type symbol and its arguments, you can
optionally write keyword-value pairs (@pxref{Type Keywords}).

  Some type symbols do not use any arguments; those are called
@dfn{simple types}.  For a simple type, if you do not use any
keyword-value pairs, you can omit the parentheses around the type
symbol.  For example just @code{string} as a customization type is
equivalent to @code{(string)}.

  All customization types are implemented as widgets; see @ref{Top, ,
Introduction, widget, The Emacs Widget Library}, for details.

@menu
* Simple Types::            Simple customization types: sexp, integer, etc.
* Composite Types::         Build new types from other types or data.
* Splicing into Lists::     Splice elements into list with @code{:inline}.
* Type Keywords::           Keyword-argument pairs in a customization type.
* Defining New Types::      Give your type a name.
@end menu

@node Simple Types
@subsection Simple Types

  This section describes all the simple customization types.  For
several of these customization types, the customization widget
provides inline completion with @kbd{C-M-i} or @kbd{M-@key{TAB}}.

@table @code
@item sexp
The value may be any Lisp object that can be printed and read back.
You can use @code{sexp} as a fall-back for any option, if you don't
want to take the time to work out a more specific type to use.

@item integer
The value must be an integer.

@item number
The value must be a number (floating point or integer).

@item float
The value must be a floating point number.

@item string
The value must be a string.  The customization buffer shows the string
without delimiting @samp{"} characters or @samp{\} quotes.

@item regexp
Like @code{string} except that the string must be a valid regular
expression.

@item character
The value must be a character code.  A character code is actually an
integer, but this type shows the value by inserting the character in the
buffer, rather than by showing the number.

@item file
The value must be a file name.  The widget provides completion.

@item (file :must-match t)
The value must be a file name for an existing file.  The widget
provides completion.

@item directory
The value must be a directory name.  The widget provides completion.

@item hook
The value must be a list of functions.  This customization type is
used for hook variables.  You can use the @code{:options} keyword in a
hook variable's @code{defcustom} to specify a list of functions
recommended for use in the hook; @xref{Variable Definitions}.

@item symbol
The value must be a symbol.  It appears in the customization buffer as
the symbol name.  The widget provides completion.

@item function
The value must be either a lambda expression or a function name.  The
widget provides completion for function names.

@item variable
The value must be a variable name.  The widget provides completion.

@item face
The value must be a symbol which is a face name.  The widget provides
completion.

@item boolean
The value is boolean---either @code{nil} or @code{t}.  Note that by
using @code{choice} and @code{const} together (see the next section),
you can specify that the value must be @code{nil} or @code{t}, but also
specify the text to describe each value in a way that fits the specific
meaning of the alternative.

@item key-sequence
The value is a key sequence.  The customization buffer shows the key
sequence using the same syntax as the @kbd{kbd} function.  @xref{Key
Sequences}.

@item coding-system
The value must be a coding-system name, and you can do completion with
@kbd{M-@key{TAB}}.

@item color
The value must be a valid color name.  The widget provides completion
for color names, as well as a sample and a button for selecting a
color name from a list of color names shown in a @file{*Colors*}
buffer.
@end table

@node Composite Types
@subsection Composite Types
@cindex composite types (customization)

  When none of the simple types is appropriate, you can use composite
types, which build new types from other types or from specified data.
The specified types or data are called the @dfn{arguments} of the
composite type.  The composite type normally looks like this:

@example
(@var{constructor} @var{arguments}@dots{})
@end example

@noindent
but you can also add keyword-value pairs before the arguments, like
this:

@example
(@var{constructor} @r{@{}@var{keyword} @var{value}@r{@}}@dots{} @var{arguments}@dots{})
@end example

  Here is a table of constructors and how to use them to write
composite types:

@table @code
@item (cons @var{car-type} @var{cdr-type})
The value must be a cons cell, its @sc{car} must fit @var{car-type}, and
its @sc{cdr} must fit @var{cdr-type}.  For example, @code{(cons string
symbol)} is a customization type which matches values such as
@code{("foo" . foo)}.

In the customization buffer, the @sc{car} and @sc{cdr} are displayed
and edited separately, each according to their specified type.

@item (list @var{element-types}@dots{})
The value must be a list with exactly as many elements as the
@var{element-types} given; and each element must fit the
corresponding @var{element-type}.

For example, @code{(list integer string function)} describes a list of
three elements; the first element must be an integer, the second a
string, and the third a function.

In the customization buffer, each element is displayed and edited
separately, according to the type specified for it.

@item (group @var{element-types}@dots{})
This works like @code{list} except for the formatting
of text in the Custom buffer.  @code{list} labels each
element value with its tag; @code{group} does not.

@item (vector @var{element-types}@dots{})
Like @code{list} except that the value must be a vector instead of a
list.  The elements work the same as in @code{list}.

@item (alist :key-type @var{key-type} :value-type @var{value-type})
The value must be a list of cons-cells, the @sc{car} of each cell
representing a key of customization type @var{key-type}, and the
@sc{cdr} of the same cell representing a value of customization type
@var{value-type}.  The user can add and delete key/value pairs, and
edit both the key and the value of each pair.

If omitted, @var{key-type} and @var{value-type} default to
@code{sexp}.

The user can add any key matching the specified key type, but you can
give some keys a preferential treatment by specifying them with the
@code{:options} (see @ref{Variable Definitions}).  The specified keys
will always be shown in the customize buffer (together with a suitable
value), with a checkbox to include or exclude or disable the key/value
pair from the alist.  The user will not be able to edit the keys
specified by the @code{:options} keyword argument.

The argument to the @code{:options} keywords should be a list of
specifications for reasonable keys in the alist.  Ordinarily, they are
simply atoms, which stand for themselves.  For example:

@example
:options '("foo" "bar" "baz")
@end example

@noindent
specifies that there are three ``known'' keys, namely @code{"foo"},
@code{"bar"} and @code{"baz"}, which will always be shown first.

You may want to restrict the value type for specific keys, for
example, the value associated with the @code{"bar"} key can only be an
integer.  You can specify this by using a list instead of an atom in
the list.  The first element will specify the key, like before, while
the second element will specify the value type.  For example:

@example
:options '("foo" ("bar" integer) "baz")
@end example

Finally, you may want to change how the key is presented.  By default,
the key is simply shown as a @code{const}, since the user cannot change
the special keys specified with the @code{:options} keyword.  However,
you may want to use a more specialized type for presenting the key, like
@code{function-item} if you know it is a symbol with a function binding.
This is done by using a customization type specification instead of a
symbol for the key.

@example
:options '("foo"
           ((function-item some-function) integer)
           "baz")
@end example

Many alists use lists with two elements, instead of cons cells.  For
example,

@example
(defcustom list-alist
  '(("foo" 1) ("bar" 2) ("baz" 3))
  "Each element is a list of the form (KEY VALUE).")
@end example

@noindent
instead of

@example
(defcustom cons-alist
  '(("foo" . 1) ("bar" . 2) ("baz" . 3))
  "Each element is a cons-cell (KEY . VALUE).")
@end example

Because of the way lists are implemented on top of cons cells, you can
treat @code{list-alist} in the example above as a cons cell alist, where
the value type is a list with a single element containing the real
value.

@example
(defcustom list-alist '(("foo" 1) ("bar" 2) ("baz" 3))
  "Each element is a list of the form (KEY VALUE)."
  :type '(alist :value-type (group integer)))
@end example

The @code{group} widget is used here instead of @code{list} only because
the formatting is better suited for the purpose.

Similarly, you can have alists with more values associated with each
key, using variations of this trick:

@example
(defcustom person-data '(("brian"  50 t)
                         ("dorith" 55 nil)
                         ("ken"    52 t))
  "Alist of basic info about people.
Each element has the form (NAME AGE MALE-FLAG)."
  :type '(alist :value-type (group integer boolean)))
@end example

@item (plist :key-type @var{key-type} :value-type @var{value-type})
This customization type is similar to @code{alist} (see above), except
that (i) the information is stored as a property list,
(@pxref{Property Lists}), and (ii) @var{key-type}, if omitted,
defaults to @code{symbol} rather than @code{sexp}.

@item (choice @var{alternative-types}@dots{})
The value must fit one of @var{alternative-types}.  For example,
@code{(choice integer string)} allows either an integer or a string.

In the customization buffer, the user selects an alternative
using a menu, and can then edit the value in the usual way for that
alternative.

Normally the strings in this menu are determined automatically from the
choices; however, you can specify different strings for the menu by
including the @code{:tag} keyword in the alternatives.  For example, if
an integer stands for a number of spaces, while a string is text to use
verbatim, you might write the customization type this way,

@example
(choice (integer :tag "Number of spaces")
        (string :tag "Literal text"))
@end example

@noindent
so that the menu offers @samp{Number of spaces} and @samp{Literal text}.

In any alternative for which @code{nil} is not a valid value, other than
a @code{const}, you should specify a valid default for that alternative
using the @code{:value} keyword.  @xref{Type Keywords}.

If some values are covered by more than one of the alternatives,
customize will choose the first alternative that the value fits.  This
means you should always list the most specific types first, and the
most general last.  Here's an example of proper usage:

@example
(choice (const :tag "Off" nil)
        symbol (sexp :tag "Other"))
@end example

@noindent
This way, the special value @code{nil} is not treated like other
symbols, and symbols are not treated like other Lisp expressions.

@item (radio @var{element-types}@dots{})
This is similar to @code{choice}, except that the choices are displayed
using `radio buttons' rather than a menu.  This has the advantage of
displaying documentation for the choices when applicable and so is often
a good choice for a choice between constant functions
(@code{function-item} customization types).

@item (const @var{value})
The value must be @var{value}---nothing else is allowed.

The main use of @code{const} is inside of @code{choice}.  For example,
@code{(choice integer (const nil))} allows either an integer or
@code{nil}.

@code{:tag} is often used with @code{const}, inside of @code{choice}.
For example,

@example
(choice (const :tag "Yes" t)
        (const :tag "No" nil)
        (const :tag "Ask" foo))
@end example

@noindent
describes a variable for which @code{t} means yes, @code{nil} means no,
and @code{foo} means ``ask''.

@item (other @var{value})
This alternative can match any Lisp value, but if the user chooses this
alternative, that selects the value @var{value}.

The main use of @code{other} is as the last element of @code{choice}.
For example,

@example
(choice (const :tag "Yes" t)
        (const :tag "No" nil)
        (other :tag "Ask" foo))
@end example

@noindent
describes a variable for which @code{t} means yes, @code{nil} means no,
and anything else means ``ask''.  If the user chooses @samp{Ask} from
the menu of alternatives, that specifies the value @code{foo}; but any
other value (not @code{t}, @code{nil} or @code{foo}) displays as
@samp{Ask}, just like @code{foo}.

@item (function-item @var{function})
Like @code{const}, but used for values which are functions.  This
displays the documentation string as well as the function name.
The documentation string is either the one you specify with
@code{:doc}, or @var{function}'s own documentation string.

@item (variable-item @var{variable})
Like @code{const}, but used for values which are variable names.  This
displays the documentation string as well as the variable name.  The
documentation string is either the one you specify with @code{:doc}, or
@var{variable}'s own documentation string.

@item (set @var{types}@dots{})
The value must be a list, and each element of the list must match one of
the @var{types} specified.

This appears in the customization buffer as a checklist, so that each of
@var{types} may have either one corresponding element or none.  It is
not possible to specify two different elements that match the same one
of @var{types}.  For example, @code{(set integer symbol)} allows one
integer and/or one symbol in the list; it does not allow multiple
integers or multiple symbols.  As a result, it is rare to use
nonspecific types such as @code{integer} in a @code{set}.

Most often, the @var{types} in a @code{set} are @code{const} types, as
shown here:

@example
(set (const :bold) (const :italic))
@end example

Sometimes they describe possible elements in an alist:

@example
(set (cons :tag "Height" (const height) integer)
     (cons :tag "Width" (const width) integer))
@end example

@noindent
That lets the user specify a height value optionally
and a width value optionally.

@item (repeat @var{element-type})
The value must be a list and each element of the list must fit the type
@var{element-type}.  This appears in the customization buffer as a
list of elements, with @samp{[INS]} and @samp{[DEL]} buttons for adding
more elements or removing elements.

@item (restricted-sexp :match-alternatives @var{criteria})
This is the most general composite type construct.  The value may be
any Lisp object that satisfies one of @var{criteria}.  @var{criteria}
should be a list, and each element should be one of these
possibilities:

@itemize @bullet
@item
A predicate---that is, a function of one argument that has no side
effects, and returns either @code{nil} or non-@code{nil} according to
the argument.  Using a predicate in the list says that objects for which
the predicate returns non-@code{nil} are acceptable.

@item
A quoted constant---that is, @code{'@var{object}}.  This sort of element
in the list says that @var{object} itself is an acceptable value.
@end itemize

For example,

@example
(restricted-sexp :match-alternatives
                 (integerp 't 'nil))
@end example

@noindent
allows integers, @code{t} and @code{nil} as legitimate values.

The customization buffer shows all legitimate values using their read
syntax, and the user edits them textually.
@end table

  Here is a table of the keywords you can use in keyword-value pairs
in a composite type:

@table @code
@item :tag @var{tag}
Use @var{tag} as the name of this alternative, for user communication
purposes.  This is useful for a type that appears inside of a
@code{choice}.

@item :match-alternatives @var{criteria}
@kindex match-alternatives@r{, customization keyword}
Use @var{criteria} to match possible values.  This is used only in
@code{restricted-sexp}.

@item :args @var{argument-list}
@kindex args@r{, customization keyword}
Use the elements of @var{argument-list} as the arguments of the type
construct.  For instance, @code{(const :args (foo))} is equivalent to
@code{(const foo)}.  You rarely need to write @code{:args} explicitly,
because normally the arguments are recognized automatically as
whatever follows the last keyword-value pair.
@end table

@node Splicing into Lists
@subsection Splicing into Lists

  The @code{:inline} feature lets you splice a variable number of
elements into the middle of a @code{list} or @code{vector}
customization type.  You use it by adding @code{:inline t} to a type
specification which is contained in a @code{list} or @code{vector}
specification.

  Normally, each entry in a @code{list} or @code{vector} type
specification describes a single element type.  But when an entry
contains @code{:inline t}, the value it matches is merged directly
into the containing sequence.  For example, if the entry matches a
list with three elements, those become three elements of the overall
sequence.  This is analogous to @samp{,@@} in a backquote construct
(@pxref{Backquote}).

  For example, to specify a list whose first element must be @code{baz}
and whose remaining arguments should be zero or more of @code{foo} and
@code{bar}, use this customization type:

@example
(list (const baz) (set :inline t (const foo) (const bar)))
@end example

@noindent
This matches values such as @code{(baz)}, @code{(baz foo)}, @code{(baz bar)}
and @code{(baz foo bar)}.

  When the element-type is a @code{choice}, you use @code{:inline} not
in the @code{choice} itself, but in (some of) the alternatives of the
@code{choice}.  For example, to match a list which must start with a
file name, followed either by the symbol @code{t} or two strings, use
this customization type:

@example
(list file
      (choice (const t)
              (list :inline t string string)))
@end example

@noindent
If the user chooses the first alternative in the choice, then the
overall list has two elements and the second element is @code{t}.  If
the user chooses the second alternative, then the overall list has three
elements and the second and third must be strings.

@node Type Keywords
@subsection Type Keywords

You can specify keyword-argument pairs in a customization type after the
type name symbol.  Here are the keywords you can use, and their
meanings:

@table @code
@item :value @var{default}
Provide a default value.

If @code{nil} is not a valid value for the alternative, then it is
essential to specify a valid default with @code{:value}.

If you use this for a type that appears as an alternative inside of
@code{choice}; it specifies the default value to use, at first, if and
when the user selects this alternative with the menu in the
customization buffer.

Of course, if the actual value of the option fits this alternative, it
will appear showing the actual value, not @var{default}.

@item :format @var{format-string}
@kindex format@r{, customization keyword}
This string will be inserted in the buffer to represent the value
corresponding to the type.  The following @samp{%} escapes are available
for use in @var{format-string}:

@table @samp
@item %[@var{button}%]
Display the text @var{button} marked as a button.  The @code{:action}
attribute specifies what the button will do if the user invokes it;
its value is a function which takes two arguments---the widget which
the button appears in, and the event.

There is no way to specify two different buttons with different
actions.

@item %@{@var{sample}%@}
Show @var{sample} in a special face specified by @code{:sample-face}.

@item %v
Substitute the item's value.  How the value is represented depends on
the kind of item, and (for variables) on the customization type.

@item %d
Substitute the item's documentation string.

@item %h
Like @samp{%d}, but if the documentation string is more than one line,
add a button to control whether to show all of it or just the first line.

@item %t
Substitute the tag here.  You specify the tag with the @code{:tag}
keyword.

@item %%
Display a literal @samp{%}.
@end table

@item :action @var{action}
@kindex action@r{, customization keyword}
Perform @var{action} if the user clicks on a button.

@item :button-face @var{face}
@kindex button-face@r{, customization keyword}
Use the face @var{face} (a face name or a list of face names) for button
text displayed with @samp{%[@dots{}%]}.

@item :button-prefix @var{prefix}
@itemx :button-suffix @var{suffix}
@kindex button-prefix@r{, customization keyword}
@kindex button-suffix@r{, customization keyword}
These specify the text to display before and after a button.
Each can be:

@table @asis
@item @code{nil}
No text is inserted.

@item a string
The string is inserted literally.

@item a symbol
The symbol's value is used.
@end table

@item :tag @var{tag}
Use @var{tag} (a string) as the tag for the value (or part of the value)
that corresponds to this type.

@item :doc @var{doc}
@kindex doc@r{, customization keyword}
Use @var{doc} as the documentation string for this value (or part of the
value) that corresponds to this type.  In order for this to work, you
must specify a value for @code{:format}, and use @samp{%d} or @samp{%h}
in that value.

The usual reason to specify a documentation string for a type is to
provide more information about the meanings of alternatives inside a
@code{:choice} type or the parts of some other composite type.

@item :help-echo @var{motion-doc}
@kindex help-echo@r{, customization keyword}
When you move to this item with @code{widget-forward} or
@code{widget-backward}, it will display the string @var{motion-doc} in
the echo area.  In addition, @var{motion-doc} is used as the mouse
@code{help-echo} string and may actually be a function or form evaluated
to yield a help string.  If it is a function, it is called with one
argument, the widget.

@item :match @var{function}
@kindex match@r{, customization keyword}
Specify how to decide whether a value matches the type.  The
corresponding value, @var{function}, should be a function that accepts
two arguments, a widget and a value; it should return non-@code{nil} if
the value is acceptable.

@item :validate @var{function}
Specify a validation function for input.  @var{function} takes a
widget as an argument, and should return @code{nil} if the widget's
current value is valid for the widget.  Otherwise, it should return
the widget containing the invalid data, and set that widget's
@code{:error} property to a string explaining the error.

@ignore
@item :indent @var{columns}
Indent this item by @var{columns} columns.  The indentation is used for
@samp{%n}, and automatically for group names, for checklists and radio
buttons, and for editable lists.  It affects the whole of the
item except for the first line.

@item :offset @var{extra}
Indent the subitems of this item @var{extra} columns more than this
item itself.  By default, subitems are indented the same as their
parent.

@item :extra-offset @var{n}
Add @var{n} extra spaces to this item's indentation, compared to its
parent's indentation.

@item :notify @var{function}
Call @var{function} each time the item or a subitem is changed.  The
function gets two or three arguments.  The first argument is the item
itself, the second argument is the item that was changed, and the
third argument is the event leading to the change, if any.

@item :menu-tag @var{tag-string}
Use @var{tag-string} in the menu when the widget is used as an option
in a @code{menu-choice} widget.

@item :menu-tag-get
A function used for finding the tag when the widget is used as an option
in a @code{menu-choice} widget.  By default, the tag used will be either the
@code{:menu-tag} or @code{:tag} property if present, or the @code{princ}
representation of the @code{:value} property if not.

@item :tab-order
Specify the order in which widgets are traversed with
@code{widget-forward} or @code{widget-backward}.  This is only partially
implemented.

@enumerate a
@item
Widgets with tabbing order @code{-1} are ignored.

@item
(Unimplemented) When on a widget with tabbing order @var{n}, go to the
next widget in the buffer with tabbing order @var{n+1} or @code{nil},
whichever comes first.

@item
When on a widget with no tabbing order specified, go to the next widget
in the buffer with a positive tabbing order, or @code{nil}
@end enumerate

@item :parent
The parent of a nested widget (e.g., a @code{menu-choice} item or an
element of a @code{editable-list} widget).

@item :sibling-args
This keyword is only used for members of a @code{radio-button-choice} or
@code{checklist}.  The value should be a list of extra keyword
arguments, which will be used when creating the @code{radio-button} or
@code{checkbox} associated with this item.
@end ignore
@end table

@node Defining New Types
@subsection Defining New Types

In the previous sections we have described how to construct elaborate
type specifications for @code{defcustom}.  In some cases you may want
to give such a type specification a name.  The obvious case is when
you are using the same type for many user options: rather than repeat
the specification for each option, you can give the type specification
a name, and use that name each @code{defcustom}.  The other case is
when a user option's value is a recursive data structure.  To make it
possible for a datatype to refer to itself, it needs to have a name.

Since custom types are implemented as widgets, the way to define a new
customize type is to define a new widget.  We are not going to describe
the widget interface here in details, see @ref{Top, , Introduction,
widget, The Emacs Widget Library}, for that.  Instead we are going to
demonstrate the minimal functionality needed for defining new customize
types by a simple example.

@example
(define-widget 'binary-tree-of-string 'lazy
  "A binary tree made of cons-cells and strings."
  :offset 4
  :tag "Node"
  :type '(choice (string :tag "Leaf" :value "")
                 (cons :tag "Interior"
                       :value ("" . "")
                       binary-tree-of-string
                       binary-tree-of-string)))

(defcustom foo-bar ""
  "Sample variable holding a binary tree of strings."
  :type 'binary-tree-of-string)
@end example

The function to define a new widget is called @code{define-widget}.  The
first argument is the symbol we want to make a new widget type.  The
second argument is a symbol representing an existing widget, the new
widget is going to be defined in terms of difference from the existing
widget.  For the purpose of defining new customization types, the
@code{lazy} widget is perfect, because it accepts a @code{:type} keyword
argument with the same syntax as the keyword argument to
@code{defcustom} with the same name.  The third argument is a
documentation string for the new widget.  You will be able to see that
string with the @kbd{M-x widget-browse @key{RET} binary-tree-of-string
@key{RET}} command.

After these mandatory arguments follow the keyword arguments.  The most
important is @code{:type}, which describes the data type we want to match
with this widget.  Here a @code{binary-tree-of-string} is described as
being either a string, or a cons-cell whose car and cdr are themselves
both @code{binary-tree-of-string}.  Note the reference to the widget
type we are currently in the process of defining.  The @code{:tag}
attribute is a string to name the widget in the user interface, and the
@code{:offset} argument is there to ensure that child nodes are
indented four spaces relative to the parent node, making the tree
structure apparent in the customization buffer.

The @code{defcustom} shows how the new widget can be used as an ordinary
customization type.

The reason for the name @code{lazy} is that the other composite
widgets convert their inferior widgets to internal form when the
widget is instantiated in a buffer.  This conversion is recursive, so
the inferior widgets will convert @emph{their} inferior widgets.  If
the data structure is itself recursive, this conversion is an infinite
recursion.  The @code{lazy} widget prevents the recursion: it convert
its @code{:type} argument only when needed.

@node Applying Customizations
@section Applying Customizations

The following functions are responsible for installing the user's
customization settings for variables and faces, respectively.  When
the user invokes @samp{Save for future sessions} in the Customize
interface, that takes effect by writing a @code{custom-set-variables}
and/or a @code{custom-set-faces} form into the custom file, to be
evaluated the next time Emacs starts.

@defun custom-set-variables &rest args
This function installs the variable customizations specified by
@var{args}.  Each argument in @var{args} should have the form

@example
(@var{var} @var{expression} [@var{now} [@var{request} [@var{comment}]]])
@end example

@noindent
@var{var} is a variable name (a symbol), and @var{expression} is an
expression which evaluates to the desired customized value.

If the @code{defcustom} form for @var{var} has been evaluated prior to
this @code{custom-set-variables} call, @var{expression} is immediately
evaluated, and the variable's value is set to the result.  Otherwise,
@var{expression} is stored into the variable's @code{saved-value}
property, to be evaluated when the relevant @code{defcustom} is called
(usually when the library defining that variable is loaded into
Emacs).

The @var{now}, @var{request}, and @var{comment} entries are for
internal use only, and may be omitted.  @var{now}, if non-@code{nil},
means to set the variable's value now, even if the variable's
@code{defcustom} form has not been evaluated.  @var{request} is a list
of features to be loaded immediately (@pxref{Named Features}).
@var{comment} is a string describing the customization.
@end defun

@defun custom-set-faces &rest args
This function installs the face customizations specified by
@var{args}.  Each argument in @var{args} should have the form

@example
(@var{face} @var{spec} [@var{now} [@var{comment}]])
@end example

@noindent
@var{face} is a face name (a symbol), and @var{spec} is the customized
face specification for that face (@pxref{Defining Faces}).

The @var{now} and @var{comment} entries are for internal use only, and
may be omitted.  @var{now}, if non-@code{nil}, means to install the
face specification now, even if the @code{defface} form has not been
evaluated.  @var{comment} is a string describing the customization.
@end defun

@node Custom Themes
@section Custom Themes

  @dfn{Custom themes} are collections of settings that can be enabled
or disabled as a unit.  @xref{Custom Themes,,, emacs, The GNU Emacs
Manual}.  Each Custom theme is defined by an Emacs Lisp source file,
which should follow the conventions described in this section.
(Instead of writing a Custom theme by hand, you can also create one
using a Customize-like interface; @pxref{Creating Custom Themes,,,
emacs, The GNU Emacs Manual}.)

  A Custom theme file should be named @file{@var{foo}-theme.el}, where
@var{foo} is the theme name.  The first Lisp form in the file should
be a call to @code{deftheme}, and the last form should be a call to
@code{provide-theme}.

@defmac deftheme theme &optional doc
This macro declares @var{theme} (a symbol) as the name of a Custom
theme.  The optional argument @var{doc} should be a string describing
the theme; this is the description shown when the user invokes the
@code{describe-theme} command or types @kbd{?} in the @samp{*Custom
Themes*} buffer.

Two special theme names are disallowed (using them causes an error):
@code{user} is a ``dummy'' theme that stores the user's direct
customization settings, and @code{changed} is a ``dummy'' theme that
stores changes made outside of the Customize system.
@end defmac

@defmac provide-theme theme
This macro declares that the theme named @var{theme} has been fully
specified.
@end defmac

  In between @code{deftheme} and @code{provide-theme} are Lisp forms
specifying the theme settings: usually a call to
@code{custom-theme-set-variables} and/or a call to
@code{custom-theme-set-faces}.

@defun custom-theme-set-variables theme &rest args
This function specifies the Custom theme @var{theme}'s variable
settings.  @var{theme} should be a symbol.  Each argument in
@var{args} should be a list of the form

@example
(@var{var} @var{expression} [@var{now} [@var{request} [@var{comment}]]])
@end example

@noindent
where the list entries have the same meanings as in
@code{custom-set-variables}.  @xref{Applying Customizations}.
@end defun

@defun custom-theme-set-faces theme &rest args
This function specifies the Custom theme @var{theme}'s face settings.
@var{theme} should be a symbol.  Each argument in @var{args} should be
a list of the form

@example
(@var{face} @var{spec} [@var{now} [@var{comment}]])
@end example

@noindent
where the list entries have the same meanings as in
@code{custom-set-faces}.  @xref{Applying Customizations}.
@end defun

  In theory, a theme file can also contain other Lisp forms, which
would be evaluated when loading the theme, but that is ``bad form''.
To protect against loading themes containing malicious code, Emacs
displays the source file and asks for confirmation from the user
before loading any non-built-in theme for the first time.

  The following functions are useful for programmatically enabling and
disabling themes:

@defun custom-theme-p theme
This function return a non-@code{nil} value if @var{theme} (a symbol)
is the name of a Custom theme (i.e., a Custom theme which has been
loaded into Emacs, whether or not the theme is enabled).  Otherwise,
it returns @code{nil}.
@end defun

@deffn Command load-theme theme &optional no-confirm no-enable
This function loads the Custom theme named @var{theme} from its source
file, looking for the source file in the directories specified by the
variable @code{custom-theme-load-path}.  @xref{Custom Themes,,, emacs,
The GNU Emacs Manual}.  It also @dfn{enables} the theme (unless the
optional argument @var{no-enable} is non-@code{nil}), causing its
variable and face settings to take effect.  It prompts the user for
confirmation before loading the theme, unless the optional argument
@var{no-confirm} is non-@code{nil}.
@end deffn

@deffn Command enable-theme theme
This function enables the Custom theme named @var{theme}.  It signals
an error if no such theme has been loaded.
@end deffn

@deffn Command disable-theme theme
This function disables the Custom theme named @var{theme}.  The theme
remains loaded, so that a subsequent call to @code{enable-theme} will
re-enable it.
@end deffn