aboutsummaryrefslogtreecommitdiffstats
path: root/lib/stdlib/doc/src/gen_statem.xml
blob: f66f399137a6e9c02d926cb2c615a64b32ed59e1 (plain) (blame)
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
<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE erlref SYSTEM "erlref.dtd">

<erlref>
  <header>
    <copyright>
      <year>2016</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
      You may obtain a copy of the License at

          http://www.apache.org/licenses/LICENSE-2.0

      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
      See the License for the specific language governing permissions and
      limitations under the License.

    </legalnotice>

    <title>gen_statem</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
  </header>
  <module>gen_statem</module>
  <modulesummary>Generic State Machine Behaviour</modulesummary>
  <description>
    <p>A behaviour module for implementing a state machine.
      Two callback modes are supported.  One for a finite state
      machine like <seealso marker="gen_fsm">gen_fsm</seealso>
      that require the state to be an atom and use that state as
      the name of the callback function for a particular state,
      and one without restriction on the state that use the same
      callback function for all states.
    </p>
    <p>
      A generic state machine process (gen_statem) implemented using
      this module will have a standard set of interface functions
      and include functionality for tracing and error reporting.
      It will also fit into an OTP supervision tree. Refer to
      <seealso marker="doc/design_principles:gen_server_concepts">
        OTP Design Principles</seealso> for more information.
    </p>
    <p>A gen_statem assumes all specific parts to be located in a
      callback module exporting a pre-defined set of functions.
      The relationship between the behaviour functions and the callback
      functions can be illustrated as follows:</p>
    <pre>
gen_statem module            Callback module
-----------------            ---------------
gen_statem:start
gen_statem:start_link -----> Module:init/1

gen_statem:stop       -----> Module:terminate/2

gen_statem:call
gen_statem:cast
erlang:send
erlang:'!'            -----> Module:StateName/5
                             Module:handle_event/5

-                     -----> Module:terminate/3

-                     -----> Module:code_change/3</pre>
    <p>Events are of different
      <seealso marker="#type-event_type">types</seealso>
      so the callback functions can know the origin of an event
      and how to respond.
    </p>
    <p>If a callback function fails or returns a bad value,
      the gen_statem will terminate.  An exception of class
      <seealso marker="erts:erlang#throw/1"><c>throw</c></seealso>,
      however, is not regarded as an error but as a valid return.
    </p>
    <marker id="state_function" />
    <p>The "<em>state function</em>" for a specific
      <seealso marker="#type-state">state</seealso>
      in a gen_statem is the callback function that is called
      for all events in this state, and is selected depending on
      <seealso marker="#type-callback_mode">callback_mode</seealso>
      that the implementation selects during gen_statem init.
    </p>
    <p>When
      <seealso marker="#type-callback_mode">callback_mode</seealso>
      is <c>state_functions</c> the state has to be an atom and
      is used as the state function name.
      See
      <seealso marker="#Module:StateName/5">
        <c>Module:StateName/5</c>
      </seealso>.
      This naturally collects all code for a specific state
      in one function and hence dispatches on state first.
    </p>
    <p>When
      <seealso marker="#type-callback_mode">callback_mode</seealso>
      is <c>handle_event_function</c> the state can be any term
      and the state function name is
      <seealso marker="#Module:handle_event/5">
        <c>Module:handle_event/5</c>
      </seealso>.
      This makes it easy to dispatch on state or on event as
      you like but you will have to implement it.
      Also be careful about which events you handle in which
      states so you do not accidentally postpone one event
      forever creating an infinite busy loop.
    </p>
    <p>Any state name or any state value (depending on
      <seealso marker="#type-callback_mode">callback_mode</seealso>)
      is permitted with a small gotcha regarding the state
      <c>undefined</c> that is used as the previous state when
      the first gen_statem state function is called.
      You might need to know about this faked state if you
      inspect the previous state argument in your state functions.
    </p>
    <p>The gen_statem enqueues incoming events in order of arrival
      and presents these to the
      <seealso marker="#state_function">state function</seealso>
      in that order.  The state function can postpone an event
      so it is not retried in the current state.
      After a state change all enqueued events (including postponed)
      are again presented to the state function.
    </p>
    <p>The gen_statem event queue model is sufficient to emulate
      the normal process message queue and selective receive
      with postponing an event corresponding to not matching
      it in a receive statement and changing states corresponding
      to entering a new receive statement.
    </p>
    <p>The
      <seealso marker="#state_function">state function</seealso>
      can insert events using the
      <seealso marker="#type-state_operation">
	<c>state_operation()</c> <c>next_event</c>
      </seealso>
      and such an event is inserted as the next to present
      to the state function.  That is: as if it is
      the oldest incoming event.  There is a dedicated
      <seealso marker="#type-event_type">
        <c>event_type()</c> <c>internal</c>
      </seealso>
      that can be used for such events making it impossible
      to mistake for an external event.
    </p>
    <p>Inserting an event replaces the trick of calling your own
      state handling functions that you often would have to
      resort to in e.g <seealso marker="gen_fsm">gen_fsm</seealso>
      to force processing a faked event before others.
      If you for example in gen_statem postpone an event
      in one state and then call some other state function of yours,
      you have not changed states and hence the postponed event
      will not be retried, which is logical but might be confusing.
    </p>
    <p>A gen_statem handles system messages as documented in
       <seealso marker="sys">sys</seealso>.
       The <seealso marker="sys">sys</seealso> module
       can be used for debugging a gen_statem.
    </p>
    <p>Note that a gen_statem does not trap exit signals automatically,
      this must be explicitly initiated by the callback module.
    </p>
    <p>Unless otherwise stated, all functions in this module fail if
      the specified gen_statem does not exist or if bad arguments are given.
    </p>
    <p>The gen_statem process can go into hibernation (see
      <seealso marker="erts:erlang#hibernate/3">
	<c>erlang:hibernate/3</c>
      </seealso>) if a
      <seealso marker="#state_function">state function</seealso> or
      <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>
      specifies <c>'hibernate'</c> in the returned
      <seealso marker="#type-state_op"><c>StateOps</c></seealso> list.
      This might be useful if the server is expected to be idle
      for a long time. However use this feature with care
      since hibernation implies at least two garbage collections
      (when hibernating and shortly after waking up) and that is not
      something you'd want to do between each event on a busy server.
    </p>
  </description>

  <datatypes>
    <datatype>
      <name name="server_name" />
      <desc>
	<p>Name specification to use when starting a gen_statem server.
	  See <seealso marker="#start_link/3">
	    <c>start_link/3</c>
	  </seealso> and
	  <seealso marker="#type-server_ref">
	    <c>server_ref()</c>
	  </seealso> below.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="server_ref" />
      <desc>
	<p>Server specification to use when addressing a gen_statem server.
	  See <seealso marker="#call/2">call/2</seealso> and
	  <seealso marker="#type-server_name">
	    <c>server_name()</c>
	  </seealso> above.
	</p>
	<p>It can be:</p>
	<list type="bulleted">
          <item>the <c>pid()</c>,</item>
          <item><c><anno>Name</anno></c>,
	    if the gen_statem is locally registered,
	  </item>
          <item><c>{<anno>Name</anno>,<anno>Node</anno>}</c>,
	    if the gen_statem is locally registered at another node, or
	  </item>
          <item><c>{global,<anno>GlobalName</anno>}</c>,
	    if the gen_statem is globally registered.
	  </item>
          <item><c>{via,<anno>RegMod</anno>,<anno>ViaName</anno>}</c>,
	    if the gen_statem is registered through
	    an alternative process registry.
	    The registry callback module <c>RegMod</c>
	    should export the functions
	    <c>register_name/2</c>, <c>unregister_name/1</c>,
	    <c>whereis_name/1</c> and <c>send/2</c>,
	    which should behave like the corresponding functions
	    in <seealso marker="kernel:global"><c>global</c></seealso>.
	    Thus, <c>{via,global,GlobalName}</c> is the same as
	    <c>{global,GlobalName}</c>.
	  </item>
        </list>
      </desc>
    </datatype>
    <datatype>
      <name name="debug_opt" />
      <desc>
	<p>Debug option that can be used when starting
	  a gen_statem server through for example
	  <seealso marker="#enter_loop/4">enter_loop/4</seealso>.
	</p>
	<p>For every entry in <c><anno>Dbgs</anno></c>
	  the corresponding function in
	  <seealso marker="sys"><c>sys</c></seealso> will be called.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="start_opt" />
      <desc>
	<p>Options that can be used when starting
	  a gen_statem server through for example
	  <seealso marker="#start_link/3">start_link/3</seealso>.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="start_ret" />
      <desc>
	<p>Return value from the start functions for_example
	  <seealso marker="#start_link/3">start_link/3</seealso>.
	</p>
      </desc>
    </datatype>

    <datatype>
      <name name="client" />
      <desc>
	<p>Client address to use when replying through for example the
	  <seealso marker="#type-state_op">state_op()</seealso>
	  <c>{reply,Client,Reply}</c> to a client
	  that has called the gen_statem server using
	  <seealso marker="#call/2">call/2</seealso>.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="state" />
      <desc>
	<p>After a state change (<c>NewState =/= State</c>)
	  all postponed events are retried.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="state_name" />
      <desc>
	<p>If
	  <seealso marker="#type-callback_mode">
	    callback_mode
	  </seealso> is <c>state_functions</c>, which is the default,
	  the state has to be of this type i.e an <c>atom()</c>.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="state_data" />
      <desc>
	<p>A <c>term()</c> in which the state machine implementation
	  should store any state data it needs.  The difference between
	  this data and the
	  <seealso marker="#type-state">state()</seealso>
	  itself is that a change in this data does not cause
	  postponed events to be retried.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="event_type" />
      <desc>
	<p>External events are of 3 different type:
	  <c>{call,<anno>Client</anno>}</c>, <c>cast</c> or <c>info</c>.
	  Calls (synchronous) and casts (asynchronous)
	  originate from the corresponding API functions.
	  For calls the event contain whom to reply to.
	  Type <c>info</c> originates from normal messages sent
	  to the gen_statem process.
	  It is also possible for the state machine
	  implementation to insert events to itself,
	  in particular of types
	  <c>timeout</c> and <c>internal</c>.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="event_predicate" />
      <desc>
	<p>A <c>fun()</c> of arity 2 that takes an event
	  and returns a boolean.
	  When used in <c>{remove_event,RemoveEventPredicate}</c>
	  from <seealso marker="#type-state_op">state_op()</seealso>.
	  The event for which the predicate returns <c>true</c> will
	  be removed.
	</p>
	<p>
	  The predicate may <em>not</em> use a throw exception
	  to return its result.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="init_option" />
      <desc>
	<p>Option that only is valid when initializing the gen_statem
	  that is it can be returned from
	  <seealso marker="#Module:init/1">Module:init/1</seealso>
	  or given to
	  <seealso marker="#enter_loop/5">enter_loop/5,6</seealso>.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="callback_mode" />
      <desc>
	<taglist>
	  <tag><c>state_functions</c></tag>
	  <item>The state has to be of type
	    <seealso marker="#type-state_name"><c>state_name()</c></seealso>
	    and one callback function per state that is
	    <seealso marker="#Module:StateName/5">
	      <c>Module:StateName/5</c>
	    </seealso> is used.  This is the default.
	  </item>
	  <tag><c>handle_event_function</c></tag>
	  <item>The state can be any term and the callback function
	    <seealso marker="#Module:handle_event/5">
	      <c>Module:handle_event/5</c>
	    </seealso> is used for all states.
	  </item>
	</taglist>
      </desc>
    </datatype>
    <datatype>
      <name name="state_op" />
      <desc>
	<p>Either a
	  <seealso marker="#type-state_option">
	    <c>state_option()</c>
	  </seealso> of which the last occurence
	  in the containing list takes precedence, or a
	  <seealso marker="#type-state_operation">
	    <c>state_operation()</c>
	  </seealso> that are performed in order of
	  the containing list.
	</p>
	<p>These may be returned from the
	  <seealso marker="#state_function">state function</seealso>,
	  from <seealso marker="#Module:init/1">Module:init/1</seealso>
	  or given to
	  <seealso marker="#enter_loop/5">enter_loop/5,6</seealso>.
	</p>
	<p>The processing order for a state change is:</p>
	<list type="ordered">
	  <item>If the option <c>postpone</c> is <c>true</c>
	    the current event is postponed.
	  </item>
	  <item>If the state changes the queue of incoming events
	    is reset to start with the oldest postponed.
	  </item>
	  <item>All operations are processed in order of appearance.
	  </item>
	  <item>The <c>timeout</c> option is processed if present,
	    so a state timer may be started or a timeout zero event
	    may be enqueued as the newest incoming.
	  </item>
	  <item>The (possibly new)
	    <seealso marker="#state_function">state function</seealso>
	    is called with the oldest enqueued event if there is any,
	    otherwise the gen_statem goes into <c>receive</c>
	    or hibernation (if the option <c>hibernate</c> is <c>true</c>)
	    to wait for the next message.  In hibernation the next
	    non-system event awakens the gen_statem, or rather
	    the next incoming message awakens the gen_statem but if it
	    is a system event it goes back into hibernation.
	  </item>
	</list>
      </desc>
    </datatype>
    <datatype>
      <name name="state_option" />
      <desc>
	<p>If multiple state options of the same type are present
	  in the containing list these are set in the list order
	  and the last value is kept.
	</p>
	<taglist>
	  <tag><c>postpone</c></tag>
	  <tag><c>{postpone,<anno>Postpone</anno>}</c></tag>
	  <item>If <c><anno>Postpone</anno> =:= true</c>
	    or plain <c>postpone</c> postpone the current event
	    to be retried after a state change.
	    This option is ignored when returned from
	    <seealso marker="#Module:init/1">Module:init/1</seealso>
	    or given to
	    <seealso marker="#enter_loop/5">enter_loop/5,6</seealso>
	    since there is no event to postpone in those cases.
	  </item>
	  <tag><c>hibernate</c></tag>
	  <tag><c>{hibernate,<anno>Hibernate</anno>}</c></tag>
	  <item>If <c><anno>Hibernate</anno> =:= true</c>
	    or plain <c>hibernate</c> hibernate the gen_statem by calling
	    <seealso marker="proc_lib#hibernate/3">
	      <c>proc_lib:hibernate/3</c>
	    </seealso> before <c>receive</c> to wait for a new event.
	    If there are enqueued events the <c>hibernate</c> operation
	    is ignored as if an event just arrived and awakened
	    the gen_statem.
	  </item>
	  <tag>
	    <c>{timeout,<anno>Time</anno>,<anno>Msg</anno>}</c>
	  </tag>
	  <item>Generate an event of
	    <seealso marker="#type-event_type">type <c>timeout</c></seealso>
	    after <c><anno>Time</anno></c> milliseconds unless some other
	    event is received before that time.  Note that a retried
	    event counts just like a new in this respect.
	    If <c>Time =:= infinity</c> or <c>Time =:= 0</c>
	    no timer is started but for zero time the timeout
	    event is immediately enqueued as the newest received.
	    Also note that it is not possible nor needed
	    to cancel this timeout using the
	    <seealso marker="#type-state_operation">
	      <c>state_operation()</c>
	    </seealso> <c>cancel_timer</c>.
	    This timeout is cancelled automatically by any event.
	  </item>
	</taglist>
      </desc>
    </datatype>
    <datatype>
      <name name="state_operation" />
      <desc>
	<p>The state operations are executed in the containing
	 list order.  This matters for <c>next_event</c> where
	 the last one in the list will become the next event to present
	 to the state functions.  Regarding the other operations
	 it is only for <c>remove_event</c> with
	 <c><anno>EventPredicate</anno></c>
	 and for <c>reply_operation()</c> that the order may matter.
	</p>
	<taglist>
	  <tag>
	    <c>
	      {next_event,<anno>EventType</anno>,<anno>EventContent</anno>}
	    </c>
	  </tag>
	  <item>Insert the given event as the next to process.
	    An event of type
	    <seealso marker="#type-event_type">
	      <c>internal</c>
	    </seealso> should be used when you want to reliably distinguish
	    an event inserted this way from any external event.
	  </item>
	  <tag>
	    <c>
	      {remove_event,<anno>EventType</anno>,<anno>EventContent</anno>}
	    </c>
	  </tag>
	  <item>Remove the oldest queued event
	    that matches equal to the given event.
	  </item>
	  <tag>
	    <c>
	      {remove_event,<anno>EventPredicate</anno>}
	    </c>
	  </tag>
	  <item>Remove the oldest queued event for which
	  the <c><anno>EventPredicate</anno></c> returns <c>true</c>.
	  </item>
	  <tag><c>{cancel_timer,<anno>TimerRef</anno>}</c></tag>
	  <item>Uses <c><anno>TimerRef</anno></c> when calling
	    <seealso marker="erts:erlang#cancel_timer/2">
	      <c>erlang:cancel_timer/2</c>
	    </seealso> to cancel a timer, cleans the gen_statem's
	    message queue from any late timeout message from
	    the timer, and removes any late timeout message
	    from the queued events using
	    <c>{remove_event,<anno>EventPredicate</anno>}</c> above.
	    This is a convenience function that saves quite some
	    lines of code and testing time over doing it from
	    the primitives mentioned above.
	  </item>
	  <tag><c>{demonitor,<anno>MonitorRef</anno>}</c></tag>
	  <item>Like <c>{cancel_timer,_}</c> above but for
	    <seealso marker="erts:erlang#demonitor/2">
	      <c>demonitor/2</c>
	    </seealso>.
	  </item>
	  <tag><c>{unlink,<anno>Id</anno>}</c></tag>
	  <item>Like <c>{cancel_timer,_}</c> above but for
	    <seealso marker="erts:erlang#unlink/1">
	      <c>unlink/1</c>
	    </seealso>.
	  </item>
	</taglist>
      </desc>
    </datatype>
    <datatype>
      <name name="reply_operation" />
      <desc>
	<taglist>
	  <tag>
	   <c>{reply,<anno>Client</anno>,<anno>Reply</anno>}</c>
	  </tag>
	  <item>Reply to a client that called
	    <seealso marker="#call/2"><c>call/2</c></seealso>.
            <c><anno>Client</anno></c> must be the term from the
	    <seealso marker="#type-event_type">
	      <c>{call,<anno>Client</anno>}</c>
	    </seealso> argument to the
	    <seealso marker="#state_function">state function</seealso>.
	  </item>
	</taglist>
      </desc>
    </datatype>
    <datatype>
      <name name="state_callback_result" />
      <desc>
	<taglist>
	  <tag>
	   <c>{stop,<anno>Reason</anno>}</c>
	  </tag>
	  <item>The same as
	    <c>{stop,<anno>Reason</anno>,[],StateData}</c>
	    but keeps the old <c>StateData</c>.
	  </item>
	  <tag>
	   <c>{stop,<anno>Reason</anno>,<anno>NewStateData</anno>}</c>
	  </tag>
	  <item>The same as
	    <c>{stop,<anno>Reason</anno>,[],<anno>NewStateData</anno>}</c>
	  </item>
	  <tag><c>{stop,
	    <anno>Reason</anno>,
	    <anno>Replies</anno>,
	    <anno>NewStateData</anno>}</c>
	  </tag>
	  <item>The gen_statem will first send all
	    <c><anno>Replies</anno></c> and then terminate by calling
	    <seealso marker="#Module:terminate/3">
	      <c>Module:terminate/3</c>
	    </seealso> with <c>Reason</c>.
	  </item>
	  <tag>
	    <c>
	      {next_state,<anno>NewState</anno>,<anno>NewStateData</anno>}
	    </c>
	  </tag>
	  <item>The same as
	    <c>
	      {next_state,<anno>NewState</anno>,<anno>NewStateData</anno>,[]}
	    </c>
	  </item>
	  <tag>
	    <c>
	      {next_state,
	       <anno>NewState</anno>,
	       <anno>NewStateData</anno>,
	       <anno>StateOps</anno>}
	    </c>
	  </tag>
	  <item>The gen_statem will do a state transition to
	    <c><anno>NewState</anno></c>
	    (which may be the same as the current state)
	    and execute all <c><anno>StateOps</anno></c>
	  </item>
	  <tag>
	    <c>
	      {keep_state,<anno>NewStateData</anno>}}
	    </c>
	  </tag>
	  <item>The same as
	    <c>
	      {keep_state,<anno>NewStateData</anno>,[]}
	    </c>
	  </item>
	  <tag>
	    <c>
	      {keep_state,<anno>NewStateData</anno>,<anno>StateOps</anno>}
	    </c>
	  </tag>
	  <item>The gen_statem will keep the current state, or
	    do a state transition to the current state if you like,
	    and execute all <c><anno>StateOps</anno></c>
	  </item>
	  <tag>
	    <c>
	      {keep_state_and_data}
	    </c>
	  </tag>
	  <item>The same as
	    <c>
	      {keep_state_and_data,[]}
	    </c>
	  </item>
	  <tag>
	    <c>
	      {keep_state_and_data,<anno>StateOps</anno>}
	    </c>
	  </tag>
	  <item>The gen_statem will keep the current state, or
	    do a state transition to the current state if you like,
	    also keep the current state data,
	    and execute all <c><anno>StateOps</anno></c>
	  </item>
	</taglist>
      </desc>
    </datatype>
  </datatypes>

  <funcs>

    <func>
      <name name="start_link" arity="3" />
      <name name="start_link" arity="4" />
      <fsummary>Create a linked gen_statem process</fsummary>
      <desc>
        <p>Creates a gen_statem process according to OTP design principles
	  (using
          <seealso marker="proc_lib"><c>proc_lib</c></seealso>
	  primitives)
	  that is linked to the calling process.
	  This is essential when the gen_statem shall be part of
	  a supervision tree so it gets linked to its supervisor.
	</p>
        <p>The gen_statem process calls
	  <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>
	  to initialize the server. To ensure a synchronized start-up
	  procedure, <c>start_link/3,4</c> does not return until
	  <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>
          has returned.
	</p>
	<p><c><anno>ServerName</anno></c> specifies the
	  <seealso marker="#type-server_name">
	    <c>server_name()</c>
	  </seealso> to register for the gen_statem.
	  If the gen_statem is started with  <c>start_link/3</c>
	  no <c><anno>ServerName</anno></c> is provided and
	  the gen_statem is not registered.
	</p>
        <p><c><anno>Module</anno></c> is the name of the callback module.</p>
        <p><c><anno>Args</anno></c> is an arbitrary term which is passed as
          the argument to
	  <seealso marker="#Module:init/1"><c>Module:init/1</c>
	  </seealso>.
	</p>
        <p>If the option <c>{timeout,Time}</c> is present in
	  <c><anno>Options</anno></c>, the gen_statem is allowed to spend
	  <c>Time</c> milliseconds initializing or it will be
	  terminated and the start function will return
	  <seealso marker="#type-start_ret">
	    <c>{error,timeout}</c>
	  </seealso>.
	</p>
        <p>If the option
	  <seealso marker="#type-debug_opt"><c>{debug,Dbgs}</c></seealso>
	  is present in <c><anno>Options</anno></c>, debugging through
	  <seealso marker="sys"><c>sys</c></seealso> is activated.
	</p>
        <p>If the option <c>{spawn_opt,SOpts}</c> is present in
	  <c><anno>Options</anno></c>, <c>SOpts</c> will be passed
	  as option list to the <c>spawn_opt</c> BIF
	  which is used to
          <seealso marker="erts:erlang#spawn_opt/2">spawn</seealso>
	  the gen_statem.
	</p>
        <note>
          <p>Using the spawn option <c>monitor</c> is currently not
            allowed, but will cause this function to fail with reason
            <c>badarg</c>.</p>
        </note>
        <p>If the gen_statem is successfully created and initialized
          this function returns
	  <seealso marker="#type-start_ret">
	    <c>{ok,Pid}</c>,
	  </seealso> where <c>Pid</c> is the <c>pid()</c> of the gen_statem.
	  If there already exists a process with the specified
	  <c><anno>ServerName</anno></c> this function returns
	  <seealso marker="#type-start_ret">
            <c>{error,{already_started,Pid}}</c>
	  </seealso>, where <c>Pid</c> is the <c>pid()</c> of that process.
	</p>
        <p>If <c>Module:init/1</c> fails with <c>Reason</c>,
          this function returns
	  <seealso marker="#type-start_ret">
	    <c>{error,Reason}</c>
	  </seealso>. If <c>Module:init/1</c> returns
	  <seealso marker="#type-start_ret">
	    <c>{stop,Reason}</c>
	  </seealso>
	  or
	  <seealso marker="#type-start_ret">
            <c>ignore</c>
	  </seealso>, the process is terminated and this function
          returns
	  <seealso marker="#type-start_ret">
	    <c>{error,Reason}</c>
	  </seealso> or
	  <seealso marker="#type-start_ret">
	  <c>ignore</c>
	  </seealso>, respectively.
	</p>
      </desc>
    </func>


    <func>
      <name name="start" arity="3" />
      <name name="start" arity="4" />
      <fsummary>Create a stand-alone gen_statem process</fsummary>
      <desc>
        <p>Creates a stand-alone gen_statem process according to
	  OTP design principles (using
          <seealso marker="proc_lib"><c>proc_lib</c></seealso>
	  primitives).
	  Since it does not get linked to the calling process
	  this start function can not be used by a supervisor
	  to start a child.
	</p>
        <p>See <seealso marker="#start_link/3">start_link/3,4</seealso>
          for a description of arguments and return values.
	</p>
      </desc>
    </func>

    <func>
      <name name="stop" arity="1" />
      <fsummary>Synchronously stop a generic server</fsummary>
      <desc>
	<p>The same as
	  <seealso marker="#stop/3">
	    <c>stop(<anno>ServerRef</anno>, normal, infinity)</c>
	  </seealso>.
	</p>
      </desc>
    </func>
    <func>
      <name name="stop" arity="3" />
      <fsummary>Synchronously stop a generic server</fsummary>
      <desc>
	<p>Orders the gen_statem
	  <seealso marker="#type-server_ref">
	    <c><anno>ServerRef</anno></c>
	  </seealso> to exit with the given <c><anno>Reason</anno></c>
	  and waits for it to terminate.
	  The gen_statem will call
          <seealso marker="#Module:terminate/3">
	    Module:terminate/3
	  </seealso> before exiting.
	</p>
	<p>This function returns <c>ok</c> if the server terminates
	  with the expected reason. Any other reason than <c>normal</c>,
          <c>shutdown</c>, or <c>{shutdown,Term}</c> will cause an
          error report to be issued through
          <seealso marker="kernel:error_logger#format/2">
	    error_logger:format/2
	  </seealso>.
	  The default <c><anno>Reason</anno></c> is <c>normal</c>.
	</p>
	<p><c><anno>Timeout</anno></c> is an integer greater than zero
	  which specifies how many milliseconds to wait for the server to
          terminate, or the atom <c>infinity</c> to wait indefinitely.
	  The default value is <c>infinity</c>.
	  If the server has not terminated within the specified time,
	  a <c>timeout</c> exception is raised.
	</p>
	<p>If the process does not exist, a <c>noproc</c> exception
	  is raised.
	</p>
      </desc>
    </func>

    <func>
      <name name="call" arity="2" />
      <name name="call" arity="3" />
      <fsummary>Make a synchronous call to a gen_statem</fsummary>
      <desc>
        <p>Makes a synchronous call to the gen_statem
	  <seealso marker="#type-server_ref">
	    <c><anno>ServerRef</anno></c>
	  </seealso> by sending a request
	  and waiting until its reply arrives.
	  The gen_statem will call the
	  <seealso marker="#state_function">state function</seealso> with
	  <seealso marker="#type-event_type"><c>event_type()</c></seealso>
	  <c>{call,Client}</c> and event content
	  <c><anno>Request</anno></c>.
	</p>
	<p>A <c><anno>Reply</anno></c> is generated when a
	  <seealso marker="#state_function">state function</seealso>
	  returns with
	  <c>{reply,Client,<anno>Reply</anno>}</c> as one
	  <seealso marker="#type-state_op"><c>state_op()</c></seealso>,
	  and that <c><anno>Reply</anno></c> becomes the return value
	  of this function.
	</p>
        <p><c><anno>Timeout</anno></c> is an integer greater than zero
	  which specifies how many milliseconds to wait for a reply,
	  or the atom <c>infinity</c> to wait indefinitely,
	  which is the default.  If no reply is received within
	  the specified time, the function call fails.
	  <note>
	    <p>To avoid getting a late reply in the caller's
	      inbox this function spawns a proxy process that
	      does the call.  A late reply gets delivered to the
	      dead proxy process hence gets discarded.  This is
	      less efficient than using
	      <c><anno>Timeout</anno> =:= infinity</c>.
	    </p>
	  </note>
	</p>
        <p>The call may fail for example if the gen_statem dies
	  before or during this function call.
	</p>
      </desc>
    </func>

    <func>
      <name name="cast" arity="2" />
      <fsummary>Send an asynchronous event to a gen_statem</fsummary>
      <desc>
	<p>Sends an asynchronous event to the gen_statem
	  <seealso marker="#type-server_ref">
	    <c><anno>ServerRef</anno></c>
	  </seealso> and returns <c>ok</c> immediately,
	  ignoring if the destination node or gen_statem does not exist.
	  The gen_statem will call the
	  <seealso marker="#state_function">state function</seealso> with
	  <seealso marker="#type-event_type"><c>event_type()</c></seealso>
	  <c>cast</c> and event content
	  <c><anno>Msg</anno></c>.
	</p>
      </desc>
    </func>

    <func>
      <name name="reply" arity="1" />
      <name name="reply" arity="2" />
      <fsummary>Send a reply to a client</fsummary>
      <desc>
        <p>This function can be used by a gen_statem to explicitly send
          a reply to a client that called
	  <seealso marker="#call/2"><c>call/2</c></seealso>
          when the reply cannot be defined in
          the return value of the
	  <seealso marker="#state_function">state function</seealso>.
	</p>
        <p><c><anno>Client</anno></c> must be the term from the
	  <seealso marker="#type-event_type">
	    <c>{call,<anno>Client</anno>}</c>
	  </seealso> argument to the
	  <seealso marker="#state_function">state function</seealso>.
	  <c><anno>Client</anno></c> and <c><anno>Reply</anno></c>
	  an also be specified using <c><anno>ReplyOperation</anno></c>.
	</p>
	<note>
	  <p>A reply sent with this function will not be visible
	    in <seealso marker="sys">sys</seealso> debug output.
	  </p>
	</note>
      </desc>
    </func>

    <func>
      <name name="enter_loop" arity="4" />
      <fsummary>Enter the gen_statem receive loop</fsummary>
      <desc>
	<p>The same as
	  <seealso marker="#enter_loop/6"><c>enter_loop/6</c></seealso>
	  except that no
	  <seealso marker="#type-server_name">
	    <c>server_name()</c>
	  </seealso> must have been registered.
	</p>
      </desc>
    </func>
    <func>
      <name name="enter_loop" arity="5" />
      <fsummary>Enter the gen_statem receive loop</fsummary>
      <desc>
	<p>If <c><anno>Server_or_StateOps</anno></c> is a <c>list()</c>
	  the same as
	  <seealso marker="#enter_loop/6"><c>enter_loop/6</c></seealso>
	  except that no
	  <seealso marker="#type-server_name">
	    <c>server_name()</c>
	  </seealso> must have been registered and
	  <c>StateOps = <anno>Server_or_StateOps</anno></c>.
	</p>
	<p>Otherwise the same as
	  <seealso marker="#enter_loop/6"><c>enter_loop/6</c></seealso>
	  with
	  <c>Server = <anno>Server_or_StateOps</anno></c> and
	  <c>StateOps = []</c>.
	</p>
      </desc>
    </func>
    <func>
      <name name="enter_loop" arity="6" />
      <fsummary>Enter the gen_statem receive loop</fsummary>
      <desc>
        <p>Makes an the calling process become a gen_statem. Does not return,
          instead the calling process will enter the gen_statem receive
          loop and become a gen_statem server. The process
          <em>must</em> have been started using one of the start
          functions in
          <seealso marker="proc_lib"><c>proc_lib</c></seealso>.
	  The user is responsible for any initialization of the process,
	  including registering a name for it.
	</p>
        <p>This function is useful when a more complex initialization
          procedure is needed than the gen_statem behaviour provides.
	</p>
        <p><c><anno>Module</anno></c>, <c><anno>Options</anno></c> and
	  <c><anno>Server</anno></c> have the same meanings
	  as when calling
          <seealso marker="#start_link/3">
	    gen_statem:start[_link]/3,4
	  </seealso>.
          However, the
	  <seealso marker="#type-server_name">
	    <c>server_name()</c>
	  </seealso> name must have been registered accordingly
	  <em>before</em> this function is called.</p>
        <p><c><anno>State</anno></c>, <c><anno>StateData</anno></c>
	  and <c><anno>StateOps</anno></c>
	  have the same meanings as in the return value of
          <seealso marker="#Module:init/1">Module:init/1</seealso>.
          Also, the callback module <c><anno>Module</anno></c>
	  does not need to export an <c>init/1</c> function.
	</p>
        <p>Failure: If the calling process was not started by a
          <seealso marker="proc_lib"><c>proc_lib</c></seealso>
	  start function, or if it is not registered
          according to
	  <seealso marker="#type-server_name">
	    <c>server_name()</c>
	  </seealso>.
	</p>
      </desc>
    </func>

  </funcs>



  <section>
    <title>CALLBACK FUNCTIONS</title>
    <p>The following functions should be exported from a
      <c>gen_statem</c> callback module.
    </p>
  </section>
  <funcs>

    <func>
      <name>Module:init(Args) -> Result</name>
      <fsummary>Initialize process and internal state</fsummary>
      <type>
        <v>Args = term()</v>
        <v>Result = {ok,State,StateData}</v>
	<v>&nbsp;| {ok,State,StateData,StateOps}</v>
        <v>&nbsp;| {stop,Reason} | ignore</v>
	<v>State = <seealso marker="#type-state">state()</seealso></v>
	<v>StateData =
	  <seealso marker="#type-state_data">state_data()</seealso>
	</v>
	<v>StateOps =
	  [<seealso marker="#type-state_op">state_op()</seealso>
	   | <seealso marker="#type-init_option">init_option()</seealso>]
	</v>
	<v>Reason = term()</v>
      </type>
      <desc>
	<marker id="Module:init-1" />
        <p>Whenever a gen_statem is started using
          <seealso marker="#start_link/3">gen_statem:start_link/3,4</seealso>
	  or
          <seealso marker="#start/3">gen_statem:start/3,4</seealso>,
          this function is called by the new process to initialize
	  the implementation loop data.
	</p>
        <p><c>Args</c> is the <c>Args</c> argument provided to the start
          function.</p>
        <p>If the initialization is successful, the function should
          return <c>{ok,State,StateData}</c> or
	  <c>{ok,State,StateData,StateOps}</c>.
	  <c>State</c> is the <seealso marker="#type-state">state</seealso>
	  of the gen_statem.
	</p>
	<p>The <seealso marker="#type-state_op"><c>StateOps</c></seealso>
	  are executed when entering the first
	  <seealso marker="#type-state">state</seealso> just as for a
	  <seealso marker="#state_function">state function</seealso>.
	</p>
	<p>This function allows an option to select the callback mode
	  of the gen_statem.  See
	  <seealso marker="#type-init_option">init_option</seealso>.
	  This option is not allowed from the state function(s).
	</p>
        <p>If something goes wrong during the initialization
          the function should return <c>{stop,Reason}</c>
	  or <c>ignore</c>. See
          <seealso marker="#start_link/3">gen_statem:start_link/3,4</seealso>.
	</p>
      </desc>
    </func>

    <func>
      <name>Module:StateName(EventType, EventContent,
        PrevStateName, StateName, StateData) -> Result
      </name>
      <name>Module:handle_event(EventType, EventContent,
        PrevState, State, StateData) -> Result
      </name>
      <fsummary>Handle an event</fsummary>
      <type>
	<v>EventType =
	  <seealso marker="#type-event_type">event_type()</seealso>
	</v>
	<v>EventContent = term()</v>
	<v>PrevStateName =
	  <seealso marker="#type-state_name">state_name()</seealso>
	</v>
	<v>StateName =
	  <seealso marker="#type-state_name">state_name()</seealso>
	</v>
	<v>PrevState = State =
	  <seealso marker="#type-state">state()</seealso>
	</v>
	<v>StateData = NewStateData =
	  <seealso marker="#type-state_data">state_data()</seealso>
	</v>
	<v>Result =
	  <seealso marker="#type-state_callback_result">
	    state_callback_result()
	  </seealso>
	</v>
      </type>
      <desc>
        <p>Whenever a gen_statem receives an event from
          <seealso marker="#call/2">gen_statem:call/2</seealso>,
          <seealso marker="#cast/2">gen_statem:cast/2</seealso> or
	  as a normal process message this function is called.
	  If
	  <seealso marker="#type-callback_mode">callback_mode</seealso>
	  is <c>state_functions</c> then <c>Module:StateName/5</c> is called,
	  and if it is <c>handle_event_function</c>
	  then <c>Module:handle_event/5</c> is called.
	</p>
	<p>If <c>EventType</c> is
	  <seealso marker="#type-event_type"><c>{call,Client}</c></seealso>
	  the client is waiting for a reply.  The reply can be sent
	  from this or from any other
	  <seealso marker="#state_function">state function</seealso>
	  by returning with <c>{reply,Client,Reply}</c> in
	  <seealso marker="#type-state_op">StateOps</seealso>, in
	  <seealso marker="#type-reply_operation">Replies</seealso>
	  or by calling
	  <seealso marker="#reply/2">
	    <c>gen_statem:reply(Client, Reply)</c>
	  </seealso>.
	</p>
        <p><c>StateName</c> is useful in some odd cases for example
	  if you call a common event handling function from your state
	  function then you might want to pass <c>StateName</c>.
	</p>
	<p><c>PrevStateName</c> and <c>PrevState</c> are useful
	  in some odd cases for example when you want to do something
	  only at the first event in a state.
	  Note that when gen_statem enters its first state
	  this is set to <c>undefined</c>.
	</p>
	<p>If this function returns with a new state that
	  does not match equal (<c>=/=</c>) to the current state
	  all postponed events will be retried in the new state.
	</p>
	<p>See <seealso marker="#type-state_op">state_op()</seealso>
	  for options that can be set and operations that can be done
	  by gen_statem after returning from this function.
	</p>
      </desc>
    </func>

    <func>
      <name>Module:terminate(Reason, State, StateData) -> Ignored</name>
      <fsummary>Clean up before termination</fsummary>
      <type>
        <v>Reason = normal | shutdown | {shutdown,term()} | term()</v>
	<v>State = <seealso marker="#type-state">state()</seealso></v>
	<v>StateData =
	  <seealso marker="#type-state_data">
	    state_data()
	  </seealso>
	</v>
	<v>Ignored = term()</v>
      </type>
      <desc>
        <p>This function is called by a gen_statem when it is about to
          terminate. It should be the opposite of
	  <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>
          and do any necessary cleaning up. When it returns,
          the gen_statem terminates with <c>Reason</c>. The return
          value is ignored.</p>
        <p><c>Reason</c> is a term denoting the stop reason and
          <seealso marker="#type-state">State</seealso>
	  is the internal state of the gen_statem.
	</p>
        <p><c>Reason</c> depends on why the gen_statem is terminating.
          If it is because another callback function has returned a
          stop tuple <c>{stop,Reason}</c> in
	  <seealso marker="#type-state_op">StateOps</seealso>,
	  <c>Reason</c> will have the value specified in that tuple.
	  If it is due to a failure, <c>Reason</c> is the error reason.
	</p>
        <p>If the gen_statem is part of a supervision tree and is
          ordered by its supervisor to terminate, this function will be
          called with <c>Reason = shutdown</c> if the following
          conditions apply:</p>
        <list type="bulleted">
          <item>the gen_statem has been set to trap exit signals, and</item>
          <item>the shutdown strategy as defined in the supervisor's
            child specification is an integer timeout value, not
            <c>brutal_kill</c>.
	  </item>
        </list>
        <p>Even if the gen_statem is <em>not</em> part of a supervision tree,
	   this function will be called if it receives an <c>'EXIT'</c>
	   message from its parent. <c>Reason</c> will be the same as in
	   the <c>'EXIT'</c> message.
	</p>
        <p>Otherwise, the gen_statem will be immediately terminated.
	</p>
        <p>Note that for any other reason than <c>normal</c>,
          <c>shutdown</c>, or <c>{shutdown,Term}</c> the gen_statem is
	  assumed to terminate due to an error and
	  an error report is issued using
          <seealso marker="kernel:error_logger#format/2">
	    error_logger:format/2
	  </seealso>.
	</p>
      </desc>
    </func>

    <func>
      <name>Module:code_change(OldVsn, OldState, OldStateData, Extra) ->
        Result
      </name>
      <fsummary>Update the internal state during upgrade/downgrade</fsummary>
      <type>
        <v>OldVsn = Vsn | {down,Vsn}</v>
        <v>&nbsp;&nbsp;Vsn = term()</v>
        <v>OldState = NewState = term()</v>
        <v>Extra = term()</v>
	<v>Result = {ok,{NewState,NewStateData}} | Reason</v>
	<v>OldState = NewState =
	  <seealso marker="#type-state">state()</seealso>
	</v>
	<v>OldStateData = NewStateData =
	  <seealso marker="#type-state_data">state_data()</seealso>
	</v>
	<v>Reason = term()</v>
      </type>
      <desc>
        <p>This function is called by a gen_statem when it should
          update its internal state during a release upgrade/downgrade,
          i.e. when the instruction <c>{update,Module,Change,...}</c>
          where <c>Change={advanced,Extra}</c> is given in
          the <c>appup</c> file. See
          <seealso marker="doc/design_principles:release_handling#instr">
	    OTP Design Principles
	  </seealso>
          for more information.
	</p>
        <p>In the case of an upgrade, <c>OldVsn</c> is <c>Vsn</c>, and
          in the case of a downgrade, <c>OldVsn</c> is
          <c>{down,Vsn}</c>. <c>Vsn</c> is defined by the <c>vsn</c>
          attribute(s) of the old version of the callback module
          <c>Module</c>. If no such attribute is defined, the version
          is the checksum of the BEAM file.
	</p>
        <p><c>OldState</c> and <c>OldStateData</c> is the internal state
	  of the gen_statem.
	</p>
        <p><c>Extra</c> is passed as-is from the <c>{advanced,Extra}</c>
          part of the update instruction.
	</p>
        <p>If successful, the function shall return the updated
          internal state in an
	  <c>{ok,{NewState,NewStateData}}</c> tuple.
	</p>
	<p>If the function returns <c>Reason</c>, the ongoing
	  upgrade will fail and roll back to the old release.</p>
      </desc>
    </func>

    <func>
      <name>Module:format_status(Opt, [PDict,State,StateData]) ->
        Status
      </name>
      <fsummary>Optional function for providing a term describing the
        current gen_statem status</fsummary>
      <type>
        <v>Opt = normal | terminate</v>
        <v>PDict = [{Key, Value}]</v>
	<v>State =
	  <seealso marker="#type-state">state()</seealso>
	</v>
	<v>StateData =
	  <seealso marker="#type-state_data">state_data()</seealso>
	</v>
	<v>Key = term()</v>
	<v>Value = term()</v>
        <v>Status = term()</v>
      </type>
      <desc>
        <note>
          <p>This callback is optional, so callback modules need not
            export it. The gen_statem module provides a default
            implementation of this function that returns the callback
            module state.
	  </p>
        </note>
        <p>This function is called by a gen_statem process when:</p>
        <list type="bulleted">
          <item>One of
            <seealso marker="sys#get_status/1">
	      <c>sys:get_status/1,2</c>
	    </seealso>
            is invoked to get the gen_statem status. <c>Opt</c> is set
            to the atom <c>normal</c> for this case.
	  </item>
          <item>The gen_statem terminates abnormally and logs an error.
	    <c>Opt</c> is set to the atom <c>terminate</c> for this case.
	  </item>
        </list>
        <p>This function is useful for customising the form and
          appearance of the gen_statem status for these cases. A
          callback module wishing to customise the
          <seealso marker="sys#get_status/1">
	    <c>sys:get_status/1,2</c>
	  </seealso> return value as well as how
          its status appears in termination error logs exports an
          instance of <c>format_status/2</c> that returns a term
          describing the current status of the gen_statem.
	</p>
        <p><c>PDict</c> is the current value of the gen_statem's
          process dictionary.
	</p>
        <p><seealso marker="#type-state"><c>State</c></seealso>
	  is the internal state of the gen_statem.
	</p>
        <p>The function should return <c>Status</c>, a term that
          customises the details of the current state and status of
          the gen_statem. There are no restrictions on the
          form <c>Status</c> can take, but for the
          <seealso marker="sys#get_status/1">
	    <c>sys:get_status/1,2</c>
	    </seealso> case (when <c>Opt</c>
          is <c>normal</c>), the recommended form for
          the <c>Status</c> value is <c>[{data, [{"State",
          Term}]}]</c> where <c>Term</c> provides relevant details of
          the gen_statem state. Following this recommendation isn't
          required, but doing so will make the callback module status
          consistent with the rest of the
          <seealso marker="sys#get_status/1">
	    <c>sys:get_status/1,2</c>
	  </seealso> return value.
	</p>
        <p>One use for this function is to return compact alternative
          state representations to avoid having large state terms
          printed in logfiles.
	</p>
      </desc>
    </func>

  </funcs>

  <section>
    <title>SEE ALSO</title>
    <p><seealso marker="gen_event">gen_event</seealso>,
      <seealso marker="gen_fsm">gen_fsm</seealso>,
      <seealso marker="gen_server">gen_server</seealso>,
      <seealso marker="supervisor">supervisor</seealso>,
      <seealso marker="proc_lib">proc_lib</seealso>,
      <seealso marker="sys">sys</seealso></p>
  </section>
</erlref>