aboutsummaryrefslogtreecommitdiffstats
path: root/lib/stdlib/doc/src/gen_statem.xml
blob: 1aac88c308cdd4428c23cc4c86a10edfde661b4e (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
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
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
2023
2024
2025
2026
2027
2028
2029
2030
2031
2032
2033
2034
2035
2036
2037
2038
2039
2040
2041
2042
2043
2044
2045
2046
2047
2048
2049
2050
2051
2052
2053
2054
2055
2056
2057
2058
2059
2060
2061
2062
2063
2064
2065
2066
2067
2068
2069
2070
2071
2072
2073
2074
2075
2076
2077
2078
2079
2080
2081
2082
2083
2084
2085
2086
2087
2088
2089
2090
2091
2092
2093
2094
2095
2096
2097
2098
2099
2100
2101
2102
2103
2104
2105
2106
2107
2108
2109
2110
2111
2112
2113
2114
2115
2116
2117
2118
2119
2120
2121
2122
2123
2124
2125
2126
2127
2128
2129
2130
2131
2132
2133
2134
2135
2136
2137
2138
2139
2140
2141
2142
2143
2144
2145
2146
2147
2148
2149
2150
2151
2152
2153
2154
2155
2156
2157
2158
2159
2160
2161
2162
2163
2164
2165
2166
2167
2168
2169
2170
2171
2172
2173
2174
2175
2176
2177
2178
2179
<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE erlref SYSTEM "erlref.dtd">

<erlref>
  <header>
    <copyright>
      <year>2016</year><year>2017</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 behavior.</modulesummary>
  <description>
    <p>
      This behavior module provides a state machine. Two
      <seealso marker="#type-callback_mode"><em>callback modes</em></seealso>
      are supported:
    </p>
    <list type="bulleted">
      <item>
        <p>One for finite-state machines
          (<seealso marker="gen_fsm"><c>gen_fsm</c></seealso> like),
          which requires the state to be an atom and uses that state as
          the name of the current callback function
        </p>
      </item>
      <item>
        <p>One without restriction on the state data type
          that uses one callback function for all states
        </p>
      </item>
    </list>
    <note>
      <p>
        This is a new behavior in Erlang/OTP 19.0.
        It has been thoroughly reviewed, is stable enough
        to be used by at least two heavy OTP applications,
	and is here to stay.
        Depending on user feedback, we do not expect
        but can find it necessary to make minor
        not backward compatible changes into Erlang/OTP 20.0.
      </p>
    </note>
    <p>
      The <c>gen_statem</c> behavior replaces 
      <seealso marker="gen_fsm"><c>gen_fsm</c> </seealso> in Erlang/OTP 20.0.
      It has the same features and adds some really useful:
    </p>
    <list type="bulleted">
      <item>Gathered state code.</item>
      <item>Arbitrary term state.</item>
      <item>Event postponing.</item>
      <item>Self-generated events.</item>
      <item>State time-out.</item>
      <item>Multiple generic named time-outs.</item>
      <item>Absolute time-out time.</item>
      <item>Automatic state enter calls.</item>
      <item>Reply from other state than the request.</item>
      <item>Multiple <c>sys</c> traceable replies.</item>
    </list>
    <p>
      The callback model(s) for <c>gen_statem</c> differs from
      the one for <seealso marker="gen_fsm"><c>gen_fsm</c></seealso>,
      but it is still fairly easy to
      <seealso marker="gen_fsm#Migration to gen_statem">
	rewrite from
      </seealso> <c>gen_fsm</c> to <c>gen_statem</c>.
    </p>
    <p>
      A generic state machine process (<c>gen_statem</c>) implemented
      using this module has a standard set of interface functions
      and includes functionality for tracing and error reporting.
      It also fits into an OTP supervision tree. For more information, see
      <seealso marker="doc/design_principles:statem">OTP Design Principles</seealso>.
    </p>
    <p>
      A <c>gen_statem</c> assumes all specific parts to be located in a
      callback module exporting a predefined set of functions.
      The relationship between the behavior functions and the callback
      functions is as follows:</p>
    <pre>
gen_statem module            Callback module
-----------------            ---------------
gen_statem:start
gen_statem:start_link -----> Module:init/1

Server start or code change
                      -----> Module:callback_mode/0

gen_statem:stop       -----> Module:terminate/3

gen_statem:call
gen_statem:cast
erlang:send
erlang:'!'            -----> Module:StateName/3
                             Module:handle_event/4

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

-                     -----> Module:code_change/4</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 <c>gen_statem</c> terminates, unless otherwise stated.
      However, an exception of class
      <seealso marker="erts:erlang#throw/1"><c>throw</c></seealso>
      is not regarded as an error but as a valid return
      from all callback functions.
    </p>
    <marker id="state callback"/>
    <p>
      The "<em>state callback</em>" for a specific
      <seealso marker="#type-state">state</seealso>
      in a <c>gen_statem</c> is the callback function that is called
      for all events in this state. It is selected depending on which
      <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>
      that the callback module defines with the callback function
      <seealso marker="#Module:callback_mode/0"><c>Module:callback_mode/0</c></seealso>.
    </p>
    <p>
      When the
      <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>
      is <c>state_functions</c>, the state must be an atom and
      is used as the state callback name; see
      <seealso marker="#Module:StateName/3"><c>Module:StateName/3</c></seealso>.
      This gathers all code for a specific state
      in one function as the <c>gen_statem</c> engine
      branches depending on state name.
      Note the fact that the callback function
      <seealso marker="#Module:terminate/3"><c>Module:terminate/3</c></seealso>
      makes the state name <c>terminate</c> unusable in this mode.
    </p>
    <p>
      When the
      <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>
      is <c>handle_event_function</c>, the state can be any term
      and the state callback name is
      <seealso marker="#Module:handle_event/4"><c>Module:handle_event/4</c></seealso>.
      This makes it easy to branch depending on state or event as you desire.
      Be careful about which events you handle in which
      states so that you do not accidentally postpone an event
      forever creating an infinite busy loop.
    </p>
    <p>
      The <c>gen_statem</c> enqueues incoming events in order of arrival
      and presents these to the
      <seealso marker="#state callback">state callback</seealso>
      in that order. The state callback can postpone an event
      so it is not retried in the current state.
      After a state change the queue restarts with the postponed events.
    </p>
    <p>
      The <c>gen_statem</c> event queue model is sufficient
      to emulate the normal process message queue with selective receive.
      Postponing an event corresponds to not matching it
      in a receive statement, and changing states corresponds
      to entering a new receive statement.
    </p>
    <p>
      The <seealso marker="#state callback">state callback</seealso>
      can insert events using the
      <seealso marker="#type-action"><c>action()</c></seealso>
      <c>next_event</c>
      and such an event is inserted as the next to present
      to the state callback. That is, as if it is
      the oldest incoming event. A dedicated
      <seealso marker="#type-event_type"><c>event_type()</c></seealso>
      <c>internal</c> can be used for such events making them impossible
      to mistake for external events.
    </p>
    <p>
      Inserting an event replaces the trick of calling your own
      state handling functions that you often would have to
      resort to in, for example,
      <seealso marker="gen_fsm"><c>gen_fsm</c></seealso>
      to force processing an inserted event before others.
    </p>
    <p>
      The <c>gen_statem</c> engine can automatically
      make a specialized call to the
      <seealso marker="#state callback">state callback</seealso>
      whenever a new state is entered; see 
      <seealso marker="#type-state_enter"><c>state_enter()</c></seealso>.
      This is for writing code common to all state entries.
      Another way to do it is to insert events at state transitions,
      but you have to do so everywhere it is needed.
    </p>
    <note>
      <p>If you in <c>gen_statem</c>, for example, postpone
        an event in one state and then call another state callback
        of yours, you have not changed states and hence the postponed event
        is not retried, which is logical but can be confusing.
      </p>
    </note>
    <p>
      For the details of a state transition, see type
      <seealso marker="#type-transition_option"><c>transition_option()</c></seealso>.
    </p>
    <p>
      A <c>gen_statem</c> handles system messages as described in
      <seealso marker="sys"><c>sys</c></seealso>.
      The <c>sys</c> module can be used for debugging a <c>gen_statem</c>.
    </p>
    <p>
      Notice that a <c>gen_statem</c> does not trap exit signals
      automatically, this must be explicitly initiated in
      the callback module (by calling
      <seealso marker="erts:erlang#process_flag/2"><c>process_flag(trap_exit, true)</c></seealso>.
    </p>
    <p>
      Unless otherwise stated, all functions in this module fail if
      the specified <c>gen_statem</c> does not exist or
      if bad arguments are specified.
    </p>
    <p>
      The <c>gen_statem</c> process can go into hibernation; see
      <seealso marker="proc_lib#hibernate/3"><c>proc_lib:hibernate/3</c></seealso>.
      It is done when a
      <seealso marker="#state callback">state callback</seealso> or
      <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>
      specifies <c>hibernate</c> in the returned
      <seealso marker="#type-action"><c>Actions</c></seealso>
      list. This feature can be useful to reclaim process heap memory
      while the server is expected to be idle for a long time.
      However, use this feature with care,
      as hibernation can be too costly
      to use after every event; see
      <seealso marker="erts:erlang#hibernate/3"><c>erlang:hibernate/3</c></seealso>.
    </p>
  </description>

  <section>
    <title>Example</title>
    <p>
      The following example shows a simple pushbutton model
      for a toggling pushbutton implemented with
      <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>
      <c>state_functions</c>.
      You can push the button and it replies if it went on or off,
      and you can ask for a count of how many times it has been
      pushed to switch on.
    </p>
    <p>The following is the complete callback module file
      <c>pushbutton.erl</c>:</p>
    <code type="erl">
-module(pushbutton).
-behaviour(gen_statem).

-export([start/0,push/0,get_count/0,stop/0]).
-export([terminate/3,code_change/4,init/1,callback_mode/0]).
-export([on/3,off/3]).

name() -> pushbutton_statem. % The registered server name

%% API.  This example uses a registered name name()
%% and does not link to the caller.
start() ->
    gen_statem:start({local,name()}, ?MODULE, [], []).
push() ->
    gen_statem:call(name(), push).
get_count() ->
    gen_statem:call(name(), get_count).
stop() ->
    gen_statem:stop(name()).

%% Mandatory callback functions
terminate(_Reason, _State, _Data) ->
    void.
code_change(_Vsn, State, Data, _Extra) ->
    {ok,State,Data}.
init([]) ->
    %% Set the initial state + data.  Data is used only as a counter.
    State = off, Data = 0,
    {ok,State,Data}.
callback_mode() -> state_functions.

%%% state callback(s)

off({call,From}, push, Data) ->
    %% Go to 'on', increment count and reply
    %% that the resulting status is 'on'
    {next_state,on,Data+1,[{reply,From,on}]};
off(EventType, EventContent, Data) ->
    handle_event(EventType, EventContent, Data).

on({call,From}, push, Data) ->
    %% Go to 'off' and reply that the resulting status is 'off'
    {next_state,off,Data,[{reply,From,off}]};
on(EventType, EventContent, Data) ->
    handle_event(EventType, EventContent, Data).

%% Handle events common to all states
handle_event({call,From}, get_count, Data) ->
    %% Reply with the current count
    {keep_state,Data,[{reply,From,Data}]};
handle_event(_, _, Data) ->
    %% Ignore all other events
    {keep_state,Data}.
    </code>
    <p>The following is a shell session when running it:</p>
    <pre>
1> pushbutton:start().
{ok,&lt;0.36.0>}
2> pushbutton:get_count().
0
3> pushbutton:push().
on
4> pushbutton:get_count().
1
5> pushbutton:push().
off
6> pushbutton:get_count().
1
7> pushbutton:stop().
ok
8> pushbutton:push().
** exception exit: {noproc,{gen_statem,call,[pushbutton_statem,push,infinity]}}
     in function  gen:do_for_proc/2 (gen.erl, line 261)
     in call from gen_statem:call/3 (gen_statem.erl, line 386)
    </pre>
    <p>
      To compare styles, here follows the same example using
      <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>
      <c>state_functions</c>, or rather the code to replace
      after function <c>init/1</c> of the <c>pushbutton.erl</c>
      example file above:
    </p>
    <code type="erl">
callback_mode() -> handle_event_function.

%%% state callback(s)

handle_event({call,From}, push, off, Data) ->
    %% Go to 'on', increment count and reply
    %% that the resulting status is 'on'
    {next_state,on,Data+1,[{reply,From,on}]};
handle_event({call,From}, push, on, Data) ->
    %% Go to 'off' and reply that the resulting status is 'off'
    {next_state,off,Data,[{reply,From,off}]};
%%
%% Event handling common to all states
handle_event({call,From}, get_count, State, Data) ->
    %% Reply with the current count
    {next_state,State,Data,[{reply,From,Data}]};
handle_event(_, _, State, Data) ->
    %% Ignore all other events
    {next_state,State,Data}.
    </code>
  </section>

  <datatypes>
    <datatype>
      <name name="server_name"/>
      <desc>
	<p>
	  Name specification to use when starting
	  a <c>gen_statem</c> 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 <c>gen_statem</c> server.
	  See <seealso marker="#call/2"><c>call/2</c></seealso> and
	  <seealso marker="#type-server_name"><c>server_name()</c></seealso>
	  above.
	</p>
	<p>It can be:</p>
	<taglist>
	  <tag><c>pid() | LocalName</c></tag>
	  <item>
            <p>
              The <c>gen_statem</c> is locally registered.
            </p>
          </item>
	  <tag><c>{Name,Node}</c></tag>
	  <item>
            <p>
	      The <c>gen_statem</c> is locally registered
	      on another node.
            </p>
	  </item>
	  <tag><c>{global,GlobalName}</c></tag>
	  <item>
            <p>
	      The <c>gen_statem</c> is globally registered in
	      <seealso marker="kernel:global"><c>global</c></seealso>.
            </p>
	  </item>
	  <tag><c>{via,RegMod,ViaName}</c></tag>
	  <item>
            <p>
	      The <c>gen_statem</c> is registered in
	      an alternative process registry.
	      The registry callback module <c>RegMod</c>
	      is to export functions
	      <c>register_name/2</c>, <c>unregister_name/1</c>,
	      <c>whereis_name/1</c>, and <c>send/2</c>,
	      which are to 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>.
            </p>
	  </item>
	</taglist>
      </desc>
    </datatype>
    <datatype>
      <name name="debug_opt"/>
      <desc>
	<p>
	  Debug option that can be used when starting
	  a <c>gen_statem</c> server through,
	  <seealso marker="#enter_loop/4"><c>enter_loop/4-6</c></seealso>.
	</p>
	<p>
	  For every entry in <c><anno>Dbgs</anno></c>,
	  the corresponding function in
	  <seealso marker="sys"><c>sys</c></seealso> is called.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="hibernate_after_opt"/>
      <desc>
	<p>
          hibernate_after option that can be used when starting
	  a <c>gen_statem</c> server through,
	  <seealso marker="#enter_loop/4"><c>enter_loop/4-6</c></seealso>.
	</p>
    <p>If option<seealso marker="#type-hibernate_after_opt"><c>{hibernate_after,HibernateAfterTimeout}</c></seealso> is present, the <c>gen_statem</c>
        process awaits any message for <c>HibernateAfterTimeout</c> milliseconds and
        if no message is received, the process goes into hibernation automatically
        (by calling <seealso marker="proc_lib#hibernate/3"><c>proc_lib:hibernate/3</c></seealso>).
    </p>
      </desc>
    </datatype>
    <datatype>
      <name name="start_opt"/>
      <desc>
	<p>
	  Options that can be used when starting
	  a <c>gen_statem</c> server through, for example,
	  <seealso marker="#start_link/3"><c>start_link/3</c></seealso>.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="start_ret"/>
      <desc>
	<p>
	  Return value from the start functions, for example,
	  <seealso marker="#start_link/3"><c>start_link/3</c></seealso>.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="from"/>
      <desc>
	<p>
	  Destination to use when replying through, for example, the
	  <seealso marker="#type-action"><c>action()</c></seealso>
	  <c>{reply,From,Reply}</c>
	  to a process that has called the <c>gen_statem</c> server using
	  <seealso marker="#call/2"><c>call/2</c></seealso>.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="state"/>
      <desc>
	<p>
	  If the
	  <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>
	  is <c>handle_event_function</c>,
	  the state can be any term.
	  After a state change (<c>NextState =/= State</c>),
	  all postponed events are retried.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="state_name"/>
      <desc>
	<p>
	  If the
	  <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>
	  is <c>state_functions</c>,
	  the state must be of this type.
	  After a state change (<c>NextState =/= State</c>),
	  all postponed events are retried.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="data"/>
      <desc>
	<p>
	  A term in which the state machine implementation
	  is to store any server data it needs. The difference between
	  this and the <seealso marker="#type-state"><c>state()</c></seealso>
	  itself is that a change in this data does not cause
	  postponed events to be retried. Hence, if a change
	  in this data would change the set of events that
	  are handled, then that data item is to be made
	  a part of the state.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="event_type"/>
      <desc>
	<p>
	  External events are of three types:
	  <c>{call,<anno>From</anno>}</c>, <c>cast</c>, or <c>info</c>.
	  <seealso marker="#call/2">Calls</seealso>
	  (synchronous) and
	  <seealso marker="#cast/2">casts</seealso>
	  originate from the corresponding API functions.
	  For calls, the event contains whom to reply to.
	  Type <c>info</c> originates from regular process messages sent
	  to the <c>gen_statem</c>. The state machine
	  implementation can, in addition to the above,
	  generate
	  <seealso marker="#type-event_type"><c>events of types</c></seealso>
	  <c>timeout</c>, <c>{timeout,<anno>Name</anno>}</c>,
	  <c>state_timeout</c>, and <c>internal</c> to itself.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="callback_mode_result"/>
      <desc>
	<p>
	  This is the return type from
	  <seealso marker="#Module:callback_mode/0"><c>Module:callback_mode/0</c></seealso>
	  and selects 
	  <seealso marker="#type-callback_mode">callback mode</seealso>
	  and whether to do
	  <seealso marker="#type-state_enter">state enter calls</seealso>,
	  or not.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="callback_mode"/>
      <desc>
	<p>
	  The <em>callback mode</em> is selected when starting the
	  <c>gen_statem</c> and after code change
	  using the return value from
	  <seealso marker="#Module:callback_mode/0"><c>Module:callback_mode/0</c></seealso>.
	</p>
	<taglist>
	  <tag><c>state_functions</c></tag>
	  <item>
	    <p>
	      The state must 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/3"><c>Module:StateName/3</c></seealso>,
	      is used.
	    </p>
	  </item>
	  <tag><c>handle_event_function</c></tag>
	  <item>
	    <p>
	      The state can be any term and the callback function
	      <seealso marker="#Module:handle_event/4"><c>Module:handle_event/4</c></seealso>
	      is used for all states.
	    </p>
	  </item>
	</taglist>
      </desc>
    </datatype>
    <datatype>
      <name name="state_enter"/>
      <desc>
	<p>
	  Whether the state machine should use <em>state enter calls</em>
	  or not is selected when starting the  <c>gen_statem</c>
	  and after code change using the return value from
	  <seealso marker="#Module:callback_mode/0"><c>Module:callback_mode/0</c></seealso>.
	</p>
	<p>
	  If 
	  <seealso marker="#Module:callback_mode/0"><c>Module:callback_mode/0</c></seealso>
	  returns a list containing <c>state_enter</c>,
	  the <c>gen_statem</c> engine will, at every state change,
	  call the
	  <seealso marker="#state callback">state callback</seealso>
	  with arguments <c>(enter, OldState, Data)</c>.
	  This may look like an event but is really a call
	  performed after the previous state callback returned
	  and before any event is delivered to the new state callback.
	  See
	  <seealso marker="#Module:StateName/3"><c>Module:StateName/3</c></seealso>
	  and
	  <seealso marker="#Module:handle_event/4"><c>Module:handle_event/4</c></seealso>.
	  Such a call can be repeated by returning a
	  <seealso marker="#type-state_callback_result">
	    <c>repeat_state</c>
	  </seealso>
	  or
	  <seealso marker="#type-state_callback_result">
	    <c>repeat_state_and_data</c>
	  </seealso>
	  tuple from the state callback.
	</p>
	<p>
	  If 
	  <seealso marker="#Module:callback_mode/0"><c>Module:callback_mode/0</c></seealso>
	  does not return such a list, no state enter calls are done.
	</p>
	<p>
	  If 
	  <seealso marker="#Module:code_change/4"><c>Module:code_change/4</c></seealso>
	  should transform the state to a state with a different
	  name it is still regarded as the same state so this
	  does not cause a state enter call.
	</p>
	<p>
	  Note that a state enter call <em>will</em> be done
	  right before entering the initial state even though this
	  formally is not a state change.
	  In this case <c>OldState</c> will be the same as <c>State</c>,
	  which can not happen for a subsequent state change,
	  but will happen when repeating the state enter call.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="transition_option"/>
      <desc>
	<p>
	  Transition options can be set by
	  <seealso marker="#type-action">actions</seealso>
	  and they modify how the state transition is done:
	</p>
	<list type="ordered">
	  <item>
            <p>
	      If the state changes, is the initial state,
	      <seealso marker="#type-state_callback_result">
		<c>repeat_state</c>
	      </seealso>
	      or
	      <seealso marker="#type-state_callback_result">
		<c>repeat_state_and_data</c>
	      </seealso>
	      is used, and also
	      <seealso marker="#type-state_enter"><em>state enter calls</em></seealso>
	      are used, the <c>gen_statem</c> calls
	      the new state callback with arguments
	      <seealso marker="#type-state_enter">(enter, OldState, Data)</seealso>.
	      Any 
	      <seealso marker="#type-enter_action"><c>actions</c></seealso>
	      returned from this call are handled as if they were
	      appended to the actions 
	      returned by the state callback that changed states.
            </p>
	  </item>
	  <item>
	    <p>
	      All
              <seealso marker="#type-action">actions</seealso>
	      are processed in order of appearance.
            </p>
	  </item>
	  <item>
            <p>
	      If
              <seealso marker="#type-postpone"><c>postpone()</c></seealso>
	      is <c>true</c>,
	      the current event is postponed.
	    </p>
	  </item>
	  <item>
	    <p>
	      If the state changes, the queue of incoming events
              is reset to start with the oldest postponed.
	    </p>
	  </item>
	  <item>
            <p>
	      All events stored with
	      <seealso marker="#type-action"><c>action()</c></seealso>
	      <c>next_event</c>
	      are inserted to be processed before the other queued events.
            </p>
	  </item>
	  <item>
	    <p>
	      Time-out timers
              <seealso marker="#type-event_timeout"><c>event_timeout()</c></seealso>,
              <seealso marker="#type-generic_timeout"><c>generic_timeout()</c></seealso>
	      and 
              <seealso marker="#type-state_timeout"><c>state_timeout()</c></seealso>
	      are handled.  Time-outs with zero time are guaranteed to be
	      delivered to the state machine before any external
	      not yet received event so if there is such a time-out requested,
	      the corresponding time-out zero event is enqueued as
	      the newest event.
	    </p>
	    <p>
	      Any event cancels an
              <seealso marker="#type-event_timeout"><c>event_timeout()</c></seealso>
	      so a zero time event time-out is only generated
	      if the event queue is empty.
	    </p>
	    <p>
	      A state change cancels a
              <seealso marker="#type-state_timeout"><c>state_timeout()</c></seealso>
	      and any new transition option of this type
	      belongs to the new state.
	    </p>
	  </item>
	  <item>
	    <p>
	      If there are enqueued events the
	      <seealso marker="#state callback">state callback</seealso>
	      for the possibly new state
	      is called with the oldest enqueued event,
	      and we start again from the top of this list.
	    </p>
	  </item>
	  <item>
	    <p>
	      Otherwise the <c>gen_statem</c> goes into <c>receive</c>
	      or hibernation
	      (if
	      <seealso marker="#type-hibernate"><c>hibernate()</c></seealso>
	      is <c>true</c>)
	      to wait for the next message. In hibernation the next
	      non-system event awakens the <c>gen_statem</c>, or rather
	      the next incoming message awakens the <c>gen_statem</c>,
	      but if it is a system event it goes right back into hibernation.
	      When a new message arrives the 
	      <seealso marker="#state callback">state callback</seealso>
	      is called with the corresponding event,
	      and we start again from the top of this list.
	    </p>
	  </item>
	</list>
      </desc>
    </datatype>
    <datatype>
      <name name="postpone"/>
      <desc>
	<p>
	  If <c>true</c>, postpones the current event and retries
	  it when the state changes
	  (<c>NextState =/= State</c>).
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="hibernate"/>
      <desc>
	<p>
	  If <c>true</c>, hibernates the <c>gen_statem</c>
	  by calling
	  <seealso marker="proc_lib#hibernate/3"><c>proc_lib:hibernate/3</c></seealso>
	  before going into <c>receive</c>
	  to wait for a new external event.
	  If there are enqueued events,
	  to prevent receiving any new event, an
	  <seealso marker="erts:erlang#garbage_collect/0"><c>erlang:garbage_collect/0</c></seealso>
	  is done instead to simulate
	  that the <c>gen_statem</c> entered hibernation
	  and immediately got awakened by the oldest enqueued event.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="event_timeout"/>
      <desc>
	<p>
	  Starts a timer set by
	  <seealso marker="#type-enter_action"><c>enter_action()</c></seealso>
	  <c>timeout</c>.
	  When the timer expires an event of
	  <seealso marker="#type-event_type"><c>event_type()</c></seealso>
	  <c>timeout</c> will be generated.
	  See
	  <seealso marker="erts:erlang#start_timer/4"><c>erlang:start_timer/4</c></seealso>
	  for how <c>Time</c> and
	  <seealso marker="#type-timeout_option"><c>Options</c></seealso>
	  are interpreted.  Future <c>erlang:start_timer/4</c> <c>Options</c>
	  will not necessarily be supported.
	</p>
	<p>
	  Any event that arrives cancels this time-out.
	  Note that a retried or inserted event counts as arrived.
	  So does a state time-out zero event, if it was generated
	  before this time-out is requested.
	</p>
	<p>
	  If <c>Time</c> is <c>infinity</c>,
	  no timer is started, as it never would expire anyway.
	</p>
	<p>
	  If <c>Time</c> is relative and <c>0</c>
	  no timer is actually started,
	  instead the the time-out event is enqueued to ensure
	  that it gets processed before any not yet
	  received external event.
	</p>
	<p>
	  Note that it is not possible nor needed to cancel this time-out,
	  as it is cancelled automatically by any other event.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="generic_timeout"/>
      <desc>
	<p>
	  Starts a timer set by
	  <seealso marker="#type-enter_action"><c>enter_action()</c></seealso>
	  <c>{timeout,Name}</c>.
	  When the timer expires an event of
	  <seealso marker="#type-event_type"><c>event_type()</c></seealso>
	  <c>{timeout,Name}</c> will be generated.
	  See
	  <seealso marker="erts:erlang#start_timer/4"><c>erlang:start_timer/4</c></seealso>
	  for how <c>Time</c> and
	  <seealso marker="#type-timeout_option"><c>Options</c></seealso>
	  are interpreted.  Future <c>erlang:start_timer/4</c> <c>Options</c>
	  will not necessarily be supported.
	</p>
	<p>
	  If <c>Time</c> is <c>infinity</c>,
	  no timer is started, as it never would expire anyway.
	</p>
	<p>
	  If <c>Time</c> is relative and <c>0</c>
	  no timer is actually started,
	  instead the the time-out event is enqueued to ensure
	  that it gets processed before any not yet
	  received external event.
	</p>
	<p>
	  Setting a timer with the same <c>Name</c> while it is running
	  will restart it with the new time-out value.
	  Therefore it is possible to cancel
	  a specific time-out by setting it to <c>infinity</c>.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="state_timeout"/>
      <desc>
	<p>
	  Starts a timer set by
	  <seealso marker="#type-enter_action"><c>enter_action()</c></seealso>
	  <c>state_timeout</c>.
	  When the timer expires an event of
	  <seealso marker="#type-event_type"><c>event_type()</c></seealso>
	  <c>state_timeout</c> will be generated.
	  See
	  <seealso marker="erts:erlang#start_timer/4"><c>erlang:start_timer/4</c></seealso>
	  for how <c>Time</c> and
	  <seealso marker="#type-timeout_option"><c>Options</c></seealso>
	  are interpreted.  Future <c>erlang:start_timer/4</c> <c>Options</c>
	  will not necessarily be supported.
	</p>
	<p>
	  If <c>Time</c> is <c>infinity</c>,
	  no timer is started, as it never would expire anyway.
	</p>
	<p>
	  If <c>Time</c> is relative and <c>0</c>
	  no timer is actually started,
	  instead the the time-out event is enqueued to ensure
	  that it gets processed before any not yet
	  received external event.
	</p>
	<p>
	  Setting this timer while it is running will restart it with
	  the new time-out value.  Therefore it is possible to cancel
	  this time-out by setting it to <c>infinity</c>.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="timeout_option"/>
      <desc>
	<p>
	  If <c>Abs</c> is <c>true</c> an absolute timer is started,
	  and if it is <c>false</c> a relative, which is the default.
	  See
	  <seealso marker="erts:erlang#start_timer/4"><c>erlang:start_timer/4</c></seealso>
	  for details.
	</p>
	<p>
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="action"/>
      <desc>
	<p>
	  These state transition actions can be invoked by
	  returning them from the
	  <seealso marker="#state callback">state callback</seealso>
	  when it is called with an
	  <seealso marker="#type-event_type">event</seealso>,
	  from
	  <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>
	  or by giving them to
	  <seealso marker="#enter_loop/5"><c>enter_loop/5,6</c></seealso>.
	</p>
	<p>
	  Actions are executed in the containing list order.
	</p>
	<p>
	  Actions that set
	  <seealso marker="#type-transition_option">transition options</seealso>
	  override any previous of the same type,
	  so the last in the containing list wins.
	  For example, the last
	  <seealso marker="#type-postpone"><c>postpone()</c></seealso>
	  overrides any previous <c>postpone()</c> in the list.
	</p>
	<taglist>
	  <tag><c>postpone</c></tag>
	  <item>
	    <p>
	      Sets the
	      <seealso marker="#type-transition_option"><c>transition_option()</c></seealso>
	      <seealso marker="#type-postpone"><c>postpone()</c></seealso>
	      for this state transition.
	      This action is ignored when returned from
	      <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>
	      or given to
	      <seealso marker="#enter_loop/5"><c>enter_loop/5,6</c></seealso>,
	      as there is no event to postpone in those cases.
	    </p>
	  </item>
	  <tag><c>next_event</c></tag>
	  <item>
	    <p>
	      Stores the specified <c><anno>EventType</anno></c>
	      and <c><anno>EventContent</anno></c> for insertion after all
	      actions have been executed.
	    </p>
	    <p>
	      The stored events are inserted in the queue as the next to process
	      before any already queued events. The order of these stored events
	      is preserved, so the first <c>next_event</c> in the containing
	      list becomes the first to process.
	    </p>
	    <p>
	      An event of type
	      <seealso marker="#type-event_type"><c>internal</c></seealso>
	      is to be used when you want to reliably distinguish
	      an event inserted this way from any external event.
	    </p>
	  </item>
	</taglist>
      </desc>
    </datatype>
    <datatype>
      <name name="enter_action"/>
      <desc>
	<p>
	  These state transition actions can be invoked by
	  returning them from the
	  <seealso marker="#state callback">state callback</seealso>, from
	  <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>
	  or by giving them to
	  <seealso marker="#enter_loop/5"><c>enter_loop/5,6</c></seealso>.
	</p>
	<p>
	  Actions are executed in the containing list order.
	</p>
	<p>
	  Actions that set
	  <seealso marker="#type-transition_option">transition options</seealso>
	  override any previous of the same type,
	  so the last in the containing list wins.
	  For example, the last
	  <seealso marker="#type-event_timeout"><c>event_timeout()</c></seealso>
	  overrides any previous <c>event_timeout()</c> in the list.
	</p>
	<taglist>
	  <tag><c>hibernate</c></tag>
	  <item>
	    <p>
	      Sets the
	      <seealso marker="#type-transition_option"><c>transition_option()</c></seealso>
	      <seealso marker="#type-hibernate"><c>hibernate()</c></seealso>
	      for this state transition.
	    </p>
	  </item>
	  <tag><c>Timeout</c></tag>
	  <item>
	    <p>
	      Short for <c>{timeout,Timeout,Timeout}</c>, that is,
	      the time-out message is the time-out time.
	      This form exists to make the
	      <seealso marker="#state callback">state callback</seealso>
	      return value <c>{next_state,NextState,NewData,Timeout}</c>
	      allowed like for <c>gen_fsm</c>'s
	    </p>
	  </item>
	  <tag><c>timeout</c></tag>
	  <item>
	    <p>
	      Sets the
	      <seealso marker="#type-transition_option"><c>transition_option()</c></seealso>
	      <seealso marker="#type-event_timeout"><c>event_timeout()</c></seealso>
	      to <c><anno>Time</anno></c> with <c><anno>EventContent</anno></c>
	      and time-out options
	      <seealso marker="#type-timeout_option"><c><anno>Options</anno></c></seealso>.
	    </p>
	  </item>
	  <tag><c>{timeout,<anno>Name</anno>}</c></tag>
	  <item>
	    <p>
	      Sets the
	      <seealso marker="#type-transition_option"><c>transition_option()</c></seealso>
	      <seealso marker="#type-generic_timeout"><c>generic_timeout()</c></seealso>
	      to <c><anno>Time</anno></c> for <c><anno>Name</anno></c>
	      with <c><anno>EventContent</anno></c>
	      and time-out options
	      <seealso marker="#type-timeout_option"><c><anno>Options</anno></c></seealso>.
	    </p>
	  </item>
	  <tag><c>state_timeout</c></tag>
	  <item>
	    <p>
	      Sets the
	      <seealso marker="#type-transition_option"><c>transition_option()</c></seealso>
	      <seealso marker="#type-state_timeout"><c>state_timeout()</c></seealso>
	      to <c><anno>Time</anno></c> with <c><anno>EventContent</anno></c>
	      and time-out options
	      <seealso marker="#type-timeout_option"><c><anno>Options</anno></c></seealso>.
	    </p>
	  </item>
	</taglist>
      </desc>
    </datatype>
    <datatype>
      <name name="reply_action"/>
      <desc>
	<p>
	  This state transition action can be invoked by
	  returning it from the
	  <seealso marker="#state callback">state callback</seealso>, from
	  <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>
	  or by giving it to
	  <seealso marker="#enter_loop/5"><c>enter_loop/5,6</c></seealso>.
	</p>
	<p>
	  It replies to a caller waiting for a reply in
	  <seealso marker="#call/2"><c>call/2</c></seealso>.
	  <c><anno>From</anno></c> must be the term from argument
	  <seealso marker="#type-event_type"><c>{call,<anno>From</anno>}</c></seealso>
	  in a call to a
	  <seealso marker="#state callback">state callback</seealso>.
	</p>
	<p>
	  Note that using this action from 
	  <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>
	  or
	  <seealso marker="#enter_loop/5"><c>enter_loop/5,6</c></seealso>
	  would be weird on the border of whichcraft
	  since there has been no earlier call to a
	  <seealso marker="#state callback">state callback</seealso>
	  in this server.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="init_result"/>
      <desc>
	<p>
	  For a succesful initialization,
	  <c><anno>State</anno></c> is the initial
	  <seealso marker="#type-state"><c>state()</c></seealso>
	  and <c><anno>Data</anno></c> the initial server
	  <seealso marker="#type-data"><c>data()</c></seealso>
	  of the <c>gen_statem</c>.
	</p>
	<p>
	  The <seealso marker="#type-action"><c>Actions</c></seealso>
	  are executed when entering the first
	  <seealso marker="#type-state">state</seealso> just as for a
	  <seealso marker="#state callback">state callback</seealso>,
	  except that the action <c>postpone</c> is forced to
	  <c>false</c> since there is no event to postpone.
	</p>
	<p>
	  For an unsuccesful initialization,
          <c>{stop,<anno>Reason</anno>}</c>
	  or <c>ignore</c> should be used; see
          <seealso marker="#start_link/3"><c>start_link/3,4</c></seealso>.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="state_enter_result"/>
      <desc>
	<p>
	  <c><anno>State</anno></c> is the current state
	  and it can not be changed since the state callback
	  was called with a
	  <seealso marker="#type-state_enter"><em>state enter call</em></seealso>.
	</p>
	<taglist>
	  <tag><c>next_state</c></tag>
	  <item>
	    <p>
	      The <c>gen_statem</c> does a state transition to
	      <c><anno>State</anno></c>, which has to be
	      the current state,
	      sets <c><anno>NewData</anno></c>,
	      and executes all <c><anno>Actions</anno></c>.
	    </p>
	  </item>
	</taglist>
      </desc>
    </datatype>
    <datatype>
      <name name="event_handler_result"/>
      <desc>
	<p>
	  <c><anno>StateType</anno></c> is
	  <seealso marker="#type-state_name"><c>state_name()</c></seealso>
	  if
	  <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>
	  is <c>state_functions</c>, or
	  <seealso marker="#type-state"><c>state()</c></seealso>
	  if
	  <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>
	  is <c>handle_event_function</c>.
	</p>
	<taglist>
	  <tag><c>next_state</c></tag>
	  <item>
	    <p>
	      The <c>gen_statem</c> does a state transition to
	      <c><anno>NextState</anno></c>
	      (which can be the same as the current state),
	      sets <c><anno>NewData</anno></c>,
	      and executes all <c><anno>Actions</anno></c>.
	    </p>
	  </item>
	</taglist>
      </desc>
    </datatype>
    <datatype>
      <name name="state_callback_result"/>
      <desc>
	<p>
	  <c><anno>ActionType</anno></c> is
	  <seealso marker="#type-enter_action"><c>enter_action()</c></seealso>
	  if the state callback was called with a
	  <seealso marker="#type-state_enter"><em>state enter call</em></seealso>
	  and
	  <seealso marker="#type-action"><c>action()</c></seealso>
	  if the state callback was called with an event.
	</p>
	<taglist>
	  <tag><c>keep_state</c></tag>
	  <item>
	    <p>
	      The <c>gen_statem</c> keeps the current state, or
	      does a state transition to the current state if you like,
	      sets <c><anno>NewData</anno></c>,
	      and executes all <c><anno>Actions</anno></c>.
	      This is the same as
	      <c>{next_state,CurrentState,<anno>NewData</anno>,<anno>Actions</anno>}</c>.
	    </p>
	  </item>
	  <tag><c>keep_state_and_data</c></tag>
	  <item>
	    <p>
	      The <c>gen_statem</c> keeps the current state or
	      does a state transition to the current state if you like,
	      keeps the current server data,
	      and executes all <c><anno>Actions</anno></c>.
	      This is the same as
	      <c>{next_state,CurrentState,CurrentData,<anno>Actions</anno>}</c>.
	    </p>
	  </item>
	  <tag><c>repeat_state</c></tag>
	  <item>
	    <p>
	      The <c>gen_statem</c> keeps the current state, or
	      does a state transition to the current state if you like,
	      sets <c><anno>NewData</anno></c>,
	      and executes all <c><anno>Actions</anno></c>.
	      If the <c>gen_statem</c> runs with
	      <seealso marker="#type-state_enter"><em>state enter calls</em></seealso>,
	      the state enter call is repeated, see type
	      <seealso marker="#type-transition_option"><c>transition_option()</c></seealso>,
	      otherwise <c>repeat_state</c> is the same as
	      <c>keep_state</c>.
	    </p>
	  </item>
	  <tag><c>repeat_state_and_data</c></tag>
	  <item>
	    <p>
	      The <c>gen_statem</c> keeps the current state and data, or
	      does a state transition to the current state if you like,
	      and executes all <c><anno>Actions</anno></c>.
	      This is the same as
	      <c>{repeat_state,CurrentData,<anno>Actions</anno>}</c>.
	      If the <c>gen_statem</c> runs with
	      <seealso marker="#type-state_enter"><em>state enter calls</em></seealso>,
	      the state enter call is repeated, see type
	      <seealso marker="#type-transition_option"><c>transition_option()</c></seealso>,
	      otherwise <c>repeat_state_and_data</c> is the same as
	      <c>keep_state_and_data</c>.
	    </p>
	  </item>
	  <tag><c>stop</c></tag>
	  <item>
	    <p>
	      Terminates the <c>gen_statem</c> by calling
	      <seealso marker="#Module:terminate/3"><c>Module:terminate/3</c></seealso>
	      with <c>Reason</c> and
	      <c><anno>NewData</anno></c>, if specified.
	    </p>
	  </item>
	  <tag><c>stop_and_reply</c></tag>
	  <item>
	    <p>
	      Sends all <c><anno>Replies</anno></c>,
	      then terminates the <c>gen_statem</c> by calling
	      <seealso marker="#Module:terminate/3"><c>Module:terminate/3</c></seealso>
	      with <c>Reason</c> and
	      <c><anno>NewData</anno></c>, if specified.
	    </p>
	  </item>
	</taglist>
	<p>
	  All these terms are tuples or atoms and this property
	  will hold in any future version of <c>gen_statem</c>.
	</p>
      </desc>
    </datatype>
  </datatypes>

  <funcs>
    <func>
      <name name="call" arity="2"/>
      <name name="call" arity="3"/>
      <fsummary>Make a synchronous call to a <c>gen_statem</c>.</fsummary>
      <desc>
        <p>
	  Makes a synchronous call to the <c>gen_statem</c>
	  <seealso marker="#type-server_ref"><c><anno>ServerRef</anno></c></seealso>
	  by sending a request
	  and waiting until its reply arrives.
	  The <c>gen_statem</c> calls the
	  <seealso marker="#state callback">state callback</seealso> with
	  <seealso marker="#type-event_type"><c>event_type()</c></seealso>
	  <c>{call,From}</c> and event content
	  <c><anno>Request</anno></c>.
	</p>
	<p>
	  A <c><anno>Reply</anno></c> is generated when a
	  <seealso marker="#state callback">state callback</seealso>
	  returns with
	  <c>{reply,From,<anno>Reply</anno>}</c> as one
	  <seealso marker="#type-action"><c>action()</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 &gt; 0,
	  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.
	</p>
	<note>
	  <p>
	    For <c><anno>Timeout</anno> &lt; infinity</c>,
	    to avoid getting a late reply in the caller's
	    inbox if the caller should catch exceptions,
	    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>
	  <c><anno>Timeout</anno></c> can also be a tuple
	  <c>{clean_timeout,<anno>T</anno>}</c> or
	  <c>{dirty_timeout,<anno>T</anno>}</c>, where
	  <c><anno>T</anno></c> is the time-out time.
	  <c>{clean_timeout,<anno>T</anno>}</c> works like
	  just <c>T</c> described in the note above
	  and uses a proxy process for <c>T &lt; infinity</c>,
	  while <c>{dirty_timeout,<anno>T</anno>}</c>
	  bypasses the proxy process which is more lightweight.
	</p>
	<note>
	  <p>
	    If you combine catching exceptions from this function
	    with <c>{dirty_timeout,<anno>T</anno>}</c>
	    to avoid that the calling process dies when the call
	    times out, you will have to be prepared to handle
	    a late reply.
	    So why not just let the calling process die?
	  </p>
	</note>
	<p>
	  The call can also fail, for example, if the <c>gen_statem</c>
	  dies before or during this function call.
	</p>
      </desc>
    </func>

    <func>
      <name name="cast" arity="2"/>
      <fsummary>Send an asynchronous event to a <c>gen_statem</c>.</fsummary>
      <desc>
	<p>
	  Sends an asynchronous event to the <c>gen_statem</c>
	  <seealso marker="#type-server_ref"><c><anno>ServerRef</anno></c></seealso>
	  and returns <c>ok</c> immediately,
	  ignoring if the destination node or <c>gen_statem</c>
	  does not exist.
	  The <c>gen_statem</c> calls the
	  <seealso marker="#state callback">state callback</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="enter_loop" arity="4"/>
      <fsummary>Enter the <c>gen_statem</c> receive loop.</fsummary>
      <desc>
	<p>
	  The same as
	  <seealso marker="#enter_loop/6"><c>enter_loop/6</c></seealso>
	  with <c>Actions = []</c> except that no
	  <seealso marker="#type-server_name"><c>server_name()</c></seealso>
	  must have been registered.  This creates an anonymous server.
	</p>
      </desc>
    </func>

    <func>
      <name name="enter_loop" arity="5"/>
      <fsummary>Enter the <c>gen_statem</c> receive loop.</fsummary>
      <desc>
	<p>
	  If <c><anno>Server_or_Actions</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>Actions = <anno>Server_or_Actions</anno></c>.
	  This creates an anonymous server.
	</p>
	<p>
	  Otherwise the same as
	  <seealso marker="#enter_loop/6"><c>enter_loop/6</c></seealso>
	  with
	  <c>Server = <anno>Server_or_Actions</anno></c> and
	  <c>Actions = []</c>.
	</p>
      </desc>
    </func>

    <func>
      <name name="enter_loop" arity="6"/>
      <fsummary>Enter the <c>gen_statem</c> receive loop.</fsummary>
      <desc>
	<p>
	  Makes the calling process become a <c>gen_statem</c>.
	  Does not return, instead the calling process enters
	  the <c>gen_statem</c> receive loop and becomes
	  a <c>gen_statem</c> 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 <c>gen_statem</c> behavior provides.
	</p>
	<p>
	  <c><anno>Module</anno></c>, <c><anno>Opts</anno></c>
	  have the same meaning as when calling
	  <seealso marker="#start_link/3"><c>start[_link]/3,4</c></seealso>.
	</p>
	<p>
	  If <c><anno>Server</anno></c> is <c>self()</c> an anonymous
	  server is created just as when using 
	  <seealso marker="#start_link/3"><c>start[_link]/3</c></seealso>.
	  If <c><anno>Server</anno></c> is a
	  <seealso marker="#type-server_name"><c>server_name()</c></seealso>
	  a named server is created just as when using
	  <seealso marker="#start_link/4"><c>start[_link]/4</c></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>Data</anno></c>,
	  and <c><anno>Actions</anno></c>
	  have the same meanings as in the return value of
	  <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>.
	  Also, the callback module does not need to export a
	  <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>
	  function.
	</p>
        <p>
	  The function fails 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>

    <func>
      <name name="reply" arity="1"/>
      <name name="reply" arity="2"/>
      <fsummary>Reply to a caller.</fsummary>
      <desc>
        <p>
	  This function can be used by a <c>gen_statem</c>
	  to explicitly send a reply to a process that waits in
	  <seealso marker="#call/2"><c>call/2</c></seealso>
	  when the reply cannot be defined in
	  the return value of a
	  <seealso marker="#state callback">state callback</seealso>.
	</p>
	<p>
	  <c><anno>From</anno></c> must be the term from argument
	  <seealso marker="#type-event_type"><c>{call,<anno>From</anno>}</c></seealso>
	  to the
	  <seealso marker="#state callback">state callback</seealso>.
	  A reply or multiple replies canalso be sent
	  using one or several
	  <seealso marker="#type-reply_action"><c>reply_action()</c></seealso>s
	  from a
	  <seealso marker="#state callback">state callback</seealso>.
	</p>
	<note>
	  <p>
	    A reply sent with this function is not visible
	    in <seealso marker="sys"><c>sys</c></seealso> debug output.
	  </p>
	</note>
      </desc>
    </func>

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

    <func>
      <name name="start_link" arity="3"/>
      <name name="start_link" arity="4"/>
      <fsummary>Create a linked <c>gen_statem</c> process.</fsummary>
      <desc>
        <p>
	  Creates a <c>gen_statem</c> 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 <c>gen_statem</c> must be part of
	  a supervision tree so it gets linked to its supervisor.
	</p>
	<p>
	  The <c>gen_statem</c> process calls
	  <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>
	  to initialize the server. To ensure a synchronized startup
	  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 <c>gen_statem</c>.
	  If the <c>gen_statem</c> is started with <c>start_link/3</c>,
	  no <c><anno>ServerName</anno></c> is provided and
	  the <c>gen_statem</c> 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 that is passed as
	  the argument to
	  <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>.
	</p>
        <list type="bulleted">
          <item>
	    <p>
	      If option <c>{timeout,Time}</c> is present in
	      <c><anno>Opts</anno></c>, the <c>gen_statem</c>
	      is allowed to spend <c>Time</c> milliseconds initializing
	      or it terminates and the start function returns
	      <seealso marker="#type-start_ret"><c>{error,timeout}</c></seealso>.
            </p>
          </item>
          <item>
            <p>If option<seealso marker="#type-hibernate_after_opt"><c>{hibernate_after,HibernateAfterTimeout}</c></seealso> is present, the <c>gen_statem</c>
              process awaits any message for <c>HibernateAfterTimeout</c> milliseconds and
              if no message is received, the process goes into hibernation automatically
              (by calling <seealso marker="proc_lib#hibernate/3"><c>proc_lib:hibernate/3</c></seealso>).
            </p>
          </item>
          <item>
            <p>
              If option
              <seealso marker="#type-debug_opt"><c>{debug,Dbgs}</c></seealso>
              is present in <c><anno>Opts</anno></c>, debugging through
              <seealso marker="sys"><c>sys</c></seealso> is activated.
            </p>
          </item>
          <item>
            <p>
              If option <c>{spawn_opt,SpawnOpts}</c> is present in
              <c><anno>Opts</anno></c>, <c>SpawnOpts</c> is passed
              as option list to
	      <seealso marker="erts:erlang#spawn_opt/2"><c>erlang:spawn_opt/2</c></seealso>,
              which is used to spawn the <c>gen_statem</c> process.
            </p>
          </item>
        </list>
        <note>
	  <p>
	    Using spawn option <c>monitor</c> is not
            allowed, it causes this function to fail with reason
            <c>badarg</c>.
	  </p>
        </note>
	<p>
	  If the <c>gen_statem</c> 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 <c>gen_statem</c>.
	  If a process with the specified <c><anno>ServerName</anno></c>
	  exists already, 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="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 <c>gen_statem</c>
	  <seealso marker="#type-server_ref"><c><anno>ServerRef</anno></c></seealso>
	  to exit with the specified <c><anno>Reason</anno></c>
	  and waits for it to terminate.
	  The <c>gen_statem</c> calls
	  <seealso marker="#Module:terminate/3"><c>Module:terminate/3</c></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> causes an
	  error report to be issued through
	  <seealso marker="kernel:error_logger#format/2"><c>error_logger:format/2</c></seealso>.
	  The default <c><anno>Reason</anno></c> is <c>normal</c>.
	</p>
	<p>
	  <c><anno>Timeout</anno></c> is an integer &gt; 0,
	  which specifies how many milliseconds to wait for the server to
	  terminate, or the atom <c>infinity</c> to wait indefinitely.
	  Defaults to <c>infinity</c>.
	  If the server does not terminate 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>
  </funcs>

  <section>
    <title>Callback Functions</title>
    <p>
      The following functions are to be exported from a
      <c>gen_statem</c> callback module.
    </p>
  </section>

  <funcs>
    <func>
      <name>Module:callback_mode() -> CallbackMode</name>
      <fsummary>Update the internal state during upgrade/downgrade.</fsummary>
      <type>
	<v>
	  CallbackMode =
	  <seealso marker="#type-callback_mode">callback_mode()</seealso> |
	  [ <seealso marker="#type-callback_mode">callback_mode()</seealso>
	  | <seealso marker="#type-state_enter">state_enter()</seealso> ]
	</v>
      </type>
      <desc>
        <p>
	  This function is called by a <c>gen_statem</c>
	  when it needs to find out the
	  <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>
	  of the callback module.  The value is cached by <c>gen_statem</c>
	  for efficiency reasons, so this function is only called
	  once after server start and after code change,
	  but before the first
	  <seealso marker="#state callback">state callback</seealso>
	  in the current code version is called.
	  More occasions may be added in future versions
	  of <c>gen_statem</c>.
	</p>
	<p>
	  Server start happens either when
	  <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>
	  returns or when
	  <seealso marker="#enter_loop/4"><c>enter_loop/4-6</c></seealso>
	  is called.  Code change happens when
	  <seealso marker="#Module:code_change/4"><c>Module:code_change/4</c></seealso>
	  returns.
	</p>
	<p>
	  The <c>CallbackMode</c> is either just
	  <seealso marker="#type-callback_mode"><c>callback_mode()</c></seealso>
	  or a list containing
	  <seealso marker="#type-callback_mode"><c>callback_mode()</c></seealso>
	  and possibly the atom
	  <seealso marker="#type-state_enter"><c>state_enter</c></seealso>.
	</p>
	<note>
	  <p>
	    If this function's body does not return an inline constant
	    value the callback module is doing something strange.
	  </p>
	</note>
      </desc>
    </func>

    <func>
      <name>Module:code_change(OldVsn, OldState, OldData, 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,NewData} | Reason</v>
	<v>
	  OldState = NewState =
	  <seealso marker="#type-state">state()</seealso>
	</v>
	<v>
	  OldData = NewData =
	  <seealso marker="#type-data">data()</seealso>
	</v>
	<v>Reason = term()</v>
      </type>
      <desc>
        <note>
          <p>
	    This callback is optional, so callback modules need not export it.
            If a release upgrade/downgrade with
	    <c>Change={advanced,Extra}</c>
	    specified in the <c>.appup</c> file is made
	    when <c>code_change/4</c> is not implemented
	    the process will crash with exit reason <c>undef</c>.
	  </p>
        </note>
        <p>
	  This function is called by a <c>gen_statem</c> when it is to
	  update its internal state during a release upgrade/downgrade,
	  that is, when the instruction <c>{update,Module,Change,...}</c>,
	  where <c>Change={advanced,Extra}</c>, is specified in the
	  <seealso marker="sasl:appup"><c>appup</c></seealso>
	  file. For more information, see
	  <seealso marker="doc/design_principles:release_handling#instr">OTP Design Principles</seealso>.
	</p>
	<p>
	  For an upgrade, <c>OldVsn</c> is <c>Vsn</c>, and
	  for 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>OldData</c> is the internal state
	  of the <c>gen_statem</c>.
	</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 must return the updated
	  internal state in an
	  <c>{ok,NewState,NewData}</c> tuple.
	</p>
	<p>
	  If the function returns a failure <c>Reason</c>, the ongoing
	  upgrade fails and rolls back to the old release.
	  Note that <c>Reason</c> can not be an <c>{ok,_,_}</c> tuple
	  since that will be regarded as a
	  <c>{ok,NewState,NewData}</c> tuple,
	  and that a tuple matching <c>{ok,_}</c>
	  is an also invalid failure <c>Reason</c>.
	  It is recommended to use an atom as <c>Reason</c> since
	  it will be wrapped in an <c>{error,Reason}</c> tuple.
	</p>
	<p>
	  Also note when upgrading a <c>gen_statem</c>,
	  this function and hence
	  the <c>Change={advanced,Extra}</c> parameter in the
	  <seealso marker="sasl:appup"><c>appup</c></seealso> file
	  is not only needed to update the internal state
	  or to act on the <c>Extra</c> argument.
	  It is also needed if an upgrade or downgrade should change 
	  <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>,
	  or else the callback mode after the code change
	  will not be honoured,
	  most probably causing a server crash.
	</p>
      </desc>
    </func>

    <func>
      <name>Module:init(Args) -> Result(StateType)</name>
      <fsummary>
	Initializing process and internal state.
      </fsummary>
      <type>
        <v>Args = term()</v>
	<v>
	  Result(StateType) =
	  <seealso marker="#type-init_result">init_result(StateType)</seealso>
	</v>
      </type>
      <desc>
	<marker id="Module:init-1"/>
	<p>
	  Whenever a <c>gen_statem</c> is started using
	  <seealso marker="#start_link/3"><c>start_link/3,4</c></seealso>
	  or
	  <seealso marker="#start/3"><c>start/3,4</c></seealso>,
	  this function is called by the new process to initialize
	  the implementation state and server data.
	</p>
	<p>
	  <c>Args</c> is the <c>Args</c> argument provided to that start
	  function.
	</p>
        <note>
	  <p>
	    Note that if the <c>gen_statem</c> is started trough
	    <seealso marker="proc_lib"><c>proc_lib</c></seealso>
	    and
	    <seealso marker="#enter_loop/4"><c>enter_loop/4-6</c></seealso>,
	    this callback will never be called.
	    Since this callback is not optional it can
	    in that case be implemented as:
	  </p>
	  <pre>
init(Args) -> erlang:error(not_implemented, [Args]).</pre>
        </note>
      </desc>
    </func>

    <func>
      <name>Module:format_status(Opt, [PDict,State,Data]) ->
        Status
      </name>
      <fsummary>Optional function for providing a term describing the
        current <c>gen_statem</c> status.</fsummary>
      <type>
        <v>Opt = normal | terminate</v>
        <v>PDict = [{Key, Value}]</v>
	<v>
	  State =
	  <seealso marker="#type-state">state()</seealso>
	</v>
	<v>
	  Data =
	  <seealso marker="#type-data">data()</seealso>
	</v>
	<v>Key = term()</v>
	<v>Value = term()</v>
        <v>Status = term()</v>
      </type>
      <desc>
        <note>
	  <p>
	    This callback is optional, so a callback module does not need
	    to export it. The <c>gen_statem</c> module provides a default
	    implementation of this function that returns
	    <c>{State,Data}</c>.
	  </p>
	  <p>
	    If this callback is exported but fails,
	    to hide possibly sensitive data,
	    the default function will instead return <c>{State,Info}</c>,
	    where <c>Info</c> says nothing but the fact that
	    <c>format_status/2</c> has crashed.
	  </p>
        </note>
        <p>This function is called by a <c>gen_statem</c> process when
          any of the following apply:</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 <c>gen_statem</c> status. <c>Opt</c> is set
	    to the atom <c>normal</c> for this case.
	  </item>
	  <item>
	    The <c>gen_statem</c> 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 changing the form and
          appearance of the <c>gen_statem</c> status for these cases. A
          callback module wishing to change the
          <seealso marker="sys#get_status/1"><c>sys:get_status/1,2</c></seealso>
	  return value and how
          its status appears in termination error logs exports an
          instance of <c>format_status/2</c>, which returns a term
          describing the current status of the <c>gen_statem</c>.
	</p>
	<p>
	  <c>PDict</c> is the current value of the process dictionary
          of the <c>gen_statem</c>.
	</p>
	<p>
	  <seealso marker="#type-state"><c>State</c></seealso>
	  is the internal state of the <c>gen_statem</c>.
	</p>
	<p>
	  <seealso marker="#type-data"><c>Data</c></seealso>
	  is the internal server data of the <c>gen_statem</c>.
	</p>
	<p>
	  The function is to return <c>Status</c>, a term that
	  contains the appropriate details
          of the current state and status of
          the <c>gen_statem</c>. 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 <c>gen_statem</c> state. Following this recommendation is not
          required, but it makes 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 log files. Another use is to hide sensitive data from
	  being written to the error log.
	</p>
      </desc>
    </func>

    <func>
      <name>Module:StateName(enter, OldState, Data) ->
        StateEnterResult(StateName)
      </name>
      <name>Module:StateName(EventType, EventContent, Data) ->
        StateFunctionResult
      </name>
      <name>Module:handle_event(enter, OldState, State, Data) ->
	StateEnterResult(State)
      </name>
      <name>Module:handle_event(EventType, EventContent, State, Data) ->
	HandleEventResult
      </name>
      <fsummary>Handle an event.</fsummary>
      <type>
	<v>
	  EventType =
	  <seealso marker="#type-event_type">event_type()</seealso>
	</v>
	<v>EventContent = term()</v>
	<v>
	  State =
	  <seealso marker="#type-state">state()</seealso>
	</v>
	<v>
	  Data = NewData =
	  <seealso marker="#type-data">data()</seealso>
	</v>
	<v>
	  StateEnterResult(StateName) =
	  <seealso marker="#type-state_enter_result">state_enter_result(StateName)</seealso>
	</v>
	<v>
	  StateFunctionResult =
	  <seealso marker="#type-event_handler_result">event_handler_result</seealso>(<seealso marker="#type-state_name">state_name()</seealso>)
	</v>
	<v>
	  StateEnterResult(State) =
	  <seealso marker="#type-state_enter_result">state_enter_result(State)</seealso>
	</v>
	<v>
	  HandleEventResult =
	  <seealso marker="#type-event_handler_result">event_handler_result</seealso>(<seealso marker="#type-state">state()</seealso>)
	</v>
      </type>
      <desc>
	<p>
	  Whenever a <c>gen_statem</c> receives an event from
	  <seealso marker="#call/2"><c>call/2</c></seealso>,
	  <seealso marker="#cast/2"><c>cast/2</c></seealso>, or
	  as a normal process message, one of these functions is called. If
	  <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>
	  is <c>state_functions</c>, <c>Module:StateName/3</c> is called,
	  and if it is <c>handle_event_function</c>,
	  <c>Module:handle_event/4</c> is called.
	</p>
	<p>
	  If <c>EventType</c> is
	  <seealso marker="#type-event_type"><c>{call,From}</c></seealso>,
	  the caller waits for a reply. The reply can be sent
	  from this or from any other
	  <seealso marker="#state callback">state callback</seealso>
	  by returning with <c>{reply,From,Reply}</c> in
	  <seealso marker="#type-action"><c>Actions</c></seealso>, in
	  <seealso marker="#type-reply_action"><c>Replies</c></seealso>,
	  or by calling
	  <seealso marker="#reply/2"><c>reply(From, Reply)</c></seealso>.
	</p>
	<p>
	  If this function returns with a next state that
	  does not match equal (<c>=/=</c>) to the current state,
	  all postponed events are retried in the next state.
	</p>
	<p>
	  The only difference between <c>StateFunctionResult</c> and
	  <c>HandleEventResult</c> is that for <c>StateFunctionResult</c>
	  the next state must be an atom, but for <c>HandleEventResult</c>
	  there is no restriction on the next state.
	</p>
	<p>
	  For options that can be set and actions that can be done
	  by <c>gen_statem</c> after returning from this function,
	  see <seealso marker="#type-action"><c>action()</c></seealso>.
	</p>
	<p>
	  When the <c>gen_statem</c> runs with
	  <seealso marker="#type-state_enter">state enter calls</seealso>,
	  these functions are also called with arguments
	  <c>(enter, OldState, ...)</c> whenever the state changes.
	  In this case there are some restrictions on the
	  <seealso marker="#type-enter_action">actions</seealso>
	  that may be returned:
	  <seealso marker="#type-postpone"><c>postpone()</c></seealso>
	  is not allowed since a <em>state enter call</em> is not
	  an event so there is no event to postpone, and 
	  <seealso marker="#type-action"><c>{next_event,_,_}</c></seealso>
	  is not allowed since using <em>state enter calls</em>
	  should not affect how events are consumed and produced.
	  You may also not change states from this call.
	  Should you return <c>{next_state,NextState, ...}</c>
	  with <c>NextState =/= State</c> the <c>gen_statem</c> crashes.
	  It is possible to use <c>{repeat_state, ...}</c>,
	  <c>{repeat_state_and_data,_}</c> or
	  <c>repeat_state_and_data</c> but all of them makes little
	  sense since you immediately will be called again with a new
	  <em>state enter call</em> making this just a weird way
	  of looping, and there are better ways to loop in Erlang.
	  You are advised to use <c>{keep_state,...}</c>,
	  <c>{keep_state_and_data,_}</c> or
	  <c>keep_state_and_data</c> since you can not change states
	  from a <em>state enter call</em> anyway.
	</p>
	<p>
	  Note the fact that you can use
	  <seealso marker="erts:erlang#throw/1"><c>throw</c></seealso>
	  to return the result, which can be useful.
	  For example to bail out with <c>throw(keep_state_and_data)</c>
	  from deep within complex code that can not
	  return <c>{next_state,State,Data}</c> because
	  <c>State</c> or <c>Data</c> is no longer in scope.
	</p>
      </desc>
    </func>

    <func>
      <name>Module:terminate(Reason, State, Data) -> 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>Data = <seealso marker="#type-data">data()</seealso></v>
	<v>Ignored = term()</v>
      </type>
      <desc>
        <note>
          <p>This callback is optional, so callback modules need not
          export it. The <c>gen_statem</c> module provides a default
          implementation without cleanup.</p>
        </note>
	<p>
	  This function is called by a <c>gen_statem</c>
	  when it is about to terminate. It is to 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 <c>gen_statem</c> 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"><c>State</c></seealso>
	  is the internal state of the <c>gen_statem</c>.
	</p>
	<p>
	  <c>Reason</c> depends on why the <c>gen_statem</c>
	  is terminating.
	  If it is because another callback function has returned, a
	  stop tuple <c>{stop,Reason}</c> in
	  <seealso marker="#type-action"><c>Actions</c></seealso>,
	  <c>Reason</c> has the value specified in that tuple.
	  If it is because of a failure, <c>Reason</c> is the error reason.
	</p>
        <p>
	  If the <c>gen_statem</c> is part of a supervision tree and is
          ordered by its supervisor to terminate, this function is
          called with <c>Reason = shutdown</c> if both the following
          conditions apply:</p>
        <list type="bulleted">
          <item>
            <p>
	      The <c>gen_statem</c> has been set
	      to trap exit signals.
            </p>
	  </item>
          <item>
            <p>
	      The shutdown strategy as defined in the supervisor's
              child specification is an integer time-out value, not
              <c>brutal_kill</c>.
            </p>
	  </item>
        </list>
        <p>
	  Even if the <c>gen_statem</c> is <em>not</em>
	  part of a supervision tree, this function is called
	  if it receives an <c>'EXIT'</c> message from its parent.
	  <c>Reason</c> is the same as
	  in the <c>'EXIT'</c> message.
	</p>
        <p>
	  Otherwise, the <c>gen_statem</c> is immediately terminated.
	</p>
        <p>
	  Notice that for any other reason than <c>normal</c>,
          <c>shutdown</c>, or <c>{shutdown,Term}</c>,
	  the <c>gen_statem</c> is assumed to terminate because of an error
	  and an error report is issued using
          <seealso marker="kernel:error_logger#format/2"><c>error_logger:format/2</c></seealso>.
	</p>
      </desc>
    </func>
  </funcs>

  <section>
    <title>See Also</title>
    <p>
      <seealso marker="gen_event"><c>gen_event(3)</c></seealso>,
      <seealso marker="gen_fsm"><c>gen_fsm(3)</c></seealso>,
      <seealso marker="gen_server"><c>gen_server(3)</c></seealso>,
      <seealso marker="proc_lib"><c>proc_lib(3)</c></seealso>,
      <seealso marker="supervisor"><c>supervisor(3)</c></seealso>,
      <seealso marker="sys"><c>sys(3)</c></seealso>.
    </p>
  </section>
</erlref>