1gen_statem(3) Erlang Module Definition gen_statem(3)
2
6 gen_statem - Generic state machine behavior.
7
9 gen_statem provides a generic state machine behaviour that for new code
10 replaces its predecessor gen_fsm since Erlang/OTP 20.0. The gen_fsm be‐
11 haviour remains in OTP "as is".
12 Note:
14 If you are new to gen_statem and want an overview of concepts and oper‐
15 ation the section gen_statem Behaviour located in the User's Guide
16 OTP Design Principles is recommended to read before this reference
17 manual, possibly after the Description section you are reading here.
18 This reference manual contains type descriptions generated from types
21 in the gen_statem source code, so they are correct. However, the gener‐
22 ated descriptions also reflect the type hierarchy, which sometimes
23 makes it hard to get a good overview. If so, see the section gen_statem
24 Behaviour in the OTP Design Principles User's Guide.
25 Note:
27 * This behavior appeared in Erlang/OTP 19.0.
29 *
31 In OTP 19.1 a backwards incompatible change of the return tuple
32 from Module:init/1 was made and the mandatory callback function
33 Module:callback_mode/0 was introduced.
34 *
36 In OTP 20.0 generic time-outs were added.
37 *
39 In OTP 22.1 time-out content update and explicit time-out cancel
40 were added.
41 *
43 In OTP 22.3 the possibility to change the callback module with ac‐
44 tions change_callback_module, push_callback_module and pop_call‐
45 back_module, was added.
46 gen_statem has got the same features that gen_fsm had and adds some re‐
48 ally useful:
49 * Co-located state code
51 * Arbitrary term state
53 * Event postponing
55 * Self-generated events
57 * State time-out
59 * Multiple generic named time-outs
61 * Absolute time-out time
63 * Automatic state enter calls
65 *
67 Reply from other state than the request, sys traceable
68 * Multiple sys traceable replies
70 * Changing the callback module
72 Two callback modes are supported:
74 * One for finite-state machines (gen_fsm like), which requires the
76 state to be an atom and uses that state as the name of the current
77 callback function.
78 * One that allows the state to be any term and that uses one callback
80 function for all states.
81 The callback model(s) for gen_statem differs from the one for gen_fsm,
83 but it is still fairly easy to rewrite from gen_fsm to gen_statem.
84 A generic state machine server process (gen_statem) implemented using
86 this module has a standard set of interface functions and includes
87 functionality for tracing and error reporting. It also fits into an OTP
88 supervision tree. For more information, see OTP Design Principles.
89 A gen_statem assumes all specific parts to be located in a callback
91 module exporting a predefined set of functions. The relationship be‐
92 tween the behavior functions and the callback functions is as follows:
93 gen_statem module Callback module
95 ----------------- ---------------
96 gen_statem:start
97 gen_statem:start_monitor
98 gen_statem:start_link -----> Module:init/1
99 Server start or code change
101 -----> Module:callback_mode/0
102 gen_statem:stop -----> Module:terminate/3
104 gen_statem:call
106 gen_statem:cast
107 gen_statem:send_request
108 erlang:send
109 erlang:'!' -----> Module:StateName/3
110 Module:handle_event/4
111 - -----> Module:terminate/3
113 - -----> Module:code_change/4
115 Events are of different types, so the callback functions can know the
117 origin of an event and how to respond.
118 If a callback function fails or returns a bad value, the gen_statem
120 terminates, unless otherwise stated. However, an exception of class
121 throw is not regarded as an error but as a valid return from all call‐
122 back functions.
123 The state callback for a specific state in a gen_statem is the callback
125 function that is called for all events in this state. It is selected
126 depending on which callback mode that the callback module defines with
127 the callback function Module:callback_mode/0.
128 When the callback mode is state_functions, the state must be an atom
130 and is used as the state callback name; see Module:StateName/3. This
131 co-locates all code for a specific state in one function as the
132 gen_statem engine branches depending on state name. Note the fact that
133 the callback function Module:terminate/3 makes the state name terminate
134 unusable in this mode.
135 When the callback mode is handle_event_function, the state can be any
137 term and the state callback name is Module:handle_event/4. This makes
138 it easy to branch depending on state or event as you desire. Be careful
139 about which events you handle in which states so that you do not acci‐
140 dentally postpone an event forever creating an infinite busy loop.
141 When gen_statem receives a process message it is converted into an
143 event and the state callback is called with the event as two arguments:
144 type and content. When the state callback has processed the event it
145 returns to gen_statem which does a state transition. If this state
146 transition is to a different state, that is: NextState =/= State, it is
147 a state change.
148 The state callback may return transition actions for gen_statem to exe‐
150 cute during the state transition, for example to reply to a
151 gen_statem:call/2,3.
152 One of the possible transition actions is to postpone the current
154 event. Then it is not retried in the current state. The gen_statem en‐
155 gine keeps a queue of events divided into the postponed events and the
156 events still to process. After a state change the queue restarts with
157 the postponed events.
158 The gen_statem event queue model is sufficient to emulate the normal
160 process message queue with selective receive. Postponing an event cor‐
161 responds to not matching it in a receive statement, and changing states
162 corresponds to entering a new receive statement.
163 The state callback can insert events using the transition actions
165 next_event and such an event is inserted in the event queue as the next
166 to call the state callback with. That is, as if it is the oldest incom‐
167 ing event. A dedicated event_type() internal can be used for such
168 events making them impossible to mistake for external events.
169 Inserting an event replaces the trick of calling your own state han‐
171 dling functions that you often would have to resort to in, for example,
172 gen_fsm to force processing an inserted event before others.
173 The gen_statem engine can automatically make a specialized call to the
175 state callback whenever a new state is entered; see state_enter(). This
176 is for writing code common to all state entries. Another way to do it
177 is to explicitly insert an event at the state transition, and/or to use
178 a dedicated state transition function, but that is something you will
179 have to remember at every state transition to the state(s) that need
180 it.
181 Note:
183 If you in gen_statem, for example, postpone an event in one state and
184 then call another state callback of yours, you have not done a state
185 change and hence the postponed event is not retried, which is logical
186 but can be confusing.
187 For the details of a state transition, see type transition_option().
190 A gen_statem handles system messages as described in sys. The sys mod‐
192 ule can be used for debugging a gen_statem.
193 Notice that a gen_statem does not trap exit signals automatically, this
195 must be explicitly initiated in the callback module (by calling
196 process_flag(trap_exit, true).
197 Unless otherwise stated, all functions in this module fail if the spec‐
199 ified gen_statem does not exist or if bad arguments are specified.
200 The gen_statem process can go into hibernation; see proc_lib:hiber‐
202 nate/3. It is done when a state callback or Module:init/1 specifies hi‐
203 bernate in the returned Actions list. This feature can be useful to re‐
204 claim process heap memory while the server is expected to be idle for a
205 long time. However, use this feature with care, as hibernation can be
206 too costly to use after every event; see erlang:hibernate/3.
207 There is also a server start option {hibernate_after, Timeout} for
209 start/3,4, start_monitor/3,4, start_link/3,4 or enter_loop/4,5,6, that
210 may be used to automatically hibernate the server.
211 If the gen_statem process terminates, e.g. as a result of a function in
213 the callback module returning {stop,Reason}, an exit signal with this
214 Reason is sent to linked processes and ports. See Processes in the
215 Reference Manual for details regarding error handling using exit sig‐
216 nals.
217
219 The following example shows a simple pushbutton model for a toggling
220 pushbutton implemented with callback mode state_functions. You can push
221 the button and it replies if it went on or off, and you can ask for a
222 count of how many times it has been pushed to switch on.
223 The following is the complete callback module file pushbutton.erl:
225 -module(pushbutton).
227 -behaviour(gen_statem).
228 -export([start/0,push/0,get_count/0,stop/0]).
230 -export([terminate/3,code_change/4,init/1,callback_mode/0]).
231 -export([on/3,off/3]).
232 name() -> pushbutton_statem. % The registered server name
234 %% API. This example uses a registered name name()
236 %% and does not link to the caller.
237 start() ->
238 gen_statem:start({local,name()}, ?MODULE, [], []).
239 push() ->
240 gen_statem:call(name(), push).
241 get_count() ->
242 gen_statem:call(name(), get_count).
243 stop() ->
244 gen_statem:stop(name()).
245 %% Mandatory callback functions
247 terminate(_Reason, _State, _Data) ->
248 void.
249 code_change(_Vsn, State, Data, _Extra) ->
250 {ok,State,Data}.
251 init([]) ->
252 %% Set the initial state + data. Data is used only as a counter.
253 State = off, Data = 0,
254 {ok,State,Data}.
255 callback_mode() -> state_functions.
256 %%% state callback(s)
258 off({call,From}, push, Data) ->
260 %% Go to 'on', increment count and reply
261 %% that the resulting status is 'on'
262 {next_state,on,Data+1,[{reply,From,on}]};
263 off(EventType, EventContent, Data) ->
264 handle_event(EventType, EventContent, Data).
265 on({call,From}, push, Data) ->
267 %% Go to 'off' and reply that the resulting status is 'off'
268 {next_state,off,Data,[{reply,From,off}]};
269 on(EventType, EventContent, Data) ->
270 handle_event(EventType, EventContent, Data).
271 %% Handle events common to all states
273 handle_event({call,From}, get_count, Data) ->
274 %% Reply with the current count
275 {keep_state,Data,[{reply,From,Data}]};
276 handle_event(_, _, Data) ->
277 %% Ignore all other events
278 {keep_state,Data}.
279 The following is a shell session when running it:
282 1> pushbutton:start().
284 {ok,<0.36.0>}
285 2> pushbutton:get_count().
286 0
287 3> pushbutton:push().
288 on
289 4> pushbutton:get_count().
290 1
291 5> pushbutton:push().
292 off
293 6> pushbutton:get_count().
294 1
295 7> pushbutton:stop().
296 ok
297 8> pushbutton:push().
298 ** exception exit: {noproc,{gen_statem,call,[pushbutton_statem,push,infinity]}}
299 in function gen:do_for_proc/2 (gen.erl, line 261)
300 in call from gen_statem:call/3 (gen_statem.erl, line 386)
301 To compare styles, here follows the same example using callback mode
304 handle_event_function, or rather the code to replace after function
305 init/1 of the pushbutton.erl example file above:
306 callback_mode() -> handle_event_function.
308 %%% state callback(s)
310 handle_event({call,From}, push, off, Data) ->
312 %% Go to 'on', increment count and reply
313 %% that the resulting status is 'on'
314 {next_state,on,Data+1,[{reply,From,on}]};
315 handle_event({call,From}, push, on, Data) ->
316 %% Go to 'off' and reply that the resulting status is 'off'
317 {next_state,off,Data,[{reply,From,off}]};
318 %%
319 %% Event handling common to all states
320 handle_event({call,From}, get_count, State, Data) ->
321 %% Reply with the current count
322 {next_state,State,Data,[{reply,From,Data}]};
323 handle_event(_, _, State, Data) ->
324 %% Ignore all other events
325 {next_state,State,Data}.
326
329 server_name() =
330 {global, GlobalName :: term()} |
331 {via, RegMod :: module(), Name :: term()} |
332 {local, atom()}
333 Name specification to use when starting a gen_statem server. See
335 start_link/3 and server_ref() below.
336 server_ref() =
338 pid() |
339 (LocalName :: atom()) |
340 {Name :: atom(), Node :: atom()} |
341 {global, GlobalName :: term()} |
342 {via, RegMod :: module(), ViaName :: term()}
343 Server specification to use when addressing a gen_statem server.
345 See call/2 and server_name() above.
346 It can be:
348 pid() | LocalName:
350 The gen_statem is locally registered.
351 {Name,Node}:
353 The gen_statem is locally registered on another node.
354 {global,GlobalName}:
356 The gen_statem is globally registered in global.
357 {via,RegMod,ViaName}:
359 The gen_statem is registered in an alternative process reg‐
360 istry. The registry callback module RegMod is to export
361 functions register_name/2, unregister_name/1,
362 whereis_name/1, and send/2, which are to behave like the
363 corresponding functions in global. Thus, {via,global,Global‐
364 Name} is the same as {global,GlobalName}.
365 start_opt() =
367 {timeout, Time :: timeout()} |
368 {spawn_opt, [proc_lib:start_spawn_option()]} |
369 enter_loop_opt()
370 Options that can be used when starting a gen_statem server
372 through, for example, start_link/3.
373 start_ret() = {ok, pid()} | ignore | {error, term()}
375 Return value from the start() and start_link() functions, for
377 example, start_link/3.
378 start_mon_ret() =
380 {ok, {pid(), reference()}} | ignore | {error, term()}
381 Return value from the start_monitor() functions.
383 enter_loop_opt() =
385 {hibernate_after, HibernateAfterTimeout :: timeout()} |
386 {debug, Dbgs :: [sys:debug_option()]}
387 Options that can be used when starting a gen_statem server
389 through, enter_loop/4-6.
390 hibernate_after:
392 HibernateAfterTimeout specifies that the gen_statem process
393 awaits any message for HibernateAfterTimeout milliseconds
394 and if no message is received, the process goes into hiber‐
395 nation automatically (by calling proc_lib:hibernate/3).
396 debug:
398 For every entry in Dbgs, the corresponding function in sys
399 is called.
400 from() = {To :: pid(), Tag :: term()}
402 Destination to use when replying through, for example, the ac‐
404 tion() {reply,From,Reply} to a process that has called the
405 gen_statem server using call/2.
406 state() = state_name() | term()
408 If the callback mode is handle_event_function, the state can be
410 any term. After a state change (NextState =/= State), all post‐
411 poned events are retried.
412 state_name() = atom()
414 If the callback mode is state_functions, the state must be an
416 atom. After a state change (NextState =/= State), all postponed
417 events are retried. Note that the state terminate is not possi‐
418 ble to use since it would collide with the optional callback
419 function Module:terminate/3.
420 data() = term()
422 A term in which the state machine implementation is to store any
424 server data it needs. The difference between this and the
425 state() itself is that a change in this data does not cause
426 postponed events to be retried. Hence, if a change in this data
427 would change the set of events that are handled, then that data
428 item is to be made a part of the state.
429 event_type() =
431 external_event_type() | timeout_event_type() | internal
432 There are 3 categories of events: external, timeout, and inter‐
434 nal.
435 internal events can only be generated by the state machine it‐
437 self through the transition action next_event.
438 external_event_type() = {call, From :: from()} | cast | info
440 External events are of 3 types: {call,From}, cast, or info. Type
442 call originates from the API functions call/2 and send_re‐
443 quest/2. For calls, the event contains whom to reply to. Type
444 cast originates from the API function cast/2. Type info origi‐
445 nates from regular process messages sent to the gen_statem.
446 timeout_event_type() =
448 timeout | {timeout, Name :: term()} | state_timeout
449 There are 3 types of time-out events that the state machine can
451 generate for itself with the corresponding timeout_action()s.
452 callback_mode_result() =
454 callback_mode() | [callback_mode() | state_enter()]
455 This is the return type from Module:callback_mode/0 and selects
457 callback mode and whether to do state enter calls, or not.
458 callback_mode() = state_functions | handle_event_function
460 The callback mode is selected with the return value from Mod‐
462 ule:callback_mode/0:
463 state_functions:
465 The state must be of type state_name() and one callback
466 function per state, that is, Module:StateName/3, is used.
467 handle_event_function:
469 The state can be any term and the callback function Mod‐
470 ule:handle_event/4 is used for all states.
471 The function Module:callback_mode/0 is called when starting the
473 gen_statem, after code change and after changing the callback
474 module with any of the actions change_callback_module,
475 push_callback_module or pop_callback_module. The result is
476 cached for subsequent calls to state callbacks.
477 state_enter() = state_enter
479 Whether the state machine should use state enter calls or not is
481 selected when starting the gen_statem and after code change us‐
482 ing the return value from Module:callback_mode/0.
483 If Module:callback_mode/0 returns a list containing state_enter,
485 the gen_statem engine will, at every state change, call the
486 state callback with arguments (enter, OldState, Data) or (enter,
487 OldState, State, Data), depending on the callback mode. This may
488 look like an event but is really a call performed after the pre‐
489 vious state callback returned and before any event is delivered
490 to the new state callback. See Module:StateName/3 and Mod‐
491 ule:handle_event/4. Such a call can be repeated by returning a
492 repeat_state or repeat_state_and_data tuple from the state call‐
493 back.
494 If Module:callback_mode/0 does not return such a list, no state
496 enter calls are done.
497 If Module:code_change/4 should transform the state, it is re‐
499 garded as a state rename and not a state change, which will not
500 cause a state enter call.
501 Note that a state enter call will be done right before entering
503 the initial state even though this actually is not a state
504 change. In this case OldState =:= State, which cannot happen for
505 a subsequent state change, but will happen when repeating the
506 state enter call.
507 transition_option() =
509 postpone() |
510 hibernate() |
511 event_timeout() |
512 generic_timeout() |
513 state_timeout()
514 Transition options can be set by actions and modify the state
516 transition. The state transition takes place when the state
517 callback has processed an event and returns. Here are the se‐
518 quence of steps for a state transition:
519 * All returned actions are processed in order of appearance.
521 In this step all replies generated by any reply_action() are
522 sent. Other actions set transition_option()s that come into
523 play in subsequent steps.
524 * If state enter calls are used, and either it is the initial
526 state or one of the callback results repeat_state_and_data
527 or repeat_state_and_data is used the gen_statem engine calls
528 the current state callback with arguments (enter, State,
529 Data) or (enter, State, State, Data) (depending on callback
530 mode) and when it returns starts again from the top of this
531 sequence.
532 If state enter calls are used, and the state changes the
534 gen_statem engine calls the new state callback with argu‐
535 ments (enter, OldState, Data) or (enter, OldState, State,
536 Data) (depending on callback mode) and when it returns
537 starts again from the top of this sequence.
538 * If postpone() is true, the current event is postponed.
540 * If this is a state change, the queue of incoming events is
542 reset to start with the oldest postponed.
543 * All events stored with action() next_event are inserted to
545 be processed before previously queued events.
546 * Time-out timers event_timeout(), generic_timeout() and
548 state_timeout() are handled. Time-outs with zero time are
549 guaranteed to be delivered to the state machine before any
550 external not yet received event so if there is such a time-
551 out requested, the corresponding time-out zero event is en‐
552 queued as the newest received event; that is after already
553 queued events such as inserted and postponed events.
554 Any event cancels an event_timeout() so a zero time event
556 time-out is only generated if the event queue is empty.
557 A state change cancels a state_timeout() and any new transi‐
559 tion option of this type belongs to the new state, that is;
560 a state_timeout() applies to the state the state machine en‐
561 ters.
562 * If there are enqueued events the state callback for the pos‐
564 sibly new state is called with the oldest enqueued event,
565 and we start again from the top of this sequence.
566 * Otherwise the gen_statem goes into receive or hibernation
568 (if hibernate() is true) to wait for the next message. In
569 hibernation the next non-system event awakens the
570 gen_statem, or rather the next incoming message awakens the
571 gen_statem, but if it is a system event it goes right back
572 into hibernation. When a new message arrives the state call‐
573 back is called with the corresponding event, and we start
574 again from the top of this sequence.
575 postpone() = boolean()
577 If true, postpones the current event and retries it after a
579 state change (NextState =/= State).
580 hibernate() = boolean()
582 If true, hibernates the gen_statem by calling proc_lib:hiber‐
584 nate/3 before going into receive to wait for a new external
585 event.
586 Note:
588 If there are enqueued events to process when hibrnation is re‐
589 quested, this is optimized by not hibernating but instead call‐
590 ing erlang:garbage_collect/0 to simulate that the gen_statem en‐
591 tered hibernation and immediately got awakened by an enqueued
592 event.
593 event_timeout() = timeout() | integer()
596 Starts a timer set by enter_action() timeout. When the timer ex‐
598 pires an event of event_type() timeout will be generated. See
599 erlang:start_timer/4 for how Time and Options are interpreted.
600 Future erlang:start_timer/4 Options will not necessarily be sup‐
601 ported.
602 Any event that arrives cancels this time-out. Note that a re‐
604 tried or inserted event counts as arrived. So does a state time-
605 out zero event, if it was generated before this time-out is re‐
606 quested.
607 If Time is infinity, no timer is started, as it never would ex‐
609 pire anyway.
610 If Time is relative and 0 no timer is actually started, instead
612 the the time-out event is enqueued to ensure that it gets pro‐
613 cessed before any not yet received external event, but after al‐
614 ready queued events.
615 Note that it is not possible nor needed to cancel this time-out,
617 as it is cancelled automatically by any other event.
618 generic_timeout() = timeout() | integer()
620 Starts a timer set by enter_action() {timeout,Name}. When the
622 timer expires an event of event_type() {timeout,Name} will be
623 generated. See erlang:start_timer/4 for how Time and Options are
624 interpreted. Future erlang:start_timer/4 Options will not neces‐
625 sarily be supported.
626 If Time is infinity, no timer is started, as it never would ex‐
628 pire anyway.
629 If Time is relative and 0 no timer is actually started, instead
631 the the time-out event is enqueued to ensure that it gets pro‐
632 cessed before any not yet received external event.
633 Setting a timer with the same Name while it is running will
635 restart it with the new time-out value. Therefore it is possible
636 to cancel a specific time-out by setting it to infinity.
637 state_timeout() = timeout() | integer()
639 Starts a timer set by enter_action() state_timeout. When the
641 timer expires an event of event_type() state_timeout will be
642 generated. See erlang:start_timer/4 for how Time and Options are
643 interpreted. Future erlang:start_timer/4 Options will not neces‐
644 sarily be supported.
645 If Time is infinity, no timer is started, as it never would ex‐
647 pire anyway.
648 If Time is relative and 0 no timer is actually started, instead
650 the the time-out event is enqueued to ensure that it gets pro‐
651 cessed before any not yet received external event.
652 Setting this timer while it is running will restart it with the
654 new time-out value. Therefore it is possible to cancel this
655 time-out by setting it to infinity.
656 timeout_option() = {abs, Abs :: boolean()}
658 If Abs is true an absolute timer is started, and if it is false
660 a relative, which is the default. See erlang:start_timer/4 for
661 details.
662 action() =
664 postpone |
665 {postpone, Postpone :: postpone()} |
666 {next_event,
667 EventType :: event_type(),
668 EventContent :: term()} |
669 {change_callback_module, NewModule :: module()} |
670 {push_callback_module, NewModule :: module()} |
671 pop_callback_module |
672 enter_action()
673 These transition actions can be invoked by returning them from
675 the state callback when it is called with an event, from Mod‐
676 ule:init/1 or by giving them to enter_loop/5,6.
677 Actions are executed in the containing list order.
679 Actions that set transition options override any previous of
681 the same type, so the last in the containing list wins. For ex‐
682 ample, the last postpone() overrides any previous postpone() in
683 the list.
684 postpone:
686 Sets the transition_option() postpone() for this state tran‐
687 sition. This action is ignored when returned from Mod‐
688 ule:init/1 or given to enter_loop/5,6, as there is no event
689 to postpone in those cases.
690 next_event:
692 This action does not set any transition_option() but instead
693 stores the specified EventType and EventContent for inser‐
694 tion after all actions have been executed.
695 The stored events are inserted in the queue as the next to
697 process before any already queued events. The order of these
698 stored events is preserved, so the first next_event in the
699 containing list becomes the first to process.
700 An event of type internal is to be used when you want to re‐
702 liably distinguish an event inserted this way from any ex‐
703 ternal event.
704 change_callback_module:
706 Changes the callback module to NewModule which will be used
707 when calling all subsequent state callbacks.
708 The gen_statem engine will find out the callback mode of
710 NewModule by calling NewModule:callback_mode/0 before the
711 next state callback.
712 Changing the callback module does not affect the state tran‐
714 sition in any way, it only changes which module that handles
715 the events. Be aware that all relevant callback functions in
716 NewModule such as the state callback, NewMod‐
717 ule:code_change/4, NewModule:format_status/2 and NewMod‐
718 ule:terminate/3 must be able to handle the state and data
719 from the old module.
720 push_callback_module:
722 Pushes the current callback module to the top of an internal
723 stack of callback modules and changes the callback module to
724 NewModule. Otherwise like {change_callback_module, NewMod‐
725 ule} above.
726 pop_callback_module:
728 Pops the top module from the internal stack of callback
729 modules and changes the callback module to be the popped
730 module. If the stack is empty the server fails. Otherwise
731 like {change_callback_module, NewModule} above.
732 enter_action() =
734 hibernate |
735 {hibernate, Hibernate :: hibernate()} |
736 timeout_action() |
737 reply_action()
738 These transition actions can be invoked by returning them from
740 the state callback, from Module:init/1 or by giving them to en‐
741 ter_loop/5,6.
742 Actions are executed in the containing list order.
744 Actions that set transition options override any previous of the
746 same type, so the last in the containing list wins. For example,
747 the last event_timeout() overrides any previous event_timeout()
748 in the list.
749 hibernate:
751 Sets the transition_option() hibernate() for this state
752 transition.
753 timeout_action() =
755 (Time :: event_timeout()) |
756 {timeout, Time :: event_timeout(), EventContent :: term()} |
757 {timeout,
758 Time :: event_timeout(),
759 EventContent :: term(),
760 Options :: timeout_option() | [timeout_option()]} |
761 {{timeout, Name :: term()},
762 Time :: generic_timeout(),
763 EventContent :: term()} |
764 {{timeout, Name :: term()},
765 Time :: generic_timeout(),
766 EventContent :: term(),
767 Options :: timeout_option() | [timeout_option()]} |
768 {state_timeout,
769 Time :: state_timeout(),
770 EventContent :: term()} |
771 {state_timeout,
772 Time :: state_timeout(),
773 EventContent :: term(),
774 Options :: timeout_option() | [timeout_option()]} |
775 timeout_cancel_action() |
776 timeout_update_action()
777 These transition actions can be invoked by returning them from
779 the state callback, from Module:init/1 or by giving them to en‐
780 ter_loop/5,6.
781 These time-out actions sets time-out transition options.
783 Time:
785 Short for {timeout,Time,Time}, that is, the time-out message
786 is the time-out time. This form exists to make the state
787 callback return value {next_state,NextState,NewData,Time}
788 allowed like for gen_fsm.
789 timeout:
791 Sets the transition_option() event_timeout() to Time with
792 EventContent and time-out options Options.
793 {timeout,Name}:
795 Sets the transition_option() generic_timeout() to Time for
796 Name with EventContent and time-out options Options.
797 state_timeout:
799 Sets the transition_option() state_timeout() to Time with
800 EventContent and time-out options Options.
801 timeout_cancel_action() =
803 {timeout, cancel} |
804 {{timeout, Name :: term()}, cancel} |
805 {state_timeout, cancel}
806 This is a shorter and clearer form of timeout_action() with
808 Time = infinity which cancels a time-out.
809 timeout_update_action() =
811 {timeout, update, EventContent :: term()} |
812 {{timeout, Name :: term()}, update, EventContent :: term()} |
813 {state_timeout, update, EventContent :: term()}
814 Updates a time-out with a new EventContent. See timeout_ac‐
816 tion() for how to start a time-out.
817 If no time-out of the same type is active instead insert the
819 time-out event just like when starting a time-out with relative
820 Time = 0.
821 reply_action() = {reply, From :: from(), Reply :: term()}
823 This transition action can be invoked by returning it from the
825 state callback, from Module:init/1 or by giving it to en‐
826 ter_loop/5,6.
827 It does not set any transition_option() but instead replies to a
829 caller waiting for a reply in call/2. From must be the term from
830 argument {call,From} in a call to a state callback.
831 Note that using this action from Module:init/1 or enter_loop/5,6
833 would be weird on the border of witchcraft since there has been
834 no earlier call to a state callback in this server.
835 init_result(StateType) =
837 {ok, State :: StateType, Data :: data()} |
838 {ok,
839 State :: StateType,
840 Data :: data(),
841 Actions :: [action()] | action()} |
842 ignore |
843 {stop, Reason :: term()}
844 For a succesful initialization, State is the initial state() and
846 Data the initial server data() of the gen_statem.
847 The Actions are executed when entering the first state just as
849 for a state callback, except that the action postpone is forced
850 to false since there is no event to postpone.
851 For an unsuccesful initialization, {stop,Reason} or ignore
853 should be used; see start_link/3,4.
854 state_enter_result(State) =
856 {next_state, State, NewData :: data()} |
857 {next_state, State,
858 NewData :: data(),
859 Actions :: [enter_action()] | enter_action()} |
860 state_callback_result(enter_action())
861 State is the current state and it cannot be changed since the
863 state callback was called with a state enter call.
864 next_state:
866 The gen_statem does a state transition to State, which has
867 to be the current state, sets NewData, and executes all Ac‐
868 tions.
869 event_handler_result(StateType) =
871 {next_state, NextState :: StateType, NewData :: data()} |
872 {next_state,
873 NextState :: StateType,
874 NewData :: data(),
875 Actions :: [action()] | action()} |
876 state_callback_result(action())
877 StateType is state_name() if callback mode is state_functions,
879 or state() if callback mode is handle_event_function.
880 next_state:
882 The gen_statem does a state transition to NextState (which
883 can be the same as the current state), sets NewData, and ex‐
884 ecutes all Actions. If NextState =/= CurrentState the state
885 transition is a state change.
886 state_callback_result(ActionType) =
888 {keep_state, NewData :: data()} |
889 {keep_state,
890 NewData :: data(),
891 Actions :: [ActionType] | ActionType} |
892 keep_state_and_data |
893 {keep_state_and_data, Actions :: [ActionType] | ActionType} |
894 {repeat_state, NewData :: data()} |
895 {repeat_state,
896 NewData :: data(),
897 Actions :: [ActionType] | ActionType} |
898 repeat_state_and_data |
899 {repeat_state_and_data, Actions :: [ActionType] | ActionType} |
900 stop |
901 {stop, Reason :: term()} |
902 {stop, Reason :: term(), NewData :: data()} |
903 {stop_and_reply,
904 Reason :: term(),
905 Replies :: [reply_action()] | reply_action()} |
906 {stop_and_reply,
907 Reason :: term(),
908 Replies :: [reply_action()] | reply_action(),
909 NewData :: data()}
910 ActionType is enter_action() if the state callback was called
912 with a state enter call and action() if the state callback was
913 called with an event.
914 keep_state:
916 The same as {next_state,CurrentState,NewData,Actions}.
917 keep_state_and_data:
919 The same as {keep_state,CurrentData,Actions}.
920 repeat_state:
922 If the gen_statem runs with state enter calls, the state en‐
923 ter call is repeated, see type transition_option(), other
924 than that repeat_state is the same as keep_state.
925 repeat_state_and_data:
927 The same as {repeat_state,CurrentData,Actions}.
928 stop:
930 Terminates the gen_statem by calling Module:terminate/3 with
931 Reason and NewData, if specified. An exit signal with this
932 reason is sent to linked processes and ports. The default
933 Reason is normal.
934 stop_and_reply:
936 Sends all Replies, then terminates the gen_statem by calling
937 Module:terminate/3 with Reason and NewData, if specified. An
938 exit signal with this reason is sent to linked processes and
939 ports.
940 All these terms are tuples or atoms and this property will hold
942 in any future version of gen_statem.
943 request_id() = term()
945 A request handle, see send_request/2 for details.
947
949 call(ServerRef :: server_ref(), Request :: term()) ->
950 Reply :: term()
951 call(ServerRef :: server_ref(),
953 Request :: term(),
954 Timeout ::
955 timeout() |
956 {clean_timeout, T :: timeout()} |
957 {dirty_timeout, T :: timeout()}) ->
958 Reply :: term()
959 Makes a synchronous call to the gen_statem ServerRef by sending
961 a request and waiting until its reply arrives. The gen_statem
962 calls the state callback with event_type() {call,From} and event
963 content Request.
964 A Reply is generated when a state callback returns with {re‐
966 ply,From,Reply} as one action(), and that Reply becomes the re‐
967 turn value of this function.
968 Timeout is an integer > 0, which specifies how many milliseconds
970 to wait for a reply, or the atom infinity to wait indefinitely,
971 which is the default. If no reply is received within the speci‐
972 fied time, the function call fails.
973 Note:
975 For Timeout < infinity, to avoid getting a late reply in the
976 caller's inbox if the caller should catch exceptions, this func‐
977 tion spawns a proxy process that does the call. A late reply
978 gets delivered to the dead proxy process, hence gets discarded.
979 This is less efficient than using Timeout == infinity.
980 Timeout can also be a tuple {clean_timeout,T} or {dirty_time‐
983 out,T}, where T is the time-out time. {clean_timeout,T} works
984 like just T described in the note above and uses a proxy process
985 while {dirty_timeout,T} bypasses the proxy process which is more
986 lightweight.
987 Note:
989 If you combine catching exceptions from this function with
990 {dirty_timeout,T} to avoid that the calling process dies when
991 the call times out, you will have to be prepared to handle a
992 late reply. Note that there is an odd chance to get a late reply
993 even with {dirty_timeout,infinity} or infinity for example in
994 the event of network problems. So why not just let the calling
995 process die by not catching the exception?
996 The call can also fail, for example, if the gen_statem dies be‐
999 fore or during this function call.
1000 cast(ServerRef :: server_ref(), Msg :: term()) -> ok
1002 Sends an asynchronous event to the gen_statem ServerRef and re‐
1004 turns ok immediately, ignoring if the destination node or
1005 gen_statem does not exist. The gen_statem calls the state call‐
1006 back with event_type() cast and event content Msg.
1007 check_response(Msg :: term(), RequestId :: request_id()) ->
1009 {reply, Reply :: term()} |
1010 no_reply |
1011 {error, {term(), server_ref()}}
1012 This function is used to check if a previously received message,
1014 for example by receive or handle_info/2, is a result of a re‐
1015 quest made with send_request/2. If Msg is a reply to the handle
1016 RequestId the result of the request is returned in Reply. Other‐
1017 wise returns no_reply and no cleanup is done, and thus the func‐
1018 tion shall be invoked repeatedly until a reply is returned.
1019 The return value Reply is generated when a state callback re‐
1021 turns with {reply,From,Reply} as one action(), and that Reply
1022 becomes the return value of this function.
1023 The function returns an error if the gen_statem dies before or
1025 during this request.
1026 enter_loop(Module :: module(),
1028 Opts :: [enter_loop_opt()],
1029 State :: state(),
1030 Data :: data()) ->
1031 no_return()
1032 The same as enter_loop/6 with Actions = [] except that no
1034 server_name() must have been registered. This creates an anony‐
1035 mous server.
1036 enter_loop(Module :: module(),
1038 Opts :: [enter_loop_opt()],
1039 State :: state(),
1040 Data :: data(),
1041 Server_or_Actions :: server_name() | pid() | [action()]) ->
1042 no_return()
1043 If Server_or_Actions is a list(), the same as enter_loop/6 ex‐
1045 cept that no server_name() must have been registered and Actions
1046 = Server_or_Actions. This creates an anonymous server.
1047 Otherwise the same as enter_loop/6 with Server = Server_or_Ac‐
1049 tions and Actions = [].
1050 enter_loop(Module :: module(),
1052 Opts :: [enter_loop_opt()],
1053 State :: state(),
1054 Data :: data(),
1055 Server :: server_name() | pid(),
1056 Actions :: [action()] | action()) ->
1057 no_return()
1058 Makes the calling process become a gen_statem. Does not return,
1060 instead the calling process enters the gen_statem receive loop
1061 and becomes a gen_statem server. The process must have been
1062 started using one of the start functions in proc_lib. The user
1063 is responsible for any initialization of the process, including
1064 registering a name for it.
1065 This function is useful when a more complex initialization pro‐
1067 cedure is needed than the gen_statem behavior provides.
1068 Module, Opts have the same meaning as when calling
1070 start[_link|_monitor]/3,4.
1071 If Server is self() an anonymous server is created just as when
1073 using start[_link|_monitor]/3. If Server is a server_name() a
1074 named server is created just as when using start[_link|_moni‐
1075 tor]/4. However, the server_name() name must have been regis‐
1076 tered accordingly before this function is called.
1077 State, Data, and Actions have the same meanings as in the return
1079 value of Module:init/1. Also, the callback module does not need
1080 to export a Module:init/1 function.
1081 The function fails if the calling process was not started by a
1083 proc_lib start function, or if it is not registered according to
1084 server_name().
1085 receive_response(RequestId :: request_id()) ->
1087 {reply, Reply :: term()} |
1088 {error, {term(), server_ref()}}
1089 receive_response(RequestId :: request_id(), Timeout :: timeout()) ->
1091 {reply, Reply :: term()} |
1092 timeout |
1093 {error, {term(), server_ref()}}
1094 This function is used to receive for a reply of a request made
1096 with send_request/2 to the gen_statem process. This function
1097 must be called from the same process from which send_request/2
1098 was made.
1099 Timeout is an integer greater then or equal to zero that speci‐
1101 fies how many milliseconds to wait for an reply, or the atom in‐
1102 finity to wait indefinitely. Defaults to infinity. If no reply
1103 is received within the specified time, the function returns
1104 timeout. Assuming that the server executes on a node supporting
1105 aliases (introduced in OTP 24) no response will be received af‐
1106 ter a timeout. Otherwise, a garbage response might be received
1107 at a later time.
1108 The return value Reply is generated when a state callback re‐
1110 turns with {reply,From,Reply} as one action(), and that Reply
1111 becomes the return value of this function.
1112 The function returns an error if the gen_statem dies before or
1114 during this function call.
1115 The difference between wait_response() and receive_response() is
1117 that receive_response() abandons the request at timeout so that
1118 a potential future response is ignored, while wait_response()
1119 does not.
1120 reply(Replies :: [reply_action()] | reply_action()) -> ok
1122 reply(From :: from(), Reply :: term()) -> ok
1124 This function can be used by a gen_statem to explicitly send a
1126 reply to a process that waits in call/2 when the reply cannot be
1127 defined in the return value of a state callback.
1128 From must be the term from argument {call,From} to the state
1130 callback. A reply or multiple replies canalso be sent using one
1131 or several reply_action()s from a state callback.
1132 Note:
1134 A reply sent with this function is not visible in sys debug out‐
1135 put.
1136 send_request(ServerRef :: server_ref(), Request :: term()) ->
1139 RequestId :: request_id()
1140 Sends a request to the gen_statem ServerRef and returns a handle
1142 RequestId.
1143 The return value RequestId shall later be used with receive_re‐
1145 sponse/1,2, wait_response/1,2, or check_response/2 to fetch the
1146 actual result of the request.
1147 The call gen_statem:wait_response(gen_statem:send_re‐
1149 quest(ServerRef,Request), Timeout) can be seen as equivalent to
1150 gen_statem:call(Server,Request,Timeout), ignoring the error han‐
1151 dling.
1152 The gen_statem calls the state callback with event_type()
1154 {call,From} and event content Request.
1155 A Reply is generated when a state callback returns with {re‐
1157 ply,From,Reply} as one action(), and that Reply becomes the re‐
1158 turn value of receive_response/1,2, wait_response/1,2, or
1159 check_response/2 function.
1160 start(Module :: module(), Args :: term(), Opts :: [start_opt()]) ->
1162 start_ret()
1163 start(ServerName :: server_name(),
1165 Module :: module(),
1166 Args :: term(),
1167 Opts :: [start_opt()]) ->
1168 start_ret()
1169 Creates a standalone gen_statem process according to OTP design
1171 principles (using proc_lib primitives). As it does not get
1172 linked to the calling process, this start function cannot be
1173 used by a supervisor to start a child.
1174 For a description of arguments and return values, see
1176 start_link/3,4.
1177 start_link(Module :: module(),
1179 Args :: term(),
1180 Opts :: [start_opt()]) ->
1181 start_ret()
1182 start_link(ServerName :: server_name(),
1184 Module :: module(),
1185 Args :: term(),
1186 Opts :: [start_opt()]) ->
1187 start_ret()
1188 Creates a gen_statem process according to OTP design principles
1190 (using proc_lib primitives) that is linked to the calling
1191 process. This is essential when the gen_statem must be part of a
1192 supervision tree so it gets linked to its supervisor.
1193 The gen_statem process calls Module:init/1 to initialize the
1195 server. To ensure a synchronized startup procedure,
1196 start_link/3,4 does not return until Module:init/1 has returned.
1197 ServerName specifies the server_name() to register for the
1199 gen_statem. If the gen_statem is started with start_link/3, no
1200 ServerName is provided and the gen_statem is not registered.
1201 Module is the name of the callback module.
1203 Args is an arbitrary term that is passed as the argument to Mod‐
1205 ule:init/1.
1206 * If option {timeout,Time} is present in Opts, the gen_statem
1208 is allowed to spend Time milliseconds initializing or it
1209 terminates and the start function returns {error,timeout}.
1210 * If option {hibernate_after,HibernateAfterTimeout} is
1212 present, the gen_statem process awaits any message for Hi‐
1213 bernateAfterTimeout milliseconds and if no message is re‐
1214 ceived, the process goes into hibernation automatically (by
1215 calling proc_lib:hibernate/3).
1216 * If option {debug,Dbgs} is present in Opts, debugging through
1218 sys is activated.
1219 * If option {spawn_opt,SpawnOpts} is present in Opts,
1221 SpawnOpts is passed as option list to erlang:spawn_opt/2,
1222 which is used to spawn the gen_statem process.
1223 Note:
1225 Using spawn option monitor is not allowed, it causes this func‐
1226 tion to fail with reason badarg.
1227 If the gen_statem is successfully created and initialized, this
1230 function returns {ok,Pid}, where Pid is the pid() of the
1231 gen_statem. If a process with the specified ServerName exists
1232 already, this function returns {error,{already_started,Pid}},
1233 where Pid is the pid() of that process.
1234 If Module:init/1 fails with Reason, this function returns {er‐
1236 ror,Reason}. If Module:init/1 returns {stop,Reason} or ignore,
1237 the process is terminated and this function returns {error,Rea‐
1238 son} or ignore, respectively. An exit signal with the same Rea‐
1239 son (or normal if Module:init/1 returns ignore) is set to linked
1240 processes and ports, including the process calling
1241 start_link/3,4.
1242 start_monitor(Module :: module(),
1244 Args :: term(),
1245 Opts :: [start_opt()]) ->
1246 start_mon_ret()
1247 start_monitor(ServerName :: server_name(),
1249 Module :: module(),
1250 Args :: term(),
1251 Opts :: [start_opt()]) ->
1252 start_mon_ret()
1253 Creates a standalone gen_statem process according to OTP design
1255 principles (using proc_lib primitives) and atomically sets up a
1256 monitor to the newly created process. As it does not get linked
1257 to the calling process, this start function cannot be used by a
1258 supervisor to start a child.
1259 For a description of arguments and return values, see
1261 start_link/3,4. Note that the return value on successful start
1262 differs from start_link/3,4. start_monitor/3,4 will return
1263 {ok,{Pid,Mon}} where Pid is the process identifier of the
1264 process, and Mon is a reference to the monitor set up to monitor
1265 the process. If the start is not successful, the caller will be
1266 blocked until the DOWN message has been received and removed
1267 from the message queue.
1268 stop(ServerRef :: server_ref()) -> ok
1270 The same as stop(ServerRef, normal, infinity).
1272 stop(ServerRef :: server_ref(),
1274 Reason :: term(),
1275 Timeout :: timeout()) ->
1276 ok
1277 Orders the gen_statem ServerRef to exit with the specified Rea‐
1279 son and waits for it to terminate. The gen_statem calls Mod‐
1280 ule:terminate/3 before exiting.
1281 This function returns ok if the server terminates with the ex‐
1283 pected reason. Any other reason than normal, shutdown, or {shut‐
1284 down,Term} causes an error report to be issued through log‐
1285 ger(3). An exit signal with the same reason is sent to linked
1286 processes and ports. The default Reason is normal.
1287 Timeout is an integer > 0, which specifies how many milliseconds
1289 to wait for the server to terminate, or the atom infinity to
1290 wait indefinitely. Defaults to infinity. If the server does not
1291 terminate within the specified time, a timeout exception is
1292 raised.
1293 If the process does not exist, a noproc exception is raised.
1295 wait_response(RequestId :: request_id()) ->
1297 {reply, Reply :: term()} |
1298 {error, {term(), server_ref()}}
1299 wait_response(RequestId :: request_id(), Timeout :: timeout()) ->
1301 {reply, Reply :: term()} |
1302 timeout |
1303 {error, {term(), server_ref()}}
1304 This function is used to wait for a reply of a request made with
1306 send_request/2 to the gen_statem process. This function must be
1307 called from the same process from which send_request/2 was made.
1308 Timeout is an integer greater then or equal to zero that speci‐
1310 fies how many milliseconds to wait for an reply, or the atom in‐
1311 finity to wait indefinitely. Defaults to infinity. If no reply
1312 is received within the specified time, the function returns
1313 timeout and no cleanup is done, and thus the function can be in‐
1314 voked repeatedly until a reply is returned.
1315 The return value Reply is generated when a state callback re‐
1317 turns with {reply,From,Reply} as one action(), and that Reply
1318 becomes the return value of this function.
1319 The function returns an error if the gen_statem dies before or
1321 during this function call.
1322 The difference between receive_response() and wait_response() is
1324 that receive_response() abandons the request at timeout so that
1325 a potential future response is ignored, while wait_response()
1326 does not.
1327
1329 The following functions are to be exported from a gen_statem callback
1330 module.
1331
1333 Module:callback_mode() -> CallbackMode
1334 Types:
1336 CallbackMode = callback_mode() | [ callback_mode() |
1338 state_enter() ]
1339 This function is called by a gen_statem when it needs to find
1341 out the callback mode of the callback module. The value is
1342 cached by gen_statem for efficiency reasons, so this function is
1343 only called once after server start, after code change, and af‐
1344 ter changing the callback module, but before the first state
1345 callback in the current callback module's code version is
1346 called. More occasions may be added in future versions of
1347 gen_statem.
1348 Server start happens either when Module:init/1 returns or when
1350 enter_loop/4-6 is called. Code change happens when Mod‐
1351 ule:code_change/4 returns. A change of the callback module hap‐
1352 pens when a state callback returns any of the actions
1353 change_callback_module, push_callback_module or pop_call‐
1354 back_module.
1355 The CallbackMode is either just callback_mode() or a list con‐
1357 taining callback_mode() and possibly the atom state_enter.
1358 Note:
1360 If this function's body does not return an inline constant value
1361 the callback module is doing something strange.
1362 Module:code_change(OldVsn, OldState, OldData, Extra) -> Result
1365 Types:
1367 OldVsn = Vsn | {down,Vsn}
1369 Vsn = term()
1370 OldState = NewState = term()
1371 Extra = term()
1372 Result = {ok,NewState,NewData} | Reason
1373 OldState = NewState = state()
1374 OldData = NewData = data()
1375 Reason = term()
1376 Note:
1378 This callback is optional, so callback modules need not export
1379 it. If a release upgrade/downgrade with Change = {advanced,Ex‐
1380 tra} specified in the .appup file is made when code_change/4 is
1381 not implemented the process will crash with exit reason undef.
1382 This function is called by a gen_statem when it is to update its
1385 internal state during a release upgrade/downgrade, that is, when
1386 the instruction {update,Module,Change,...}, where Change = {ad‐
1387 vanced,Extra}, is specified in the appup file. For more informa‐
1388 tion, see OTP Design Principles.
1389 For an upgrade, OldVsn is Vsn, and for a downgrade, OldVsn is
1391 {down,Vsn}. Vsn is defined by the vsn attribute(s) of the old
1392 version of the callback module Module. If no such attribute is
1393 defined, the version is the checksum of the Beam file.
1394 OldState and OldData is the internal state of the gen_statem.
1396 Extra is passed "as is" from the {advanced,Extra} part of the
1398 update instruction.
1399 If successful, the function must return the updated internal
1401 state in an {ok,NewState,NewData} tuple.
1402 If the function returns a failure Reason, the ongoing upgrade
1404 fails and rolls back to the old release. Note that Reason cannot
1405 be an {ok,_,_} tuple since that will be regarded as a {ok,New‐
1406 State,NewData} tuple, and that a tuple matching {ok,_} is an
1407 also invalid failure Reason. It is recommended to use an atom as
1408 Reason since it will be wrapped in an {error,Reason} tuple.
1409 Also note when upgrading a gen_statem, this function and hence
1411 the Change = {advanced,Extra} parameter in the appup file is not
1412 only needed to update the internal state or to act on the Extra
1413 argument. It is also needed if an upgrade or downgrade should
1414 change callback mode, or else the callback mode after the code
1415 change will not be honoured, most probably causing a server
1416 crash.
1417 If the server changes callback module using any of the actions
1419 change_callback_module, push_callback_module or pop_call‐
1420 back_module, be aware that it is always the current callback
1421 module that will get this callback call. That the current call‐
1422 back module handles the current state and data update should be
1423 no surprise, but it must be able to handle even parts of the
1424 state and data that it is not familiar with, somehow.
1425 In the supervisor child specification there is a list of modules
1427 which is recommended to contain only the callback module. For a
1428 gen_statem with multiple callback modules there is no real need
1429 to list all of them, it may not even be possible since the list
1430 could change after code upgrade. If this list would contain only
1431 the start callback module, as recommended, what is important is
1432 to upgrade that module whenever a synchronized code replacement
1433 is done. Then the release handler concludes that an upgrade that
1434 upgrades that module needs to suspend, code change, and resume
1435 any server whose child specification declares that it is using
1436 that module. And again; the current callback module will get the
1437 Module:code_change/4 call.
1438 Module:init(Args) -> Result(StateType)
1440 Types:
1442 Args = term()
1444 Result(StateType) = init_result(StateType)
1445 Whenever a gen_statem is started using start_link/3,4,
1447 start_monitor/3,4, or start/3,4, this function is called by the
1448 new process to initialize the implementation state and server
1449 data.
1450 Args is the Args argument provided to that start function.
1452 Note:
1454 Note that if the gen_statem is started through proc_lib and en‐
1455 ter_loop/4-6, this callback will never be called. Since this
1456 callback is not optional it can in that case be implemented as:
1457 -spec init(_) -> no_return().
1459 init(Args) -> erlang:error(not_implemented, [Args]).
1460 Module:format_status(Opt, [PDict,State,Data]) -> Status
1463 Types:
1465 Opt = normal | terminate
1467 PDict = [{Key, Value}]
1468 State = state()
1469 Data = data()
1470 Key = term()
1471 Value = term()
1472 Status = term()
1473 Note:
1475 This callback is optional, so a callback module does not need to
1476 export it. The gen_statem module provides a default implementa‐
1477 tion of this function that returns {State,Data}.
1478 If this callback is exported but fails, to hide possibly sensi‐
1480 tive data, the default function will instead return
1481 {State,Info}, where Info says nothing but the fact that for‐
1482 mat_status/2 has crashed.
1483 This function is called by a gen_statem process when any of the
1486 following apply:
1487 *
1489 One of sys:get_status/1,2 is invoked to get the gen_statem
1490 status. Opt is set to the atom normal for this case.
1491 *
1493 The gen_statem terminates abnormally and logs an error. Opt
1494 is set to the atom terminate for this case.
1495 This function is useful for changing the form and appearance of
1497 the gen_statem status for these cases. A callback module wishing
1498 to change the sys:get_status/1,2 return value and how its status
1499 appears in termination error logs exports an instance of for‐
1500 mat_status/2, which returns a term describing the current status
1501 of the gen_statem.
1502 PDict is the current value of the process dictionary of the
1504 gen_statem.
1505 State is the internal state of the gen_statem.
1507 Data is the internal server data of the gen_statem.
1509 The function is to return Status, a term that contains the ap‐
1511 propriate details of the current state and status of the
1512 gen_statem. There are no restrictions on the form Status can
1513 take, but for the sys:get_status/1,2 case (when Opt is normal),
1514 the recommended form for the Status value is [{data, [{"State",
1515 Term}]}], where Term provides relevant details of the gen_statem
1516 state. Following this recommendation is not required, but it
1517 makes the callback module status consistent with the rest of the
1518 sys:get_status/1,2 return value.
1519 One use for this function is to return compact alternative state
1521 representations to avoid having large state terms printed in log
1522 files. Another use is to hide sensitive data from being written
1523 to the error log.
1524 Module:StateName(enter, OldState, Data) -> StateEnterResult(StateName)
1526 Module:StateName(EventType, EventContent, Data) -> StateFunctionResult
1527 Module:handle_event(enter, OldState, State, Data) -> StateEnterRe‐
1528 sult(State)
1529 Module:handle_event(EventType, EventContent, State, Data) -> Han‐
1530 dleEventResult
1531 Types:
1533 EventType = event_type()
1535 EventContent = term()
1536 State = state()
1537 Data = NewData = data()
1538 StateEnterResult(StateName) = state_enter_result(StateName)
1539 StateFunctionResult = event_handler_result(state_name())
1540 StateEnterResult(State) = state_enter_result(State)
1541 HandleEventResult = event_handler_result(state())
1542 Whenever a gen_statem receives an event from call/2, cast/2, or
1544 as a normal process message, one of these functions is called.
1545 If callback mode is state_functions, Module:StateName/3 is
1546 called, and if it is handle_event_function, Module:han‐
1547 dle_event/4 is called.
1548 If EventType is {call,From}, the caller waits for a reply. The
1550 reply can be sent from this or from any other state callback by
1551 returning with {reply,From,Reply} in Actions, in Replies, or by
1552 calling reply(From, Reply).
1553 If this function returns with a next state that does not match
1555 equal (=/=) to the current state, all postponed events are re‐
1556 tried in the next state.
1557 The only difference between StateFunctionResult and HandleEven‐
1559 tResult is that for StateFunctionResult the next state must be
1560 an atom, but for HandleEventResult there is no restriction on
1561 the next state.
1562 For options that can be set and actions that can be done by
1564 gen_statem after returning from this function, see action().
1565 When the gen_statem runs with state enter calls, these functions
1567 are also called with arguments (enter, OldState, ...) during ev‐
1568 ery state change. In this case there are some restrictions on
1569 the actions that may be returned: postpone() is not allowed
1570 since a state enter call is not an event so there is no event to
1571 postpone, and {next_event,_,_} is not allowed since using state
1572 enter calls should not affect how events are consumed and pro‐
1573 duced. You may also not change states from this call. Should you
1574 return {next_state,NextState, ...} with NextState =/= State the
1575 gen_statem crashes. Note that it is actually allowed to use {re‐
1576 peat_state, NewData, ...} although it makes little sense since
1577 you immediately will be called again with a new state enter call
1578 making this just a weird way of looping, and there are better
1579 ways to loop in Erlang. If you do not update NewData and have
1580 some loop termination condition, or if you use {re‐
1581 peat_state_and_data, _} or repeat_state_and_data you have an in‐
1582 finite loop! You are advised to use {keep_state,...},
1583 {keep_state_and_data,_} or keep_state_and_data since changing
1584 states from a state enter call is not possible anyway.
1585 Note the fact that you can use throw to return the result, which
1587 can be useful. For example to bail out with
1588 throw(keep_state_and_data) from deep within complex code that
1589 cannot return {next_state,State,Data} because State or Data is
1590 no longer in scope.
1591 Module:terminate(Reason, State, Data) -> Ignored
1593 Types:
1595 Reason = normal | shutdown | {shutdown,term()} | term()
1597 State = state()
1598 Data = data()
1599 Ignored = term()
1600 Note:
1602 This callback is optional, so callback modules need not export
1603 it. The gen_statem module provides a default implementation
1604 without cleanup.
1605 This function is called by a gen_statem when it is about to ter‐
1608 minate. It is to be the opposite of Module:init/1 and do any
1609 necessary cleaning up. When it returns, the gen_statem termi‐
1610 nates with Reason. The return value is ignored.
1611 Reason is a term denoting the stop reason and State is the in‐
1613 ternal state of the gen_statem.
1614 Reason depends on why the gen_statem is terminating. If it is
1616 because another callback function has returned, a stop tuple
1617 {stop,Reason} in Actions, Reason has the value specified in that
1618 tuple. If it is because of a failure, Reason is the error rea‐
1619 son.
1620 If the gen_statem is part of a supervision tree and is ordered
1622 by its supervisor to terminate, this function is called with
1623 Reason = shutdown if both the following conditions apply:
1624 * The gen_statem has been set to trap exit signals.
1626 * The shutdown strategy as defined in the supervisor's child
1628 specification is an integer time-out value, not brutal_kill.
1629 Even if the gen_statem is not part of a supervision tree, this
1631 function is called if it receives an 'EXIT' message from its
1632 parent. Reason is the same as in the 'EXIT' message.
1633 Otherwise, the gen_statem is immediately terminated.
1635 Notice that for any other reason than normal, shutdown, or
1637 {shutdown,Term}, the gen_statem is assumed to terminate because
1638 of an error and an error report is issued using logger(3).
1639 When the gen_statem process exits, an exit signal with the same
1641 reason is sent to linked processes and ports.
1642
1644 gen_event(3), gen_fsm(3), gen_server(3), proc_lib(3), supervisor(3),
1645 sys(3).
1646Ericsson AB stdlib 3.16.1 gen_statem(3)