path: root/lib/stdlib/doc/src/gen_statem.xml

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  <header>
    <copyright>
      <year>2016</year><year>2019</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 since="OTP 19.0">gen_statem</module>
  <modulesummary>Generic state machine behavior.</modulesummary>
  <description>
    <p>
      <c>gen_statem</c> provides a generic state machine behaviour
      that for new code replaces its predecessor
      <seealso marker="gen_fsm"><c>gen_fsm</c></seealso>
      since Erlang/OTP 20.0. The <c>gen_fsm</c> behaviour remains
      in OTP "as is".
    </p>
    <note>
      <p>
	If you are new to <c>gen_statem</c> and want an overview
	of concepts and operation the section
	<seealso marker="doc/design_principles:statem">
	  <c>gen_statem</c>&nbsp;Behaviour
	</seealso>
	located in the User's Guide
	<seealso marker="doc/design_principles:users_guide">
	  OTP Design Principles
	</seealso>
	is recommended to read before this reference manual,
	possibly after the Description section you are reading here.
      </p>
    </note>
    <p>
      This reference manual contains type descriptions generated from
      types in the <c>gen_statem</c> source code, so they are correct.
      However, the generated descriptions also reflect the type hierarchy,
      which sometimes makes it hard to get a good overview.
      If so, see the section
      <seealso marker="doc/design_principles:statem">
	<c>gen_statem</c>&nbsp;Behaviour
      </seealso>
      in the
      <seealso marker="doc/design_principles:users_guide">
	OTP Design Principles
      </seealso>
      User's Guide.
    </p>
    <note>
      <list type="bulleted">
	<item>This behavior appeared in Erlang/OTP 19.0.</item>
	<item>
	  In OTP 19.1 a backwards incompatible change of
	  the return tuple from
	  <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>
	  was made and the mandatory callback function
	  <seealso marker="#Module:callback_mode/0">
	    <c>Module:callback_mode/0</c>
	  </seealso>
	  was introduced.
	</item>
	<item>
	   In OTP 20.0
           <seealso marker="#type-generic_timeout">
	     generic time-outs
	   </seealso>
	   were added.
	</item>
	<item>
	  In OTP 22.1 time-out content
          <seealso marker="#type-timeout_update_action">
	    <c>update</c>
	  </seealso>
	  and explicit time-out
          <seealso marker="#type-timeout_cancel_action">
	    <c>cancel</c>
	  </seealso>
	  were added.
	</item>
      </list>
    </note>
    <p>
      <c>gen_statem</c> has got the same features that
      <seealso marker="gen_fsm"><c>gen_fsm</c></seealso>
      had and adds some really useful:
    </p>
    <list type="bulleted">
      <item>Co-located 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, <c>sys</c> traceable
      </item>
      <item>Multiple <c>sys</c> traceable replies</item>
    </list>


    <p>
      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>
    <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 <em>state callback</em> name; see
      <seealso marker="#Module:StateName/3"><c>Module:StateName/3</c></seealso>.
      This co-locates 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 <em>state callback</em> 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>
      When <c>gen_statem</c> receives a process message it is
      converted into an event and the
      <seealso marker="#state callback"><em>state callback</em></seealso>
      is called with the event as two arguments: type and content.
      When the
      <seealso marker="#state callback"><em>state callback</em></seealso>
      has processed the event it returns to <c>gen_statem</c>
      which does a <em>state transition</em>.
      If this <em>state transition</em> is to a different state,
      that is: <c>NextState =/= State</c>, it is a <em>state change</em>.
    </p>
    <p>
      The
      <seealso marker="#state callback"><em>state callback</em></seealso>
      may return
      <seealso marker="#type-action"><em>transition actions</em></seealso>
      for <c>gen_statem</c>
      to execute during the <em>state transition</em>,
      for example to reply to a
      <seealso marker="#call/2"><c>gen_statem:call/2,3</c></seealso>.
    </p>
    <p>
      One of the possible <em>transition actions</em>
      is to postpone the current event.
      Then it is not retried in the current state.
      The <c>gen_statem</c> engine keeps a queue of events
      divided into the postponed events
      and the events still to process.
      After a <em>state change</em> 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"><em>state callback</em></seealso>
      can insert events using the
      <seealso marker="#type-action"><em>transition actions</em></seealso>
      <c>next_event</c>
      and such an event is inserted in the event queue
      as the next to call the
      <seealso marker="#state callback"><em>state callback</em></seealso>
      with.
      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"><em>state callback</em></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 explicitly insert an event
      at the <em>state transition</em>,
      and/or to use a dedicated <em>state transition</em> function,
      but that is something you will have to remember
      at every <em>state transition</em> to the state(s) that need it.
    </p>
    <note>
      <p>If you in <c>gen_statem</c>, for example, postpone
        an event in one state and then call another <em>state callback</em>
        of yours, you have not done a <em>state change</em>
	and hence the postponed event is not retried,
	which is logical but can be confusing.
      </p>
    </note>
    <p>
      For the details of a <em>state transition</em>, 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"><em>state callback</em></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>
    <p>
      There is also a server start option
      <seealso marker="#type-enter_loop_opt">
	<c>{hibernate_after, Timeout}</c>
      </seealso>
      for
      <seealso marker="#start/3"><c>start/3,4</c></seealso>,
      <seealso marker="#start_link/3"><c>start_link/3,4</c></seealso> or
      <seealso marker="#enter_loop/4"><c>enter_loop/4,5,6</c></seealso>,
      that may be used to automatically hibernate the server.
    </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>handle_event_function</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="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="enter_loop_opt"/>
      <desc>
	<p>
	  Options 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>
	<taglist>
	  <tag><c>hibernate_after</c></tag>
	  <item>
	    <p>
	      <c>HibernateAfterTimeout</c>
	      specifies that 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>
	  <tag><c>debug</c></tag>
	  <item>
	    <p>
	      For every entry in <c><anno>Dbgs</anno></c>,
	      the corresponding function in
	      <seealso marker="sys"><c>sys</c></seealso> is called.
	    </p>
	  </item>
	</taglist>
      </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 <em>state change</em> (<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 <em>state change</em> (<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>
	  There are 3 categories of events:
	  <seealso marker="#type-external_event_type">external</seealso>,
	  <seealso marker="#type-timeout_event_type">timeout</seealso>,
	  and <c>internal</c>.
	</p>
	<p>
	  <c>internal</c> events can only be generated by the
	  state machine itself through the <em>transition action</em>
	  <seealso marker="#type-action"><c>next_event</c></seealso>.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="external_event_type"/>
      <desc>
	<p>
	  External events are of 3 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>.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="timeout_event_type"/>
      <desc>
	<p>
	  There are 3 types of time-out events that the state machine
	  can generate for itself with the corresponding
	  <seealso marker="#type-timeout_action">timeout_action()</seealso>s.
	</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"><em>callback mode</em></seealso>
	  and whether to do
	  <seealso marker="#type-state_enter"><em>state enter calls</em></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 <em>state change</em>,
	  call the
	  <seealso marker="#state callback">state callback</seealso>
	  with arguments <c>(enter, OldState, Data)</c> or
	  <c>(enter, OldState, State, Data)</c>, depending on the
	  <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>.
	  This may look like an event but is really a call
	  performed after the previous
	  <seealso marker="#state callback"><em>state callback</em></seealso>
	  returned and before any event is delivered to the new
	  <seealso marker="#state callback"><em>state callback</em></seealso>.
	  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 <em>state callback</em>.
	</p>
	<p>
	  If 
	  <seealso marker="#Module:callback_mode/0"><c>Module:callback_mode/0</c></seealso>
	  does not return such a list, no <em>state enter calls</em> are done.
	</p>
	<p>
	  If 
	  <seealso marker="#Module:code_change/4"><c>Module:code_change/4</c></seealso>
	  should transform the state,
	  it is regarded as a state rename and not a <em>state change</em>,
	  which will not cause a <em>state enter call</em>.
	</p>
	<p>
	  Note that a <em>state enter call</em> <em>will</em> be done
	  right before entering the initial state even though this
	  actually is not a <em>state change</em>.
	  In this case <c>OldState =:= State</c>,
	  which cannot happen for a subsequent state change,
	  but will happen when repeating the <em>state enter call</em>.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="transition_option"/>
      <desc>
	<p>
	  Transition options can be set by
	  <seealso marker="#type-action">actions</seealso>
	  and modify the <em>state transition</em>.
	  The <em>state transition</em> takes place when the
	  <seealso marker="#state callback"><em>state callback</em></seealso>
	  has processed an event and returns.
	  Here are the sequence of steps for a <em>state transition</em>:
	</p>
	<list type="ordered">
	  <item>
	    <p>
	      All returned
              <seealso marker="#type-action">actions</seealso>
	      are processed in order of appearance.
	      In this step all replies generated by any
	      <seealso marker="#type-reply_action"><c>reply_action()</c></seealso>
	      are sent.  Other actions set <c>transition_option()</c>s
	      that come into play in subsequent steps.
            </p>
	  </item>
	  <item>
            <p>
	      If
	      <seealso marker="#type-state_enter">
		<em>state enter calls</em>
	      </seealso>
	      are used, and either
	      it is the initial state or one of the callback results
	      <seealso marker="#type-state_callback_result">
		<c>repeat_state_and_data</c>
	      </seealso>
	      or
	      <seealso marker="#type-state_callback_result">
		<c>repeat_state_and_data</c>
	      </seealso>
	      is used the <c>gen_statem</c> engine calls
	      the current state callback with arguments
	      <seealso marker="#type-state_enter"><c>(enter, State, Data)</c></seealso>
	      or
	      <seealso marker="#type-state_enter"><c>(enter, State, State, Data)</c></seealso>
	      (depending on <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>)
	      and when it returns starts again from the top of this sequence.
	    </p>
            <p>
	      If
	      <seealso marker="#type-state_enter">
		<em>state enter calls</em>
	      </seealso>
	      are used, and the state changes
	      the <c>gen_statem</c> engine calls
	      the new state callback with arguments
	      <seealso marker="#type-state_enter"><c>(enter, OldState, Data)</c></seealso>
	      or
	      <seealso marker="#type-state_enter"><c>(enter, OldState, State, Data)</c></seealso>
	      (depending on <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>)
	      and when it returns starts again from the top of this sequence.
	    </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 this is a <em>state change</em>,
	      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 previously 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 received event;
	      that is after already queued events
	      such as inserted and postponed events.
	    </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 <em>state change</em> 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, that is; a
              <seealso marker="#type-state_timeout"><c>state_timeout()</c></seealso>
	      applies to the state the state machine enters.
	    </p>
	  </item>
	  <item>
	    <p>
	      If there are enqueued events the
	      <seealso marker="#state callback"><em>state callback</em></seealso>
	      for the possibly new state
	      is called with the oldest enqueued event,
	      and we start again from the top of this sequence.
	    </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"><em>state callback</em></seealso>
	      is called with the corresponding event,
	      and we start again from the top of this sequence.
	    </p>
	  </item>
	</list>
      </desc>
    </datatype>
    <datatype>
      <name name="postpone"/>
      <desc>
	<p>
	  If <c>true</c>, postpones the current event and retries
	  it after a <em>state change</em>
	  (<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.
	</p>
	<note>
	  <p>
	    If there are enqueued events to process
	    when hibrnation is requested,
	    this is optimized by not hibernating but instead calling
	    <seealso marker="erts:erlang#garbage_collect/0">
	      <c>erlang:garbage_collect/0</c>
	    </seealso>
	    to simulate that the <c>gen_statem</c> entered hibernation
	    and immediately got awakened by an enqueued event.
	  </p>
	</note>
      </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, but after already queued events.
	</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 <em>transition actions</em> can be invoked by
	  returning them from the
	  <seealso marker="#state callback"><em>state callback</em></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 <em>state transition</em>.
	      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>
	      This action does not set any
	      <seealso marker="#type-transition_option">
		<c>transition_option()</c>
	      </seealso>
	      but instead 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 <em>transition actions</em> can be invoked by
	  returning them from the
	  <seealso marker="#state callback"><em>state callback</em></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 <em>state transition</em>.
	    </p>
	  </item>
	</taglist>
      </desc>
    </datatype>
    <datatype>
      <name name="timeout_action"/>
      <desc>
	<p>
	  These <em>transition actions</em> can be invoked by
	  returning them from the
	  <seealso marker="#state callback"><em>state callback</em></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>
	  These time-out actions sets time-out
	  <seealso marker="#type-transition_option">transition options</seealso>.
	</p>
	<taglist>
	  <tag><c>Time</c></tag>
	  <item>
	    <p>
	      Short for <c>{timeout,Time,Time}</c>, that is,
	      the time-out message is the time-out time.
	      This form exists to make the
	      <seealso marker="#state callback"><em>state callback</em></seealso>
	      return value <c>{next_state,NextState,NewData,Time}</c>
	      allowed like for <c>gen_fsm</c>.
	    </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="timeout_cancel_action" since="OTP 22.1"/>
      <desc>
	<p>
	  This is a shorter and clearer form of
	  <seealso marker="#type-timeout_action">
	    timeout_action()
	  </seealso>
	  with <c>Time = infinity</c> which cancels a time-out.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="timeout_update_action" since="OTP 22.1"/>
      <desc>
	<p>
	  Updates a time-out with a new <c>EventContent</c>.
	  See
	  <seealso marker="#type-timeout_action">
	    timeout_action()
	  </seealso>
	  for how to start a time-out.
	</p>
	<p>
	  If no time-out of the same type is active instead
	  insert the time-out event just like when starting
	  a time-out with relative <c>Time = 0</c>.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="reply_action"/>
      <desc>
	<p>
	  This <em>transition action</em> can be invoked by
	  returning it from the
	  <seealso marker="#state callback"><em>state callback</em></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 does not set any
	  <seealso marker="#type-transition_option">
	    <c>transition_option()</c>
	  </seealso>
	  but instead 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"><em>state callback</em></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 witchcraft
	  since there has been no earlier call to a
	  <seealso marker="#state callback"><em>state callback</em></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"><em>state callback</em></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 cannot 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 <em>state transition</em> 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>.
	      If <c><anno>NextState</anno> =/= CurrentState</c>
	      the <em>state transition</em> is a <em>state change</em>.
	    </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 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 same as
	      <c>{keep_state,CurrentData,<anno>Actions</anno>}</c>.
	    </p>
	  </item>
	  <tag><c>repeat_state</c></tag>
	  <item>
	    <p>
	      If the <c>gen_statem</c> runs with
	      <seealso marker="#type-state_enter"><em>state enter calls</em></seealso>,
	      the <em>state enter call</em> is repeated, see type
	      <seealso marker="#type-transition_option"><c>transition_option()</c></seealso>,
	      other than that <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 same as
	      <c>{repeat_state,CurrentData,<anno>Actions</anno>}</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" since="OTP 19.0"/>
      <name name="call" arity="3" since="OTP 19.0"/>
      <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"><em>state callback</em></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"><em>state callback</em></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
	  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.  Note that there is an odd chance
	    to get a late reply even with
	    <c>{dirty_timeout,infinity}</c> or <c>infinity</c>
	    for example in the event of network problems.
	    So why not just let the calling process die
	    by not catching the exception?
	  </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" since="OTP 19.0"/>
      <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"><em>state callback</em></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" since="OTP 19.1"/>
      <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" since="OTP 19.0"/>
      <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" since="OTP 19.0"/>
      <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" since="OTP 19.0"/>
      <name name="reply" arity="2" since="OTP 19.0"/>
      <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"><em>state callback</em></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"><em>state callback</em></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"><em>state callback</em></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" since="OTP 19.0"/>
      <name name="start" arity="4" since="OTP 19.0"/>
      <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" since="OTP 19.0"/>
      <name name="start_link" arity="4" since="OTP 19.0"/>
      <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
	      <seealso marker="#type-start_opt">
		<c>{timeout,Time}</c>
	      </seealso>
	      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-enter_loop_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-enter_loop_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
              <seealso marker="#type-start_opt">
		<c>{spawn_opt,SpawnOpts}</c>
	      </seealso>
	      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" since="OTP 19.0"/>
      <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" since="OTP 19.0"/>
      <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:logger"><c>logger(3)</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 since="OTP 19.1">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"><em>state callback</em></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 since="OTP 19.0">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> cannot 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 <em>callback mode</em> after the code change
	  will not be honoured,
	  most probably causing a server crash.
	</p>
      </desc>
    </func>

    <func>
      <name since="OTP 19.0">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 through
	    <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 since="OTP 19.0">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 since="OTP 19.0">Module:StateName(enter, OldState, Data) ->
        StateEnterResult(StateName)
      </name>
      <name since="OTP 19.0">Module:StateName(EventType, EventContent, Data) ->
        StateFunctionResult
      </name>
      <name since="OTP 19.0">Module:handle_event(enter, OldState, State, Data) ->
	StateEnterResult(State)
      </name>
      <name since="OTP 19.0">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"><em>state callback</em></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"><em>state enter calls</em></seealso>,
	  these functions are also called with arguments
	  <c>(enter, OldState, ...)</c> during every <em>state change</em>.
	  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.
	  Note that it is actually allowed to use
	  <c>{repeat_state, NewData, ...}</c> although it 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.
	  If you do not update <c>NewData</c> and have some
	  loop termination condition, or if you use
	  <c>{repeat_state_and_data, _}</c> or
	  <c>repeat_state_and_data</c> you have an infinite loop!
	  You are advised to use <c>{keep_state,...}</c>,
	  <c>{keep_state_and_data,_}</c> or
	  <c>keep_state_and_data</c>
	  since changing states from a <em>state enter call</em>
	  is not possible 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 cannot
	  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 since="OTP 19.0">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:logger"><c>logger(3)</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>