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

<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE erlref SYSTEM "erlref.dtd">

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

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

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

    </legalnotice>

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

gen_statem:stop       -----> Module:terminate/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> will terminate.  An exception of class
      <seealso marker="erts:erlang#throw/1"><c>throw</c></seealso>,
      however, is not regarded as an error but as a valid return.
    </p>
    <marker id="state_function" />
    <p>
      The "<em>state function</em>" for a specific
      <seealso marker="#type-state">state</seealso>
      in a <c>gen_statem</c> is the callback function that is called
      for all events in this state, and is selected depending on which
      <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>
      that the implementation specifies when the the server starts.
    </p>
    <p>
      When the
      <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>
      is <c>state_functions</c>, the state has to be an atom and
      is used as the state function name.  See
      <seealso marker="#Module:StateName/3">
        <c>Module:StateName/3</c>
      </seealso>.
      This gathers all code for a specific state
      in one function and hence dispatches on state first.
      Note that in this mode the fact that there is
      a mandatory callback function
      <seealso marker="#Module:terminate/3">
        <c>Module:terminate/3</c>
      </seealso> makes the state name <c>terminate</c> unusable.
    </p>
    <p>
      When the
      <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>
      is <c>handle_event_function</c> the state can be any term
      and the state function name is
      <seealso marker="#Module:handle_event/4">
        <c>Module:handle_event/4</c>
      </seealso>.
      This makes it easy to dispatch on state or on event as you desire.
      Be careful about which events you handle in which
      states so you do not accidentally postpone one event
      forever creating an infinite busy loop.
    </p>
    <p>
      The <c>gen_statem</c> enqueues incoming events in order of arrival
      and presents these to the
      <seealso marker="#state_function">state function</seealso>
      in that order.  The state function can postpone an event
      so it is not retried in the current state.
      After a state change 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_function">state function</seealso>
      can insert events using the
      <seealso marker="#type-action">
	<c>action()</c> <c>next_event</c>
      </seealso>
      and such an event is inserted as the next to present
      to the state function.  That is: as if it is
      the oldest incoming event.  There is a dedicated
      <seealso marker="#type-event_type">
        <c>event_type()</c> <c>internal</c>
      </seealso>
      that can be used for such events making 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">gen_fsm</seealso>
      to force processing an inserted event before others.
      A warning, though: if you in <c>gen_statem</c> for example
      postpone an event in one state and then call some other state function of yours,
      you have not changed states and hence the postponed event will not be retried,
      which is logical but might be confusing.
    </p>
    <p>
      See the type
      <seealso marker="#type-transition_option">
	<c>transition_option()</c>
      </seealso>
      for the details of a state transition.
    </p>
    <p>
      A <c>gen_statem</c> handles system messages as documented in
      <seealso marker="sys">sys</seealso>.
      The <seealso marker="sys">sys</seealso> module
      can be used for debugging a <c>gen_statem</c>.
    </p>
    <p>
      Note 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 given.
    </p>
    <p>
      The <c>gen_statem</c> process can go into hibernation (see
      <seealso marker="erts:erlang#hibernate/3">
	<c>erlang:hibernate/3</c>
      </seealso>) if a
      <seealso marker="#state_function">state function</seealso> or
      <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>
      specifies <c>hibernate</c> in the returned
      <seealso marker="#type-action"><c>Actions</c></seealso>
      list. This might be useful if the server is expected to be idle
      for a long time. However use this feature with care
      since hibernation implies at least two garbage collections
      (when hibernating and shortly after waking up) and that is not
      something you would want to do between each event on a busy server.
    </p>
  </description>

  <section>
    <title>EXAMPLE</title>
    <p>
      This 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 on.
    </p>
    <p>This 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]).
-export([on/3,off/3]).

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

%% 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) ->
    {callback_mode(),State,Data}.
init([]) ->
    %% Set the callback mode and initial state + data.
    %% Data is used only as a counter.
    State = off, Data = 0,
    {callback_mode(),State,Data}.


%%% State functions

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>And this 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>
      And just to compare styles here is the same example using
      <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>
      <c>state_functions</c>, or rather here is code to replace
      from the <c>init/1</c> function of the <c>pushbutton.erl</c>
      example file above:
    </p>
    <code type="erl">
init([]) ->
    %% Set the callback mode and initial state + data.
    %% Data is used only as a counter.
    State = off, Data = 0,
    {handle_event_function,State,Data}.


%%% Event handling

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">call/2</seealso> and
	  <seealso marker="#type-server_name">
	    <c>server_name()</c>
	  </seealso>
	  above.
	</p>
	<p>It can be:</p>
	<taglist>
	  <tag><c>pid()</c><br />
	  <c>LocalName</c></tag>
	  <item>The <c>gen_statem</c> is locally registered.</item>
	  <tag><c>Name, Node</c></tag>
	  <item>
	    The <c>gen_statem</c> is locally registered
	    on another node.
	  </item>
	  <tag><c>GlobalName</c></tag>
	  <item>
	    The <c>gen_statem</c> is globally registered
	    in <seealso marker="kernel:global"><c>global</c></seealso>.
	  </item>
	  <tag><c>RegMod, ViaName</c></tag>
	  <item>
	    The <c>gen_statem</c> is registered through
	    an alternative process registry.
	    The registry callback module <c>RegMod</c>
	    should export the functions
	    <c>register_name/2</c>, <c>unregister_name/1</c>,
	    <c>whereis_name/1</c> and <c>send/2</c>,
	    which should behave like the corresponding functions
	    in <seealso marker="kernel:global"><c>global</c></seealso>.
	    Thus, <c>{via,global,GlobalName}</c> is the same as
	    <c>{global,GlobalName}</c>.
	  </item>
	</taglist>
      </desc>
    </datatype>
    <datatype>
      <name name="debug_opt" />
      <desc>
	<p>
	  Debug option that can be used when starting
	  a <c>gen_statem</c> server through for example
	  <seealso marker="#enter_loop/5">enter_loop/5</seealso>.
	</p>
	<p>
	  For every entry in <c><anno>Dbgs</anno></c>
	  the corresponding function in
	  <seealso marker="sys"><c>sys</c></seealso> will be called.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="start_opt" />
      <desc>
	<p>
	  Options that can be used when starting
	  a <c>gen_statem</c> server through for example
	  <seealso marker="#start_link/3">start_link/3</seealso>.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="start_ret" />
      <desc>
	<p>
	  Return value from the start functions for_example
	  <seealso marker="#start_link/3">start_link/3</seealso>.
	</p>
      </desc>
    </datatype>

    <datatype>
      <name name="from" />
      <desc>
	<p>
	  Destination to use when replying through for example the
	  <seealso marker="#type-action">
	    action() {reply,From,Reply}
	  </seealso>
	  to a process that has called the <c>gen_statem</c> server using
	  <seealso marker="#call/2">call/2</seealso>.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="state" />
      <desc>
	<p>
	  After a state change (<c>NextState =/= State</c>)
	  all postponed events are retried.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="state_name" />
      <desc>
	<p>
	  If the
	  <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>
	  is <c>state_functions</c>,
	  the state has to be of this type.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="data" />
      <desc>
	<p>
	  A term in which the state machine implementation
	  should store any server data it needs.  The difference between
	  this and the <seealso marker="#type-state">state()</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 than that data item should be made
	  a part of the state.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="event_type" />
      <desc>
	<p>
	  External events are of 3 different type:
	  <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 contain whom to reply to.
	  Type <c>info</c> originates from regular process messages sent
	  to the <c>gen_statem</c>. It is also possible for the state machine
	  implementation to generate events of types
	  <c>timeout</c> and <c>internal</c> to itself.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="callback_mode" />
      <desc>
	<p>
	  The <em>callback mode</em> is selected when starting the
	  <c>gen_statem</c> using the return value from
	  <seealso marker="#Module:init/1">Module:init/1</seealso>
	  or when calling
	  <seealso marker="#enter_loop/5">enter_loop/5-7</seealso>.
	</p>
	<taglist>
	  <tag><c>state_functions</c></tag>
	  <item>
	    The state has to be of type
	    <seealso marker="#type-state_name"><c>state_name()</c></seealso>
	    and one callback function per state that is
	    <seealso marker="#Module:StateName/3">
	      <c>Module:StateName/3</c>
	    </seealso>
	    is used.
	  </item>
	  <tag><c>handle_event_function</c></tag>
	  <item>
	    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.
	  </item>
	</taglist>
      </desc>
    </datatype>
    <datatype>
      <name name="transition_option" />
      <desc>
	<p>
	  Transition options may be set by
	  <seealso marker="#type-action">actions</seealso>
	  and they modify some details below in how
	  the state transition is done:
	</p>
	<list type="ordered">
	  <item>
	    All
            <seealso marker="#type-action"><c>actions</c></seealso>
	    are processed in order of appearance.
	  </item>
	  <item>
	    If
            <seealso marker="#type-postpone">
	      <c>postpone()</c>
	    </seealso>
	    is <c>true</c>
	    the current event is postponed.
	  </item>
	  <item>
	    If the state changes the queue of incoming events
            is reset to start with the oldest postponed.
	  </item>
	  <item>
	    All events stored with
	    <seealso marker="#type-action">
	      <c>action() next_event</c>
	    </seealso>
	    are inserted in the queue to be processed before
	    all other events.
	  </item>
	  <item>
	    If an
            <seealso marker="#type-event_timeout">
	      <em><c>event_timeout()</c></em>
	    </seealso>
	    is set through
            <seealso marker="#type-action">
	      <c>action() timeout</c>
	    </seealso>
	    an event timer may be started or a timeout zero event
	    may be enqueued.
	  </item>
	  <item>
	    The (possibly new)
            <seealso marker="#state_function">state function</seealso>
            is called with the oldest enqueued event if there is any,
	    otherwise the <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 back into hibernation.
	  </item>
	</list>
      </desc>
    </datatype>
    <datatype>
      <name name="postpone" />
      <desc>
	<p>
	  If <c>true</c> postpone the current event and retry
	  it when the state changes
	  (<c>NextState =/= State</c>).
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="hibernate" />
      <desc>
	<p>
	  If <c>true</c> hibernate the <c>gen_statem</c>
	  by calling
	  <seealso marker="proc_lib#hibernate/3">
	    <c>proc_lib:hibernate/3</c>
	  </seealso>
	  before going into <c>receive</c>
	  to wait for a new external event.
	  If there are enqueued events the <c>hibernate</c>
	  is ignored as if an event just arrived and awakened
	  the <c>gen_statem</c>.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="event_timeout" />
      <desc>
	<p>
	  Generate an event of
	  <seealso marker="#type-event_type">type <c>timeout</c></seealso>
	  after this time (in milliseconds) unless some other
	  event arrives in which case this timeout is cancelled.
	  Note that a retried or inserted event
	  counts just like a new in this respect.
	</p>
	<p>
	  If the value is <c>infinity</c> no timer is started since
	  it will never trigger anyway.
	</p>
	<p>
	  If the value is <c>0</c> the timeout event is immediately enqueued
	  unless there already are enqueued events since then the
	  timeout is immediately cancelled.
	  This is a feature ensuring that a timeout <c>0</c> event
	  will be processed before any not yet received external event.
	</p>
	<p>
	  Note that it is not possible nor needed to cancel this timeout
	  since it is cancelled automatically by any other event.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="action" />
      <desc>
	<p>
	  These state transition actions may be invoked by
	  returning them from the
	  <seealso marker="#state_function">state function</seealso>,
	  from <seealso marker="#Module:init/1">Module:init/1</seealso>
	  or by giving them to
	  <seealso marker="#enter_loop/6">enter_loop/6,7</seealso>.
	</p>
	<p>
	  Actions are executed in the containing list order.
	</p>
	<p>
	  Actions that set
	  <seealso marker="#type-transition_option">
	    transition options
	  </seealso>
	  overrides 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 other <c>event_timeout()</c> in the list.
	</p>
	<taglist>
	  <tag><c>postpone</c></tag>
	  <item>
	    Set the
	    <seealso marker="#type-transition_option">
	      <c>transition_option() postpone()</c>
	    </seealso>
	    for this state transition.
	    This action is ignored when returned from
	    <seealso marker="#Module:init/1">Module:init/1</seealso>
	    or given to
	    <seealso marker="#enter_loop/5">enter_loop/5,6</seealso>
	    since there is no event to postpone in those cases.
	  </item>
	  <tag><c>hibernate</c></tag>
	  <item>
	    Set the
	    <seealso marker="#type-transition_option">
	      <c>transition_option() hibernate()</c>
	    </seealso>
	    for this state transition.
	  </item>
	  <tag><c>Timeout</c></tag>
	  <item>
	    Short for <c>{timeout,Timeout,Timeout}</c> that is
	    the timeout message is the timeout time.
	    This form exists to make the
	    <seealso marker="#state_function">state function</seealso>
	    return value <c>{next_state,NextState,NewData,Timeout}</c>
	    allowed like for
	    <seealso marker="gen_fsm#Module:StateName/2">
	      gen_fsm Module:StateName/2</seealso>.
	  </item>
	  <tag><c>timeout</c></tag>
	  <item>
	    Set the
	    <seealso marker="#type-transition_option">
	      <c>transition_option() event_timeout()</c>
	    </seealso>
	    to <c><anno>Time</anno></c> with <c><anno>EventContent</anno></c>.
	  </item>
	  <tag><c>reply_action()</c></tag>
	  <item>Reply to a caller.</item>
	  <tag><c>next_event</c></tag>
	  <item>
	      Store the given <c><anno>EventType</anno></c>
	      and <c><anno>EventContent</anno></c> for insertion after all
	      actions have been executed.
	  </item>
	  <item>
	    <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
	      will become the first to process.
	    </p>
	  </item>
	  <item>
	    <p>
	      An event of type
	      <seealso marker="#type-event_type">
		<c>internal</c>
	      </seealso>
	      should be used when you want to reliably distinguish
	      an event inserted this way from any external event.
	    </p>
	  </item>
	</taglist>
      </desc>
    </datatype>
    <datatype>
      <name name="reply_action" />
      <desc>
	<p>
	  Reply 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 the
	  <seealso marker="#type-event_type">
	    <c>{call,<anno>From</anno>}</c>
	  </seealso>
	  argument to the
	  <seealso marker="#state_function">state function</seealso>.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="state_function_result" />
      <desc>
	<taglist>
	  <tag><c>next_state</c></tag>
	  <item>
	    The <c>gen_statem</c> will do a state transition to
	    <c><anno>NextStateName</anno></c>
	    (which may be the same as the current state),
	    set <c><anno>NewData</anno></c>
	    and execute all <c><anno>Actions</anno></c>
	  </item>
	</taglist>
	<p>
	  All these terms are tuples or atoms and this property
	  will hold in any future version of <c>gen_statem</c>,
	  just in case you need such a promise.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="handle_event_result" />
      <desc>
	<taglist>
	  <tag><c>next_state</c></tag>
	  <item>
	    The <c>gen_statem</c> will do a state transition to
	    <c><anno>NextState</anno></c>
	    (which may be the same as the current state),
	    set <c><anno>NewData</anno></c>
	    and execute all <c><anno>Actions</anno></c>
	  </item>
	</taglist>
	<p>
	  All these terms are tuples or atoms and this property
	  will hold in any future version of <c>gen_statem</c>,
	  just in case you need such a promise.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="common_state_callback_result" />
      <desc>
	<taglist>
	  <tag><c>stop</c></tag>
	  <item>
	    Terminate 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 given.
	  </item>
	  <tag><c>stop_and_reply</c></tag>
	  <item>
	    Send all <c><anno>Replies</anno></c>
	    then terminate 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 given.
	  </item>
	  <tag><c>keep_state</c></tag>
	  <item>
	    The <c>gen_statem</c> will keep the current state, or
	    do a state transition to the current state if you like,
	    set <c><anno>NewData</anno></c>
	    and execute all <c><anno>Actions</anno></c>.
	    This is the same as
	    <c>{next_state,CurrentState,<anno>NewData</anno>,<anno>Actions</anno>}</c>.
	  </item>
	  <tag><c>keep_state_and_data</c></tag>
	  <item>
	    The <c>gen_statem</c> will keep the current state or
	    do a state transition to the current state if you like,
	    keep the current server data,
	    and execute all <c><anno>Actions</anno></c>.
	    This is the same as
	    <c>{next_state,CurrentState,CurrentData,<anno>Actions</anno>}</c>.
	  </item>
	</taglist>
	<p>
	  All these terms are tuples or atoms and this property
	  will hold in any future version of <c>gen_statem</c>,
	  just in case you need such a promise.
	</p>
      </desc>
    </datatype>
  </datatypes>

  <funcs>

    <func>
      <name name="start_link" arity="3" />
      <name name="start_link" arity="4" />
      <fsummary>Create a linked <c>gen_statem</c> process</fsummary>
      <desc>
        <p>
	  Creates a <c>gen_statem</c> process according
	  to OTP design principles
	  (using
          <seealso marker="proc_lib"><c>proc_lib</c></seealso>
	  primitives)
	  that is linked to the calling process.
	  This is essential when the <c>gen_statem</c> shall 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 start-up
	  procedure, <c>start_link/3,4</c> does not return until
	  <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>
          has returned.
	</p>
	<p>
	  <c><anno>ServerName</anno></c> specifies the
	  <seealso marker="#type-server_name">
	    <c>server_name()</c>
	  </seealso>
	  to register for the <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 which is passed as
          the argument to
	  <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>.
	</p>
        <p>
	  If the option <c>{timeout,Time}</c> is present in
	  <c><anno>Opts</anno></c>, the <c>gen_statem</c>
	  is allowed to spend <c>Time</c> milliseconds initializing
	  or it will be terminated and the start function will return
	  <seealso marker="#type-start_ret"><c>{error,timeout}</c></seealso>.
	</p>
        <p>
	  If the option
	  <seealso marker="#type-debug_opt"><c>{debug,Dbgs}</c></seealso>
	  is present in <c><anno>Opts</anno></c>, debugging through
	  <seealso marker="sys"><c>sys</c></seealso> is activated.
	</p>
        <p>
	  If the option <c>{spawn_opt,SpawnOpts}</c> is present in
	  <c><anno>Opts</anno></c>, <c>SpawnOpts</c> will be passed
	  as option list to
          <seealso marker="erts:erlang#spawn_opt/2">spawn_opt/2</seealso>
	  which is used to spawn the <c>gen_statem</c> process.
	</p>
        <note>
          <p>
	    Using the spawn option <c>monitor</c> is currently not
            allowed, but will cause this function to fail with reason
            <c>badarg</c>.
	  </p>
        </note>
        <p>
	  If the <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 there already exists a process with the specified
	  <c><anno>ServerName</anno></c> this function returns
	  <seealso marker="#type-start_ret"><c>{error,{already_started,Pid}}</c></seealso>,
	  where <c>Pid</c> is the <c>pid()</c> of that process.
	</p>
        <p>
	  If <c>Module:init/1</c> fails with <c>Reason</c>,
          this function returns
	  <seealso marker="#type-start_ret"><c>{error,Reason}</c></seealso>.
	  If <c>Module:init/1</c> returns
	  <seealso marker="#type-start_ret">
	    <c>{stop,Reason}</c>
	  </seealso>
	  or
	  <seealso marker="#type-start_ret"><c>ignore</c></seealso>,
	  the process is terminated and this function
          returns
	  <seealso marker="#type-start_ret">
	    <c>{error,Reason}</c>
	  </seealso>
	  or
	  <seealso marker="#type-start_ret"><c>ignore</c></seealso>,
	  respectively.
	</p>
      </desc>
    </func>


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

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

    <func>
      <name name="call" arity="2" />
      <name name="call" arity="3" />
      <fsummary>Make a synchronous call to a <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> will call the
	  <seealso marker="#state_function">state function</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_function">state function</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 greater than zero
	  which specifies how many milliseconds to wait for a reply,
	  or the atom <c>infinity</c> to wait indefinitely,
	  which is the default.  If no reply is received within
	  the specified time, the function call fails.
	</p>
	<note>
	  <p>
	    To avoid getting a late reply in the caller's
	    inbox this function spawns a proxy process that
	    does the call.  A late reply gets delivered to the
	    dead proxy process hence gets discarded.  This is
	    less efficient than using
	    <c><anno>Timeout</anno> =:= infinity</c>.
	  </p>
	</note>
        <p>
	  The call may fail for example if the <c>gen_statem</c> dies
	  before or during this function call.
	</p>
      </desc>
    </func>

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

    <func>
      <name name="reply" arity="1" />
      <name name="reply" arity="2" />
      <fsummary>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_function">state function</seealso>.
	</p>
        <p>
	  <c><anno>From</anno></c> must be the term from the
	  <seealso marker="#type-event_type">
	    <c>{call,<anno>From</anno>}</c>
	  </seealso>
	  argument to the
	  <seealso marker="#state_function">state function</seealso>.
	  <c><anno>From</anno></c> and <c><anno>Reply</anno></c>
	  can also be specified using a
	  <seealso marker="#type-reply_action">
	    <c>reply_action()</c>
	  </seealso>
	  and multiple replies with a list of them.
	</p>
	<note>
	  <p>
	    A reply sent with this function will not be visible
	    in <seealso marker="sys">sys</seealso> debug output.
	  </p>
	</note>
      </desc>
    </func>

    <func>
      <name name="enter_loop" arity="5" />
      <fsummary>Enter the <c>gen_statem</c> receive loop</fsummary>
      <desc>
	<p>
	  The same as
	  <seealso marker="#enter_loop/7"><c>enter_loop/7</c></seealso>
	  except that no
	  <seealso marker="#type-server_name">
	    <c>server_name()</c>
	  </seealso>
	  must have been registered.
	</p>
      </desc>
    </func>
    <func>
      <name name="enter_loop" arity="6" />
      <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/7"><c>enter_loop/7</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>.
	</p>
	<p>
	  Otherwise the same as
	  <seealso marker="#enter_loop/7"><c>enter_loop/7</c></seealso>
	  with
	  <c>Server = <anno>Server_or_Actions</anno></c> and
	  <c>Actions = []</c>.
	</p>
      </desc>
    </func>
    <func>
      <name name="enter_loop" arity="7" />
      <fsummary>Enter the <c>gen_statem</c> receive loop</fsummary>
      <desc>
        <p>
	  Makes an the calling process become a <c>gen_statem</c>.
	  Does not return, instead the calling process will enter
	  the <c>gen_statem</c> receive loop and become
	  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> behaviour provides.
	</p>
        <p>
	  <c><anno>Module</anno></c>, <c><anno>Opts</anno></c> and
	  <c><anno>Server</anno></c> have the same meanings
	  as when calling
          <seealso marker="#start_link/3"><c>gen_statem</c>:start[_link]/3,4</seealso>.
          However, the
	  <seealso marker="#type-server_name">
	    <c>server_name()</c>
	  </seealso>
	  name must have been registered accordingly
	  <em>before</em> this function is called.</p>
        <p>
	  <c><anno>CallbackMode</anno></c>, <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">Module:init/1</seealso>.
          Also, the callback module <c><anno>Module</anno></c>
	  does not need to export an <c>init/1</c> function.
	</p>
        <p>
	  Failure: If the calling process was not started by a
          <seealso marker="proc_lib"><c>proc_lib</c></seealso>
	  start function, or if it is not registered
          according to
	  <seealso marker="#type-server_name"><c>server_name()</c></seealso>.
	</p>
      </desc>
    </func>

  </funcs>



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

    <func>
      <name>Module:init(Args) -> Result</name>
      <fsummary>Initialize process and internal state</fsummary>
      <type>
        <v>Args = term()</v>
        <v>Result = {CallbackMode,State,Data}</v>
	<v>&nbsp;| {CallbackMode,State,Data,Actions}</v>
        <v>&nbsp;| {stop,Reason} | ignore</v>
	<v>
	  CallbackMode =
	  <seealso marker="#type-callback_mode">callback_mode()</seealso>
	</v>
	<v>State = <seealso marker="#type-state">state()</seealso></v>
	<v>
	  Data = <seealso marker="#type-data">data()</seealso>
	</v>
	<v>
	  Actions =
	  [<seealso marker="#type-action">action()</seealso>] |
	  <seealso marker="#type-action">action()</seealso>
	</v>
	<v>Reason = term()</v>
      </type>
      <desc>
	<marker id="Module:init-1" />
        <p>
	  Whenever a <c>gen_statem</c> is started using
          <seealso marker="#start_link/3">start_link/3,4</seealso>
	  or
          <seealso marker="#start/3">start/3,4</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 the start
          function.
	</p>
        <p>
	  If the initialization is successful, the function should
          return <c>{CallbackMode,State,Data}</c> or
	  <c>{CallbackMode,State,Data,Actions}</c>.
	  <c>CallbackMode</c> selects the
	  <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>.
	  of the <c>gen_statem</c>.
	  <c>State</c> is the initial
	  <seealso marker="#type-state">state()</seealso>
	  and <c>Data</c> the initial server
	  <seealso marker="#type-data">data()</seealso>.
	</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_function">state function</seealso>.
	</p>
        <p>
	  If something goes wrong during the initialization
          the function should return <c>{stop,Reason}</c>
	  or <c>ignore</c>. See
          <seealso marker="#start_link/3">start_link/3,4</seealso>.
	</p>
	<p>
	  This function may use
	  <seealso marker="erts:erlang#throw/1"><c>throw</c></seealso>
	  to return <c>Result</c>.
	</p>
      </desc>
    </func>

    <func>
      <name>Module:StateName(EventType, EventContent, Data) ->
      StateFunctionResult
      </name>
      <name>Module:handle_event(EventType, EventContent,
      State, Data) -> HandleEventResult
      </name>
      <fsummary>Handle an event</fsummary>
      <type>
	<v>
	  EventType =
	  <seealso marker="#type-event_type">event_type()</seealso>
	</v>
	<v>EventContent = term()</v>
	<v>
	  State =
	  <seealso marker="#type-state">state()</seealso>
	</v>
	<v>
	  Data = NewData =
	  <seealso marker="#type-data">data()</seealso>
	</v>
	<v>
	  StateFunctionResult =
	  <seealso marker="#type-state_function_result">
	    state_function_result()
	  </seealso>
	</v>
	<v>
	  HandleEventResult =
	  <seealso marker="#type-handle_event_result">
	    handle_event_result()
	  </seealso>
	</v>
      </type>
      <desc>
        <p>
	  Whenever a <c>gen_statem</c> receives an event from
          <seealso marker="#call/2">call/2</seealso>,
          <seealso marker="#cast/2">cast/2</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> then <c>Module:StateName/3</c> is called,
	  and if it is <c>handle_event_function</c>
	  then <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 is waiting for a reply.  The reply can be sent
	  from this or from any other
	  <seealso marker="#state_function">state function</seealso>
	  by returning with <c>{reply,From,Reply}</c> in
	  <seealso marker="#type-action">Actions</seealso>, in
	  <seealso marker="#type-reply_action">Replies</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 will be 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 has to be an atom but for <c>HandleEventResult</c>
	  there is no restriction on the next state.
	</p>
	<p>
	  See <seealso marker="#type-action">action()</seealso>
	  for options that can be set and actions that can be done
	  by <c>gen_statem</c> after returning from this function.
	</p>
	<p>
	  These functions may use
	  <seealso marker="erts:erlang#throw/1"><c>throw</c></seealso>,
	  to return the result.
	</p>
      </desc>
    </func>

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

    <func>
      <name>Module:code_change(OldVsn, OldState, OldData, Extra) ->
        Result
      </name>
      <fsummary>Update the internal state during upgrade/downgrade</fsummary>
      <type>
        <v>OldVsn = Vsn | {down,Vsn}</v>
        <v>&nbsp;&nbsp;Vsn = term()</v>
        <v>OldState = NewState = term()</v>
        <v>Extra = term()</v>
	<v>Result = {NewCallbackMode,NewState,NewData} | Reason</v>
	<v>
	  NewCallbackMode =
	  <seealso marker="#type-callback_mode">callback_mode()</seealso>
	</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>
        <p>
	  This function is called by a <c>gen_statem</c> when it should
          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 given in the
	  <seealso marker="sasl:appup"><c>appup</c></seealso>
	  file. See
          <seealso marker="doc/design_principles:release_handling#instr">
	    OTP Design Principles
	  </seealso>
          for more information.
	</p>
        <p>
	  In the case of an upgrade, <c>OldVsn</c> is <c>Vsn</c>, and
          in the case of a downgrade, <c>OldVsn</c> is
          <c>{down,Vsn}</c>. <c>Vsn</c> is defined by the <c>vsn</c>
          attribute(s) of the old version of the callback module
          <c>Module</c>. If no such attribute is defined, the version
          is the checksum of the BEAM file.
	</p>
	<note>
	  <p>
	    If you would dare to change
	    <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>
	    during release upgrade/downgrade, the upgrade is no problem
	    since the new code surely knows what <em>callback mode</em>
	    it needs, but for a downgrade this function will have to
	    know from the <c>Extra</c> argument that comes from the
	    <seealso marker="sasl:appup"><c>appup</c></seealso>
	    file what <em>callback mode</em> the old code did use.
	    It may also be possible to figure this out
	    from the <c>{down,Vsn}</c> argument since <c>Vsn</c>
	    in effect defines the old callback module version.
	  </p>
	</note>
        <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 shall return the updated
          internal state in an
	  <c>{NewCallbackMode,NewState,NewData}</c> tuple.
	</p>
	<p>
	  If the function returns <c>Reason</c>, the ongoing
	  upgrade will fail and roll back to the old release.</p>
	<p>
	  This function may use
	  <seealso marker="erts:erlang#throw/1"><c>throw</c></seealso>
	  to return <c>Result</c> or <c>Reason</c>.
	</p>
      </desc>
    </func>

    <func>
      <name>Module:format_status(Opt, [PDict,State,Data]) ->
        Status
      </name>
      <fsummary>Optional function for providing a term describing the
        current <c>gen_statem</c> status</fsummary>
      <type>
        <v>Opt = normal | terminate</v>
        <v>PDict = [{Key, Value}]</v>
	<v>
	  State =
	  <seealso marker="#type-state">state()</seealso>
	</v>
	<v>
	  Data =
	  <seealso marker="#type-data">data()</seealso>
	</v>
	<v>Key = term()</v>
	<v>Value = term()</v>
        <v>Status = term()</v>
      </type>
      <desc>
        <note>
          <p>
	    This callback is optional, so a callback module need not
            export it. The <c>gen_statem</c> module provides a default
            implementation of this function that returns
            <c>{State,Data}</c>.  If this callback fails the default
	    function will return <c>{State,Info}</c>
	    where <c>Info</c> informs of the crash but no details,
	    to hide possibly sensitive data.
	  </p>
        </note>
        <p>This function is called by a <c>gen_statem</c> process when:</p>
        <list type="bulleted">
          <item>
	    One of
            <seealso marker="sys#get_status/1">
	      <c>sys:get_status/1,2</c>
	    </seealso>
            is invoked to get the <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 customising the form and
          appearance of the <c>gen_statem</c> status for these cases. A
          callback module wishing to customise the
          <seealso marker="sys#get_status/1">
	    <c>sys:get_status/1,2</c>
	  </seealso>
	  return value as well as how
          its status appears in termination error logs exports an
          instance of <c>format_status/2</c> that returns a term
          describing the current status of the <c>gen_statem</c>.
	</p>
        <p>
	  <c>PDict</c> is the current value of the <c>gen_statem</c>'s
          process dictionary.
	</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 should return <c>Status</c>, a term that
          customises the 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 isn't
          required, but doing so will make the callback module status
          consistent with the rest of the
          <seealso marker="sys#get_status/1">
	    <c>sys:get_status/1,2</c>
	  </seealso>
	  return value.
	</p>
        <p>
	  One use for this function is to return compact alternative
          state representations to avoid having large state terms
          printed in logfiles.  Another is to hide sensitive data from
	  being written to the error log.
	</p>
	<p>
	  This function may use
	  <seealso marker="erts:erlang#throw/1"><c>throw</c></seealso>
	  to return <c>Status</c>.
	</p>
      </desc>
    </func>

  </funcs>

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