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diff --git a/lib/stdlib/doc/src/gen_statem.xml b/lib/stdlib/doc/src/gen_statem.xml new file mode 100644 index 0000000000..ec7f267c64 --- /dev/null +++ b/lib/stdlib/doc/src/gen_statem.xml @@ -0,0 +1,1724 @@ +<?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 finite state machines + (<seealso marker="gen_fsm"><c>gen_fsm</c></seealso> like) + that requires the state to be an atom and uses that state as + the name of the current callback function, + and one without restriction on the state data type + that uses one callback function for all states. + </p> + <p> + This is a new behaviour in OTP-19.0. + It has been thoroughly reviewed, is stable enough + to be used by at least two heavy OTP applications, and is here to stay. + But depending on user feedback, we do not expect + but might find it necessary to make minor + not backwards compatible changes into OTP-20.0, + so its state can be designated as "not quite experimental"... + </p> + <p> + The <c>gen_statem</c> behaviour is intended to replace + <seealso marker="gen_fsm"><c>gen_fsm</c></seealso> for new code. + It has the same features and add some really useful: + </p> + <list type="bulleted"> + <item>State code is gathered</item> + <item>The state can be any term</item> + <item>Events can be postponed</item> + <item>Events can be self generated</item> + <item>A reply can be sent from a later state</item> + <item>There can be multiple sys traceable replies</item> + </list> + <p> + The callback model(s) for <c>gen_statem</c> differs from + the one for <seealso marker="gen_fsm"><c>gen_fsm</c></seealso>, + but it is still fairly easy to rewrite + from <c>gen_fsm</c> to <c>gen_statem</c>. + </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> + </seealso> + <c>internal</c> 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"><c>gen_fsm</c></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"><c>sys</c></seealso>. + The <c>sys</c>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="proc_lib#hibernate/3"> + <c>proc_lib:hibernate/3</c>. + </seealso> + It is done when 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 feature might 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 + since hibernation can be too costly + to use after every event; see + <seealso marker="erts:erlang#hibernate/3"> + <c>erlang:hibernate/3</c>. + </seealso> + </p> + </description> + + <section> + <title>EXAMPLE</title> + <p> + 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,<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"><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()</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"><c>enter_loop/5</c></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"><c>start_link/3</c></seealso>. + </p> + </desc> + </datatype> + <datatype> + <name name="start_ret" /> + <desc> + <p> + Return value from the start functions for_example + <seealso marker="#start_link/3"><c>start_link/3</c></seealso>. + </p> + </desc> + </datatype> + + <datatype> + <name name="from" /> + <desc> + <p> + Destination to use when replying through for example the + <seealso marker="#type-action"> + <c>action()</c> + </seealso> + <c>{reply,From,Reply}</c> + to a process that has called the <c>gen_statem</c> server using + <seealso marker="#call/2"><c>call/2</c></seealso>. + </p> + </desc> + </datatype> + <datatype> + <name name="state" /> + <desc> + <p> + 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"><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 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"><c>Module:init/1</c></seealso> + or when calling + <seealso marker="#enter_loop/5"><c>enter_loop/5-7</c></seealso>, + and with the return value from + <seealso marker="#Module:code_change/4"> + <c>Module:code_change/4</c>. + </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">actions</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()</c> + </seealso> + <c>next_event</c> + are inserted in the queue to be processed before + all other events. + </item> + <item> + If an + <seealso marker="#type-event_timeout"> + <c>event_timeout()</c> + </seealso> + is set through + <seealso marker="#type-action"> + <c>action()</c> + </seealso> + <c>timeout</c> + 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 right 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, + to prevent receiving any new event; a + <seealso marker="erts:erlang#garbage_collect/0"> + <c>garbage_collect/0</c> + </seealso> is done instead to simulate + that the <c>gen_statem</c> entered hibernation + and immediately got awakened by the oldest enqueued event. + </p> + </desc> + </datatype> + <datatype> + <name name="event_timeout" /> + <desc> + <p> + Generate an event of + <seealso marker="#type-event_type"><c>event_type()</c></seealso> + <c>timeout</c> + 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"><c>Module:init/1</c></seealso> + or by giving them to + <seealso marker="#enter_loop/6"><c>enter_loop/6,7</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> + 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()</c> + </seealso> + <seealso marker="#type-postpone"> + <c>postpone()</c> + </seealso> + for this state transition. + This action is ignored when returned from + <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso> + or given to + <seealso marker="#enter_loop/5"><c>enter_loop/5,6</c></seealso> + 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()</c> + </seealso> + <seealso marker="#type-hibernate"> + <c>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"> + <c>gen_fsm Module:StateName/2</c>. + </seealso> + </item> + <tag><c>timeout</c></tag> + <item> + Set 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>. + </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"><c>spawn_opt/2</c></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"><c>start_link/3,4</c></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"> + <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> will cause an + error report to be issued through + <seealso marker="kernel:error_logger#format/2"> + <c>error_logger:format/2</c>. + </seealso> + The default <c><anno>Reason</anno></c> is <c>normal</c>. + </p> + <p> + <c><anno>Timeout</anno></c> is an integer 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"><c>sys</c></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:start[_link]/3,4</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>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"><c>Module:init/1</c></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> | {CallbackMode,State,Data,Actions}</v> + <v> | {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"><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 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"><c>state()</c></seealso> + and <c>Data</c> the initial server + <seealso marker="#type-data"><c>data()</c></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"><c>start_link/3,4</c></seealso>. + </p> + <p> + This function may use + <seealso marker="erts:erlang#throw/1"><c>throw/1</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"><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> 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"><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 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"><c>action()</c></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/1</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"><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> 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"> + <c>error_logger:format/2</c>. + </seealso> + </p> + <p> + This function may use + <seealso marker="erts:erlang#throw/1"><c>throw/1</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> 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/1</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/1</c></seealso> + to return <c>Status</c>. + </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="supervisor"><c>supervisor(3)</c></seealso>, + <seealso marker="proc_lib"><c>proc_lib(3)</c></seealso>, + <seealso marker="sys"><c>sys(3)</c></seealso></p> + </section> +</erlref> |