Ditch PrevState

StateName/4 -> StateName/3
handle_event/5 -> handle_event/4

3 files changed, 73 insertions, 101 deletions

diff --git a/lib/stdlib/doc/src/gen_statem.xml b/lib/stdlib/doc/src/gen_statem.xml
index 1e41d616e9..f21610d14c 100644
--- a/lib/stdlib/doc/src/gen_statem.xml
+++ b/lib/stdlib/doc/src/gen_statem.xml
@@ -62,12 +62,12 @@ gen_statem:stop       -----> Module:terminate/2
 gen_statem:call
 gen_statem:cast
 erlang:send
-erlang:'!'            -----> Module:StateName/4
-                             Module:handle_event/5
+erlang:'!'            -----> Module:StateName/3
+                             Module:handle_event/4
 
 -                     -----> Module:terminate/3
 
--                     -----> Module:code_change/3</pre>
+-                     -----> 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
@@ -91,28 +91,22 @@ erlang:'!'            -----> Module:StateName/4
       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/4">
-        <c>Module:StateName/4</c>
+      <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 callback function
-      <seealso marker="#Module:code_change/4">
-        <c>Module:code_change/4</c>
-      </seealso> makes the state name <c>code_change</c>
-      unusable.  Actually you might get away with using it
-      as a state name but you will have to find a way to separate
-      the two callback roles e.g by observing that you can force
-      argument 3 into having different types for the different roles.
-      This is a slippery slope, though.  It is easier
-      to simply not use <c>code_change</c> as a state name.
+      <seealso marker="#Module:terminate/3">
+        <c>Module:terminate/3</c>
+      </seealso> makes the state name <c>terminate</c> unusable.
     </p>
     <p>When
       <seealso marker="#type-callback_mode">callback_mode</seealso>
       is <c>handle_event_function</c> the state can be any term
       and the state function name is
-      <seealso marker="#Module:handle_event/5">
-        <c>Module:handle_event/5</c>
+      <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 like but you will have to implement it.
@@ -120,12 +114,6 @@ erlang:'!'            -----> Module:StateName/4
       states so you do not accidentally postpone one event
       forever creating an infinite busy loop.
     </p>
-    <p>There is a small gotcha in both callback modes regarding
-      the state <c>undefined</c> that is used as the previous state
-      when the first state function is called.  So either do not
-      use <c>undefined</c> as a state, or figure out
-      the implications yourself.
-    </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>
@@ -378,14 +366,14 @@ erlang:'!'            -----> Module:StateName/4
 	  <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/4">
-	      <c>Module:StateName/4</c>
-	    </seealso> is used.  This is the default.
+	    <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/5">
-	      <c>Module:handle_event/5</c>
+	    <seealso marker="#Module:handle_event/4">
+	      <c>Module:handle_event/4</c>
 	    </seealso> is used for all states.
 	  </item>
 	</taglist>
@@ -1054,9 +1042,10 @@ erlang:'!'            -----> Module:StateName/4
 	  <c>CallbackMode</c> selects the
 	  <seealso marker="#type-callback_mode">callback_mode()</seealso>.
 	  of the <c>gen_statem</c>.
-	  <c>State</c> is the <seealso marker="#type-state">state()</seealso>
-	  of the <c>gen_statem</c> and
-	  <c>Data</c> the server <seealso marker="#type-data">data()</seealso>
+	  <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
@@ -1076,11 +1065,10 @@ erlang:'!'            -----> Module:StateName/4
     </func>
 
     <func>
-      <name>Module:StateName(EventType, EventContent,
-      PrevStateName, Data) -> Result
+      <name>Module:StateName(EventType, EventContent, Data) -> Result
       </name>
       <name>Module:handle_event(EventType, EventContent,
-        PrevState, State, Data) -> Result
+      State, Data) -> Result
       </name>
       <fsummary>Handle an event</fsummary>
       <type>
@@ -1088,10 +1076,7 @@ erlang:'!'            -----> Module:StateName/4
 	  <seealso marker="#type-event_type">event_type()</seealso>
 	</v>
 	<v>EventContent = term()</v>
-	<v>PrevStateName =
-	  <seealso marker="#type-state_name">state_name()</seealso>
-	</v>
-	<v>PrevState = State =
+	<v>State = NewState =
 	  <seealso marker="#type-state">state()</seealso>
 	</v>
 	<v>Data = NewData =
@@ -1109,9 +1094,9 @@ erlang:'!'            -----> Module:StateName/4
           <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">callback_mode</seealso>
-	  is <c>state_functions</c> then <c>Module:StateName/4</c> is called,
+	  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/5</c> is called.
+	  then <c>Module:handle_event/4</c> is called.
 	</p>
 	<p>If <c>EventType</c> is
 	  <seealso marker="#type-event_type"><c>{call,Caller}</c></seealso>
@@ -1126,13 +1111,6 @@ erlang:'!'            -----> Module:StateName/4
 	    <c>reply(Caller, Reply)</c>
 	  </seealso>.
 	</p>
-	<p><c>PrevStateName</c> and <c>PrevState</c> are useful
-	  in some odd cases for example when you want to do something
-	  only at the first event in a state, or when you need to
-	  handle events differently depending on the previous state.
-	  Note that when <c>gen_statem</c> enters its first state
-	  this is set to <c>undefined</c>.
-	</p>
 	<p>If this function returns with a new state that
 	  does not match equal (<c>=/=</c>) to the current state
 	  all postponed events will be retried in the new state.
diff --git a/lib/stdlib/src/gen_statem.erl b/lib/stdlib/src/gen_statem.erl
index 7fbc1e0f0d..c844b76653 100644
--- a/lib/stdlib/src/gen_statem.erl
+++ b/lib/stdlib/src/gen_statem.erl
@@ -184,7 +184,6 @@
 -callback state_name(
 	    event_type(),
 	    EventContent :: term(),
-	    PrevStateName :: state_name(), % Previous state name
 	    Data :: data()) ->
     state_callback_result().
 %%
@@ -195,7 +194,6 @@
 -callback handle_event(
 	    event_type(),
 	    EventContent :: term(),
-	    PrevState :: state(), % Previous state
 	    State :: state(), % Current state
 	    Data :: data()) ->
     state_callback_result().
@@ -234,10 +232,10 @@
    [init/1, % One may use enter_loop/5,6,7 instead
     format_status/2, % Has got a default implementation
     %%
-    state_name/4, % Example for callback_mode =:= state_functions:
+    state_name/3, % Example for callback_mode =:= state_functions:
     %% there has to be a StateName/5 callback function for every StateName.
     %%
-    handle_event/5]). % For callback_mode =:= handle_event_function
+    handle_event/4]). % For callback_mode =:= handle_event_function
 
 %% Type validation functions
 callback_mode(CallbackMode) ->
@@ -518,7 +516,7 @@ enter(Module, Opts, CallbackMode, State, Data, Server, Actions, Parent)
 	true ->
 	    Name = gen:get_proc_name(Server),
 	    Debug = gen:debug_options(Name, Opts),
-	    PrevState = undefined,
+	    OldState = make_ref(),
 	    NewActions =
 		if
 		    is_list(Actions) ->
@@ -530,8 +528,7 @@ enter(Module, Opts, CallbackMode, State, Data, Server, Actions, Parent)
 	      callback_mode => CallbackMode,
 	      module => Module,
 	      name => Name,
-	      prev_state => PrevState,
-	      state => PrevState, % Discarded by loop_event_actions
+	      state => OldState, % Discarded by loop_event_actions
 	      data => Data,
 	      timer => undefined,
 	      postponed => [],
@@ -539,7 +536,7 @@ enter(Module, Opts, CallbackMode, State, Data, Server, Actions, Parent)
 	    loop_event_actions(
 	      Parent, Debug, S, [],
 	      {event,undefined}, % Discarded due to {postpone,false}
-	      PrevState, State, Data, NewActions);
+	      OldState, State, Data, NewActions);
 	false ->
 	    erlang:error(
 	      badarg,
@@ -771,7 +768,6 @@ loop_events(
   Parent, Debug,
   #{callback_mode := CallbackMode,
     module := Module,
-    prev_state := PrevState,
     state := State,
     data := Data} = S,
   [{Type,Content} = Event|Events] = Q, Timer) ->
@@ -780,9 +776,9 @@ loop_events(
     try
 	case CallbackMode of
 	    state_functions ->
-		Module:State(Type, Content, PrevState, Data);
+		Module:State(Type, Content, Data);
 	    handle_event_function ->
-		Module:handle_event(Type, Content, PrevState, State, Data)
+		Module:handle_event(Type, Content, State, Data)
 	end of
 	Result ->
 	    loop_event_result(
@@ -796,7 +792,7 @@ loop_events(
 	    %% of calling a nonexistent state function
 	    case erlang:get_stacktrace() of
 		[{Module,State,
-		  [Type,Content,PrevState,Data]=Args,
+		  [Type,Content,Data]=Args,
 		  _}
 		 |Stacktrace]
 		when CallbackMode =:= state_functions ->
@@ -806,7 +802,7 @@ loop_events(
 		      Stacktrace,
 		      Debug, S, Q);
 		[{Module,handle_event,
-		  [Type,Content,PrevState,State,Data]=Args,
+		  [Type,Content,State,Data]=Args,
 		  _}
 		 |Stacktrace]
 		when CallbackMode =:= handle_event_function ->
@@ -932,7 +928,6 @@ loop_event_actions(
     loop_events(
       Parent, NewDebug,
       S#{
-	prev_state := State,
 	state := NewState,
 	data := NewData,
 	timer := Timer,
diff --git a/lib/stdlib/test/gen_statem_SUITE.erl b/lib/stdlib/test/gen_statem_SUITE.erl
index 3302171329..27b042c0d8 100644
--- a/lib/stdlib/test/gen_statem_SUITE.erl
+++ b/lib/stdlib/test/gen_statem_SUITE.erl
@@ -1160,26 +1160,26 @@ terminate(_Reason, _State, _Data) ->
 
 %% State functions
 
-idle(cast, {connect,Pid}, _, Data) ->
+idle(cast, {connect,Pid}, Data) ->
     Pid ! accept,
     {next_state,wfor_conf,Data};
-idle({call,From}, connect, _, Data) ->
+idle({call,From}, connect, Data) ->
     gen_statem:reply(From, accept),
     {next_state,wfor_conf,Data};
-idle(cast, badreturn, _, _Data) ->
+idle(cast, badreturn, _Data) ->
     badreturn;
-idle({call,_From}, badreturn, _, _Data) ->
+idle({call,_From}, badreturn, _Data) ->
     badreturn;
-idle({call,From}, {delayed_answer,T}, _, Data) ->
+idle({call,From}, {delayed_answer,T}, Data) ->
     receive
     after T ->
 	    gen_statem:reply({reply,From,delayed}),
 	    throw({keep_state,Data})
     end;
-idle({call,From}, {timeout,Time}, _, _Data) ->
+idle({call,From}, {timeout,Time}, _Data) ->
     {next_state,timeout,{From,Time},
      [{timeout,Time,idle}]};
-idle(Type, Content, PrevState, Data) ->
+idle(Type, Content, Data) ->
     case handle_common_events(Type, Content, idle, Data) of
 	undefined ->
 	    case Type of
@@ -1187,45 +1187,45 @@ idle(Type, Content, PrevState, Data) ->
 		    throw({keep_state,Data,[{reply,From,'eh?'}]});
 		_ ->
 		    throw(
-		      {stop,{unexpected,idle,Type,Content,PrevState}})
+		      {stop,{unexpected,idle,Type,Content}})
 	    end;
 	Result ->
 	    Result
     end.
 
-timeout(timeout, idle, idle, {From,Time}) ->
+timeout(timeout, idle, {From,Time}) ->
     TRef2 = erlang:start_timer(Time, self(), ok),
     TRefC1 = erlang:start_timer(Time, self(), cancel1),
     TRefC2 = erlang:start_timer(Time, self(), cancel2),
     {next_state,timeout2,{From,Time,TRef2},
      [{cancel_timer, TRefC1},
       {next_event,internal,{cancel_timer,TRefC2}}]};
-timeout(_, _, _, Data) ->
+timeout(_, _, Data) ->
     {keep_state,Data}.
 
 timeout2(
-  internal, {cancel_timer,TRefC2}, timeout, {From,Time,TRef2}) ->
+  internal, {cancel_timer,TRefC2}, {From,Time,TRef2}) ->
     Time4 = Time * 4,
     receive after Time4 -> ok end,
     {next_state,timeout3,{From,TRef2},
      [{cancel_timer,TRefC2}]};
-timeout2(_, _, _, Data) ->
+timeout2(_, _, Data) ->
     {keep_state,Data}.
 
-timeout3(info, {timeout,TRef2,Result}, _, {From,TRef2}) ->
+timeout3(info, {timeout,TRef2,Result}, {From,TRef2}) ->
     gen_statem:reply([{reply,From,Result}]),
     {next_state,idle,state};
-timeout3(_, _, _, Data) ->
+timeout3(_, _, Data) ->
     {keep_state,Data}.
 
-wfor_conf({call,From}, confirm, _, Data) ->
+wfor_conf({call,From}, confirm, Data) ->
     {next_state,connected,Data,
      [{reply,From,yes}]};
-wfor_conf(cast, {ping,_,_}, _, _) ->
+wfor_conf(cast, {ping,_,_}, _) ->
     {keep_state_and_data,[postpone]};
-wfor_conf(cast, confirm, _, Data) ->
+wfor_conf(cast, confirm, Data) ->
     {next_state,connected,Data};
-wfor_conf(Type, Content, _, Data) ->
+wfor_conf(Type, Content, Data) ->
     case handle_common_events(Type, Content, wfor_conf, Data) of
 	undefined ->
 	    case Type of
@@ -1239,21 +1239,21 @@ wfor_conf(Type, Content, _, Data) ->
 	    Result
     end.
 
-connected({call,From}, {msg,Ref}, _, Data) ->
+connected({call,From}, {msg,Ref}, Data) ->
     {keep_state,Data,
      [{reply,From,{ack,Ref}}]};
-connected(cast, {msg,From,Ref}, _, Data) ->
+connected(cast, {msg,From,Ref}, Data) ->
     From ! {ack,Ref},
     {keep_state,Data};
-connected({call,From}, disconnect, _, Data) ->
+connected({call,From}, disconnect, Data) ->
     {next_state,idle,Data,
      [{reply,From,yes}]};
-connected(cast, disconnect, _, Data) ->
+connected(cast, disconnect, Data) ->
     {next_state,idle,Data};
-connected(cast, {ping,Pid,Tag}, _, Data) ->
+connected(cast, {ping,Pid,Tag}, Data) ->
     Pid ! {pong,Tag},
     {keep_state,Data};
-connected(Type, Content, _, Data) ->
+connected(Type, Content, Data) ->
     case handle_common_events(Type, Content, connected, Data) of
 	undefined ->
 	    case Type of
@@ -1267,9 +1267,9 @@ connected(Type, Content, _, Data) ->
 	    Result
     end.
 
-state0({call,From}, stop, _, Data) ->
+state0({call,From}, stop, Data) ->
     {stop_and_reply,normal,[{reply,From,stopped}],Data};
-state0(Type, Content, _, Data) ->
+state0(Type, Content, Data) ->
     case handle_common_events(Type, Content, state0, Data) of
 	undefined ->
 	    {keep_state,Data};
@@ -1277,26 +1277,26 @@ state0(Type, Content, _, Data) ->
 	    Result
     end.
 
-hiber_idle({call,From}, 'alive?', _, Data) ->
+hiber_idle({call,From}, 'alive?', Data) ->
     {keep_state,Data,
      [{reply,From,'alive!'}]};
-hiber_idle({call,From}, hibernate_sync, _, Data) ->
+hiber_idle({call,From}, hibernate_sync, Data) ->
     {next_state,hiber_wakeup,Data,
      [{reply,From,hibernating},
       hibernate]};
-hiber_idle(info, hibernate_later, _, _) ->
+hiber_idle(info, hibernate_later, _) ->
     Tref = erlang:start_timer(1000, self(), hibernate),
     {keep_state,Tref};
-hiber_idle(info, hibernate_now, _, Data) ->
+hiber_idle(info, hibernate_now, Data) ->
     {keep_state,Data,
      [hibernate]};
-hiber_idle(info, {timeout,Tref,hibernate}, _, Tref) ->
+hiber_idle(info, {timeout,Tref,hibernate}, Tref) ->
     {keep_state,[],
      [hibernate]};
-hiber_idle(cast, hibernate_async, _, Data) ->
+hiber_idle(cast, hibernate_async, Data) ->
     {next_state,hiber_wakeup,Data,
      [hibernate]};
-hiber_idle(Type, Content, _, Data) ->
+hiber_idle(Type, Content, Data) ->
     case handle_common_events(Type, Content, hiber_idle, Data) of
 	undefined ->
 	    {keep_state,Data};
@@ -1304,19 +1304,19 @@ hiber_idle(Type, Content, _, Data) ->
 	    Result
     end.
 
-hiber_wakeup({call,From}, wakeup_sync, _, Data) ->
+hiber_wakeup({call,From}, wakeup_sync, Data) ->
     {next_state,hiber_idle,Data,
      [{reply,From,good_morning}]};
-hiber_wakeup({call,From}, snooze_sync, _, Data) ->
+hiber_wakeup({call,From}, snooze_sync, Data) ->
     {keep_state,Data,
      [{reply,From,please_just_five_more},
       hibernate]};
-hiber_wakeup(cast, wakeup_async, _, Data) ->
+hiber_wakeup(cast, wakeup_async, Data) ->
     {next_state,hiber_idle,Data};
-hiber_wakeup(cast, snooze_async, _, Data) ->
+hiber_wakeup(cast, snooze_async, Data) ->
     {keep_state,Data,
      [hibernate]};
-hiber_wakeup(Type, Content, _, Data) ->
+hiber_wakeup(Type, Content, Data) ->
     case handle_common_events(Type, Content, hiber_wakeup, Data) of
 	undefined ->
 	    {keep_state,Data};
@@ -1353,10 +1353,9 @@ handle_common_events(_, _, _, _) ->
 %% Wrap the state in a 1 element list just to test non-atom
 %% states.  Note that the state from init/1 is not wrapped
 %% so both atom and non-atom states are tested.
-handle_event(Type, Event, PrevState, State, Data) ->
-    PrevStateName = unwrap_state(PrevState),
+handle_event(Type, Event, State, Data) ->
     StateName = unwrap_state(State),
-    try ?MODULE:StateName(Type, Event, PrevStateName, Data) of
+    try ?MODULE:StateName(Type, Event, Data) of
 	Result ->
 	    wrap_result(Result)
     catch