Ditch State so StateName/5 -> StateName/4

3 files changed, 136 insertions, 119 deletions

diff --git a/lib/stdlib/doc/src/gen_statem.xml b/lib/stdlib/doc/src/gen_statem.xml
index 096be3025e..8934d912c6 100644
--- a/lib/stdlib/doc/src/gen_statem.xml
+++ b/lib/stdlib/doc/src/gen_statem.xml
@@ -62,7 +62,7 @@ gen_statem:stop       -----> Module:terminate/2
 gen_statem:call
 gen_statem:cast
 erlang:send
-erlang:'!'            -----> Module:StateName/5
+erlang:'!'            -----> Module:StateName/4
                              Module:handle_event/5
 
 -                     -----> Module:terminate/3
@@ -84,18 +84,28 @@ erlang:'!'            -----> Module:StateName/5
       in a <c>gen_statem</c> is the callback function that is called
       for all events in this state, and is selected depending on
       <seealso marker="#type-callback_mode">callback_mode</seealso>
-      that the implementation selects during <c>gen_statem</c> init.
+      that the implementation specifies during <c>gen_statem</c> init.
     </p>
     <p>When
       <seealso marker="#type-callback_mode">callback_mode</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/5">
-        <c>Module:StateName/5</c>
+      <seealso marker="#Module:StateName/4">
+        <c>Module:StateName/4</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.
     </p>
     <p>When
       <seealso marker="#type-callback_mode">callback_mode</seealso>
@@ -110,13 +120,11 @@ erlang:'!'            -----> Module:StateName/5
       states so you do not accidentally postpone one event
       forever creating an infinite busy loop.
     </p>
-    <p>Any state name or any state value (depending on
-      <seealso marker="#type-callback_mode">callback_mode</seealso>)
-      is permitted with a small gotcha regarding the state
-      <c>undefined</c> that is used as the previous state when
-      the first <c>gen_statem</c> state function is called.
-      You might need to know about this faked state if you
-      inspect the previous state argument in your state functions.
+    <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
@@ -181,7 +189,7 @@ erlang:'!'            -----> Module:StateName/5
       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'd want to do between each event on a busy server.
+      something you would want to do between each event on a busy server.
     </p>
   </description>
 
@@ -350,13 +358,20 @@ erlang:'!'            -----> Module:StateName/5
     <datatype>
       <name name="callback_mode" />
       <desc>
+	<p>
+	  The <c>callback_mode</c> 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/5">
-	      <c>Module:StateName/5</c>
+	    <seealso marker="#Module:StateName/4">
+	      <c>Module:StateName/4</c>
 	    </seealso> is used.  This is the default.
 	  </item>
 	  <tag><c>handle_event_function</c></tag>
@@ -384,7 +399,7 @@ erlang:'!'            -----> Module:StateName/5
 	  <seealso marker="#state_function">state function</seealso>,
 	  from <seealso marker="#Module:init/1">Module:init/1</seealso>
 	  or given to
-	  <seealso marker="#enter_loop/5">enter_loop/5,6</seealso>.
+	  <seealso marker="#enter_loop/6">enter_loop/6,7</seealso>.
 	</p>
 	<p>The processing order for a state change is:</p>
 	<list type="ordered">
@@ -990,7 +1005,7 @@ erlang:'!'            -----> Module:StateName/5
 
     <func>
       <name>Module:StateName(EventType, EventContent,
-        PrevStateName, StateName, Data) -> Result
+      PrevStateName, Data) -> Result
       </name>
       <name>Module:handle_event(EventType, EventContent,
         PrevState, State, Data) -> Result
@@ -1004,9 +1019,6 @@ erlang:'!'            -----> Module:StateName/5
 	<v>PrevStateName =
 	  <seealso marker="#type-state_name">state_name()</seealso>
 	</v>
-	<v>StateName =
-	  <seealso marker="#type-state_name">state_name()</seealso>
-	</v>
 	<v>PrevState = State =
 	  <seealso marker="#type-state">state()</seealso>
 	</v>
@@ -1023,10 +1035,9 @@ erlang:'!'            -----> Module:StateName/5
         <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 this function is called.
-	  If
-	  <seealso marker="#type-callback_mode">callback_mode</seealso>
-	  is <c>state_functions</c> then <c>Module:StateName/5</c> is called,
+	  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,
 	  and if it is <c>handle_event_function</c>
 	  then <c>Module:handle_event/5</c> is called.
 	</p>
@@ -1043,13 +1054,10 @@ erlang:'!'            -----> Module:StateName/5
 	    <c>reply(Caller, Reply)</c>
 	  </seealso>.
 	</p>
-        <p><c>StateName</c> is useful in some odd cases for example
-	  if you call a common event handling function from your state
-	  function then you might want to pass <c>StateName</c>.
-	</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.
+	  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>
diff --git a/lib/stdlib/src/gen_statem.erl b/lib/stdlib/src/gen_statem.erl
index 02c8d60c61..fe84a428f6 100644
--- a/lib/stdlib/src/gen_statem.erl
+++ b/lib/stdlib/src/gen_statem.erl
@@ -152,8 +152,7 @@
 -callback state_name(
 	    event_type(),
 	    EventContent :: term(),
-	    PrevStateName :: state_name() | reference(),
-	    StateName :: state_name(), % Current state
+	    PrevStateName :: state_name(), % Previous state name
 	    Data :: data()) ->
     state_callback_result().
 %%
@@ -164,7 +163,7 @@
 -callback handle_event(
 	    event_type(),
 	    EventContent :: term(),
-	    PrevState :: state(),
+	    PrevState :: state(), % Previous state
 	    State :: state(), % Current state
 	    Data :: data()) ->
     state_callback_result().
@@ -203,7 +202,7 @@
    [init/1, % One may use enter_loop/5,6,7 instead
     format_status/2, % Has got a default implementation
     %%
-    state_name/5, % Example for callback_mode =:= state_functions:
+    state_name/4, % 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
@@ -740,14 +739,13 @@ loop_events(
   [{Type,Content} = Event|Events] = Q, Timer) ->
     _ = (Timer =/= undefined) andalso
 	cancel_timer(Timer),
-    Func =
+    try
 	case CallbackMode of
-	    handle_event_function ->
-		handle_event;
 	    state_functions ->
-		State
-	end,
-    try Module:Func(Type, Content, PrevState, State, Data) of
+		Module:State(Type, Content, PrevState, Data);
+	    handle_event_function ->
+		Module:handle_event(Type, Content, PrevState, State, Data)
+	end of
 	Result ->
 	    loop_event_result(
 	      Parent, Debug, S, Events, Event, Result)
@@ -759,15 +757,27 @@ loop_events(
 	    %% Process an undef to check for the simple mistake
 	    %% of calling a nonexistent state function
 	    case erlang:get_stacktrace() of
-		[{Module,Func,
-		  [Type,Content,PrevState,State,Data]=Args,
+		[{Module,State,
+		  [Type,Content,PrevState,Data]=Args,
 		  _}
-		 |Stacktrace] ->
+		 |Stacktrace]
+		when CallbackMode =:= state_functions ->
 		    terminate(
 		      error,
 		      {undef_state_function,{Module,State,Args}},
 		      Stacktrace,
 		      Debug, S, Q);
+		[{Module,handle_event,
+		  [Type,Content,PrevState,State,Data]=Args,
+		  _}
+		 |Stacktrace]
+		when CallbackMode =:= handle_event_function ->
+		    terminate(
+		      error,
+		      {undef_state_function,
+		       {Module,handle_event,Args}},
+		      Stacktrace,
+		      Debug, S, Q);
 		Stacktrace ->
 		    terminate(error, undef, Stacktrace, Debug, S, Q)
 	    end;
diff --git a/lib/stdlib/test/gen_statem_SUITE.erl b/lib/stdlib/test/gen_statem_SUITE.erl
index 81182eff71..3302171329 100644
--- a/lib/stdlib/test/gen_statem_SUITE.erl
+++ b/lib/stdlib/test/gen_statem_SUITE.erl
@@ -1160,73 +1160,73 @@ 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}, _, State, _Data) ->
+idle({call,From}, {timeout,Time}, _, _Data) ->
     {next_state,timeout,{From,Time},
-     [{timeout,Time,State}]};
-idle(Type, Content, PrevState, State, Data) ->
-    case handle_common_events(Type, Content, PrevState, State, Data) of
+     [{timeout,Time,idle}]};
+idle(Type, Content, PrevState, Data) ->
+    case handle_common_events(Type, Content, idle, Data) of
 	undefined ->
 	    case Type of
 		{call,From} ->
 		    throw({keep_state,Data,[{reply,From,'eh?'}]});
 		_ ->
 		    throw(
-		      {stop,{unexpected,State,PrevState,Type,Content}})
+		      {stop,{unexpected,idle,Type,Content,PrevState}})
 	    end;
 	Result ->
 	    Result
     end.
 
-timeout(timeout, idle, idle, timeout, {From,Time}) ->
+timeout(timeout, idle, 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(_, _, _, State, Data) ->
-    {next_state,State,Data}.
+timeout(_, _, _, Data) ->
+    {keep_state,Data}.
 
 timeout2(
-  internal, {cancel_timer,TRefC2}, timeout, _, {From,Time,TRef2}) ->
+  internal, {cancel_timer,TRefC2}, timeout, {From,Time,TRef2}) ->
     Time4 = Time * 4,
     receive after Time4 -> ok end,
     {next_state,timeout3,{From,TRef2},
      [{cancel_timer,TRefC2}]};
-timeout2(_, _, _, State, Data) ->
-    {next_state,State,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(_, _, _, State, Data) ->
-    {next_state,State,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, PrevState, State, Data) ->
-    case handle_common_events(Type, Content, PrevState, State, Data) of
+wfor_conf(Type, Content, _, Data) ->
+    case handle_common_events(Type, Content, wfor_conf, Data) of
 	undefined ->
 	    case Type of
 		{call,From} ->
@@ -1239,113 +1239,113 @@ wfor_conf(Type, Content, PrevState, State, Data) ->
 	    Result
     end.
 
-connected({call,From}, {msg,Ref}, _, State, Data) ->
-    {next_state,State,Data,
+connected({call,From}, {msg,Ref}, _, Data) ->
+    {keep_state,Data,
      [{reply,From,{ack,Ref}}]};
-connected(cast, {msg,From,Ref}, _, State, Data) ->
+connected(cast, {msg,From,Ref}, _, Data) ->
     From ! {ack,Ref},
-    {next_state,State,Data};
-connected({call,From}, disconnect, _, _, Data) ->
+    {keep_state,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}, _, State, Data) ->
+connected(cast, {ping,Pid,Tag}, _, Data) ->
     Pid ! {pong,Tag},
-    {next_state,State,Data};
-connected(Type, Content, PrevState, State, Data) ->
-    case handle_common_events(Type, Content, PrevState, State, Data) of
+    {keep_state,Data};
+connected(Type, Content, _, Data) ->
+    case handle_common_events(Type, Content, connected, Data) of
 	undefined ->
 	    case Type of
 		{call,From} ->
-		    {next_state,State,Data,
+		    {keep_state,Data,
 		     [{reply,From,'eh?'}]};
 		_ ->
-		    {next_state,State,Data}
+		    {keep_state,Data}
 	    end;
 	Result ->
 	    Result
     end.
 
-state0({call,From}, stop, _, _, Data) ->
+state0({call,From}, stop, _, Data) ->
     {stop_and_reply,normal,[{reply,From,stopped}],Data};
-state0(Type, Content, PrevState, State, Data) ->
-    case handle_common_events(Type, Content, PrevState, State, Data) of
+state0(Type, Content, _, Data) ->
+    case handle_common_events(Type, Content, state0, Data) of
 	undefined ->
-	    {next_state,State,Data};
+	    {keep_state,Data};
 	Result ->
 	    Result
     end.
 
-hiber_idle({call,From}, 'alive?', _, State, Data) ->
-    {next_state,State,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, _, State, _) ->
+hiber_idle(info, hibernate_later, _, _) ->
     Tref = erlang:start_timer(1000, self(), hibernate),
-    {next_state,State,Tref};
-hiber_idle(info, hibernate_now, _, State, Data) ->
-    {next_state,State,Data,
+    {keep_state,Tref};
+hiber_idle(info, hibernate_now, _, Data) ->
+    {keep_state,Data,
      [hibernate]};
-hiber_idle(info, {timeout,Tref,hibernate}, _, State, Tref) ->
-    {next_state,State,[],
+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, PrevState, State, Data) ->
-    case handle_common_events(Type, Content, PrevState, State, Data) of
+hiber_idle(Type, Content, _, Data) ->
+    case handle_common_events(Type, Content, hiber_idle, Data) of
 	undefined ->
-	    {next_state,State,Data};
+	    {keep_state,Data};
 	Result ->
 	    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, _, State, Data) ->
-    {next_state,State,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, _, State, Data) ->
-    {next_state,State,Data,
+hiber_wakeup(cast, snooze_async, _, Data) ->
+    {keep_state,Data,
      [hibernate]};
-hiber_wakeup(Type, Content, PrevState, State, Data) ->
-    case handle_common_events(Type, Content, PrevState, State, Data) of
+hiber_wakeup(Type, Content, _, Data) ->
+    case handle_common_events(Type, Content, hiber_wakeup, Data) of
 	undefined ->
-	    {next_state,State,Data};
+	    {keep_state,Data};
 	Result ->
 	    Result
     end.
 
 
-handle_common_events({call,From}, get, _, State, Data) ->
-    {next_state,State,Data,
+handle_common_events({call,From}, get, State, Data) ->
+    {keep_state,Data,
      [{reply,From,{state,State,Data}}]};
-handle_common_events(cast, {get,Pid}, _, State, Data) ->
+handle_common_events(cast, {get,Pid}, State, Data) ->
     Pid ! {state,State,Data},
-    {next_state,State,Data};
-handle_common_events({call,From}, stop, _, _, Data) ->
+    {keep_state,Data};
+handle_common_events({call,From}, stop, _, Data) ->
     {stop_and_reply,normal,[{reply,From,stopped}],Data};
-handle_common_events(cast, stop, _, _, _) ->
+handle_common_events(cast, stop, _, _) ->
     {stop,normal};
-handle_common_events({call,From}, {stop,Reason}, _, _, Data) ->
+handle_common_events({call,From}, {stop,Reason}, _, Data) ->
     {stop_and_reply,Reason,{reply,From,stopped},Data};
-handle_common_events(cast, {stop,Reason}, _, _, _) ->
-     {stop,Reason};
-handle_common_events({call,From}, 'alive?', _, State, Data) ->
-    {next_state,State,Data,
+handle_common_events(cast, {stop,Reason}, _, _) ->
+    {stop,Reason};
+handle_common_events({call,From}, 'alive?', _, Data) ->
+    {keep_state,Data,
      [{reply,From,yes}]};
-handle_common_events(cast, {'alive?',Pid}, _, State, Data) ->
+handle_common_events(cast, {'alive?',Pid}, _, Data) ->
     Pid ! yes,
-    {next_state,State,Data};
-handle_common_events(_, _, _, _, _) ->
+    {keep_state,Data};
+handle_common_events(_, _, _, _) ->
     undefined.
 
 %% Dispatcher to test callback_mode handle_event_function
@@ -1356,8 +1356,7 @@ handle_common_events(_, _, _, _, _) ->
 handle_event(Type, Event, PrevState, State, Data) ->
     PrevStateName = unwrap_state(PrevState),
     StateName = unwrap_state(State),
-    try ?MODULE:StateName(
-	   Type, Event, PrevStateName, StateName, Data) of
+    try ?MODULE:StateName(Type, Event, PrevStateName, Data) of
 	Result ->
 	    wrap_result(Result)
     catch