From 26a7af61fbffae90c0968d945ae8b146582ba068 Mon Sep 17 00:00:00 2001
From: Raimo Niskanen <raimo@erlang.org>
Date: Thu, 18 Feb 2016 16:38:57 +0100
Subject: Change initial PrevState to 'undefined'

---
 lib/stdlib/doc/src/gen_statem.xml | 27 ++++++++++++++++++++++++---
 lib/stdlib/src/gen_statem.erl     |  7 ++++---
 2 files changed, 28 insertions(+), 6 deletions(-)

diff --git a/lib/stdlib/doc/src/gen_statem.xml b/lib/stdlib/doc/src/gen_statem.xml
index 5fbedb12f8..9d98763973 100644
--- a/lib/stdlib/doc/src/gen_statem.xml
+++ b/lib/stdlib/doc/src/gen_statem.xml
@@ -110,6 +110,14 @@ 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 gen_statem 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>
     <p>The gen_statem enqueues incoming events in order of arrival
       and presents these to the
       <seealso marker="#state_function">state function</seealso>
@@ -118,6 +126,12 @@ erlang:'!'            -----> Module:StateName/5
       After a state change all enqueued events (including postponed)
       are again presented to the state function.
     </p>
+    <p>The gen_statem event queue model is sufficient to emulate
+      the normal process message queue and selective receive
+      with postponing an event corresponding to not matching
+      it in a receive statement and changing states corresponding
+      to entering a new receive statement.
+    </p>
     <p>The
       <seealso marker="#state_function">state function</seealso>
       can insert events using the
@@ -133,6 +147,15 @@ erlang:'!'            -----> Module:StateName/5
       that can be used for such events making it impossible
       to mistake for an external event.
     </p>
+    <p>Inserting an event replaces the trick of calling your own
+      state handling functions that you often would have to
+      resort to in e.g <seealso marker="gen_fsm">gen_fsm</seealso>
+      to force processing a faked event before others.
+      If you for example in gen_statem 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>A gen_statem handles system messages as documented in
        <seealso marker="sys">sys</seealso>.
        The <seealso marker="sys">sys</seealso> module
@@ -1011,7 +1034,6 @@ erlang:'!'            -----> Module:StateName/5
 	<v>EventContent = term()</v>
 	<v>PrevStateName =
 	  <seealso marker="#type-state_name">state_name()</seealso>
-	  | reference()
 	</v>
 	<v>StateName =
 	  <seealso marker="#type-state_name">state_name()</seealso>
@@ -1060,8 +1082,7 @@ erlang:'!'            -----> Module:StateName/5
 	  in some odd cases for example when you want to do something
 	  only at the first event in a state.
 	  Note that when gen_statem enters its first state
-	  this is set to a <c>reference()</c>
-	  since that can not match equal to any 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
diff --git a/lib/stdlib/src/gen_statem.erl b/lib/stdlib/src/gen_statem.erl
index 486f61b1ed..e03e22b087 100644
--- a/lib/stdlib/src/gen_statem.erl
+++ b/lib/stdlib/src/gen_statem.erl
@@ -459,13 +459,13 @@ do_send(Proc, Msg) ->
 enter(Module, Options, State, StateData, Server, InitOps, Parent) ->
     Name = gen:get_proc_name(Server),
     Debug = gen:debug_options(Name, Options),
-    PrevState = make_ref(),
+    PrevState = undefined,
     S = #{
       callback_mode => state_functions,
       module => Module,
       name => Name,
       prev_state => PrevState,
-      state => PrevState,
+      state => PrevState, % Will be discarded by loop_event_state_ops
       state_data => StateData,
       timer => undefined,
       postponed => [],
@@ -475,7 +475,8 @@ enter(Module, Options, State, StateData, Server, InitOps, Parent) ->
 	    loop_event_state_ops(
 	      Parent, Debug,
 	      S#{callback_mode := CallbackMode},
-	      [], {event,undefined},
+	      [],
+	      {event,undefined}, % Will be discarded by {postpone,false}
 	      State, StateData,
 	      StateOps++[{postpone,false}]);
 	[Reason] ->
-- 
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