Merge branch 'raimo/stdlib/gen_statem-dev/OTP-14015'

* raimo/stdlib/gen_statem-dev/OTP-14015:
  Fix after feedback
  Improve pointer to User's Guide
  Fix after feedback on 'When to use'
  Add a 'When to use' section
  Fix timeout parsing and doc feedback
  Improve doc, change images to .svg
  erl_docgen: Implement width in image tag
  Update User's Guide and pointers to it
  Improve error reasons from state enter call

1 files changed, 151 insertions, 68 deletions

diff --git a/lib/stdlib/doc/src/gen_statem.xml b/lib/stdlib/doc/src/gen_statem.xml
index be0d64feba..e918e83df7 100644
--- a/lib/stdlib/doc/src/gen_statem.xml
+++ b/lib/stdlib/doc/src/gen_statem.xml
@@ -32,7 +32,68 @@
   <modulesummary>Generic state machine behavior.</modulesummary>
   <description>
     <p>
-      This behavior module provides a state machine. Two
+      <c>gen_statem</c> provides a generic state machine behaviour
+      and replaces its predecessor
+      <seealso marker="gen_fsm"><c>gen_fsm</c></seealso>
+      since Erlang/OTP 20.0.
+    </p>
+    <p>
+      This reference manual describes types generated from the types
+      in the <c>gen_statem</c> source code, so they are correct.
+      However, the generated descriptions also reflect the type hiearchy,
+      which makes them kind of hard to read.
+    </p>
+    <p>
+      To get an overview of the concepts and operation of <c>gen_statem</c>,
+      do read the
+      <seealso marker="doc/design_principles:statem">
+	<c>gen_statem</c>&nbsp;Behaviour
+      </seealso>
+      in
+      <seealso marker="doc/design_principles:users_guide">
+	OTP Design Principles
+      </seealso>
+      which frequently links back to this reference manual to avoid
+      containing detailed facts that may rot by age.
+    </p>
+    <note>
+      <p>
+        This behavior appeared in Erlang/OTP 19.0.
+	In OTP 19.1 a backwards incompatible change of
+	the return tuple from
+	<seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>
+	was made and the mandatory callback function
+	<seealso marker="#Module:callback_mode/0">
+	  <c>Module:callback_mode/0</c>
+	</seealso>
+	was introduced.  In OTP 20.0 the
+        <seealso marker="#type-generic_timeout"><c>generic timeouts</c></seealso>
+	were added.
+      </p>
+    </note>
+    <p>
+      <c>gen_statem</c> has got the same features that
+      <seealso marker="gen_fsm"><c>gen_fsm</c></seealso>
+      had and adds some really useful:
+    </p>
+    <list type="bulleted">
+      <item>Co-located state code</item>
+      <item>Arbitrary term state</item>
+      <item>Event postponing</item>
+      <item>Self-generated events</item>
+      <item>State time-out</item>
+      <item>Multiple generic named time-outs</item>
+      <item>Absolute time-out time</item>
+      <item>Automatic state enter calls</item>
+      <item>
+	Reply from other state than the request, <c>sys</c> traceable
+      </item>
+      <item>Multiple <c>sys</c> traceable replies</item>
+    </list>
+
+
+    <p>
+      Two
       <seealso marker="#type-callback_mode"><em>callback modes</em></seealso>
       are supported:
     </p>
@@ -50,34 +111,6 @@
         </p>
       </item>
     </list>
-    <note>
-      <p>
-        This is a new behavior in Erlang/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.
-        Depending on user feedback, we do not expect
-        but can find it necessary to make minor
-        not backward compatible changes into Erlang/OTP 20.0.
-      </p>
-    </note>
-    <p>
-      The <c>gen_statem</c> behavior replaces 
-      <seealso marker="gen_fsm"><c>gen_fsm</c> </seealso> in Erlang/OTP 20.0.
-      It has the same features and adds some really useful:
-    </p>
-    <list type="bulleted">
-      <item>Gathered state code.</item>
-      <item>Arbitrary term state.</item>
-      <item>Event postponing.</item>
-      <item>Self-generated events.</item>
-      <item>State time-out.</item>
-      <item>Multiple generic named time-outs.</item>
-      <item>Absolute time-out time.</item>
-      <item>Automatic state enter calls.</item>
-      <item>Reply from other state than the request.</item>
-      <item>Multiple <c>sys</c> 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>,
@@ -148,7 +181,7 @@ erlang:'!'            -----> Module:StateName/3
       is <c>state_functions</c>, the state must be an atom and
       is used as the state callback name; see
       <seealso marker="#Module:StateName/3"><c>Module:StateName/3</c></seealso>.
-      This gathers all code for a specific state
+      This co-locates all code for a specific state
       in one function as the <c>gen_statem</c> engine
       branches depending on state name.
       Note the fact that the callback function
@@ -207,8 +240,10 @@ erlang:'!'            -----> Module:StateName/3
       whenever a new state is entered; see 
       <seealso marker="#type-state_enter"><c>state_enter()</c></seealso>.
       This is for writing code common to all state entries.
-      Another way to do it is to insert events at state transitions,
-      but you have to do so everywhere it is needed.
+      Another way to do it is to insert an event at the state transition,
+      and/or to use a dedicated state transition function,
+      but that is something you will have to remember
+      at every state transition to the state(s) that need it.
     </p>
     <note>
       <p>If you in <c>gen_statem</c>, for example, postpone
@@ -252,6 +287,16 @@ erlang:'!'            -----> Module:StateName/3
       to use after every event; see
       <seealso marker="erts:erlang#hibernate/3"><c>erlang:hibernate/3</c></seealso>.
     </p>
+    <p>
+      There is also a server start option
+      <seealso marker="#type-hibernate_after_opt">
+	<c>{hibernate_after, Timeout}</c>
+      </seealso>
+      for
+      <seealso marker="#start/3"><c>start/3,4</c></seealso> or
+      <seealso marker="#start_link/3"><c>start_link/3,4</c></seealso>
+      that may be used to automatically hibernate the server.
+    </p>
   </description>
 
   <section>
@@ -668,9 +713,9 @@ handle_event(_, _, State, Data) ->
 	<p>
 	  If 
 	  <seealso marker="#Module:code_change/4"><c>Module:code_change/4</c></seealso>
-	  should transform the state to a state with a different
-	  name it is still regarded as the same state so this
-	  does not cause a state enter call.
+	  should transform the state,
+	  it is regarded as a state rename and not a state change,
+	  which will not cause a state enter call.
 	</p>
 	<p>
 	  Note that a state enter call <em>will</em> be done
@@ -688,12 +733,19 @@ handle_event(_, _, State, Data) ->
 	<p>
 	  Transition options can be set by
 	  <seealso marker="#type-action">actions</seealso>
-	  and they modify how the state transition is done:
+	  and modify the state transition.
+	  Here are the sequence of steps for a state transition:
 	</p>
 	<list type="ordered">
 	  <item>
             <p>
-	      If the state changes, is the initial state,
+	      If
+	      <seealso marker="#type-state_enter">
+		<em>state enter calls</em>
+	      </seealso>
+	      are used, and either:
+	      the state changes, it is the initial state,
+	      or one of the callback results
 	      <seealso marker="#type-state_callback_result">
 		<c>repeat_state</c>
 	      </seealso>
@@ -701,16 +753,21 @@ handle_event(_, _, State, Data) ->
 	      <seealso marker="#type-state_callback_result">
 		<c>repeat_state_and_data</c>
 	      </seealso>
-	      is used, and also
-	      <seealso marker="#type-state_enter"><em>state enter calls</em></seealso>
-	      are used, the <c>gen_statem</c> calls
+	      is used; the <c>gen_statem</c> calls
 	      the new state callback with arguments
-	      <seealso marker="#type-state_enter">(enter, OldState, Data)</seealso>.
+	      <seealso marker="#type-state_enter"><c>(enter, OldState, Data)</c></seealso>.
+	    </p>
+	    <p>
 	      Any 
-	      <seealso marker="#type-enter_action"><c>actions</c></seealso>
+	      <seealso marker="#type-enter_action">actions</seealso>
 	      returned from this call are handled as if they were
-	      appended to the actions 
-	      returned by the state callback that changed states.
+	      appended to the actions
+	      returned by the state callback that caused the state entry.
+	    </p>
+	    <p>
+	      Should this state enter call return any of
+	      the mentioned <c>repeat_*</c> callback results
+	      it is repeated again, with the updated <c>Data</c>.
             </p>
 	  </item>
 	  <item>
@@ -739,7 +796,7 @@ handle_event(_, _, State, Data) ->
 	      All events stored with
 	      <seealso marker="#type-action"><c>action()</c></seealso>
 	      <c>next_event</c>
-	      are inserted to be processed before the other queued events.
+	      are inserted to be processed before previously queued events.
             </p>
 	  </item>
 	  <item>
@@ -753,7 +810,9 @@ handle_event(_, _, State, Data) ->
 	      delivered to the state machine before any external
 	      not yet received event so if there is such a time-out requested,
 	      the corresponding time-out zero event is enqueued as
-	      the newest event.
+	      the newest received event;
+	      that is after already queued events
+	      such as inserted and postponed events.
 	    </p>
 	    <p>
 	      Any event cancels an
@@ -791,7 +850,7 @@ handle_event(_, _, State, Data) ->
 	      When a new message arrives the 
 	      <seealso marker="#state callback">state callback</seealso>
 	      is called with the corresponding event,
-	      and we start again from the top of this list.
+	      and we start again from the top of this sequence.
 	    </p>
 	  </item>
 	</list>
@@ -816,13 +875,19 @@ handle_event(_, _, State, Data) ->
 	  <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, an
-	  <seealso marker="erts:erlang#garbage_collect/0"><c>erlang: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>
+	<note>
+	  <p>
+	    If there are enqueued events to process
+	    when hibrnation is requested,
+	    this is optimized by not hibernating but instead calling
+	    <seealso marker="erts:erlang#garbage_collect/0">
+	      <c>erlang:garbage_collect/0</c>
+	    </seealso>
+	    to simulate that the <c>gen_statem</c> entered hibernation
+	    and immediately got awakened by an enqueued event.
+	  </p>
+	</note>
       </desc>
     </datatype>
     <datatype>
@@ -857,7 +922,7 @@ handle_event(_, _, State, Data) ->
 	  no timer is actually started,
 	  instead the the time-out event is enqueued to ensure
 	  that it gets processed before any not yet
-	  received external event.
+	  received external event, but after already queued events.
 	</p>
 	<p>
 	  Note that it is not possible nor needed to cancel this time-out,
@@ -943,7 +1008,9 @@ handle_event(_, _, State, Data) ->
 	  If <c>Abs</c> is <c>true</c> an absolute timer is started,
 	  and if it is <c>false</c> a relative, which is the default.
 	  See
-	  <seealso marker="erts:erlang#start_timer/4"><c>erlang:start_timer/4</c></seealso>
+	  <seealso marker="erts:erlang#start_timer/4">
+	    <c>erlang:start_timer/4</c>
+	  </seealso>
 	  for details.
 	</p>
 	<p>
@@ -969,7 +1036,9 @@ handle_event(_, _, State, Data) ->
 	</p>
 	<p>
 	  Actions that set
-	  <seealso marker="#type-transition_option">transition options</seealso>
+	  <seealso marker="#type-transition_option">
+	    transition options
+	  </seealso>
 	  override any previous of the same type,
 	  so the last in the containing list wins.
 	  For example, the last
@@ -981,7 +1050,9 @@ handle_event(_, _, State, Data) ->
 	  <item>
 	    <p>
 	      Sets the
-	      <seealso marker="#type-transition_option"><c>transition_option()</c></seealso>
+	      <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
@@ -994,7 +1065,11 @@ handle_event(_, _, State, Data) ->
 	  <tag><c>next_event</c></tag>
 	  <item>
 	    <p>
-	      Stores the specified <c><anno>EventType</anno></c>
+	      This action does not set any
+	      <seealso marker="#type-transition_option">
+		<c>transition_option()</c>
+	      </seealso>
+	      but instead stores the specified <c><anno>EventType</anno></c>
 	      and <c><anno>EventContent</anno></c> for insertion after all
 	      actions have been executed.
 	    </p>
@@ -1066,15 +1141,15 @@ handle_event(_, _, State, Data) ->
 	  <seealso marker="#type-transition_option">transition options</seealso>.
 	</p>
 	<taglist>
-	  <tag><c>Timeout</c></tag>
+	  <tag><c>Time</c></tag>
 	  <item>
 	    <p>
-	      Short for <c>{timeout,Timeout,Timeout}</c>, that is,
+	      Short for <c>{timeout,Time,Time}</c>, that is,
 	      the time-out message is the time-out time.
 	      This form exists to make the
 	      <seealso marker="#state callback">state callback</seealso>
-	      return value <c>{next_state,NextState,NewData,Timeout}</c>
-	      allowed like for <c>gen_fsm</c>'s
+	      return value <c>{next_state,NextState,NewData,Time}</c>
+	      allowed like for <c>gen_fsm</c>.
 	    </p>
 	  </item>
 	  <tag><c>timeout</c></tag>
@@ -1126,7 +1201,11 @@ handle_event(_, _, State, Data) ->
 	  <seealso marker="#enter_loop/5"><c>enter_loop/5,6</c></seealso>.
 	</p>
 	<p>
-	  It replies to a caller waiting for a reply in
+	  It does not set any
+	  <seealso marker="#type-transition_option">
+	    <c>transition_option()</c>
+	  </seealso>
+	  but instead replies 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 argument
 	  <seealso marker="#type-event_type"><c>{call,<anno>From</anno>}</c></seealso>
@@ -2109,16 +2188,20 @@ init(Args) -> erlang:error(not_implemented, [Args]).</pre>
 	  You may also not change states from this call.
 	  Should you return <c>{next_state,NextState, ...}</c>
 	  with <c>NextState =/= State</c> the <c>gen_statem</c> crashes.
-	  It is possible to use <c>{repeat_state, ...}</c>,
-	  <c>{repeat_state_and_data,_}</c> or
-	  <c>repeat_state_and_data</c> but all of them makes little
+	  Note that it is actually allowed to use
+	  <c>{repeat_state, NewData, ...}</c> although it makes little
 	  sense since you immediately will be called again with a new
 	  <em>state enter call</em> making this just a weird way
 	  of looping, and there are better ways to loop in Erlang.
+	  If you do not update <c>NewData</c> and have some
+	  loop termination condition, or if you use
+	  <c>{repeat_state_and_data, _}</c> or
+	  <c>repeat_state_and_data</c> you have an infinite loop!
 	  You are advised to use <c>{keep_state,...}</c>,
 	  <c>{keep_state_and_data,_}</c> or
-	  <c>keep_state_and_data</c> since you can not change states
-	  from a <em>state enter call</em> anyway.
+	  <c>keep_state_and_data</c>
+	  since changing states from a <em>state enter call</em>
+	  is not possible anyway.
 	</p>
 	<p>
 	  Note the fact that you can use