Merge branch 'xsipewe_gen_statem1' of https://github.com/xsipewe/otp into raimo/polish-gen_statem/OTP-13065

Conflicts:
	lib/stdlib/doc/src/gen_statem.xml

1 files changed, 349 insertions, 347 deletions

diff --git a/system/doc/design_principles/statem.xml b/system/doc/design_principles/statem.xml
index ca0fce55e2..8b0fbed7c0 100644
--- a/system/doc/design_principles/statem.xml
+++ b/system/doc/design_principles/statem.xml
@@ -5,7 +5,7 @@
   <header>
     <copyright>
       <year>2016</year>
-      <holder>Ericsson AB.  All Rights Reserved.</holder>
+      <holder>Ericsson AB. All Rights Reserved.</holder>
     </copyright>
     <legalnotice>
       Licensed under the Apache License, Version 2.0 (the "License");
@@ -22,7 +22,7 @@
 
     </legalnotice>
 
-    <title>gen_statem Behaviour</title>
+    <title>gen_statem Behavior</title>
     <prepared></prepared>
     <docno></docno>
     <date></date>
@@ -33,63 +33,60 @@
   <p>
     This section is to be read with the
     <seealso marker="stdlib:gen_statem"><c>gen_statem(3)</c></seealso>
-    manual page in STDLIB, where all interface functions and callback
+    manual page in <c>STDLIB</c>, where all interface functions and callback
     functions are described in detail.
   </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>
+  <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>
 
 <!-- =================================================================== -->
 
   <section>
-    <title>Event Driven State Machines</title>
+    <title>Event-Driven State Machines</title>
     <p>
       Established Automata theory does not deal much with
       how a state transition is triggered,
-      but in general assumes that the output is a function
+      but assumes that the output is a function
       of the input (and the state) and that they are
       some kind of values.
     </p>
     <p>
-      For an Event Driven State Machine the input is an event
+      For an Event-Driven State Machine, the input is an event
       that triggers a state transition and the output
       is actions executed during the state transition.
       It can analogously to the mathematical model of a
-      Finite State Machine be described as
-      a set of relations of the form:
+      Finite-State Machine be described as
+      a set of relations of the following form:
     </p>
     <pre>
 State(S) x Event(E) -> Actions(A), State(S')</pre>
-    <p>These relations are interpreted as meaning:</p>
-    <p>
-      If we are in state <c>S</c> and event <c>E</c> occurs, we
+    <p>These relations are interpreted as follows:
+      if we are in state <c>S</c> and event <c>E</c> occurs, we
       are to perform actions <c>A</c> and make a transition to
-      state <c>S'</c>.
-    </p>
-    <p>
-      Note that <c>S'</c> may be equal to <c>S</c>.
+      state <c>S'</c>. Notice that <c>S'</c> can be equal to <c>S</c>.
     </p>
     <p>
-      Since <c>A</c> and <c>S'</c> depend only on
-      <c>S</c> and <c>E</c> the kind of state machine described
-      here is a Mealy Machine.
-      (See for example the corresponding Wikipedia article)
+      As <c>A</c> and <c>S'</c> depend only on
+      <c>S</c> and <c>E</c>, the kind of state machine described
+      here is a Mealy Machine
+      (see, for example, the corresponding Wikipedia article).
     </p>
     <p>
-      Like most <c>gen_</c> behaviours, <c>gen_statem</c> keeps
-      a server <c>Data</c> besides the state.  This and the fact that
+      Like most <c>gen_</c> behaviors, <c>gen_statem</c> keeps
+      a server <c>Data</c> besides the state. Because of this, and as
       there is no restriction on the number of states
-      (assuming enough virtual machine memory)
-      or on the number of distinct input events actually makes
-      a state machine implemented with this behaviour Turing complete.
-      But it feels mostly like an Event Driven Mealy Machine.
+      (assuming that there is enough virtual machine memory)
+      or on the number of distinct input events, makes
+      a state machine implemented with this behavior Turing complete.
+      But it feels mostly like an Event-Driven Mealy Machine.
     </p>
   </section>
 
@@ -99,38 +96,40 @@ State(S) x Event(E) -> Actions(A), State(S')</pre>
     <marker id="callback_modes" />
     <title>Callback Modes</title>
     <p>
-      The <c>gen_statem</c> behaviour supports two different callback modes.
-      In the mode
-      <seealso marker="stdlib:gen_statem#type-callback_mode">
-	<c>state_functions</c>,
-      </seealso>
-      the state transition rules are written as a number of Erlang
-      functions, which conform to the following convention:
+      The <c>gen_statem</c> behavior supports two callback modes:
     </p>
-    <pre>
+    <list type="bulleted">
+      <item>
+        <p>
+          In mode <seealso marker="stdlib:gen_statem#type-callback_mode">
+          <c>state_functions</c></seealso>,
+          the state transition rules are written as some Erlang
+          functions, which conform to the following convention:
+        </p>
+        <pre>
 StateName(EventType, EventContent, Data) ->
     .. code for actions here ...
     {next_state, NewStateName, NewData}.</pre>
-    <p>
-      In the mode
-      <seealso marker="stdlib:gen_statem#type-callback_mode">
-	<c>handle_event_function</c>
-      </seealso>
-      there is only one
-      Erlang function that implements all state transition rules:
-    </p>
-    <pre>
+      </item>
+      <item>
+        <p>
+          In mode <seealso marker="stdlib:gen_statem#type-callback_mode">
+          <c>handle_event_function</c></seealso>,
+          only one Erlang function provides all state transition rules:
+        </p>
+        <pre>
 handle_event(EventType, EventContent, State, Data) ->
     .. code for actions here ...
     {next_state, State', Data'}</pre>
+      </item>
+    </list>
     <p>
-      Both these modes allow other return tuples
-      that you can find in the
+      Both these modes allow other return tuples; see
       <seealso marker="stdlib:gen_statem#Module:StateName/3">
-	reference manual.
-      </seealso>
-      These other return tuples can for example stop the machine,
-      execute state transition actions on the machine engine itself
+      <c>Module:StateName/3</c></seealso>
+      in the <c>gen_statem</c> manual page.
+      These other return tuples can, for example, stop the machine,
+      execute state transition actions on the machine engine itself,
       and send replies.
     </p>
 
@@ -139,54 +138,55 @@ handle_event(EventType, EventContent, State, Data) ->
       <p>
 	The two
 	<seealso marker="#callback_modes">callback modes</seealso>
-	gives different possibilities
+	give different possibilities
 	and restrictions, but one goal remains:
 	you want to handle all possible combinations of
 	events and states.
       </p>
       <p>
-	You can for example do this by focusing on one state at the time
-	and for every state ensure that all events are handled,
-	or the other way around focus on one event at the time
-	and ensure that it is handled in every state,
-	or mix these strategies.
+	This can be done, for example, by focusing on one state at the time
+	and for every state ensure that all events are handled.
+	Alternatively, you can focus on one event at the time
+	and ensure that it is handled in every state.
+	You can also use a mix of these strategies.
       </p>
       <p>
-	With <c>state_functions</c> you are restricted to use
-	atom only states, and the <c>gen_statem</c> engine dispatches
-	on state name for you.  This encourages the callback module
+	With <c>state_functions</c>, you are restricted to use
+	atom-only states, and the <c>gen_statem</c> engine dispatches
+	on state name for you. This encourages the callback module
 	to gather the implementation of all event actions particular
-	to one state in the same place in the code
+	to one state in the same place in the code,
 	hence to focus on one state at the time.
       </p>
       <p>
-	This mode fits well when you have a regular state diagram
-	like the ones in this chapter that describes all events and actions
+	This mode fits well when you have a regular state diagram,
+	like the ones in this chapter, which describes all events and actions
 	belonging to a state visually around that state,
 	and each state has its unique name.
       </p>
       <p>
-	With <c>handle_event_function</c> you are free to mix strategies
-	as you like because all events and states
-	are handled in the the same callback function.
+	With <c>handle_event_function</c>, you are free to mix strategies,
+	as all events and states are handled in the same callback function.
       </p>
       <p>
 	This mode works equally well when you want to focus on
-	one event at the time or when you want to focus on
-	one state at the time, but the <c>handle_event/4</c> function
+	one event at the time or on
+	one state at the time, but function
+	<seealso marker="stdlib:gen_statem#Module:handle_event/4">
+	<c>Module:handle_event/4</c></seealso>
 	quickly grows too large to handle without introducing dispatching.
       </p>
       <p>
-	The mode enables the use of non-atom states for example
-	complex states or even hiearchical states.
+	The mode enables the use of non-atom states, for example,
+	complex states or even hierarchical states.
 	If, for example, a state diagram is largely alike
-	for the client and for the server side of a protocol;
-	then you can have a state <c>{StateName,server}</c> or
-	<c>{StateName,client}</c> and since you do the dispatching
-	yourself you make <c>StateName</c> decide where in the code
+	for the client side and the server side of a protocol,
+	you can have a state <c>{StateName,server}</c> or
+	<c>{StateName,client}</c>. Also, as you do the dispatching
+	yourself, you make <c>StateName</c> decide where in the code
 	to handle most events in the state.
-	The second element of the tuple	is then used to select
-	whether to handle special client side or server side events.
+	The second element of the tuple is then used to select
+	whether to handle special client-side or server-side events.
       </p>
     </section>
   </section>
@@ -196,31 +196,28 @@ handle_event(EventType, EventContent, State, Data) ->
   <section>
     <title>Example</title>
     <p>
-      This is an example starting off as equivalent to the the example in the
-      <seealso marker="fsm"><c>gen_fsm</c> behaviour</seealso>
-      description.  In later chapters additions and tweaks are made
+      This example starts off as equivalent to the example in section
+      <seealso marker="fsm"><c>gen_fsm</c> Behavior</seealso>.
+      In later sections, additions and tweaks are made
       using features in <c>gen_statem</c> that <c>gen_fsm</c> does not have.
-      At the end of this section you can find the example again
+      The end of this chapter provides the example again
       with all the added features.
     </p>
     <p>
-      A door with a code lock can be viewed as a state machine.
-      Initially, the door is locked.  Anytime someone presses a button,
-      this generates an event.
+      A door with a code lock can be seen as a state machine.
+      Initially, the door is locked. When someone presses a button,
+      an event is generated.
       Depending on what buttons have been pressed before,
       the sequence so far can be correct, incomplete, or wrong.
-    </p>
-    <p>
-      If it is correct, the door is unlocked for 10 seconds (10000 ms).
-      If it is incomplete, we wait for another button to be pressed.  If
-      it is is wrong, we start all over,
-      waiting for a new button sequence.
+      If correct, the door is unlocked for 10 seconds (10,000 milliseconds).
+      If incomplete, we wait for another button to be pressed. If
+      wrong, we start all over, waiting for a new button sequence.
     </p>
     <image file="../design_principles/code_lock.png">
-      <icaption>Code lock state diagram</icaption>
+      <icaption>Code Lock State Diagram</icaption>
     </image>
     <p>
-      We can implement such a code lock state machine using
+      This code lock state machine can be implemented using
       <c>gen_statem</c> with the following callback module:
     </p>
     <marker id="ex"></marker>
@@ -241,7 +238,6 @@ start_link(Code) ->
 button(Digit) ->
     gen_statem:cast(?NAME, {button,Digit}).
 
-
 init(Code) ->
     do_lock(),
     Data = #{code => Code, remaining => Code},
@@ -286,7 +282,7 @@ code_change(_Vsn, State, Data, _Extra) ->
   <section>
     <title>Starting gen_statem</title>
     <p>
-      In the example in the previous section, the <c>gen_statem</c> is
+      In the example in the previous section, <c>gen_statem</c> is
       started by calling <c>code_lock:start_link(Code)</c>:
     </p>
     <code type="erl"><![CDATA[
@@ -294,63 +290,64 @@ start_link(Code) ->
     gen_statem:start_link({local,?NAME}, ?MODULE, Code, []).
     ]]></code>
     <p>
-      <c>start_link</c> calls the function
+      <c>start_link</c> calls function
       <seealso marker="stdlib:gen_statem#start_link/4">
 	<c>gen_statem:start_link/4</c>
-      </seealso>
-      which spawns and links to a new process; a <c>gen_statem</c>.
+      </seealso>,
+      which spawns and links to a new process, a <c>gen_statem</c>.
     </p>
     <list type="bulleted">
       <item>
         <p>
-	  The first argument, <c>{local,?NAME}</c>, specifies
-          the name.  In this case, the <c>gen_statem</c> is locally
-	  registered as <c>code_lock</c> through the macro <c>?NAME</c>.
-	</p>
+          The first argument, <c>{local,?NAME}</c>, specifies
+          the name. In this case, the <c>gen_statem</c> is locally
+          registered as <c>code_lock</c> through the macro <c>?NAME</c>.
+        </p>
         <p>
-	  If the name is omitted, the <c>gen_statem</c> is not registered.
-          Instead its pid must be used.  The name can also be given
-	  as <c>{global,Name}</c>, in which case the <c>gen_statem</c> is
-	  registered using
-	  <seealso marker="kernel:global#register_name/2">
-	    <c>global:register_name/2</c>.
-	  </seealso>
-	</p>
+          If the name is omitted, the <c>gen_statem</c> is not registered.
+          Instead its pid must be used. The name can also be specified
+          as <c>{global,Name}</c>, then the <c>gen_statem</c> is
+          registered using
+          <seealso marker="kernel:global#register_name/2">
+            <c>global:register_name/2</c>
+          </seealso>
+          in <c>Kernel</c>.
+        </p>
       </item>
       <item>
         <p>
-	  The second argument, <c>?MODULE</c>, is the name of
-          the callback module, that is; the module where the callback
+          The second argument, <c>?MODULE</c>, is the name of
+          the callback module, that is, the module where the callback
           functions are located, which is this module.
-	</p>
+        </p>
         <p>
-	  The interface functions (<c>start_link/1</c> and <c>button/1</c>)
-	  are located in the same module as the callback functions
-	  (<c>init/1</c>, <c>locked/3</c>, and <c>open/3</c>).
-	  It is normally good programming practice to have the client
-	  side and the server side code contained in one module.
-	</p>
+          The interface functions (<c>start_link/1</c> and <c>button/1</c>)
+          are located in the same module as the callback functions
+          (<c>init/1</c>, <c>locked/3</c>, and <c>open/3</c>).
+          It is normally good programming practice to have the client-side
+          code and the server-side code contained in one module.
+        </p>
       </item>
       <item>
         <p>
-	  The third argument, <c>Code</c>, is a list of digits that
-	  is the correct unlock code which is passsed
-	  to the callback function <c>init/1</c>.
+          The third argument, <c>Code</c>, is a list of digits, which
+          is the correct unlock code that is passed
+          to callback function <c>init/1</c>.
 	</p>
       </item>
       <item>
         <p>
-	  The fourth argument, <c>[]</c>, is a list of options.  See the
-	  <seealso marker="stdlib:gen_statem#start_link/3">
-	    <c>gen_statem:start_link/3</c>
-	  </seealso>
-	  manual page for available options.
+          The fourth argument, <c>[]</c>, is a list of options.
+          For the available options, see
+          <seealso marker="stdlib:gen_statem#start_link/3">
+            <c>gen_statem:start_link/3</c>
+          </seealso>.
 	</p>
       </item>
     </list>
     <p>
       If name registration succeeds, the new <c>gen_statem</c> process
-      calls the callback function <c>code_lock:init(Code)</c>.
+      calls callback function <c>code_lock:init(Code)</c>.
       This function is expected to return <c>{CallbackMode,State,Data}</c>,
       where
       <seealso marker="#callback_modes">
@@ -360,14 +357,14 @@ start_link(Code) ->
       <seealso marker="stdlib:gen_statem#type-callback_mode">
 	<c>state_functions</c>
       </seealso>
-      through the macro <c>?CALLBACK_MODE</c> that is; each state
+      through macro <c>?CALLBACK_MODE</c>. That is, each state
       has got its own handler function.
       <c>State</c> is the initial state of the <c>gen_statem</c>,
-      in this case <c>locked</c>; assuming the door is locked to begin with.
-      <c>Data</c> is the internal server data of the <c>gen_statem</c>.
+      in this case <c>locked</c>; assuming that the door is locked to begin
+      with. <c>Data</c> is the internal server data of the <c>gen_statem</c>.
       Here the server data is a <seealso marker="stdlib:maps">map</seealso>
-      with the key <c>code</c> that stores
-      the correct button sequence and the key <c>remaining</c>
+      with key <c>code</c> that stores
+      the correct button sequence, and key <c>remaining</c>
       that stores the remaining correct button sequence
       (the same as the <c>code</c> to begin with).
     </p>
@@ -377,24 +374,25 @@ init(Code) ->
     Data = #{code => Code, remaining => Code},
     {?CALLBACK_MODE,locked,Data}.
     ]]></code>
-    <p>
+    <p>Function
       <seealso marker="stdlib:gen_statem#start_link/3">
 	<c>gen_statem:start_link</c>
       </seealso>
-      is synchronous.  It does not return until the <c>gen_statem</c>
-      has been initialized and is ready to receive events.
+      is synchronous. It does not return until the <c>gen_statem</c>
+      is initialized and is ready to receive events.
     </p>
     <p>
+      Function
       <seealso marker="stdlib:gen_statem#start_link/3">
 	<c>gen_statem:start_link</c>
       </seealso>
       must be used if the <c>gen_statem</c>
-      is part of a supervision tree, that is; started by a supervisor.
-      There is another function;
+      is part of a supervision tree, that is, started by a supervisor.
+      Another function,
       <seealso marker="stdlib:gen_statem#start/3">
 	<c>gen_statem:start</c>
       </seealso>
-      to start a standalone <c>gen_statem</c>, that is;
+      can be used to start a standalone <c>gen_statem</c>, that is,
       a <c>gen_statem</c> that is not part of a supervision tree.
     </p>
   </section>
@@ -402,7 +400,7 @@ init(Code) ->
 <!-- =================================================================== -->
 
   <section>
-    <title>Events and Handling them</title>
+    <title>Handling Events</title>
     <p>The function notifying the code lock about a button event is
       implemented using
       <seealso marker="stdlib:gen_statem#cast/2">
@@ -415,9 +413,9 @@ button(Digit) ->
     ]]></code>
     <p>
       The first argument is the name of the <c>gen_statem</c> and must
-      agree with the name used to start it so therefore we use the
+      agree with the name used to start it. So, we use the
       same macro <c>?NAME</c> as when starting.
-      <c>{button,Digit}</c> is the actual event content.
+      <c>{button,Digit}</c> is the event content.
     </p>
     <p>
       The event is made into a message and sent to the <c>gen_statem</c>.
@@ -452,19 +450,19 @@ open(cast, {button,_}, Data) ->
     ]]></code>
     <p>
       If the door is locked and a button is pressed, the pressed
-      button is compared with the next correct button and,
-      depending on the result, the door is either unlocked
+      button is compared with the next correct button.
+      Depending on the result, the door is either unlocked
       and the <c>gen_statem</c> goes to state <c>open</c>,
       or the door remains in state <c>locked</c>.
     </p>
     <p>
-      If the pressed button is incorrect the server data
+      If the pressed button is incorrect, the server data
       restarts from the start of the code sequence.
     </p>
     <p>
-      In state <c>open</c> any button locks the door since
-      any event cancels the event timer so we will not get
-      a timeout event after a button event.
+      In state <c>open</c>, any button locks the door, as
+      any event cancels the event timer, so no
+      time-out event occurs after a button event.
     </p>
   </section>
 
@@ -478,11 +476,11 @@ open(cast, {button,_}, Data) ->
 {next_state,open,Data#{remaining := Code},10000};
     ]]></code>
     <p>
-      10000 is a time-out value in milliseconds.
-      After this time, that is; 10 seconds, a time-out occurs.
+      10,000 is a time-out value in milliseconds.
+      After this time (10 seconds), a time-out occurs.
       Then, <c>StateName(timeout, 10000, Data)</c> is called.
       The time-out occurs when the door has been in state <c>open</c>
-      for 10 seconds.  After that the door is locked again:
+      for 10 seconds. After that the door is locked again:
     </p>
     <code type="erl"><![CDATA[
 open(timeout, _,  Data) ->
@@ -496,16 +494,16 @@ open(timeout, _,  Data) ->
   <section>
     <title>All State Events</title>
     <p>
-      Sometimes an event can arrive in any state of the <c>gen_statem</c>.
+      Sometimes events can arrive in any state of the <c>gen_statem</c>.
       It is convenient to handle these in a common state handler function
       that all state functions call for events not specific to the state.
     </p>
     <p>
-      Let's introduce a <c>code_length/0</c> function that returns
+      Consider a <c>code_length/0</c> function that returns
       the length of the correct code
-      (that should not be sensitive to reveal...).
-      We'll dispatch all events that are not state specific
-      to the common function <c>handle_event/3</c>.
+      (that should not be sensitive to reveal).
+      We dispatch all events that are not state-specific
+      to the common function <c>handle_event/3</c>:
     </p>
     <code type="erl"><![CDATA[
 ...
@@ -532,7 +530,7 @@ handle_event({call,From}, code_length, #{code := Code} = Data) ->
       This example uses
       <seealso marker="stdlib:gen_statem#call/2">
 	<c>gen_statem:call/2</c>
-      </seealso>
+      </seealso>,
       which waits for a reply from the server.
       The reply is sent with a <c>{reply,From,Reply}</c> tuple
       in an action list in the <c>{keep_state,...}</c> tuple
@@ -545,9 +543,12 @@ handle_event({call,From}, code_length, #{code := Code} = Data) ->
   <section>
     <title>One Event Handler</title>
     <p>
-      If you use the mode <c>handle_event_function</c>
-      all events are handled in <c>handle_event/4</c> and we
-      may (but do not have to) use an event-centered approach
+      If mode <c>handle_event_function</c> is used,
+      all events are handled in
+      <seealso marker="stdlib:gen_statem#Module:handle_event/4">
+        <c>Module:handle_event/4</c>
+      </seealso>
+      and we can (but do not have to) use an event-centered approach
       where we dispatch on event first and then state:
     </p>
     <code type="erl"><![CDATA[
@@ -596,19 +597,18 @@ handle_event(timeout, _, open, Data) ->
 	The <c>gen_statem</c> is automatically terminated by its supervisor.
 	Exactly how this is done is defined by a
 	<seealso marker="sup_princ#shutdown">shutdown strategy</seealso>
-        set in the supervisor.
+	set in the supervisor.
       </p>
       <p>
 	If it is necessary to clean up before termination, the shutdown
-        strategy must be a time-out value and the <c>gen_statem</c> must
-	in the <c>init/1</c> function set itself to trap exit signals
+	strategy must be a time-out value and the <c>gen_statem</c> must
+	in function <c>init/1</c> set itself to trap exit signals
 	by calling
 	<seealso marker="erts:erlang#process_flag/2">
-	  <c>process_flag(trap_exit, true)</c>.
-	</seealso>
-	When ordered to shutdown, the <c>gen_statem</c> then calls
-	the callback function
-	<c>terminate(shutdown, State, Data)</c>:
+	  <c>process_flag(trap_exit, true)</c>
+	</seealso>.
+	When ordered to shut down, the <c>gen_statem</c> then calls
+	callback function <c>terminate(shutdown, State, Data)</c>:
       </p>
       <code type="erl"><![CDATA[
 init(Args) ->
@@ -617,9 +617,9 @@ init(Args) ->
     ...
       ]]></code>
       <p>
-	In this example we let the <c>terminate/3</c> function
-	lock the door if it is open so we do not accidentally leave the door
-	open when the supervision tree terminates.
+	In the following example, function <c>terminate/3</c>
+	locks the door if it is open, so we do not accidentally leave the door
+	open when the supervision tree terminates:
       </p>
       <code type="erl"><![CDATA[
 terminate(_Reason, State, _Data) ->
@@ -634,8 +634,8 @@ terminate(_Reason, State, _Data) ->
 	If the <c>gen_statem</c> is not part of a supervision tree,
 	it can be stopped using
 	<seealso marker="stdlib:gen_statem#stop/1">
-	  <c>gen_statem:stop</c>,
-	</seealso>
+	  <c>gen_statem:stop</c>
+	</seealso>,
 	preferably through an API function:
       </p>
       <code type="erl"><![CDATA[
@@ -647,8 +647,8 @@ stop() ->
     gen_statem:stop(?NAME).
       ]]></code>
       <p>
-	This makes the <c>gen_statem</c> call the <c>terminate/3</c>
-	callback function just like for a supervised server
+	This makes the <c>gen_statem</c> call callback function
+	<c>terminate/3</c> just like for a supervised server
 	and waits for the process to terminate.
       </p>
     </section>
@@ -659,15 +659,15 @@ stop() ->
   <section>
     <title>Actions</title>
     <p>
-      In the first chapters we mentioned actions as a part of
-      the general state machine model, and these actions
-      are implemented with the code the <c>gen_statem</c>
-      callback module executes in an event handling
+      In the first sections actions were mentioned as a part of
+      the general state machine model. These actions
+      are implemented with the code that callback module
+      <c>gen_statem</c> executes in an event-handling
       callback function before returning
       to the <c>gen_statem</c> engine.
     </p>
     <p>
-      There are more specific state transition actions
+      There are more specific state-transition actions
       that a callback function can order the <c>gen_statem</c>
       engine to do after the callback function return.
       These are ordered by returning a list of
@@ -680,27 +680,28 @@ stop() ->
       </seealso>
       from the
       <seealso marker="stdlib:gen_statem#Module:StateName/3">
-	callback function.
-      </seealso>
+	callback function
+      </seealso>.
       These state transition actions affect the <c>gen_statem</c>
-      engine itself.  They can:
+      engine itself and can do the following:
     </p>
     <list type="bulleted">
-      <item>Postpone the current event.</item>
-      <item>Hibernate the <c>gen_statem</c>.</item>
-      <item>Start an event timeout.</item>
-      <item>Reply to a caller.</item>
-      <item>Generate the next event to handle.</item>
+      <item>Postpone the current event</item>
+      <item>Hibernate the <c>gen_statem</c></item>
+      <item>Start an event time-out</item>
+      <item>Reply to a caller</item>
+      <item>Generate the next event to handle</item>
     </list>
     <p>
-      We have mentioned the event timeout
-      and replying to a caller in the example above.
-      An example of event postponing comes in later in this chapter.
-      See the
+      In the example earlier was mentioned the event time-out
+      and replying to a caller.
+      An example of event postponing is included later in this chapter.
+      For details, see the
       <seealso marker="stdlib:gen_statem#type-action">
-	reference manual
+	<c>gen_statem(3)</c>
       </seealso>
-      for details.  You can for example actually reply to several callers
+      manual page.
+      You can, for example, reply to many callers
       and generate multiple next events to handle.
     </p>
   </section>
@@ -710,16 +711,16 @@ stop() ->
   <section>
     <title>Event Types</title>
     <p>
-      So far we have mentioned a few
+      The previous sections mentioned a few
       <seealso marker="stdlib:gen_statem#type-event_type">
-	event types.
-      </seealso>
+	event types
+      </seealso>.
       Events of all types are handled in the same callback function,
       for a given state, and the function gets
       <c>EventType</c> and <c>EventContent</c> as arguments.
     </p>
     <p>
-      Here is the complete list of event types and where
+      The following is a complete list of event types and where
       they come from:
     </p>
     <taglist>
@@ -735,13 +736,13 @@ stop() ->
 	Generated by
 	<seealso marker="stdlib:gen_statem#call/2">
 	  <c>gen_statem:call</c>
-	</seealso>
+	</seealso>,
 	where <c>From</c> is the reply address to use
 	when replying either through the state transition action
 	<c>{reply,From,Msg}</c> or by calling
 	<seealso marker="stdlib:gen_statem#reply/1">
-	  <c>gen_statem:reply</c>.
-	</seealso>
+	  <c>gen_statem:reply</c>
+	</seealso>.
       </item>
       <tag><c>info</c></tag>
       <item>
@@ -750,15 +751,15 @@ stop() ->
       </item>
       <tag><c>timeout</c></tag>
       <item>
-	Generated by the state transition action
+	Generated by state transition action
 	<c>{timeout,Time,EventContent}</c> (or its short form <c>Time</c>)
 	timer timing out.
       </item>
       <tag><c>internal</c></tag>
       <item>
-	Generated by the state transition action
+	Generated by state transition action
 	<c>{next_event,internal,EventContent}</c>.
-	In fact all event types above can be generated using
+	All event types above can be generated using
 	<c>{next_event,EventType,EventContent}</c>.
       </item>
     </taglist>
@@ -767,34 +768,34 @@ stop() ->
 <!-- =================================================================== -->
 
   <section>
-    <title>State Timeouts</title>
+    <title>State Time-Outs</title>
     <p>
-      The timeout event generated by the state transition action
-      <c>{timeout,Time,EventContent}</c> is an event timeout,
-      that is; if an event arrives the timer is cancelled.
-      You get either an event or a timeout but not both.
+      The time-out event generated by state transition action
+      <c>{timeout,Time,EventContent}</c> is an event time-out,
+      that is, if an event arrives the timer is cancelled.
+      You get either an event or a time-out, but not both.
     </p>
     <p>
-      Often you want a timer to not be cancelled by any event
+      Often you want a timer not to be cancelled by any event
       or you want to start a timer in one state and respond
-      to the timeout in another.  This can be accomplished
-      with a regular erlang timer:
+      to the time-out in another. This can be accomplished
+      with a regular Erlang timer:
       <seealso marker="erts:erlang#start_timer/4">
 	<c>erlang:start_timer</c>.
       </seealso>
     </p>
     <p>
-      Looking at the example in this chapter so far; using the
+      For the example so far in this chapter: using the
       <c>gen_statem</c> event timer has the consequence that
       if a button event is generated while in the <c>open</c> state,
-      the timeout is cancelled and the button event is delivered.
-      Therefore we chose to lock the door if this happended.
+      the time-out is cancelled and the button event is delivered.
+      So, we choose to lock the door if this occurred.
     </p>
     <p>
-      Suppose we do not want a button to lock the door,
+      Suppose that we do not want a button to lock the door,
       instead we want to ignore button events in the <c>open</c> state.
       Then we start a timer when entering the <c>open</c> state
-      and wait for it to expire while ignoring button events:
+      and waits for it to expire while ignoring button events:
     </p>
     <code type="erl"><![CDATA[
 ...
@@ -816,17 +817,17 @@ open(cast, {button,_}, Data) ->
 ...
     ]]></code>
     <p>
-      If you need to cancel a timer due to some other event you can use
+      If you need to cancel a timer because of some other event, you can use
       <seealso marker="erts:erlang#cancel_timer/2">
-	<c>erlang:cancel_timer(Tref)</c>.
-      </seealso>
-      Note that a timeout message can not arrive after this,
+	<c>erlang:cancel_timer(Tref)</c>
+      </seealso>.
+      Notice that a time-out message cannot arrive after this,
       unless you have postponed it (see the next section) before,
-      so make sure you do not accidentally postpone such messages.
+      so ensure that you do not accidentally postpone such messages.
     </p>
     <p>
-      Another way to cancel a timer is to not cancel it,
-      but instead to ignore it if it arrives in a state
+      Another way to cancel a timer is not to cancel it,
+      but to ignore it if it arrives in a state
       where it is known to be late.
     </p>
   </section>
@@ -839,7 +840,7 @@ open(cast, {button,_}, Data) ->
       If you want to ignore a particular event in the current state
       and handle it in a future state, you can postpone the event.
       A postponed event is retried after the state has
-      changed i.e <c>OldState =/= NewState</c>.
+      changed, that is, <c>OldState =/= NewState</c>.
     </p>
     <p>
       Postponing is ordered by the state transition
@@ -850,8 +851,8 @@ open(cast, {button,_}, Data) ->
     </p>
     <p>
       In this example, instead of ignoring button events
-      while in the <c>open</c> state we can postpone them
-      and they will be queued and later handled in the <c>locked</c> state:
+      while in the <c>open</c> state, we can postpone them
+      and they are queued and later handled in the <c>locked</c> state:
     </p>
     <code type="erl"><![CDATA[
 ...
@@ -860,17 +861,17 @@ open(cast, {button,_}, Data) ->
 ...
     ]]></code>
     <p>
-      The fact that a postponed event is only retried after a state change
-      translates into a requirement on the event and state space:
-      if you have a choice between storing a state data item
-      in the <c>State</c> or in the <c>Data</c>;
-      should a change in the item value affect which events that
-      are handled, then this item ought to be part of the state.
+      A postponed event is only retried after a state change
+      translates into a requirement on the event and state space.
+      If you have a choice between storing a state data item
+      in the <c>State</c> or in the <c>Data</c>:
+      if a change in the item value affects which events that
+      are handled, then this item is to be part of the state.
     </p>
     <p>
-      What you want to avoid is that you maybe much later decide
-      to postpone an event in one state and by misfortune it is never retried
-      because the code only changes the <c>Data</c> but not the <c>State</c>.
+      You want to avoid that you maybe much later decide
+      to postpone an event in one state and by misfortune it is never retried,
+      as the code only changes the <c>Data</c> but not the <c>State</c>.
     </p>
 
     <section>
@@ -883,7 +884,7 @@ open(cast, {button,_}, Data) ->
 	or from the context.
       </p>
       <p>
-	Possible actions may be; ignore as in drop the event
+	Possible actions: ignore as in drop the event
 	(maybe log it) or deal with the event in some other state
 	as in postpone it.
       </p>
@@ -892,10 +893,10 @@ open(cast, {button,_}, Data) ->
     <section>
       <title>Selective Receive</title>
       <p>
-	Erlang's selective receive statement is often used to
-	describe simple state machine examples in straightforward
-	Erlang code.  Here is a possible implementation of
-	the first example:
+        Erlang's selective receive statement is often used to
+        describe simple state machine examples in straightforward
+        Erlang code. The following is a possible implementation of
+        the first example:
       </p>
     <code type="erl"><![CDATA[
 -module(code_lock).
@@ -937,16 +938,16 @@ do_unlock() ->
     io:format("Open~n", []).
     ]]></code>
     <p>
-      The selective receive in this case causes <c>open</c>
-      to implicitly postpone any events to the <c>locked</c> state.
+      The selective receive in this case causes implicitly <c>open</c>
+      to postpone any events to the <c>locked</c> state.
     </p>
     <p>
-      A selective receive can not be used from a <c>gen_statem</c>
-      behaviour just as for any <c>gen_*</c> behavior
-      since the receive statement is within the <c>gen_*</c> engine itself.
-      It has to be there because all
+      A selective receive cannot be used from a <c>gen_statem</c>
+      behavior as for any <c>gen_*</c> behavior,
+      as the receive statement is within the <c>gen_*</c> engine itself.
+      It must be there because all
       <seealso marker="stdlib:sys"><c>sys</c></seealso>
-      compatible behaviours must respond to system messages and therefore
+      compatible behaviors must respond to system messages and therefore
       do that in their engine receive loop,
       passing non-system messages to the callback module.
     </p>
@@ -955,15 +956,15 @@ do_unlock() ->
       <seealso marker="stdlib:gen_statem#type-action">
 	action
       </seealso>
-      <c>postpone</c> is designed to be able to model
-      selective receives.  A selective receive implicitly postpones
+      <c>postpone</c> is designed to model
+      selective receives. A selective receive implicitly postpones
       any not received events, but the <c>postpone</c>
       state transition action explicitly postpones one received event.
     </p>
     <p>
-      Other than that both mechanisms have got the same theoretical
+      Both mechanisms have the same theoretical
       time and memory complexity, while the selective receive
-      language construct has got smaller constant factors.
+      language construct has smaller constant factors.
     </p>
     </section>
   </section>
@@ -971,9 +972,9 @@ do_unlock() ->
 <!-- =================================================================== -->
 
   <section>
-    <title>Self Generated Events</title>
+    <title>Self-Generated Events</title>
     <p>
-      It may be beneficial in some cases to be able to generate events
+      It can sometimes be beneficial to be able to generate events
       to your own state machine.
       This can be done with the state transition
       <seealso marker="stdlib:gen_statem#type-action">
@@ -984,30 +985,30 @@ do_unlock() ->
     <p>
       You can generate events of any existing
       <seealso marker="stdlib:gen_statem#type-action">
-	type,
-      </seealso>
-      but the <c>internal</c> type can only be generated through the
-      <c>next_event</c> action and hence can not come from an external source,
+	type
+      </seealso>,
+      but the <c>internal</c> type can only be generated through action
+      <c>next_event</c>. Hence, it cannot come from an external source,
       so you can be certain that an <c>internal</c> event is an event
       from your state machine to itself.
     </p>
     <p>
-      One example of using self generated events may be when you have
+      One example of using self-generated events can be when you have
       a state machine specification that uses state entry actions.
-      That you could code using a dedicated function
-      to do the state transition.  But if you want that code to be
-      visible besides the other state logic you can insert
+      You can code that using a dedicated function
+      to do the state transition. But if you want that code to be
+      visible besides the other state logic, you can insert
       an <c>internal</c> event that does the entry actions.
       This has the same unfortunate consequence as using
-      state transition functions that everywhere you go to
-      the state in question you will have to explicitly
+      state transition functions: everywhere you go to
+      the state, you must explicitly
       insert the <c>internal</c> event
-      or use state transition function.
+      or use a state transition function.
     </p>
     <p>
-      Here is an implementation of entry actions
+      The following is an implementation of entry actions
       using <c>internal</c> events with content <c>enter</c>
-      utilizing a helper function <c>enter/3</c> for state entry:
+      using a helper function <c>enter/3</c> for state entry:
     </p>
     <code type="erl"><![CDATA[
 ...
@@ -1051,20 +1052,20 @@ enter(Tag, State, Data) ->
   <section>
     <title>Example Revisited</title>
     <p>
-      Here is the example after all mentioned modifications
-      and some more utilizing the entry actions,
+      This section includes the example after all mentioned modifications
+      and some more using the entry actions,
       which deserves a new state diagram:
     </p>
     <image file="../design_principles/code_lock_2.png">
-      <icaption>Code lock state diagram revisited</icaption>
+      <icaption>Code Lock State Diagram Revisited</icaption>
     </image>
     <p>
-      Note that this state diagram does not specify how to handle
-      a button event in the state <c>open</c>, so you will have to
-      read some other place that is here that unspecified events
-      shall be ignored as in not consumed but handled in some other state.
-      Nor does it show that the <c>code_length/0</c> call shall be
-      handled in every state.
+      Notice that this state diagram does not specify how to handle
+      a button event in the state <c>open</c>. So, you need to
+      read somewhere else that unspecified events
+      must be ignored as in not consumed but handled in some other state.
+      Also, the state diagram does not show that the <c>code_length/0</c>
+      call must be handled in every state.
     </p>
 
     <section>
@@ -1147,10 +1148,11 @@ code_change(_Vsn, State, Data, _Extra) ->
     <section>
       <title>Callback Mode: handle_event_function</title>
       <p>
-	What to change to use one <c>handle_event/4</c> function.
-	Here a clean first-dispatch-on-event approach
-	does not work that well due to the generated
-	entry actions:
+        This section describes what to change in the example
+        to use one <c>handle_event/4</c> function.
+        The following clean first-dispatch-on-event approach
+        does not work that well because of the generated
+        entry actions:
       </p>
       <code type="erl"><![CDATA[
 ...
@@ -1195,7 +1197,7 @@ handle_event({call,From}, code_length, _State, #{code := Code}) ->
       ]]></code>
     </section>
     <p>
-      Note that postponing buttons from the <c>locked</c> state
+      Notice that postponing buttons from the <c>locked</c> state
       to the <c>open</c> state feels like the wrong thing to do
       for a code lock, but it at least illustrates event postponing.
     </p>
@@ -1206,33 +1208,33 @@ handle_event({call,From}, code_length, _State, #{code := Code}) ->
   <section>
     <title>Filter the State</title>
     <p>
-      The example servers so far in this chapter will for example
-      when killed by an exit signal or due to an internal error
-      print out the full internal state in the error log.
+      The example servers so far in this chapter
+      print the full internal state in the error log, for example,
+      when killed by an exit signal or because of an internal error.
       This state contains both the code lock code
-      and which digits that remains to unlock.
+      and which digits that remain to unlock.
     </p>
     <p>
       This state data can be regarded as sensitive,
       and maybe not what you want in the error log
-      because of something unpredictable happening.
+      because of some unpredictable event.
     </p>
     <p>
       Another reason to filter the state can be
-      that the state is too big to print out since it fills
+      that the state is too large to print, as it fills
       the error log with uninteresting details.
     </p>
     <p>
-      To avoid this you can format the internal state
+      To avoid this, you can format the internal state
       that gets in the error log and gets returned from
       <seealso marker="stdlib:sys#get_status/1">
 	<c>sys:get_status/1,2</c>
       </seealso>
-      by implementing the
+      by implementing function
       <seealso marker="stdlib:gen_statem#Module:format_status/2">
 	<c>Module:format_status/2</c>
-      </seealso>
-      function, for example like this:
+      </seealso>,
+      for example like this:
     </p>
     <code type="erl"><![CDATA[
 ...
@@ -1260,9 +1262,9 @@ format_status(Opt, [_PDict,State,Data]) ->
       <seealso marker="stdlib:gen_statem#Module:format_status/2">
 	<c>Module:format_status/2</c>
       </seealso>
-      function.  If you do not a default implementation is used that
+      function. If you do not, a default implementation is used that
       does the same as this example function without filtering
-      the <c>Data</c> term that is: <c>StateData = {State,Data}</c>.
+      the <c>Data</c> term, that is, <c>StateData = {State,Data}</c>.
     </p>
   </section>
 
@@ -1275,54 +1277,56 @@ format_status(Opt, [_PDict,State,Data]) ->
       <seealso marker="stdlib:gen_statem#type-callback_mode">
 	<c>handle_event_function</c>
       </seealso>
-      enables using a non-atom state as described in
+      enables using a non-atom state as described in section
       <seealso marker="#callback_modes">
-	Callback Modes,
-      </seealso>
-      for example a complex state term like a tuple.
+	Callback Modes
+      </seealso>,
+      for example, a complex state term like a tuple.
     </p>
     <p>
       One reason to use this is when you have
-      a state item that affects the event handling
-      in particular when combining that with postponing events.
-      Let us complicate the previous example
+      a state item that affects the event handling,
+      in particular in combination with postponing events.
+      We complicate the previous example
       by introducing a configurable lock button
-      (this is the state item in question)
-      that in the <c>open</c> state immediately locks the door,
+      (this is the state item in question),
+      which in the <c>open</c> state immediately locks the door,
       and an API function <c>set_lock_button/1</c> to set the lock button.
     </p>
     <p>
       Suppose now that we call <c>set_lock_button</c>
       while the door is open,
       and have already postponed a button event
-      that up until now was not the lock button;
-      the sensible thing might be to say that
-      the button was pressed too early so it should
-      not be recognized as the lock button,
-      but then it might be surprising that a button event
+      that until now was not the lock button.
+      The sensible thing can be to say that
+      the button was pressed too early so it is
+      not to be recognized as the lock button.
+      However, then it can be surprising that a button event
       that now is the lock button event arrives (as retried postponed)
       immediately after the state transits to <c>locked</c>.
     </p>
     <p>
-      So let us make the <c>button/1</c> function synchronous
-      by using <c>gen_statem:call</c>,
+      So we make the <c>button/1</c> function synchronous
+      by using
+      <seealso marker="stdlib:gen_statem#call/2">
+        <c>gen_statem:call</c></seealso>
       and still postpone its events in the <c>open</c> state.
       Then a call to <c>button/1</c> during the <c>open</c>
-      state will not return until the state transits to <c>locked</c>
-      since it is there the event is handled and the reply is sent.
+      state does not return until the state transits to <c>locked</c>,
+      as it is there the event is handled and the reply is sent.
     </p>
     <p>
-      If now one process calls <c>set_lock_button/1</c>
-      to change the lock button while some other process
-      hangs in <c>button/1</c> with the new lock button
-      it could be expected that the hanging lock button call
+      If a process now calls <c>set_lock_button/1</c>
+      to change the lock button while another process
+      hangs in <c>button/1</c> with the new lock button,
+      it can be expected that the hanging lock button call
       immediately takes effect and locks the lock.
-      Therefore we make the current lock button a part of the state
-      so when we change the lock button the state will change
-      and all postponed events will be retried.
+      Therefore, we make the current lock button a part of the state,
+      so that when we change the lock button, the state changes
+      and all postponed events are retried.
     </p>
     <p>
-      We define the state as <c>{StateName,LockButton}</c>
+      We define the state as <c>{StateName,LockButton}</c>,
       where <c>StateName</c> is as before
       and <c>LockButton</c> is the current lock button:
     </p>
@@ -1441,10 +1445,9 @@ format_status(Opt, [_PDict,State,Data]) ->
     end.
     ]]></code>
     <p>
-      It may be an ill-fitting model for a physical code lock
-      that the <c>button/1</c> call might hang until the lock
-      is locked.  But for an API in general it is really not
-      that strange.
+      It can be an ill-fitting model for a physical code lock
+      that the <c>button/1</c> call can hang until the lock
+      is locked. But for an API in general it is not that strange.
     </p>
   </section>
 
@@ -1457,26 +1460,25 @@ format_status(Opt, [_PDict,State,Data]) ->
       and they have some state(s) in their lifetime in which
       the servers can be expected to idle for a while,
       and the amount of heap memory all these servers need
-      is a problem; then it is possible to minimize
-      the memory footprint of a server by hibernating it through
+      is a problem, then the memory footprint of a server
+      can be mimimized by hibernating it through
       <seealso marker="stdlib:proc_lib#hibernate/3">
 	<c>proc_lib:hibernate/3</c>.
       </seealso>
     </p>
     <note>
       <p>
-	To hibernate a process is rather costly. See
-	<seealso marker="erts:erlang#hibernate/3">
-	  <c>erlang:hibernate/3</c>.
-	</seealso>
-	It is in general not something you want to do
-	after every event.
+        It is rather costly to hibernate a process; see
+        <seealso marker="erts:erlang#hibernate/3">
+          <c>erlang:hibernate/3</c>
+        </seealso>.
+        It is not something you want to do after every event.
       </p>
     </note>
     <p>
-      We can in this example hibernate in the <c>{open,_}</c> state
-      since what normally happens in that state is that
-      the state timeout after a while
+      We can in this example hibernate in the <c>{open,_}</c> state,
+      because what normally occurs in that state is that
+      the state time-out after a while
       triggers a transition to <c>{locked,_}</c>:
     </p>
     <code type="erl"><![CDATA[
@@ -1498,24 +1500,24 @@ handle_event(
       </seealso>
       action list on the last line
       when entering the <c>{open,_}</c> state is the only change.
-      If any event arrives in the <c>{open,_},</c> state we
-      do not bother to re-hibernate, so the server stays
+      If any event arrives in the <c>{open,_},</c> state, we
+      do not bother to rehibernate, so the server stays
       awake after any event.
     </p>
     <p>
       To change that we would need to insert
-      the <c>hibernate</c> action in more places,
-      for example for the state independent <c>set_lock_button</c>
+      action <c>hibernate</c> in more places.
+      For example, for the state-independent <c>set_lock_button</c>
       and <c>code_length</c> operations that then would have to
       be aware of using <c>hibernate</c> while in the
-      <c>{open,_}</c> state which would clutter the code.
+      <c>{open,_}</c> state, which would clutter the code.
     </p>
     <p>
-      This server probably does not use an amount of
+      This server probably does not use
       heap memory worth hibernating for.
-      To gain anything from hibernation your server would
-      have to actually produce some garbage during callback execution,
-      for which this example server may serve as a bad example.
+      To gain anything from hibernation, your server would
+      have to produce some garbage during callback execution,
+      for which this example server can serve as a bad example.
     </p>
   </section>