File 2186-Add-a-When-to-use-section.patch of Package erlang

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File 2186-Add-a-When-to-use-section.patch of Package erlang

From b2a68e1e20b9ae41490e5d2777ed5c4f1147b26b Mon Sep 17 00:00:00 2001
From: Raimo Niskanen <raimo@erlang.org>
Date: Thu, 12 Apr 2018 11:01:55 +0200
Subject: [PATCH 6/9] Add a 'When to use' section

---
 system/doc/design_principles/statem.xml | 107 ++++++++++++++++++++++++++------
 1 file changed, 89 insertions(+), 18 deletions(-)

diff --git a/system/doc/design_principles/statem.xml b/system/doc/design_principles/statem.xml
index b44e169a9a..c44f240098 100644
--- a/system/doc/design_principles/statem.xml
+++ b/system/doc/design_principles/statem.xml
@@ -82,6 +82,69 @@ State(S) x Event(E) -> Actions(A), State(S')</pre>
 
 </section>
 
+
+
+ <section>
+ <marker id="When to use gen_statem" />
+ <title>When to use gen_statem</title>
+ 
+ If your process logic is convenient to describe as a state machine,
+ and you want any of these <c>gen_statem</c> key features:
+ 
+ <list type="bulleted">
+ <item>
+	Gathered callback code for each state,
+	regardless of
+	<seealso marker="#Event Types">Event Types</seealso>.
+	(such as calls, casts and infos)
+ </item>
+ <item>
+	<seealso marker="#Postponing Events">
+	 Postponing Events
+	</seealso>
+	(a substitute for selective receive)
+ </item>
+ <item>
+	<seealso marker="#Inserted Events">
+	 Inserted Events
+	</seealso>
+	(in particular purely internal events)
+ </item>
+ <item>
+	<seealso marker="#State Enter Calls">
+	 State Enter Calls
+	</seealso>
+	(callback on state entry gathered with the rest
+	of the state callback code)
+ </item>
+ <item>
+	Higher level timeouts
+	(
+	<seealso marker="#State Time-Outs">State Time-Outs</seealso>,
+	<seealso marker="#Event Time-Outs">Event Time-Outs</seealso>
+	or
+	<seealso marker="#Generic Time-Outs">Generic Time-outs</seealso>
+	(named time-outs))
+ </item>
+ </list>
+ 
+ If so, or if possibly needed in future versions,
+ then you should consider using <c>gen_statem</c> over
+ <seealso marker="stdlib:gen_server"><c>gen_server</c></seealso>.
+ 
+ 
+ For simple state machines not needing these fetures
+ <seealso marker="stdlib:gen_server"><c>gen_server</c></seealso>
+ works just fine.
+ It also has got smaller call overhead,
+ but we are talking about something like 2 vs 3.3 microseconds
+ call roundtrip time here, so if the server callback
+ does just a little bit more than just replying,
+ or if the callback is not extremely frequent,
+ that difference will be hard to notice.
+ 
+ </section>
+
 
 
 <section>
@@ -160,7 +223,7 @@ State(S) x Event(E) -> Actions(A), State(S')</pre>
 </seealso>
 function may also return a list containing the callback mode
 and the atom <c>state_enter</c> in which case
- <seealso marker="#State Enter Calls">state enter calls</seealso>
+ <seealso marker="#State Enter Calls">State Enter Calls</seealso>
 are activated for the callback mode.
 
 
@@ -176,7 +239,7 @@ State(S) x Event(E) -> Actions(A), State(S')</pre>
 
 
 	The two
-	<seealso marker="#Callback Modes">callback modes</seealso>
+	<seealso marker="#Callback Modes">Callback Modes</seealso>
 	give different possibilities
 	and restrictions, but one goal remains:
 	you want to handle all possible combinations of
@@ -221,6 +284,8 @@ State(S) x Event(E) -> Actions(A), State(S')</pre>
 
 	The mode enables the use of non-atom states, for example,
 	complex states or even hierarchical states.
+	See section
+	<seealso marker="#Complex State">Complex State</seealso>.
 	If, for example, a state diagram is largely alike
 	for the client side and the server side of a protocol,
 	you can have a state <c>{StateName,server}</c> or
@@ -344,7 +409,10 @@ State(S) x Event(E) -> Actions(A), State(S')</pre>
 <item>
 	
 	 Same as the <c>keep_state</c> or <c>keep_state_and_data</c> values,
-	 and if <seealso marker="#State Enter Calls">state enter calls</seealso>
+	 and if
+	 <seealso marker="#State Enter Calls">
+	 State Enter Calls
+	 </seealso>
 	 are enabled, repeat that call.
 	
 </item>
@@ -379,7 +447,7 @@ State(S) x Event(E) -> Actions(A), State(S')</pre>
 	 <c>Module:init(Args)</c>
 	</seealso>
 	callback function is called before any
-	<seealso marker="#Event Handler">event handler</seealso>
+	<seealso marker="#Event Handler">Event Handler</seealso>
 	is called. This function behaves like an event handler
 	function, but gets its only argument <c>Args</c> from 
 	the <c>gen_statem</c>
@@ -407,8 +475,10 @@ State(S) x Event(E) -> Actions(A), State(S')</pre>
 <marker id="State Transition Actions" />
 <title>State Transition Actions</title>
 
- In the first
- <seealso marker="#Event-Driven State Machines">section</seealso>
+ In the first section
+ <seealso marker="#Event-Driven State Machines">
+	Event-Driven State Machines
+ </seealso>
 actions were mentioned as a part of
 the general state machine model. These general actions
 are implemented with the code that callback module
@@ -644,7 +714,7 @@ StateName(EventType, EventContent, Data) ->
 
 Since the state enter call is not an event there are restrictions
 on the allowed return value and
- <seealso marker="#State Transition Actions">state transition actions</seealso>.
+ <seealso marker="#State Transition Actions">State Transition Actions</seealso>.
 You may not change the state,
 <seealso marker="#Postponing Events">postpone</seealso>
 this non-event, or
@@ -657,7 +727,7 @@ StateName(EventType, EventContent, Data) ->
 
 You may repeat the state enter call using the <c>{repeat_state,...}</c>
 return value from the 
- <seealso marker="#Event Handler">event handler</seealso>.
+ <seealso marker="#Event Handler">Event Handler</seealso>.
 In this case <c>OldState</c> will also be equal to the current state.
 
 
@@ -1301,8 +1371,8 @@ open(cast, {button,_}, Data) ->
 ...
 ]]></code>
 
- An specific generic time-out can just as a
- <seealso marker="#State Time-Outs">state time-out</seealso>
+ Specific generic time-outs can just as
+ <seealso marker="#State Time-Outs">State Time-Outs</seealso>
 be restarted or cancelled
 by setting it to a new time or <c>infinity</c>.
 
@@ -1397,7 +1467,7 @@ open(cast, {button,_}, Data) ->
 
 Postponing is ordered by the state transition
 <seealso marker="#State Transition Actions">
-	state transition action
+	State Transition Action
 </seealso>
 <c>postpone</c>.
 
@@ -1422,7 +1492,7 @@ open(cast, {button,_}, Data) ->
 (see section
 <seealso marker="#Complex State">Complex State</seealso>)
 with
- <seealso marker="#Callback Modes">callback mode</seealso>
+ <seealso marker="#Callback Modes">Callback Mode</seealso>
 <seealso marker="stdlib:gen_statem#type-callback_mode"><c>handle_event_function</c></seealso>.
 If a change in the value changes the set of events that is handled,
 then the value should be kept in the State.
@@ -1527,8 +1597,8 @@ do_unlock() ->
 
 
 The
- <seealso marker="#State Transition Action">
-	state transition action
+ <seealso marker="#State Transition Actions">
+	State Transition Action
 </seealso>
 <c>postpone</c> is designed to model
 selective receives. A selective receive implicitly postpones
@@ -1555,7 +1625,7 @@ do_unlock() ->
 (described in the next section), especially if just
 one or a few states has got state enter actions,
 this is a perfect use case for the built in
- <seealso marker="#State Enter Calls">state enter calls</seealso>.
+ <seealso marker="#State Enter Calls">State Enter Calls</seealso>.
 
 
 You return a list containing <c>state_enter</c> from your
@@ -1617,7 +1687,7 @@ open(state_timeout, lock, Data) ->
 to your own state machine.
 This can be done with the
 <seealso marker="#State Transition Action">
-	state transition action
+	State Transition Action
 </seealso>
 <c>{next_event,EventType,EventContent}</c>.
 
@@ -1970,7 +2040,7 @@ format_status(Opt, [_PDict,State,Data]) ->
 
 One reason to use this is when you have a state item
 that when changed should cancel the
- <seealso marker="#State Time-Outs">state time-out</seealso>,
+ <seealso marker="#State Time-Outs">State Time-Out</seealso>,
 or one that affects the event handling
 in combination with postponing events.
 We will go for the latter and complicate the previous example
@@ -2160,7 +2230,8 @@ handle_event(enter, _OldState, {open,_}, _Data) ->
 <c>{open,_}</c> state, which would clutter the code.
 
 
- Another not uncommon scenario is to use the event time-out
+ Another not uncommon scenario is to use the
+ <seealso marker="#Event Time-Outs">Event Time-Out</seealso>
 to trigger hibernation after a certain time of inactivity.
 There is also a server start option
 <seealso marker="stdlib:gen_statem#type-hibernate_after_opt">
-- 
2.16.3

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