From eff1ee5ebf1d767d610cd6bc059e5f4dea57d2af Mon Sep 17 00:00:00 2001
From: Zandra Norman <zandra@erlang.org>
Date: Mon, 23 Jan 2017 17:06:48 +0100
Subject: stdlib: Make gen_statem callbacks optional

---
 lib/stdlib/doc/src/gen_statem.xml | 10 ++++++++++
 1 file changed, 10 insertions(+)

(limited to 'lib/stdlib/doc/src')

diff --git a/lib/stdlib/doc/src/gen_statem.xml b/lib/stdlib/doc/src/gen_statem.xml
index 1b99b65e09..bc86415d28 100644
--- a/lib/stdlib/doc/src/gen_statem.xml
+++ b/lib/stdlib/doc/src/gen_statem.xml
@@ -1733,6 +1733,16 @@ handle_event(_, _, State, Data) ->
 	<v>Reason = term()</v>
       </type>
       <desc>
+        <note>
+          <p>
+	    This callback is optional, so callback modules need not export it.
+            If a release upgrade/downgrade with
+	    <c>Change={advanced,Extra}</c>
+	    specified in the <c>.appup</c> file is made
+	    when <c>code_change/4</c> is not implemented
+	    the process will crash with exit reason <c>undef</c>.
+	  </p>
+        </note>
         <p>
 	  This function is called by a <c>gen_statem</c> when it is to
 	  update its internal state during a release upgrade/downgrade,
-- 
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