From 9dfc9857ff5089282f97656fe59012c26ccae52a Mon Sep 17 00:00:00 2001
From: Hans Bolinder <hasse@erlang.org>
Date: Tue, 25 Mar 2014 14:08:30 +0100
Subject: refman: Clarify the '-callback' attribute

---
 system/doc/design_principles/spec_proc.xml | 16 ++++++++--------
 system/doc/reference_manual/modules.xml    | 19 ++++++++++++++++---
 2 files changed, 24 insertions(+), 11 deletions(-)

diff --git a/system/doc/design_principles/spec_proc.xml b/system/doc/design_principles/spec_proc.xml
index 69bf2e0448..e4fb5fdca7 100644
--- a/system/doc/design_principles/spec_proc.xml
+++ b/system/doc/design_principles/spec_proc.xml
@@ -4,7 +4,7 @@
 <chapter>
   <header>
     <copyright>
-      <year>1997</year><year>2013</year>
+      <year>1997</year><year>2014</year>
       <holder>Ericsson AB. All Rights Reserved.</holder>
     </copyright>
     <legalnotice>
@@ -431,11 +431,11 @@ loop(...) ->
 
   <section>
     <title>User-Defined Behaviours</title>
-    <p>To implement a user-defined behaviour, write code similar to
+    <p><marker id="behaviours"/>To implement a user-defined behaviour, write code similar to
       code for a special process but calling functions in a callback
       module for handling specific tasks.</p>
     <p>If it is desired that the compiler should warn for missing callback
-    functions, as it does for the OTP behaviours, add callback attributes in the
+    functions, as it does for the OTP behaviours, add <c>-callback</c> attributes in the
     behaviour module to describe the expected callbacks:</p>
     <code type="none">
 -callback Name1(Arg1_1, Arg1_2, ..., Arg1_N1) -> Res1.
@@ -445,15 +445,15 @@ loop(...) ->
     <p>where <c>NameX</c> are the names of the expected callbacks and
     <c>ArgX_Y</c>, <c>ResX</c> are types as they are described in Specifications
     for functions in <seealso marker="../reference_manual/typespec">Types and
-    Function Specifications</seealso>. The whole syntax of spec attributes is
-    supported by callback attributes.</p>
+    Function Specifications</seealso>. The whole syntax of <c>-spec</c> attribute is
+    supported by <c>-callback</c> attribute.</p>
     <p>Alternatively you may directly implement and export the function:</p>
     <code type="none">
 behaviour_info(callbacks) ->
-    [{Name1,Arity1},...,{NameN,ArityN}].</code>
-    <p>where each <c>{Name,Arity}</c> specifies the name and arity of a callback
+    [{Name1, Arity1},...,{NameN, ArityN}].</code>
+    <p>where each <c>{Name, Arity}</c> specifies the name and arity of a callback
     function. This function is otherwise automatically generated by the compiler
-    using the callback attributes.</p>
+    using the <c>-callback</c> attributes.</p>
     <p>When the compiler encounters the module attribute
     <c>-behaviour(Behaviour).</c> in a module <c>Mod</c>, it will call
     <c>Behaviour:behaviour_info(callbacks)</c> and compare the result with the
diff --git a/system/doc/reference_manual/modules.xml b/system/doc/reference_manual/modules.xml
index 9e5f4de385..cd4c3a1b1b 100644
--- a/system/doc/reference_manual/modules.xml
+++ b/system/doc/reference_manual/modules.xml
@@ -4,7 +4,7 @@
 <chapter>
   <header>
     <copyright>
-      <year>2003</year><year>2013</year>
+      <year>2003</year><year>2014</year>
       <holder>Ericsson AB. All Rights Reserved.</holder>
     </copyright>
     <legalnotice>
@@ -134,8 +134,21 @@ fact(0) ->           %  |
         standard behaviours <c>gen_server</c>, <c>gen_fsm</c>,
         <c>gen_event</c> or <c>supervisor</c>.</p>
       <p>The spelling <c>behavior</c> is also accepted.</p>
-      <p>Read more about behaviours and callback modules in OTP Design
-        Principles.</p>
+      <p>The callback functions of the module can be specified either
+        directly by the exported function <c>behaviour_info/1</c>:</p>
+        <pre>
+behaviour_info(callbacks) -> Callbacks.</pre>
+      <p>or by a <c>-callback</c> attribute for each callback
+        function:</p>
+      <pre>
+-callback Name(Arguments) -> Result.</pre>
+      <p>where <c>Arguments</c> is a list of zero or more arguments.
+       The <c>-callback</c> attribute is to be preferred since the
+       extra type information can be used by tools to produce
+       documentation or find discrepancies.</p>
+      <p>Read more about behaviours and callback modules in
+        <seealso marker="doc/design_principles:spec_proc#behaviours">
+        OTP Design Principles</seealso>.</p>
     </section>
 
     <section>
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