Update the documentation with information on the callback attribute

1 files changed, 33 insertions, 15 deletions

diff --git a/system/doc/design_principles/spec_proc.xml b/system/doc/design_principles/spec_proc.xml
index f0f62891b6..a1c862e004 100644
--- a/system/doc/design_principles/spec_proc.xml
+++ b/system/doc/design_principles/spec_proc.xml
@@ -411,30 +411,48 @@ loop(...) ->
     <p>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, implement
-      and export the function:</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
+    behaviour module to describe the expected callbacks:</p>
+    <code type="none">
+-callback Name1(Arg1_1, Arg1_2, ..., Arg1_N1) -> Res1.
+-callback Name2(Arg2_1, Arg2_2, ..., Arg2_N2) -> Res2.
+...
+-callback NameM(ArgM_1, ArgM_2, ..., ArgM_NM) -> ResM.</code>
+    <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>
+    <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 function.</p>
+    <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>
     <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 set of functions actually exported from <c>Mod</c>, and
-      issue a warning if any callback function is missing.</p>
+    <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
+    set of functions actually exported from <c>Mod</c>, and issue a warning if
+    any callback function is missing.</p>
     <p>Example:</p>
     <code type="none">
 %% User-defined behaviour module
 -module(simple_server).
 -export([start_link/2,...]).
--export([behaviour_info/1]).
 
-behaviour_info(callbacks) ->
-    [{init,1},
-     {handle_req,1},
-     {terminate,0}].
+-callback init(State :: term()) -> 'ok'.
+-callback handle_req(Req :: term(), State :: term()) -> {'ok', Reply :: term()}.
+-callback terminate() -> 'ok'.
+
+%% Alternatively you may define:
+%%
+%% -export([behaviour_info/1]).
+%% behaviour_info(callbacks) ->
+%%     [{init,1},
+%%      {handle_req,2},
+%%      {terminate,0}].
 
 start_link(Name, Module) ->
     proc_lib:start_link(?MODULE, init, [self(), Name, Module]).
@@ -452,7 +470,7 @@ init(Parent, Name, Module) ->
 -module(db).
 -behaviour(simple_server).
 
--export([init/0, handle_req/1, terminate/0]).
+-export([init/0, handle_req/2, terminate/0]).
 
 ...</code>
   </section>