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Update the documentation with information on the callback attribute

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commit 8ea0c7c566d2c356f273806c42615f31b9e946d3 1 parent 493262e
@aronisstav aronisstav authored proxyles committed
Showing with 33 additions and 15 deletions.
  1. +33 −15 system/doc/design_principles/spec_proc.xml
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48 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>
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