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Diffstat (limited to 'system/doc/design_principles/spec_proc.xml')
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1 files changed, 56 insertions, 20 deletions
diff --git a/system/doc/design_principles/spec_proc.xml b/system/doc/design_principles/spec_proc.xml index e4fb5fdca7..e849388a38 100644 --- a/system/doc/design_principles/spec_proc.xml +++ b/system/doc/design_principles/spec_proc.xml @@ -431,43 +431,79 @@ loop(...) -> <section> <title>User-Defined Behaviours</title> - <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 <c>-callback</c> attributes in the - behaviour module to describe the expected callbacks:</p> + + <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 + <c>-callback</c> attributes in the behaviour module to describe + the expected callback functions:</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 <c>-spec</c> attribute is - supported by <c>-callback</c> attribute.</p> - <p>Alternatively you may directly implement and export the function:</p> + + <p>where each <c>Name</c> is the name of a callback function and + <c>Arg</c> and <c>Res</c> are types as described in + Specifications for functions in <seealso + marker="../reference_manual/typespec">Types and Function + Specifications</seealso>. The whole syntax of the + <c>-spec</c> attribute is supported by <c>-callback</c> + attribute.</p> + <p>Callback functions that are optional for the user of the + behaviour to implement are specified by use of the + <c>-optional_callbacks</c> attribute:</p> + +<code type="none"> +-optional_callbacks([OptName1/OptArity1, ..., OptNameK/OptArityK]).</code> + + <p>where each <c>OptName/OptArity</c> specifies the name and arity + of a callback function. Note that the <c>-optional_callbacks</c> + attribute is to be used together with the <c>-callback</c> + attribute; it cannot be combined with the + <c>behaviour_info()</c> function described below.</p> + <p>Tools that need to know about optional callback functions can + call <c>Behaviour:behaviour_info(optional_callbacks)</c> to get + a list of all optional callback functions.</p> + + <note><p>We recommend using the <c>-callback</c> attribute rather + than the <c>behaviour_info()</c> function. The reason is that + the extra type information can be used by tools to produce + documentation or find discrepancies.</p></note> + + <p>As an alternative to the <c>-callback</c> and + <c>-optional_callbacks</c> attributes you may directly implement + and export <c>behaviour_info()</c>:</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. This function is otherwise automatically generated by the compiler - using the <c>-callback</c> attributes.</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 <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 - 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([start_link/2, init/3, ...]). -callback init(State :: term()) -> 'ok'. -callback handle_req(Req :: term(), State :: term()) -> {'ok', Reply :: term()}. -callback terminate() -> 'ok'. +-callback format_state(State :: term()) -> term(). + +-optional_callbacks([format_state/1]). %% Alternatively you may define: %% |