How do you define and use a custom behavior with the -callback attribute?

Beyond OTP's built-in behaviors (gen_server, supervisor, and so on), you can define your own reusable process pattern by declaring the required callbacks a module implementing your behavior must export, using the -callback attribute.

%% my_plugin.erl - the behavior definition
-module(my_plugin).
-callback handle_event(Event :: term(), State :: term()) ->
    {ok, NewState :: term()}.

%% logger_plugin.erl - a module implementing it
-module(logger_plugin).
-behaviour(my_plugin).
-export([handle_event/2]).

handle_event(Event, State) ->
    io:format("event: ~p~n", [Event]),
    {ok, State}.

The -callback declaration lets Dialyzer and the compiler verify that any module claiming -behaviour(my_plugin) actually exports a matching handle_event/2, warning at compile time if it's missing — giving you the same "pluggable module with an enforced contract" pattern OTP's own behaviors use, for your own domain-specific extension points.

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