Custom Behaviors In Erlang

October 29, 2008

Erlang behaviors are similar to interfaces in other languages. They are essentially a required set of callbacks. The OTP libraries use them to separate the functional parts of a server from the non-functional parts, letting developers quickly leverage common design patterns. I wrote about Erlang behaviors in my ejabberd module tutorial where we implemented ejabberd’s gen_mod behavior. Sometimes you’ll want to craft your own behaviors that other developers can use.

:EXTENDED:

Making Custom Behaviors

Making your own behaviors is very simple, but for some reason is completely undocumented. Aside from the gen_mod behavior of ejabberd as an example, the only information I could find was a mailing list post.

To create your own behavior, you must make a module that exports a function, behaviour_info/1. Note that while Erlang is American-English-friendly in regards to declaring behaviors, it is not as friendly when you are defining them. For example, you can use either -behavior(gen_server). or -behaviour(gen_server). when you declare you module implements the gen_server behavior, but if you name the behaviour_info/1 function without the ‘u’ it will not work.

behaviour_info/1 just needs to return the list of required exports of an implementation. Here’s an example.

-module(gen_foo).
-export([behaviour_info/1]).

behaviour_info(callbacks) ->
    [{foo, 0}, {bar, 1}, {baz, 2}];
behavior_info(_) ->
    undefined.

This declares three callbacks for the gen_foo behavior: foo/0, bar/1, and baz/2. behaviour_info/1 simply returns an array of 2-tuples with the function name and arity.

When a module declares that it implements gen_foo now, the Erlang compiler will check to make sure it exports and implements the required callbacks, and it will print warnings if this is not the case.

Behavior Helpers

gen_foo is just a toy example. A real behavior implementation is likely to have helper functions just like the OTP behaviors. For example, ejabberd’s gen_mod has helper functions like gen_mod:get_opt/2 which make it easy to access a gen_mod module’s options.