The using macro

Elixir also has a special macro, <module>.__using__/1, which can be invoked by just calling the use/2 keyword. This is a special macro that doesn’t need us to explicitly call require with the module first before calling the macro.

For example, let’s update BehaviorInjector to define a __using__/1 macro:

behavior_injector.ex

defmodule BehaviorInjector do
  defmacro __using__(_options) do
    quote do
      def hello, do: IO.puts "Hello world!"
    end
  end
end

Now, in order to call that macro, all we need to do is use the use/2 macro, as follows:

test_subject.ex

defmodule TestSubject do
  use BehaviorInjector
end

In the preceding code, we only passed one argument to the use/2 macro because the second argument is optional. Now, we can see that the final result and the final behavior of TestSubject remained unchanged:

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