Code injection using evaluation

Now that we understand how Code.eval_quoted/3 works, we can use that function to inject behavior into a module from another module.

The following is a module that defines a function that returns a quoted literal. The quoted literal, when evaluated, further defines a hello function inside the module it is evaluated in. The function is defined as a simple hello world function:

behavior_injector.ex

defmodule BehaviorInjector do
  def behaviour_quoted_expr do
    quote do
      def hello, do: IO.puts "Hello world!"
    end
  end
end

Now, let’s use the previously defined module’s function inside another module:

test_subject.ex

defmodule TestSubject do
  Code.eval_quoted(BehaviorInjector.behaviour_quoted_expr(), [], __ENV__)
end

In the preceding code snippet, we need to provide __ENV__ in order to pass compile-time...