Code injection using macros

We learned that we can use Code.eval_quoted/3 to dynamically add behavior to a module, but Elixir provides a cleaner and more consistent way of doing this, using macro.

Code.eval_quoted/3 allows us to inject code using a set of variable bindings and an environment. macro, on the other hand, allows us to define a set of quoted literals at the time of their compilation and evaluate the expressions, by simply invoking it like a function inside another module. A macro can be defined by using defmacro/2, which itself is a macro.

To understand the preceding distinction better, let us use macro to implement the same BehaviorInjector module and inject behavior in the TestSubject module:

behavior_injector.ex

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

In the preceding...