module_attributes.ex

# file1.ex
defmodule A do
  defmacro __using__(_env) do
    quote do
      Module.register_attribute __MODULE__, :rules, accumulate: true
    end
  end

  defmacro register() do
    @rules
    |> Enum.each fn(x) ->
    #....
    #....
    end
    quote do
      # something
    end
  end
  
  defmacro define_rule() do
    quote do
      @rules {"rule"}
    end
  end
end

require A

# file2.ex
defmodule B do
  use A
  
  A.define_rule
  A.register # @rules is not yet defined when this macro is expanded
end