Dynamic proxy pass in elixir

proxy_pass.ex

defmodule ProxyPass do
  @docs """
  Dynamically delegate function calls to another module.
  
  Usage:

  defmodule RepoA do
    def insert(), do: IO.puts("Called from RepoA")
  end

  defmodule RepoB do
    def insert(), do: IO.puts("Called from RepoB")
  end

  defmodule Repo do
    use ProxyPass, to: get_module()

    def get_module do
      if System.get_env("USE_MODULE_A") == "true", do: RepoA, else: RepoB
    end
  end

  Repo.insert()
  """
  
  defmacro __using__(to: target) do
    quote do
      def unquote(:"$handle_undefined_function")(func, args) do
        apply(unquote(target), func, args)
      end
    end
  end
end