Generate accessors for module attributes in Elixir

attre_accessor.ex

defmodule AttrAccessor do
  defmacro __using__(opts) do
    quote do
      unquote(opts[:attrs])
      |> Enum.each(fn attr ->
        Module.register_attribute(unquote(opts[:target]), attr, persist: true)
      end)

      def method_missing(func, _args) do
        if value = __MODULE__.__info__(:attributes)[func] do
          Enum.at(value, 0)
        else
          raise("no such attribute")
        end
      end

      def unquote(:"$handle_undefined_function")(func, args), do: method_missing(func, args)
    end
  end
end

defmodule Foo do
  use AttrAccessor, target: __MODULE__, attrs: [:a, :b]

  @a :value_a
  @b :value_b
  @c :value_c
end

Foo.a |> IO.inspect
Foo.b |> IO.inspect
Foo.c |> IO.inspect # raise an error