Macro-based constants in Elixir

constant.ex

defmodule Constant do
  @moduledoc """
  Allows generating dynamic functions for the given list of values.
  Accepts an enumerable with the of keys and values to be generated as functions.
  """
  defmacro __using__(constants) do
    [define_constants(constants), define_helpers()]
  end

  defp define_constants(constants) when is_list(constants) do
    for {key, value} <- constants do
      quote do
        defp fetch_const(unquote(key)), do: unquote(value)
      end
    end
  end

  defp define_helpers() do
    quote do
      def const(name) when is_atom(name), do: fetch_const(name)
      def const(name) when is_list(name), do: List.flatten(Enum.map(name, &fetch_const/1))
    end
  end
end

demo.ex

defmodule Colors do
  use Constant, rgb: [:red, :green, :blue]   
end

defmodule Greeter do
  def greet(name) do
    Enum.each(Colors.const(:rgb), fn color -> 
      IO.puts(IO.ANSI.format([color, "Hello #{name}"]))
    end)
  end
end