Elixir: override Mix config with environment variables

config.ex

defmodule Config do
  defmacro __using__(_) do
    quote do
      import unquote(__MODULE__), only: :macros
    end
  end

  defmacro cfg(key, type \\ :string) do
    quote do
      defp unquote(key)() do
        unquote(__MODULE__).get(__MODULE__, unquote(key), unquote(type))
      end
    end
  end

  @app_name :my_app

  def get(module, key, type \\ :string) do
    case System.get_env(env_var(module, key)) do
      nil -> Application.get_env(@app_name, module)[key]
      str -> parse_val(str, type)
    end
  end

  def get_all do
    Application.get_all_env(@app_name)
    |> Enum.map(fn {module, _} -> {module_name(module), get_module(module)} end)
    |> Enum.into(Map.new)
  end

  def get_module(module) do
    Application.get_env(@app_name, module)
    |> Enum.map(fn {key, _} -> {key, get(module, key)} end)
    |> Enum.into(Map.new)
  end

  defp env_var(module, key) do
    String.upcase "#{stringify module}_#{stringify key}"
  end

  defp module_name(module) do
    module
    |> to_string
    |> String.replace_leading("Elixir.", "")
  end

  defp stringify(atom) do
    atom
    |> module_name
    |> String.replace(".", "_")
    |> String.replace_trailing("?", "")
  end

  defp parse_val(str, type) do
    fun =
      case type do
        :atom -> &String.to_atom/1
        :boolean -> &to_boolean/1
        :float -> &String.to_float/1
        :integer -> &String.to_integer/1
        {:list, element_type} -> &to_list(&1, element_type)
        {:set, element_type} -> &to_set(&1, element_type)
        :string -> &(&1)
      end

    fun.(str)
  end

  defp to_boolean(str) do
    String.downcase(str) == "true"
  end

  defp to_list(str, element_type) do
    str |> String.split(",") |> Enum.map(&parse_val(&1, element_type))
  end

  defp to_set(str, element_type) do
    str |> to_list(element_type) |> MapSet.new
  end
end

Enhanced yours so one doesn't have to edit config.ex in order to change the app that is being configured. Renamed to "confix.ex" as well as the module, because there's already a "config.ex" in my (and everyone's) project. Put usage code in a new file into same gist so it's all in one place. Cheers, thanks for making this!

https://gist.github.com/seeekr/9f781ee081836e081638df209ac60928