A nice config wrapper which allows using the {:system, "VAR"} convention comfortably

config.ex

defmodule Config do
  @moduledoc """
  This module handles fetching values from the config with some additional niceties
  """

  @doc """
  Fetches a value from the config, or from the environment if {:system, "VAR"}
  is provided.

  An optional default value can be provided if desired.

  ## Example

      iex> {test_var, expected_value} = System.get_env |> Enum.take(1) |> List.first
      ...> Application.put_env(:myapp, :test_var, {:system, test_var})
      ...> ^expected_value = #{__MODULE__}.get(:myapp, :test_var)
      ...> :ok
      :ok

      iex> Application.put_env(:myapp, :test_var2, 1)
      ...> 1 = #{__MODULE__}.get(:myapp, :test_var2)
      1

      iex> :default = #{__MODULE__}.get(:myapp, :missing_var, :default)
      :default
  """
  @spec get(atom, atom, term | nil) :: term
  def get(app, key, default \\ nil) when is_atom(app) and is_atom(key) do
    case Application.get_env(app, key) do
      {:system, env_var} ->
        case System.get_env(env_var) do
          nil -> default
          val -> val
        end
      {:system, env_var, preconfigured_default} ->
        case System.get_env(env_var) do
          nil -> preconfigured_default
          val -> val
        end
      nil ->
        default
      val ->
        val
    end
  end
end