Environment Variable based Config Provider

environment.ex

defmodule Huddle.Environment do
  use GenServer
  
  @config_entries [
    {:huddle_web, HuddleWeb.Endpoint, ~w(http port)a, {:system, "PORT"}}
  ]
  
  def start_link(args) do
    GenServer.start_link(__MODULE__, args)
  end
  
  def init(args) do
    with :ok <- configure(@config_entries) do
      {:ok, args}
    end
  end

  defp configure([]), do: :ok
  defp configure([h|t]) do
    with :ok <- configure_entry(h) do
      configure(t)
    end
  end

  defp configure_entry({app, key, path, source}) do
    with \
      from_env <- Application.get_env(app, key),
      {:ok, value} <- resolve_source(source),
      to_env <- put_in(from_env, path, value),
      :ok <- Application.put_env(app, key, to_env, persistent: true)
    do
      :ok
    end
  end

  defp resolve_source({:system, envar}) do
    case System.get_env(envar) do
      nil -> {:error, {:missing_envar, envar}}
      "" -> {:error, {:missing_envar, envar}}
      value -> {:ok, value}
    end
  end
end