exercise.exs

@proto_version "1.0"

  def process_options(opts) do
    env_args()
    |> (&process_in(opts, &1)).()
    |> (&process_err(opts, &1)).()
    |> (&process_dir(opts, &1)).()
  end

  defp process_in(opts, args) do
    case Keyword.get(opts, :in, false) do
     true -> ["-in" | args]
     false -> args
    end
  end

  defp process_err(opts, args) do
    case Keyword.get(opts, :err, false) do
      :out -> ["-err", "out" | args]
      :err -> ["-err", "err" | args]
      false -> args
    end
  end

  defp process_dir(opts, args) do
    case Keyword.get(opts, :dir, nil) do
      dir when is_binary(dir) -> ["-dir", dir | args]
      nil -> args
    end
  end

  defp env_args() do
    ["-proto", @proto_version | log_args()]
  end

  defp log_args() do
    case Application.fetch_env(:porcelain, :goon_driver_log) do
      :error ->
        []
      {:ok, val} ->
        ["-log", val]
    end
  end

exercise2.ex

@proto_version "1.0"

  def process_options(opts) do
    Enum.reverse(opts)
    |> Enum.reduce(%{in: [], err: [], dir: []}, &process_options/2)
    |> Enum.reduce(env_args(), fn({_key, args}, acc) -> args ++ acc end)
  end

  defp process_options({key, value}, map) do
    Map.put(map, key, process_option(key, value))
  end

  defp process_option(:in, true), do: ["-in"]
  defp process_option(:in, false), do: []
  defp process_option(:err, :out), do: ["-err", "out"]
  defp process_option(:err, :err), do: ["-err", "err"]
  defp process_option(:err, false), do: []
  defp process_option(:dir, path) when is_binary(path), do: ["-dir", path]
  defp process_option(:dir, nil), do: []
  defp process_option(key, value) do
    raise ArgumentError, "invalid option: #{inspect(key)}: #{inspect(value)}"
  end

  defp env_args() do
    ["-proto", @proto_version | log_args()]
  end

  defp log_args() do
    case Application.fetch_env(:porcelain, :goon_driver_log) do
      :error ->
        []
      {:ok, val} ->
        ["-log", val]
    end
  end