file_store.ex

defmodule Someapp.FileStore do
  @type key :: String.t()
  @type data :: binary
  @type options :: keyword

  @callback put(key, data, options) :: :ok | {:error, atom}
  @callback get(key) :: {:ok, data} | {:error, atom}
  @callback download(key, Path.t()) :: :ok | {:error, atom}
  @callback upload(key, Path.t(), options) :: :ok | {:error, atom}
  @callback stream_down(Plug.Conn.t(), key, options) :: Plug.Conn.t() | no_return

  @spec config() :: keyword
  def config do
    Application.get_env(:someapp, __MODULE__, [])
  end

  @spec adapter() :: __MODULE__
  def adapter do
    config() |> Keyword.get(:adapter, Shareup.MissingFileStore)
  end
end

defmodule Shareup.MissingFileStore do
  @behaviour Shareup.FileStore

  @error {:error, :no_file_store_adapter_specified_in_config}

  def put(_key, _data, _options), do: @error
  def get(_key), do: @error
  def download(_key, _path), do: @error
  def upload(_key, _path, _options), do: @error

  def stream_down(conn, _key, _options) do
    Plug.Conn.send_resp(conn, 500, inspect(@error))
  end
end