super_plug

defmodule SuperPlug do
  @moduledoc """
  Give your Plug superpowers!

  ## Usage

      defmodule FooPlug do
        use SuperPlug

        def init(opts) do
          ...
        end

        def call(conn, opts) do
          ...
        end
      end

  ## Primary Superpower

  `call/1` initializes default options.

      iex> conn = %{assigns: %{}}
      ...> FooPlug.call(conn)
      %{assigns: %{fooed?: false}}

  ## Secondary Superpower

  `call/2` initializes keyword options.

      iex> conn = %{assigns: %{}}
      ...> FooPlug.call(conn, foo?: true)
      %{assigns: %{fooed?: true}}

  ## Tertiary Superpower

  `call/2` short circuits if an error is assigned.

      iex> conn = %{assigns: %{error: :not_found}}
      ...> FooPlug.call(conn)
      %{assigns: %{error: :not_found}}

  It can be configured to return the error as a tuple.

      defmodule BarPlug do
        use SuperPlug, return_error_as_tuple: true
        ...
      end

      iex> conn = %{assigns: %{error: :not_found}}
      ...> BarPlug.call(conn)
      {:error, :not_found, %{assigns: %{error: :not_found}}}
  """

  defmacro __using__(return_error_as_tuple: true) do
    quote do
      def call(conn), do: call(conn, init([]))
      def call(conn, opts) when is_list(opts), do: call(conn, init(opts))
      def call(conn = %{assigns: %{error: error}}, _), do: {:error, error, conn}
    end
  end

  defmacro __using__(_opts) do
    quote do
      def call(conn), do: call(conn, init([]))
      def call(conn, opts) when is_list(opts), do: call(conn, init(opts))
      def call(conn = %{assigns: %{error: _}}, _), do: conn
    end
  end
end