flash.ex

defmodule AcmeApp.Flash do
  import Plug.Conn

  defmodule UnknownFlashException do
    defexception key: nil

    def message(exception), do: "unknown flash " <> inspect(exception.key)
  end

  defmodule Item do
    defstruct value: nil, age: 0

    def grow_old(%Item{ value: value, age: age }), do: %Item{ value: value, age: (age + 1) }
  end

  defp get_registry(conn), do: get_session(conn, :flash) || []
  defp put_registry(reg, conn), do: put_session(conn, :flash, reg)

  def get(conn, key) when is_atom(key) do
    case get_registry(conn) do
      [] -> { :error, { :unknown_key, key } }
      reg -> { :ok, reg[key] }
    end
  end

  def get!(conn, key) when is_atom(key) do
    case get_registry(conn) do
      [] -> raise UnknownFlashException, key: key
      reg -> reg[key]
    end
  end

  def put(conn, key, value) when is_atom(key) do
    get_registry(conn)
      |> Keyword.put(key, %Item{value: value})
      |> put_registry(conn)
  end

  def clean(conn) do
    get_registry(conn)
      |> Enum.filter(fn %Item{age: age} -> age == 0 end)
      |> Enum.map(&Item.grow_old/1)
      |> put_registry(conn)
  end
  
end