switch.ex

defmodule Switch do  
  def start_link(n \\ 10, callback \\ fn (_) -> IO.puts "Turned on" end, opts \\ []) do
    :gen_fsm.start_link(__MODULE__, [n, callback], opts)
  end

  def init([n, callback]) do
    {:ok, :off, %{n: n, received: 0, on_callback: callback}}    
  end

  def off({:event}, state) do
    if state.received + 1 > state.n do
      {:next_state, :on, %{state | received: state.received + 1}}
    else
      {:next_state, :off, %{state | received: state.received + 1}}  
    end  
  end

  def on(_, state) do
    state.on_callback()
    {:next_state, :on, state}
  end
end