Example of decoupling entities using message passing

client.ex

defmodule Player.Client do
  def effect(pid, effect) do
    GenServer.cast(pid, {:effect, effect})
  end
end

impl.ex

defmodule Player.Impl do
  def effect(state, effect) do
    effect.(state) |>
      update() # This is an internal function that tells web clients about this update.
  end
end

server.ex

defmodule Player.Server do
  use GenServer
  
  def handle_cast({:effect, effect}, state) do
    {:noreply, Impl.effect(state, effect)}
  end
end