Separate state

fake_dux.ex

defmodule Reducer do
  use GenServer
  def start_link(state_pid, options \\ []) do
    GenServer.start_link(__MODULE__, state_pid, options)
  end
  
  def init(state_pid) do
    {:ok, %{state_pid: state_pid}}
  end
  
  def run(callback) do
    GenServer.cast(__MODULE__, {:run, callback})
  end
  
  def get_state do
    GenServer.call(__MODULE__, :get_state)
  end
  
  def handle_cast({:run, callback}, state = %{state_pid: state_pid}) do
    Agent.update(state_pid, callback) 
    {:noreply, state}
  end
  
  def handle_call(:get_state, _from, state = %{state_pid: state_pid}) do
    {:reply, Agent.get(state_pid, &(&1)), state}
  end
end

{:ok, state} = Agent.start_link(fn -> 0 end)
Reducer.start_link(state, name: Reducer)
Reducer.run(fn (num) -> num + 1 end)
Reducer.get_state