stack_agent.ex

defmodule StackAgent do
    use Agent
  
    def start(initial_value) do
      Agent.start(fn -> initial_value end, name: __MODULE__)
    end
  
    def value() do
      Agent.get(__MODULE__, fn state -> state end)
    end
  
    def set_value(value) do
      Agent.update(__MODULE__, fn _ -> value end)
    end
  end
  

defmodule Stack do
    use GenServer
    def init(init_arg) do
        state = case StackAgent.start(init_arg) do
            {:ok, agent_pid} ->
                init_arg
            {:error, {:already_started, agent_pid}} ->
                StackAgent.value()
        end
        {:ok, state}
    end

    def pop(pid), do: GenServer.call(pid, :pop)
    def push(pid, value), do: GenServer.cast(pid, {:push, value})
    def handle_call(:pop, _from, state) do
        [head | tail] = state
        StackAgent.set_value(tail)
        {:reply, head, tail}
    end
    def handle_cast({:push, value}, state) do
        StackAgent.set_value([value | state])
        {:noreply, [value | state]}
    end
end

{:ok, pid} = GenServer.start_link(Stack, [])
IO.puts(Stack.push(pid, 1))
IO.puts(Stack.pop(pid))