counter.ex

defmodule Counter do
  use GenServer

  def increment() do
    GenServer.cast(__MODULE__, :increment)
  end

  def decrement() do
    GenServer.cast(__MODULE__, :decrement)
  end

  def value() do
    GenServer.call(__MODULE__, :value)
  end

  def start_link(initial_value \\ 0) do
    GenServer.start_link(__MODULE__, initial_value, name: __MODULE__)
  end

  def init(initial_value) do
    {:ok, initial_value}
  end

  def handle_cast(:increment, val) do
    {:noreply, val + 1}
  end

  def handle_cast(:decrement, val) do
    {:noreply, val - 1}
  end

  def handle_call(:value, _from, val) do
    {:reply, val, val}
  end
end