counter.exs

defmodule Counter do
  def start_link do
    Agent.start_link(fn -> 0 end)
  end

  def set(pid, value) do
    Agent.update(pid, fn _ -> value end)
  end

  def increment(pid) do
    Agent.get_and_update(pid, fn state ->
      {state + 1, state + 1}
    end)
  end

  def decrement(pid) do
    Agent.get_and_update(pid, fn state ->
      {state - 1, state - 1}
    end)
  end
end

{:ok, pid} = Counter.start_link

Counter.increment(pid)
Counter.increment(pid)
Counter.increment(pid)

Counter.set(pid, 5)

Counter.decrement(pid)
Counter.increment(pid)

Agent.get(pid, &(&1))