Elixir counter

counter.ex

defmodule Counter do
  use GenServer

  # Client

  def decrement(pid) do
    GenServer.call(pid, :decrement)
  end

  def count(pid) when is_pid(pid) do
    GenServer.call(pid, :count)
    |> elem(1)
  end

  def increment(pid) do
    GenServer.call(pid, :increment)
  end

  def start_link do
    start_link(0)
  end

  def start_link(integer, opts \\ []) when is_integer(integer) do
    GenServer.start_link(__MODULE__, integer, opts)
  end

  # Server

  def handle_call(:count, _from, state) do
    {:reply, state, state}
  end

  def handle_call(:difference, _from, state) do
    difference = state - 1
    {:reply, difference, difference}
  end

  def handle_call(:increment, _from, state) do
    sum = state + 1
    {:reply, sum, sum}
  end
end

import Supervisor.Spec
children = [
  worker(Counter, [])
]
{:ok, sup_pid} = Supervisor.start_link(children, strategy: :simple_one_for_one)
{:ok, pid } = Supervisor.start_child(sup_pid, [10])
IO.puts Counter.increment pid # 11
IO.puts Counter.increment pid # 12
IO.puts Counter.increment pid # 13