gen_increment.ex

defmodule GenIncrement do
  use GenServer
  # Public
  def start_link do
    # GenServer.start_link(GenIncrement, [], name: __MODULE__)
    {:ok, pid} = GenServer.start_link(GenIncrement, [])
    Process.register(pid, __MODULE__)
    {:ok, pid}
  end

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

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

  def reset do
    GenServer.cast(__MODULE__, {:reset})
  end

  def status do
    GenServer.call(__MODULE__, {:status})
  end

  # Private

  def init(_) do
    {:ok, 0}
  end

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

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

  def handle_cast({:reset}, _total) do
    {:noreply, 0}
  end

  def handle_call({:status}, _pid, total) do
    reply_with = total
    next_state = total
    {:reply, reply_with, next_state}
  end
end

increment.ex

defmodule Increment do
  def loop(total \\ 0) do
    receive do
      {:increment} -> loop(total + 1)
      {:decrement} -> loop(total - 1)
      {:reset} -> loop
      {:status, pid} ->
        send(pid, {:result, total})
        loop(total)
    end
  end
end

processes.ex

defmodule ProcessTest do
  def loop do
    receive do
      {:ok, msg} -> IO.puts("I got a message!!! #{msg}")
      anything ->
        IO.puts "I don't understand that"
        IO.inspect anything
    end
    loop
  end
end