hello_world.ex

defmodule HelloWorld do

  def say do
    world_pid = spawn(HelloWorld, :world, [])
    spawn(HelloWorld, :hello, [1000, world_pid])
  end

  def hello(n, world_pid) when n > 0 do
    IO.puts "Hello, "
    world_pid <- {:world, Process.self}
    receive do
      {:hello} -> hello(n - 1, world_pid)
    end
  end

  def hello(0, world_pid) do
    world_pid <- :done
  end

  def world do
    receive do
      {:world, hello_pid} ->
        IO.puts "World"
        hello_pid <- {:hello}
        world
      :quit -> :quit
    end
  end

end

hello_world_registered.ex

defmodule HelloWorld do

  def say do
    Process.register world_pid_name, spawn(HelloWorld, :world, [])
    Process.register hello_pid_name, spawn(HelloWorld, :hello, [])
  end

  def hello, do: hello(1000)

  def hello(n) when n > 0 do
    IO.puts "Hello, "
    world_pid <- :world
    receive do
      :hello -> hello(n - 1)
    end
  end

  def hello(0) do
    world_pid <- :quit
  end

  def world do
    receive do
      :world ->
        IO.puts "World"
        hello_pid <- :hello
        world
      :quit -> :quit
    end
  end

  defp world_pid_name, do: :hello_world_world_pid
  defp hello_pid_name, do: :hello_world_hello_pid
  defp world_pid, do: Process.whereis(world_pid_name)
  defp hello_pid, do: Process.whereis(hello_pid_name)

end