Genserver Implementation

genserver.ex

defmodule Server do
  def start(callback_module, state \\ nil) do
    parent = self
    spawn fn ->
      loop(callback_module, parent, state)
    end
  end

  def loop(callback_module, parent, state) do
    receive do
      message ->
        state = callback_module.handle_message(message, parent, state)
        loop(callback_module, parent, state)
    end
  end
end

## And test

defmodule Speaker do
  def handle_message({:say, msg}, _from, _state) do
    IO.puts msg
  end

  def handle_message(_other, _from, _state) do
    false
  end
end

server = Server.start(Speaker)
send server, {:say, "Hello world!"}