Suck it immutable state!

mutable.ex

defrecord Mutant, pid: nil, _first_name: nil do
  def init do
    pid = spawn(Mutant, :run, [])
    current_state(pid)
  end

  def current_state(pid) do
    pid <- {self, :state }
    receive do
      { :ok, record } -> record
    end
  end

  def run do
    record = Mutant.new(pid: self)
    run(record)
  end

  def run(record) do
    receive do
      { sender, :state } ->
        sender <- { :ok, record }
        run(record)
      { sender, :state, new_record } ->
        sender <- :ok
        run(new_record)
    end
  end

  def first_name(record) do
    real_record = current_state(record.pid)
    real_record._first_name
  end

  def set_first_name(name, record) do
    record.pid <- { self, :state, record._first_name(name) }
  end
end

record = Mutant.init
record.set_first_name("bob")
IO.puts record.first_name
IO.puts record.first_name
record.set_first_name("Joe")
IO.puts record.first_name
IO.puts record.first_name
record.set_first_name("Thijs")
IO.puts record.first_name