Naive promise implementation for Elixir

naive_promise.ex

defmodule Promise do
  defstruct functions: []

  def new(), do: %Promise{}
  def new(func), do: %Promise{functions: [func]}

  def then(%Promise{functions: functions} = promise, func) do
    Map.put promise, :functions, [func | functions]
  end

  def run(promise) do
    spawn_link fn ->
      promise.functions
      |> Enum.reverse
      |> Enum.reduce(nil, & do_run(&1, &2))
    end
  end

  defp do_run(fun, last_result) do
    if :erlang.fun_info(fun)[:arity] == 0 do
      fun.()
    else
      fun.(last_result)
    end
  end
end