future.ex

defmodule Future do

  def new(fun) do
    fn(x) ->
      spawn_link fn ->
        value = try do
          { :ok, fun.(x) }
        rescue
          e -> { :error, e }
        end
        receive do
          pid -> pid <- {self, value}
        end
      end
    end
  end

  def value(pid, timeout // :infinity, default // {:error, :timeout}) do
    pid <- self
    receive do
      {^pid, {:ok, value}} -> value
      {^pid, {:error, e}}  -> raise e
    after
      timeout -> default
    end
  end

end

defmodule Parallel do

  def pmap(collection, fun) do
    collection
    |> Enum.map(Future.new(fun))
    |> Enum.map(Future.value(&1))
  end

end

IO.inspect Parallel.pmap([1, 2, 3, 4], fn x -> :timer.sleep(1000); x * x end)