Limited parallel map in Elixir

lpmap.exs

defmodule Parallel do
  def run(funs, max) do
    tasks = for fun <- funs, do: {make_ref, fun}
    results = do_run(tasks, max, 0, %{})
    for {ref, _} <- tasks, do: results[ref]
  end

  defp do_run([], _max, 0, results) do
    results
  end

  defp do_run([{ref, fun} | tasks], max, workers, results) when workers < max do
    caller = self
    spawn_link fn ->
      send caller, {ref, fun.()}
    end
    do_run(tasks, max, workers + 1, results)
  end

  defp do_run(tasks, max, workers, results) do
    results = receive do
      {ref, result} -> Map.put(results, ref, result)
    end
    do_run(tasks, max, workers - 1, results)
  end
end

Inspired by http://jmilet.github.io/processes/elixir/erlang/2016/04/04/limited-pmap.html