Lazy permutations in Elixir

lazy_permutations.exs

defmodule LazyPermutations do
  def permutations(list) do
    list
    |> Enum.sort
    |> Stream.unfold fn
      [] -> nil
      p -> {p, next_permutation(p)}
    end
  end

  defp next_permutation(permutation) do
    if permutation == permutation |> Enum.sort |> Enum.reverse do
      []
    else
      permutation
      |> split
      |> heal
    end
  end

  defp split(permutation) do
    permutation
    |> Enum.reverse
    |> Enum.reduce({0, false, [], []}, fn(x, {prev, split, first, last}) ->
      case split do
        false -> {x, x < prev, first, [x|last]}
        true -> {x, true, [x|first], last}
      end
    end)
    |> fn({_, _,first, last}) -> {first, last} end.()
  end

  defp heal({first, [h|_] = last}) do
    next = last |> Enum.filter(&(&1 > h)) |> Enum.min
    rest = last -- [next] |> Enum.sort
    first ++ [next] ++ rest
  end
end