Elixir Flatten array (with tail recursion)

flatten_array.ex

defmodule FlattenArray do
  @doc """
    Accept a list and return the list flattened without nil values.

    ## Examples

      iex> FlattenArray.flatten([1, [2], 3, nil])
      [1,2,3]

      iex> FlattenArray.flatten([nil, nil])
      []

  """

  @spec flatten(list) :: list
  def flatten([]), do: [] # optimization
  def flatten(l), do: flatten([], [], l)

  def flatten(acc, [], []), do: Enum.reverse(acc) # reversing an accumulated list is fast in EVM
  def flatten(acc, queue, []), do: flatten(acc, [], queue) # process queue once element is empty
  def flatten(acc, queue, [nil | t]), do: flatten(acc, queue, t) # skip nil
  def flatten(acc, queue, [[]  | t]), do: flatten(acc, queue, t) # skip empty list
  def flatten(acc, queue, [h   | t]) when is_list(h), do: flatten(acc, [t | queue], h) # push tail into queue, and process element
  def flatten(acc, queue, [h   | t]), do: flatten([h | acc], queue, t) # put element into accumulator
end

Important reading: https://www.erlang.org/doc/efficiency_guide/listhandling