flatten.ex

defmodule Flatten do
  @moduledoc """
  Documentation for Flatten.
  """

  @doc """
  Flattens an arbitrarily sized list

  ## Examples

      iex> Flatten.flatten([])
      []

      iex> Flatten.flatten([[1,2,[3]],4])
      [1,2,3,4]

      iex> Flatten.flatten([[[1],[2],[3]],4, 5, 6, 10, "apple"])
      [1,2,3,4, 5, 6, 10, "apple"]

      # This test shows that this implementation is the same as the built-in List.flatten/1 method
      iex> large_range = Enum.reduce(0..10000, [], fn(x, acc) -> acc ++ [x] end)
      ...> flattened = List.flatten(large_range)
      ...> flattened == Flatten.flatten(large_range)
      true   
  """
  def flatten([]), do: []
  def flatten([head|tail]), do: flatten(head) ++ flatten(tail)
  def flatten(item) when not is_list(item), do: [item]
end

flatten_test.exs

defmodule FlattenTest do
  use ExUnit.Case
  doctest Flatten
end