flatten.exs

defmodule Flatten do
  @spec flatten(List.t()) :: List.t()
  def flatten([]), do: []
  def flatten([head | tail]) when is_list(head), do: flatten(head) ++ flatten(tail)
  def flatten([head | tail]), do: [head | flatten(tail)]
  def flatten(_), do: raise("argument must be a list")
end

ExUnit.start()

defmodule FlattenTest do
  import Flatten
  import ExUnit.Assertions, only: [assert_raise: 2]
  use ExUnit.Case

  test "returns an  empty array" do
    assert flatten([]) == []
  end

  test "returns an array of one element" do
    assert flatten(["a"]) == ["a"]
  end

  test "returns an array of two elements" do
    assert flatten(["a", "b"]) == ["a", "b"]
  end

  test "can flatten one element" do
    assert flatten([["a"]]) == ["a"]
  end

  test "raises on non list argument" do
    assert_raise RuntimeError, fn -> flatten("foo") end
  end

  test "big lists work too" do
    assert flatten([[1, 2, 3], [4, [[5, 6], [7], 8]], 9, 10, [[[[[11]]]]]]) == Enum.to_list(1..11)
  end
end