Flatten Exercise

flatten.ex

defmodule Flatten do
  def flat(list) do
    cond do
      is_list(list) -> flat(list, []) |> Enum.reverse
      true -> {:error, "Must pass a List"}
    end
  end

  def flat([], acc), do: acc

  def flat([h|t], acc) when is_list(h) do
    flat(t, flat(h, acc))
  end

  def flat([h|t], acc), do: flat(t, [h | acc])
end


ExUnit.start()
defmodule FlattenTest do
  use ExUnit.Case

  test "returns tuple with error and reason" do
    assert Flatten.flat("hello") == {:error, "Must pass a List"}
  end

  test "returns list if empty list passed" do
    assert Flatten.flat([]) == []
  end

  test "flats a nested list" do
    assert Flatten.flat([1,[2],3,[4], [1,2,3]]) == [1,2,3,4,1,2,3]
  end

  test "flats really big nested list" do
    assert Flatten.flat([1,[2],3,[4], [1,2,3], [3], [34, [5,5,6]]]) == [1,2,3,4,1,2,3,3,34,5,5,6]
  end

  test "flats even when empty list inside" do
    assert Flatten.flat([[1], 2, [[3, 4], 5], [[[]]], [[[6]]], 7, 8, []]) == [1,2,3,4,5,6,7,8]
  end
end