Intuition into recursion as alternative to loops in FP

p7.exs

defmodule P7 do
  # public function that calls a private one with accumulator
  def flatten(list), do: Enum.reverse(do_flatten(list, []))
  # base case - empty list
  defp do_flatten([], result), do: result
  # next two clauses for list with many elements
  defp do_flatten([head | tail], result) when is_list(head), do: do_flatten(tail, do_flatten(head, result))
  defp do_flatten([head | tail], result), do: do_flatten(tail, [head | result])
end

ExUnit.start

defmodule P7Test do
  use ExUnit.Case

  test "P7.flatten" do
    # step 1 - empty list
    assert P7.flatten([]) == []

    # step 2 - list with single element - either list or value
    assert P7.flatten([1]) == [1]
    assert P7.flatten([[1]]) == [1]
    assert P7.flatten([[[1]]]) == [1]

    # step 3 - list with many elements - either list or value
    assert P7.flatten([[1, [2, 3]], 4, [5, 6]]) == [1, 2, 3, 4, 5, 6]
    assert P7.flatten([1, [2, 3, [4, 5]], 6]) == [1, 2, 3, 4, 5, 6]
  end
end