defmodule Factorial do
  @moduledoc """
  Computes the factorial of a non‑negative integer.
  Returns :error for negative inputs.
  """

  @spec factorial(integer) :: integer | :error
  def factorial(n) when is_integer(n) and n >= 0 do
    do_factorial(n, 1)
  end

  def factorial(_), do: :error

  @spec do_factorial(integer, integer) :: integer
  defp do_factorial(0, acc), do: acc
  defp do_factorial(n, acc) when n > 0, do: do_factorial(n - 1, acc * n)
end