I’m looking at an existing project with code like this:

  @spec default_rating :: List.t()
  @spec default_rating(non_neg_integer()) :: List.t()
  def default_rating(rating_type_id \\ nil) do
    {skill, uncertainty} = Openskill.rating()
    rating_value = calculate_rating_value(skill, uncertainty)
    leaderboard_rating = calculate_leaderboard_rating(skill, uncertainty)

    %{
      rating_type_id: rating_type_id,
      skill: skill,
      uncertainty: uncertainty,
      rating_value: rating_value,
      leaderboard_rating: leaderboard_rating
    }
  end

I don’t understand why the return type is List.t().
Can I use :: map as the return type? What’s the difference?