The point of the article is that it doesn't have to fail gracefully.

Consider the API:

    data Date = Date { getYear :: Int, getMonth :: Int, getDay :: Int }
        deriving (Eq, Ord, Show)

    addDays :: Date -> Int -> Date
    addMonthsRounded :: Date -> Int -> Date

Someone who does

    d `addMonthsRounded` 3

immediately has a contextual clue that there might be something fishy going on, and has a string they can google to get to the docs to find out that this "Rounded" business is all about "hey, the code

    let y = (x `addMonthsRounded` 1) `addMonthsRounded` (-1)
      in y == x

might sometimes return False because it truncates if your day doesn't fit in the given month."