lib/Data/Time/Calendar/WeekDate.hs

    -- | ISO 8601 Week Date format
    module Data.Time.Calendar.WeekDate where

    import Data.Time.Calendar.OrdinalDate
    import Data.Time.Calendar.Days
    import Data.Time.Calendar.Private

    -- | Convert to ISO 8601 Week Date format. First element of result is year, second week number (1-53), third day of week (1 for Monday to 7 for Sunday).
    -- Note that \"Week\" years are not quite the same as Gregorian years, as the first day of the year is always a Monday.
    -- The first week of a year is the first week to contain at least four days in the corresponding Gregorian year.
    toWeekDate :: Day -> (Integer,Int,Int)
    toWeekDate date@(ModifiedJulianDay mjd) = (y1,fromInteger (w1 + 1),fromInteger d_mod_7 + 1) where
        (d_div_7, d_mod_7) = d `divMod` 7
        (y0,yd) = toOrdinalDate date
        d = mjd + 2
        foo :: Integer -> Integer
        foo y = bar (toModifiedJulianDay (fromOrdinalDate y 6))
        bar k = d_div_7 - k `div` 7
        (y1,w1) = case bar (d - toInteger yd + 4) of
                    -1 -> (y0 - 1, foo (y0 - 1))
                    52 -> if foo (y0 + 1) == 0
                          then (y0 + 1, 0)
                          else (y0, 52)
                    w0  -> (y0, w0)

    -- | Convert from ISO 8601 Week Date format. First argument is year, second week number (1-52 or 53), third day of week (1 for Monday to 7 for Sunday).
    -- Invalid week and day values will be clipped to the correct range.
    fromWeekDate :: Integer -> Int -> Int -> Day
    fromWeekDate y w d = ModifiedJulianDay (k - (mod k 7) + (toInteger (((clip 1 (if longYear then 53 else 52) w) * 7) + (clip 1 7 d))) - 10) where
            k = toModifiedJulianDay (fromOrdinalDate y 6)
            longYear = case toWeekDate (fromOrdinalDate y 365) of
                (_,53,_) -> True
                _ -> False

    -- | Convert from ISO 8601 Week Date format. First argument is year, second week number (1-52 or 53), third day of week (1 for Monday to 7 for Sunday).
    -- Invalid week and day values will return Nothing.
    fromWeekDateValid :: Integer -> Int -> Int -> Maybe Day
    fromWeekDateValid y w d = do
        d' <- clipValid 1 7 d
        let
            longYear = case toWeekDate (fromOrdinalDate y 365) of
                (_,53,_) -> True
                _ -> False
        w' <- clipValid 1 (if longYear then 53 else 52) w
        let
            k = toModifiedJulianDay (fromOrdinalDate y 6)
        return (ModifiedJulianDay (k - (mod k 7) + (toInteger ((w' * 7) + d')) - 10))

    -- | Show in ISO 8601 Week Date format as yyyy-Www-d (e.g. \"2006-W46-3\").
    showWeekDate :: Day -> String
    showWeekDate date = (show4 y) ++ "-W" ++ (show2 w) ++ "-" ++ (show d) where
        (y,w,d) = toWeekDate date