lib/Data/Time/Format/Format/Class.hs

    module Data.Time.Format.Format.Class
        (
            -- * Formatting
            formatTime,
            FormatNumericPadding,
            FormatOptions(..),
            FormatTime(..),
            ShowPadded,PadOption,
            formatGeneral,formatString,formatNumber,formatNumberStd,
            showPaddedFixed,showPaddedFixedFraction,
            quotBy,remBy,
        )
        where

    import Data.Char
    import Data.Maybe
    import Data.Fixed
    import Data.Time.Calendar.Private
    import Data.Time.Format.Locale

    type FormatNumericPadding = Maybe Char

    data FormatOptions = MkFormatOptions {
        foLocale :: TimeLocale,
        foPadding :: Maybe FormatNumericPadding,
        foWidth :: Maybe Int
    }

    -- <http://www.opengroup.org/onlinepubs/007908799/xsh/strftime.html>
    class FormatTime t where
        -- | @since 1.9.1
        formatCharacter :: Bool -> Char -> Maybe (FormatOptions -> t -> String)

    -- the weird UNIX logic is here
    getPadOption :: Bool -> Bool -> Int -> Char -> Maybe FormatNumericPadding -> Maybe Int -> PadOption
    getPadOption trunc fdef idef cdef mnpad mi = let
        c = case mnpad of
            Just (Just c') -> c'
            Just Nothing -> ' '
            _ -> cdef
        i = case mi of
            Just i' -> case mnpad of
                Just Nothing -> i'
                _ -> if trunc then i' else max i' idef
            Nothing -> idef
        f = case mi of
            Just _ -> True
            Nothing -> case mnpad of
                Nothing -> fdef
                Just Nothing -> False
                Just (Just _) -> True
        in if f then Pad i c else NoPad

    formatGeneral :: Bool -> Bool -> Int -> Char -> (TimeLocale -> PadOption -> t -> String) -> (FormatOptions -> t -> String)
    formatGeneral trunc fdef idef cdef ff fo = ff (foLocale fo) $ getPadOption trunc fdef idef cdef (foPadding fo) (foWidth fo)

    formatString :: (TimeLocale -> t -> String) -> (FormatOptions -> t -> String)
    formatString ff = formatGeneral False False 1 ' ' $ \locale pado -> showPadded pado . ff locale

    formatNumber :: (ShowPadded i) => Bool -> Int -> Char -> (t -> i) -> (FormatOptions -> t -> String)
    formatNumber fdef idef cdef ff = formatGeneral False fdef idef cdef $ \_ pado -> showPaddedNum pado . ff

    formatNumberStd :: Int -> (t -> Integer) -> (FormatOptions -> t -> String)
    formatNumberStd n = formatNumber False n '0'

    showPaddedFixed :: HasResolution a => PadOption -> PadOption -> Fixed a -> String
    showPaddedFixed padn padf x | x < 0 = '-' : showPaddedFixed padn padf (negate x)
    showPaddedFixed padn padf x = let
        ns = showPaddedNum padn $ (floor x :: Integer)
        fs = showPaddedFixedFraction padf x
        ds = if null fs then "" else "."
        in ns ++ ds ++ fs

    showPaddedFixedFraction :: HasResolution a => PadOption -> Fixed a -> String
    showPaddedFixedFraction pado x = let
        digits = dropWhile (=='.') $ dropWhile (/='.') $ showFixed True x
        n = length digits
        in case pado of
            NoPad -> digits
            Pad i c -> if i < n
                then take i digits
                else digits ++ replicate (i - n) c

    -- | Substitute various time-related information for each %-code in the string, as per 'formatCharacter'.
    --
    -- The general form is @%\<modifier\>\<width\>\<alternate\>\<specifier\>@, where @\<modifier\>@, @\<width\>@, and @\<alternate\>@ are optional.
    --
    -- == @\<modifier\>@
    -- glibc-style modifiers can be used before the specifier (here marked as @z@):
    --
    -- [@%-z@] no padding
    --
    -- [@%_z@] pad with spaces
    --
    -- [@%0z@] pad with zeros
    --
    -- [@%^z@] convert to upper case
    --
    -- [@%#z@] convert to lower case (consistently, unlike glibc)
    --
    -- == @\<width\>@
    -- Width digits can also be used after any modifiers and before the specifier (here marked as @z@), for example:
    --
    -- [@%4z@] pad to 4 characters (with default padding character)
    --
    -- [@%_12z@] pad with spaces to 12 characters
    --
    -- == @\<alternate\>@
    -- An optional @E@ character indicates an alternate formatting. Currently this only affects @%Z@ and @%z@.
    --
    -- [@%Ez@] alternate formatting
    --
    -- == @\<specifier\>@
    --
    -- For all types (note these three are done by 'formatTime', not by 'formatCharacter'):
    --
    -- [@%%@] @%@
    --
    -- [@%t@] tab
    --
    -- [@%n@] newline
    --
    -- === 'TimeZone'
    -- For 'TimeZone' (and 'ZonedTime' and 'UTCTime'):
    --
    -- [@%z@] timezone offset in the format @±HHMM@
    --
    -- [@%Ez@] timezone offset in the format @±HH:MM@
    --
    -- [@%Z@] timezone name (or else offset in the format @±HHMM@)
    --
    -- [@%EZ@] timezone name (or else offset in the format @±HH:MM@)
    --
    -- === 'LocalTime'
    -- For 'LocalTime' (and 'ZonedTime' and 'UTCTime' and 'UniversalTime'):
    --
    -- [@%c@] as 'dateTimeFmt' @locale@ (e.g. @%a %b %e %H:%M:%S %Z %Y@)
    --
    -- === 'TimeOfDay'
    -- For 'TimeOfDay' (and 'LocalTime' and 'ZonedTime' and 'UTCTime' and 'UniversalTime'):
    --
    -- [@%R@] same as @%H:%M@
    --
    -- [@%T@] same as @%H:%M:%S@
    --
    -- [@%X@] as 'timeFmt' @locale@ (e.g. @%H:%M:%S@)
    --
    -- [@%r@] as 'time12Fmt' @locale@ (e.g. @%I:%M:%S %p@)
    --
    -- [@%P@] day-half of day from ('amPm' @locale@), converted to lowercase, @am@, @pm@
    --
    -- [@%p@] day-half of day from ('amPm' @locale@), @AM@, @PM@
    --
    -- [@%H@] hour of day (24-hour), 0-padded to two chars, @00@ - @23@
    --
    -- [@%k@] hour of day (24-hour), space-padded to two chars, @ 0@ - @23@
    --
    -- [@%I@] hour of day-half (12-hour), 0-padded to two chars, @01@ - @12@
    --
    -- [@%l@] hour of day-half (12-hour), space-padded to two chars, @ 1@ - @12@
    --
    -- [@%M@] minute of hour, 0-padded to two chars, @00@ - @59@
    --
    -- [@%S@] second of minute (without decimal part), 0-padded to two chars, @00@ - @60@
    --
    -- [@%q@] picosecond of second, 0-padded to twelve chars, @000000000000@ - @999999999999@.
    --
    -- [@%Q@] decimal point and fraction of second, up to 12 second decimals, without trailing zeros.
    -- For a whole number of seconds, @%Q@ omits the decimal point unless padding is specified.
    --
    -- === 'UTCTime' and 'ZonedTime'
    -- For 'UTCTime' and 'ZonedTime':
    --
    -- [@%s@] number of whole seconds since the Unix epoch. For times before
    -- the Unix epoch, this is a negative number. Note that in @%s.%q@ and @%s%Q@
    -- the decimals are positive, not negative. For example, 0.9 seconds
    -- before the Unix epoch is formatted as @-1.1@ with @%s%Q@.
    --
    -- === 'DayOfWeek'
    -- For 'DayOfWeek' (and 'Day' and 'LocalTime' and 'ZonedTime' and 'UTCTime' and 'UniversalTime'):
    --
    -- [@%u@] day of week number for Week Date format, @1@ (= Monday) - @7@ (= Sunday)
    --
    -- [@%w@] day of week number, @0@ (= Sunday) - @6@ (= Saturday)
    --
    -- [@%a@] day of week, short form ('snd' from 'wDays' @locale@), @Sun@ - @Sat@
    --
    -- [@%A@] day of week, long form ('fst' from 'wDays' @locale@), @Sunday@ - @Saturday@
    --
    -- === 'Day'
    -- For 'Day' (and 'LocalTime' and 'ZonedTime' and 'UTCTime' and 'UniversalTime'):
    --
    -- [@%D@] same as @%m\/%d\/%y@
    --
    -- [@%F@] same as @%Y-%m-%d@
    --
    -- [@%x@] as 'dateFmt' @locale@ (e.g. @%m\/%d\/%y@)
    --
    -- [@%Y@] year, no padding. Note @%0Y@ and @%_Y@ pad to four chars
    --
    -- [@%y@] year of century, 0-padded to two chars, @00@ - @99@
    --
    -- [@%C@] century, no padding. Note @%0C@ and @%_C@ pad to two chars
    --
    -- [@%B@] month name, long form ('fst' from 'months' @locale@), @January@ - @December@
    --
    -- [@%b@, @%h@] month name, short form ('snd' from 'months' @locale@), @Jan@ - @Dec@
    --
    -- [@%m@] month of year, 0-padded to two chars, @01@ - @12@
    --
    -- [@%d@] day of month, 0-padded to two chars, @01@ - @31@
    --
    -- [@%e@] day of month, space-padded to two chars,  @ 1@ - @31@
    --
    -- [@%j@] day of year, 0-padded to three chars, @001@ - @366@
    --
    -- [@%f@] century for Week Date format, no padding. Note @%0f@ and @%_f@ pad to two chars
    --
    -- [@%V@] week of year for Week Date format, 0-padded to two chars, @01@ - @53@
    --
    -- [@%U@] week of year where weeks start on Sunday (as 'sundayStartWeek'), 0-padded to two chars, @00@ - @53@
    --
    -- [@%W@] week of year where weeks start on Monday (as 'mondayStartWeek'), 0-padded to two chars, @00@ - @53@
    --
    -- == Duration types
    -- The specifiers for 'DiffTime', 'NominalDiffTime', 'CalendarDiffDays', and 'CalendarDiffTime' are semantically
    -- separate from the other types.
    -- Specifiers on negative time differences will generally be negative (think 'rem' rather than 'mod').
    --
    -- === 'NominalDiffTime' and 'DiffTime'
    -- Note that a "minute" of 'DiffTime' is simply 60 SI seconds, rather than a minute of civil time.
    -- Use 'NominalDiffTime' to work with civil time, ignoring any leap seconds.
    --
    -- For 'NominalDiffTime' and 'DiffTime':
    --
    -- [@%w@] total whole weeks
    --
    -- [@%d@] total whole days
    --
    -- [@%D@] whole days of week
    --
    -- [@%h@] total whole hours
    --
    -- [@%H@] whole hours of day
    --
    -- [@%m@] total whole minutes
    --
    -- [@%M@] whole minutes of hour
    --
    -- [@%s@] total whole seconds
    --
    -- [@%Es@] total seconds, with decimal point and up to \<width\> (default 12) decimal places, without trailing zeros.
    -- For a whole number of seconds, @%Es@ omits the decimal point unless padding is specified.
    --
    -- [@%0Es@] total seconds, with decimal point and \<width\> (default 12) decimal places.
    --
    -- [@%S@] whole seconds of minute
    --
    -- [@%ES@] seconds of minute, with decimal point and up to \<width\> (default 12) decimal places, without trailing zeros.
    -- For a whole number of seconds, @%ES@ omits the decimal point unless padding is specified.
    --
    -- [@%0ES@] seconds of minute as two digits, with decimal point and \<width\> (default 12) decimal places.
    --
    -- === 'CalendarDiffDays'
    -- For 'CalendarDiffDays' (and 'CalendarDiffTime'):
    --
    -- [@%y@] total years
    --
    -- [@%b@] total months
    --
    -- [@%B@] months of year
    --
    -- [@%w@] total weeks, not including months
    --
    -- [@%d@] total days, not including months
    --
    -- [@%D@] days of week
    --
    -- === 'CalendarDiffTime'
    -- For 'CalendarDiffTime':
    --
    -- [@%h@] total hours, not including months
    --
    -- [@%H@] hours of day
    --
    -- [@%m@] total minutes, not including months
    --
    -- [@%M@] minutes of hour
    --
    -- [@%s@] total whole seconds, not including months
    --
    -- [@%Es@] total seconds, not including months, with decimal point and up to \<width\> (default 12) decimal places, without trailing zeros.
    -- For a whole number of seconds, @%Es@ omits the decimal point unless padding is specified.
    --
    -- [@%0Es@] total seconds, not including months, with decimal point and \<width\> (default 12) decimal places.
    --
    -- [@%S@] whole seconds of minute
    --
    -- [@%ES@] seconds of minute, with decimal point and up to \<width\> (default 12) decimal places, without trailing zeros.
    -- For a whole number of seconds, @%ES@ omits the decimal point unless padding is specified.
    --
    -- [@%0ES@] seconds of minute as two digits, with decimal point and \<width\> (default 12) decimal places.
    formatTime :: (FormatTime t) => TimeLocale -> String -> t -> String
    formatTime _ [] _ = ""
    formatTime locale ('%':cs) t = case formatTime1 locale cs t of
        Just result -> result
        Nothing -> '%':(formatTime locale cs t)
    formatTime locale (c:cs) t = c:(formatTime locale cs t)

    formatTime1 :: (FormatTime t) => TimeLocale -> String -> t -> Maybe String
    formatTime1 locale ('_':cs) t = formatTime2 locale id (Just (Just ' ')) cs t
    formatTime1 locale ('-':cs) t = formatTime2 locale id (Just Nothing) cs t
    formatTime1 locale ('0':cs) t = formatTime2 locale id (Just (Just '0')) cs t
    formatTime1 locale ('^':cs) t = formatTime2 locale (fmap toUpper) Nothing cs t
    formatTime1 locale ('#':cs) t = formatTime2 locale (fmap toLower) Nothing cs t
    formatTime1 locale cs t = formatTime2 locale id Nothing cs t

    getDigit :: Char -> Maybe Int
    getDigit c | c < '0' = Nothing
    getDigit c | c > '9' = Nothing
    getDigit c = Just $ (ord c) - (ord '0')

    pullNumber :: Maybe Int -> String -> (Maybe Int,String)
    pullNumber mx [] = (mx,[])
    pullNumber mx s@(c:cs) = case getDigit c of
        Just i -> pullNumber (Just $ (fromMaybe 0 mx)*10+i) cs
        Nothing -> (mx,s)

    formatTime2 :: (FormatTime t) => TimeLocale -> (String -> String) -> Maybe FormatNumericPadding -> String -> t -> Maybe String
    formatTime2 locale recase mpad cs t = let
        (mwidth,rest) = pullNumber Nothing cs
        in formatTime3 locale recase mpad mwidth rest t

    formatTime3 :: (FormatTime t) => TimeLocale -> (String -> String) -> Maybe FormatNumericPadding -> Maybe Int -> String -> t -> Maybe String
    formatTime3 locale recase mpad mwidth ('E':cs) = formatTime4 True recase (MkFormatOptions locale mpad mwidth) cs
    formatTime3 locale recase mpad mwidth cs = formatTime4 False recase (MkFormatOptions locale mpad mwidth) cs

    formatTime4 :: (FormatTime t) => Bool -> (String -> String) -> FormatOptions -> String -> t -> Maybe String
    formatTime4 alt recase fo (c:cs) t = Just $ (recase (formatChar alt c fo t)) ++ (formatTime (foLocale fo) cs t)
    formatTime4 _alt _recase _fo [] _t = Nothing

    formatChar :: (FormatTime t) => Bool -> Char -> FormatOptions -> t -> String
    formatChar _ '%' = formatString $ \_ _ -> "%"
    formatChar _ 't' = formatString $ \_ _ -> "\t"
    formatChar _ 'n' = formatString $ \_ _ -> "\n"
    formatChar alt c = case formatCharacter alt c of
        Just f -> f
        _ -> \_ _ -> ""