+{-# LANGUAGE FlexibleContexts #-}
+module Data.Time.HTTP.Common
+ ( shortWeekDayName
+ , shortWeekDayNameP
+
+ , longWeekDayName
+ , longWeekDayNameP
+
+ , shortMonthName
+ , shortMonthNameP
+
+ , show2
+ , show4
+
+ , read2
+ , read4
+ )
+ where
+
+import Control.Monad
+import Text.Parsec
+
+shortWeekDayName :: Int -> String
+shortWeekDayName 1 = "Mon"
+shortWeekDayName 2 = "Tue"
+shortWeekDayName 3 = "Wed"
+shortWeekDayName 4 = "Thu"
+shortWeekDayName 5 = "Fri"
+shortWeekDayName 6 = "Sat"
+shortWeekDayName 7 = "Sun"
+shortWeekDayName n = error ("shortWeekDayName: unknown day number: " ++ show n)
+
+shortWeekDayNameP :: Stream s m Char => ParsecT s u m Int
+shortWeekDayNameP
+ = choice [ string "Mon" >> return 1
+ , char 'T'
+ >> choice [ string "ue" >> return 2
+ , string "hu" >> return 4
+ ]
+ , string "Wed" >> return 3
+ , string "Fri" >> return 5
+ , char 'S'
+ >> choice [ string "at" >> return 6
+ , string "un" >> return 7
+ ]
+ ]
+
+longWeekDayName :: Int -> String
+longWeekDayName 1 = "Monday"
+longWeekDayName 2 = "Tuesday"
+longWeekDayName 3 = "Wednesday"
+longWeekDayName 4 = "Thursday"
+longWeekDayName 5 = "Friday"
+longWeekDayName 6 = "Saturday"
+longWeekDayName 7 = "Sunday"
+
+longWeekDayNameP :: Stream s m Char => ParsecT s u m Int
+longWeekDayNameP
+ = choice [ string "Monday" >> return 1
+ , char 'T'
+ >> choice [ string "uesday" >> return 2
+ , string "hursday" >> return 4
+ ]
+ , string "Wednesday" >> return 3
+ , string "Friday" >> return 5
+ , char 'S'
+ >> choice [ string "aturday" >> return 6
+ , string "unday" >> return 7
+ ]
+ ]
+
+shortMonthName :: Int -> String
+shortMonthName 1 = "Jan"
+shortMonthName 2 = "Feb"
+shortMonthName 3 = "Mar"
+shortMonthName 4 = "Apr"
+shortMonthName 5 = "May"
+shortMonthName 6 = "Jun"
+shortMonthName 7 = "Jul"
+shortMonthName 8 = "Aug"
+shortMonthName 9 = "Sep"
+shortMonthName 10 = "Oct"
+shortMonthName 11 = "Nov"
+shortMonthName 12 = "Dec"
+shortMonthName n = error ("shortMonthName: unknown month number: " ++ show n)
+
+shortMonthNameP :: Stream s m Char => ParsecT s u m Int
+shortMonthNameP
+ = choice [ char 'J'
+ >> choice [ string "an" >> return 1
+ , char 'u'
+ >> choice [ char 'n' >> return 6
+ , char 'l' >> return 7
+ ]
+ ]
+ , string "Feb" >> return 2
+ , string "Ma"
+ >> choice [ char 'r' >> return 3
+ , char 'y' >> return 5
+ ]
+ , char 'A'
+ >> choice [ string "pr" >> return 4
+ , string "ug" >> return 8
+ ]
+ , string "Sep" >> return 9
+ , string "Oct" >> return 10
+ , string "Nov" >> return 11
+ , string "Dec" >> return 12
+ ]
+
+show4 :: Integral i => i -> String
+show4 i
+ | i >= 0 && i < 10 = "000" ++ show i
+ | i >= 0 && i < 100 = "00" ++ show i
+ | i >= 0 && i < 1000 = "0" ++ show i
+ | i >= 0 && i < 10000 = show i
+ | otherwise = error ("show4: the integer i must satisfy 0 <= i < 10000: " ++ show i)
+
+show2 :: Integral i => i -> String
+show2 i
+ | i >= 0 && i < 10 = "0" ++ show i
+ | i >= 0 && i < 100 = show i
+ | otherwise = error ("show2: the integer i must satisfy 0 <= i < 100: " ++ show i)
+
+read4 :: (Stream s m Char, Num n) => ParsecT s u m n
+read4 = do n1 <- digit'
+ n2 <- digit'
+ n3 <- digit'
+ n4 <- digit'
+ return (n1 * 1000 + n2 * 100 + n3 * 10 + n4)
+
+read2 :: (Stream s m Char, Num n) => ParsecT s u m n
+read2 = do n1 <- digit'
+ n2 <- digit'
+ return (n1 * 10 + n2)
+
+digit' :: (Stream s m Char, Num n) => ParsecT s u m n
+digit' = liftM fromC digit
+
+fromC :: Num n => Char -> n
+fromC '0' = 0
+fromC '1' = 1
+fromC '2' = 2
+fromC '3' = 3
+fromC '4' = 4
+fromC '5' = 5
+fromC '6' = 6
+fromC '7' = 7
+fromC '8' = 8
+fromC '9' = 9
+fromC _ = undefined