httpDateTime :: Parser ClockTime
-httpDateTime = do foldl (<|>) (fail "") (map string week)
+httpDateTime = do foldl (<|>) failP (map string week)
char ','
char ' '
day <- liftM read (count 2 digit)
char ' '
- mon <- foldl (<|>) (fail "") (map tryEqToFst (zip month [1..]))
+ mon <- foldl (<|>) failP (map tryEqToFst (zip month [1..]))
char ' '
year <- liftM read (count 4 digit)
char ' '