gistfile1.txt

data Session = Session { date :: Date, nightDay :: NightDay } deriving (Bounded, Eq, Generic, Ord)

instance Show Session where
  show (Session date Night) = show date `snoc` 'n'
  show (Session date Day) = show date

sessionParser :: (Stream a, Token a ~ Char) => Parsec () a Session
sessionParser = do
  date <- dateParser
  night <- optional (char 'n')
  pure $ Session date (bool Day Night $ isJust night)

instance Read Session where
  readPrec = do
    x <- RP.look
    case MP.runParser' sessionParser $ MP.State x (singleton $ MP.initialPos "") 0 MP.defaultTabWidth of
      (_, Left _) -> fail "Bad session"
      (MP.State _ _ processed _, Right s) -> do replicateM_ processed RP.get
                                                pure s