Haskell Quiz/Fuzzy Time/Solution Dolio

This doesn't do, strictly, what the ruby quiz specification demands (yet, at least). It makes time and fuzzy time datatypes, and a fuzzy clock monad in which you can tick off minutes, and it will report a fuzzy time at each tick. I didn't feel like messing with the system timer like the quiz suggests. However, by building around IO (and getting rid of WriterT for the reporting), one could probably get that effect if desired.

This solution makes use of the random monad.

module Main where
import Control.Arrow
import Control.Monad.Writer
import Control.Monad.State
import System
import System.Random
import MonadRandom

newtype Time = T (Int, Int) deriving (Eq)
newtype Fuzzy = F (Int, Int) deriving (Eq)

type FuzzyClock a = WriterT String (StateT (Time, Fuzzy) (Rand StdGen)) a

instance Show Time where
    show (T (h, m))
        | m < 10 = show h ++ ":0" ++ show m
        | otherwise = show h ++ ":" ++ show m

instance Read Time where
    readsPrec n s = do (h, c:s') <- readsPrec n s
                       guard (c == ':')
                       (m, s'') <- readsPrec n s'
                       return (time h m, s'')

instance Show Fuzzy where
    show (F (h, m)) = show h ++ ":" ++ show m ++ "-"

-- A safe constructor for time values
time :: Int -> Int -> Time
time hour min
    | hour < 1 || hour > 12 = error "Invalid hour."
    | min < 0  || min > 59  = error "Invalid minute."
    | otherwise             = T (hour, min)

-- Modifies a time value by adding n minutes (negative n tick backwards)
tickT :: Int -> Time -> Time
tickT n (T (hour, min)) = time h m
 where (d, m) = divMod (min + n) 60
       h = (hour + d - 1) `mod` 12 + 1

-- Modifies a fuzzy time value by adding 10n minutes (negative ...)
tickF :: Int -> Fuzzy -> Fuzzy
tickF n (F (hour, min)) = F (h, m)
 where (d, m) = divMod (min + n) 6
       h = (hour + d - 1) `mod` 12 + 1

-- Constructs a fuzzy time value from a regular time value
toFuzzy :: Time -> Fuzzy
toFuzzy (T (h, m)) = F (h, m `div` 10)

-- Given a time t, computes a random fuzzy time within a 10-minute range of t
fuzz :: MonadRandom m => Time -> m Fuzzy
fuzz t = do off <- getRandomR (-5, 5)
            return . toFuzzy $ tickT off t

-- Ticks off a minute on the fuzzy clock, reporting the current fuzzy time
tick :: FuzzyClock ()
tick = do (t, f) <- fmap (first $ tickT 1) get
          g <- fuzz t
          let h = if g == tickF (-1) f then f else g
          tell $ show t ++ "\t" ++ show h ++ "\n"
          put (t, h)

main = do [time, mins] <- getArgs
          let t = read time
              m = read mins
          f <- evalRandIO $ fuzz t
          l <- evalRandIO . flip evalStateT (t, f) 
                          . execWriterT $ replicateM_ m tick
          putStr l