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