A probabilistic cellular automaton

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{-# LANGUAGE DeriveFunctor #-}
{-# LANGUAGE DeriveFoldable #-}
import Prelude hiding (iterate,tail,repeat,sequence,take,zip,unzip)
import Data.Stream.Infinite 
    (Stream ((:>)),iterate,tail,repeat,take,zip,unzip,unfold)          
import Data.Foldable
import Data.Traversable (Traversable(..), sequence)
import Control.Applicative
import Control.Comonad
import Control.Comonad.Trans.Class (lower)
import Control.Comonad.Trans.Env (EnvT(..),ask)
import System.Random (StdGen,mkStdGen)
import Control.Monad.Random
import Control.Monad.Random.Class
-- Inspired by 
-- http://blog.sigfpe.com/2006/12/evaluating-cellular-automata-is.html
-- http://demonstrations.wolfram.com/SimpleProbabilisticCellularAutomata/
-- Streams are used instead of lists to ensure that the universe is infinite 
-- in both directions.
data U x = U (Stream x) x (Stream x) deriving (Functor,Foldable)
-- The default instance of Traversable generated by DeriveTraversable wasn't 
-- working well with MonadRandom, because U contains infinite structures. 
-- If I used the default "sequence" function to transform a U (Rand StdGen Bool) 
-- into a Rand StdGen (U Bool),   the subsequent call to evalRand would hang.  
-- I had to resort to this trick of traversing the left and right streams jointly 
-- as a "zip stream" and unzipping them afterwards.
-- Is there a standard, generally accepted way of dealing with these situations?
instance Traversable U where
    traverse f (U lstream focus rstream) = 
        let pairs =  liftA unzip 
                   . traverse (traversepair f) 
                   $ zip lstream rstream 
            traversepair f (a,b) = (,) <$> f a <*> f b
            rebuild c (u,v) = U u c v
        in rebuild <$> f focus <*> pairs
right (U a b (c:>cs)) = U (b:>a) c cs
left  (U (a:>as) b c) = U as a (b:>c)
instance Comonad U where
   extract (U _ b _) = b
   duplicate a = U (tail $ iterate left a) a (tail $ iterate right a)
type Probs = (Float,Float,Float,Float)
localRule :: EnvT Probs U Bool -> Rand StdGen Bool
localRule ca = 
    let (tt,tf,ft,ff) = ask ca
        black prob = (<prob) <$> getRandomR (0,1)
    in case lower ca of
        U (True:>_) _ (True:>_) -> black tt 
        U (True:>_) _ (False:>_) -> black tf 
        U (False:>_) _ (True:>_) -> black ft
        U (False:>_) _ (False:>_) -> black ff
-- Advances the cellular automata by one iteration, and returns a result
-- wrapped in the Rand monad, which can be later evaluated with evalRand.
-- Note the use of "sequence" to aggregate the random effects of each 
-- individual cell.
evolve :: EnvT Probs U Bool -> Rand StdGen (EnvT Probs U Bool)
evolve ca = sequence $ extend localRule ca
-- Returns an infinite stream with all the succesive states 
-- of the cellular automata.
history :: StdGen -> EnvT Probs U Bool -> Stream (EnvT Probs U Bool)
history seed initialca = 
    -- We need to split the generator because we are lazily evaluating
    -- an infinite random structure. The updated generator never "comes out"! 
    -- If we changed runRand for evalRand and tried to reuse the 
    -- updated generator for the next iteration, the call would hang. 
    let unfoldf (ca,seed) = 
            let (seed',seed'') = runRand getSplit seed 
                nextca = evalRand (evolve ca) seed'
            in  (nextca,(nextca,seed''))
    in unfold unfoldf (initialca,seed)
showca :: Int -> U Bool -> String
showca margin ca = 
    let char b = if b then '#' else '_'
        U left center right = fmap char ca
    in (reverse $ take margin left) ++ [center] ++ (take margin right)
main :: IO ()
main = do
    let probs = (0.0,0.6,0.7,0.0)
        initialca = EnvT probs $ U (repeat False) True (repeat False) 
        seed = 77
        iterations = 10
        margin = 8
    sequence . fmap (putStrLn . showca margin . lower) 
             . take iterations 
             . history (mkStdGen seed) 
             $ initialca
    return ()