# New monads/MonadRandom

### From HaskellWiki

< New monads(Difference between revisions)

BrettGiles (Talk | contribs) m (Remove redundant link) |
m (evalRandomT was missing "RandomGen g" in the type signature) |

## Revision as of 20:55, 11 October 2006

From New monads, copied from old wiki.

# MonadRandom

A simple monad transformer to allow computations in the transformed monad to generate random values.

{-# OPTIONS_GHC -fglasgow-exts #-} module MonadRandom ( MonadRandom, getRandom, getRandomR, evalRandomT, evalRand, evalRandIO, fromList, Rand, RandomT -- but not the data constructors ) where import System.Random import Control.Monad.State import Control.Monad.Identity class (Monad m) => MonadRandom m where getRandom :: (Random a) => m a getRandomR :: (Random a) => (a,a) -> m a newtype (RandomGen g) => RandomT g m a = RandomT { unRT :: StateT g m a } deriving (Functor, Monad, MonadTrans, MonadIO) liftState :: (MonadState s m) => (s -> (a,s)) -> m a liftState t = do v <- get let (x, v') = t v put v' return x instance (Monad m, RandomGen g) => MonadRandom (RandomT g m) where getRandom = (RandomT . liftState) random getRandomR (x,y) = (RandomT . liftState) (randomR (x,y)) evalRandomT :: (Monad m, RandomGen g) => RandomT g m a -> g -> m a evalRandomT x g = evalStateT (unRT x) g -- Boring random monad :) newtype Rand g a = Rand { unRand :: RandomT g Identity a } deriving (Functor, Monad, MonadRandom) evalRand :: (RandomGen g) => Rand g a -> g -> a evalRand x g = runIdentity (evalRandomT (unRand x) g) evalRandIO :: Rand StdGen a -> IO a evalRandIO x = getStdRandom (runIdentity . runStateT (unRT (unRand x))) fromList :: (MonadRandom m) => [(a,Rational)] -> m a fromList [] = error "MonadRandom.fromList called with empty list" fromList [(x,_)] = return x fromList xs = do let s = fromRational $ sum (map snd xs) -- total weight cs = scanl1 (\(x,q) (y,s) -> (y, s+q)) xs -- cumulative weight p <- liftM toRational $ getRandomR (0.0,s) return $ fst $ head $ dropWhile (\(x,q) -> q < p) cs