# New monads/MonadRandom

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From NewMonads, 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) => 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 = fmap (evalRand x) getStdGen

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
```