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module T8603 where
import Control.Monad
import Data.Functor
import Control.Monad.Trans.Class( lift )
import Control.Monad.Trans.State( StateT )
newtype RV a = RV { getPDF :: [(Rational,a)] } deriving (Show, Eq)
instance Functor RV where
fmap f = RV . map (\(x,y) -> (x, f y)) . getPDF
instance Applicative RV where
pure = return
(<*>) = ap
instance Monad RV where
return x = RV [(1,x)]
rv >>= f = RV $
do (p,a) <- getPDF rv
guard (p > 0)
(q,b) <- getPDF $ f a
guard (q > 0)
return (p*q, b)
type RVState s a = StateT s RV a
uniform :: [a] -> RV a
uniform x = RV [(1/fromIntegral (length x), y) | y <- x]
testRVState1 :: RVState s Bool
testRVState1
= do prize <- lift uniform [1,2,3]
return False
-- lift :: (MonadTrans t, Monad m) => m a -> t m a
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