fromList package:MonadRandom

fromList :: MonadRandom m => [(a, Rational)] -> m a
MonadRandom Control.Monad.Random.Class
Sample a random value from a weighted list. The list must be non-empty and the total weight must be non-zero.
fromListMay :: MonadRandom m => [(a, Rational)] -> m (Maybe a)
MonadRandom Control.Monad.Random.Class
Sample a random value from a weighted list. Return Nothing if the list is empty or the total weight is nonpositive.