module Dice (diceHard, diceSoft) where -- A toy model for dice rolling from http://dl.acm.org/citation.cfm?id=2804317 -- Exact results can be obtained using Dist monad import Control.Applicative (liftA2) import Control.Monad.Bayes.Class ( MonadCond (score), MonadInfer, MonadSample (uniformD), condition, ) -- | A toss of a six-sided die. die :: MonadSample m => m Int die = uniformD [1 .. 6] -- | A sum of outcomes of n independent tosses of six-sided dice. dice :: MonadSample m => Int -> m Int dice 1 = die dice n = liftA2 (+) die (dice (n - 1)) -- | Toss of two dice where the output is greater than 4. diceHard :: MonadInfer m => m Int diceHard = do result <- dice 2 condition (result > 4) return result -- | Toss of two dice with an artificial soft constraint. diceSoft :: MonadInfer m => m Int diceSoft = do result <- dice 2 score (1 / fromIntegral result) return result