Arbitrary-like API for randomness

Random.hs

import Control.Monad
import System.Random

class Roll k r where
  roll :: k -> IO r

instance Roll Int Int where
  roll = pure

instance (Roll k r, Random a, Integral a) => Roll (a -> k) r where
  roll f = do
    x <- randomIO
    roll $ f $ (x `mod` 6) + 1

main :: IO ()
main = do
  rolls <- replicateM 10000 $ roll $ \ x y -> x + y :: Int
  let
    mean = fromIntegral (sum (rolls :: [Int]))
         / fromIntegral (length rolls)
  print (mean :: Double)