rating function for entries using algorithm described in http://www.evanmiller.org/how-not-to-sort-by-average-rating.html

rating.hs

rating :: Integer -> Integer -> Double
rating pos neg = rating' 1 2.24 (pos+neg) (fromIntegral pos)
-- 2.24 is quantile function for 0.9875, 97.5% certainity

-- {max}-star rating system
rating' :: Double -> Double -> Integer -> Double -> Double
rating' _ _ 0 _ = 0
rating' max z cnt total
 | max > 0 && z > 0 && cnt > 0 && total >= 0 =
     let cnt' = fromIntegral cnt
         t = 2 * (total/max) / (z^2)
         ph = (total/max) / cnt'
     in max * (t*ph+1-sqrt(2*t*ph*(1-ph)+1))/(t+2)