custom_percentage.hs

data PercentageBase100 = PercentageBase100 Double deriving (Eq,Show)

instance FromField PercentageBase100 where
   parseField p = pure $ (f . read . BS8.unpack) p
     where f = checkPercentageBounds (0.0, 100.0) PercentageBase100

checkPercentageBounds :: (Double,Double) -> 
                         (Double -> a) -> Scientific -> a
checkPercentageBounds (low, high) f s
  | ((d < low) || (d > high)) = error $ "That's an invalid percentage, expected " ++ (show d) ++ " to be in the range " ++ (show (low,high))
  | otherwise          = f $ d
  where d = toRealFloat s