By some reason Data.Decimal from Decimal-0.1.0 misses instance for Fractional instance Integral i => Fractional (DecimalRaw i) where (Decimal _ 0) / (Decimal _ ym) | ym /= 0 = Decimal 0 0 (Decimal p0 m0) / (Decimal 0 d) = divRem (Decimal p0 0) m0 where divRem z 0 = z -- out of decimalPlaces divRem (Decimal p m) r | p == maxBound = Decimal p (m + dm) where -- round for last digit dm | r'*2 >= d = dm0+1 | otherwise = dm0 (dm0, r') = r `divMod` d divRem (Decimal p m) r | r < d = divRem (Decimal (p+1) (m*10)) (r*10) divRem (Decimal p m) r = divRem (Decimal p (m+dm)) r' where (dm, r') = r `divMod` d -- divide decimalPlaces (Decimal xp xm) / (Decimal yp ym) | yp <= xp = Decimal (xp-yp) xm / Decimal 0 ym | otherwise = Decimal 0 (xm * (10^(yp-xp))) / Decimal 0 ym fromRational r = fromIntegral (numerator r) / fromIntegral (denominator r) class Scalable a where scale :: Integral b => Ratio b -> a -> a ratio :: Integral b => a -> a -> Ratio b instance Integral a => Scalable (DecimalRaw a) where r `scale` x = (x * n) / d where n = fromIntegral (numerator r) d = fromIntegral (denominator r) (Decimal xp xm) `ratio` (Decimal yp ym) = (fromIntegral xm * (10^yp)) % (fromIntegral ym * (10^xp))