Hello, I recently added default generic implementations of toJSON and parseJSON to the aeson package. Now I'm optimizing them. Here are some benchmark results that compare: * th: toJSON and fromJSON generated by template-haskell. Can be compared to hand-written code. Should be the fastest of all. * syb: toJSON and fromJSON from the Data.Aeson.Generic module. Uses the Data type class. * generic: my toJSON and fromJSON using GHC Generics. The benchmark itself can be found here: https://github.com/basvandijk/aeson/blob/optimizations/benchmarks/AesonCompa... toJSON ====== D/toJSON/th 3.631734 us D/toJSON/syb 32.66679 us D/toJSON/generic 3.371868 us BigRecord/toJSON/th 8.982990 us BigRecord/toJSON/syb 48.90737 us BigRecord/toJSON/generic 8.971597 us BigProduct/toJSON/th 1.578259 us BigProduct/toJSON/syb 29.21153 us BigProduct/toJSON/generic 1.623115 us BigSum/toJSON/th 51.81214 ns BigSum/toJSON/syb 1.256708 us BigSum/toJSON/generic 71.32851 ns fromJSON ======== D/fromJSON/th 7.017204 us D/fromJSON/syb 23.46567 us D/fromJSON/generic 7.968974 us BigRecord/fromJSON/th 8.513789 us BigRecord/fromJSON/syb 36.64501 us BigRecord/fromJSON/generic 10.07809 us BigProduct/fromJSON/th 2.430677 us BigProduct/fromJSON/syb 17.97764 us BigProduct/fromJSON/generic 2.201130 us BigSum/fromJSON/th 414.8699 ns BigSum/fromJSON/syb 4.113170 us BigSum/fromJSON/generic 13.62614 us !!! As can be seen, in most cases the GHC Generics implementation is much faster than SYB and just as fast as TH. I'm impressed by how well GHC optimizes the code! Unfortunately the last benchmark, generically parsing a big sum type, is much slower. The code for parsing sums, which can be found here: https://github.com/basvandijk/aeson/blob/optimizations/Data/Aeson/Types/Inte... is basically this: instance (GFromSum a, GFromSum b) => GFromJSON (a :+: b) where gParseJSON (Object (M.toList -> [keyVal])) = gParseSum keyVal gParseJSON v = typeMismatch "sum (:+:)" v {-# INLINE gParseJSON #-} class GFromSum f where gParseSum :: Pair -> Parser (f a) instance (GFromSum a, GFromSum b) => GFromSum (a :+: b) where gParseSum keyVal = (L1 <$> gParseSum keyVal) <|> (R1 <$> gParseSum keyVal) {-# INLINE gParseSum #-} instance (Constructor c, GFromJSON a, ConsFromJSON a) => GFromSum (C1 c a) where gParseSum (key, value) | key == pack (conName (undefined :: t c a p)) = gParseJSON value | otherwise = notFound $ unpack key {-# INLINE gParseSum #-} notFound :: String -> Parser a notFound key = fail $ "The key \"" ++ key ++ "\" was not found" {-# INLINE notFound #-} Any idea how to make it faster? Regards, Bas