GHC.Generics or Data.Data?

Question

There are currently 2 (3 if you count TemplateHaskell) options for generic programming using GHC, Data.Data / Data.Typeable and GHC.Generics, both available from the base package. So, what are the advantages and disadvantages of each? Is GHC.Generics the "modern" way and Data.Data obsolete and just kept for backwards compatibility?

Answer 1

GHC.Generics is the modern way and it is much faster than SYB. It however exposes a different approach to generic programming to the end user, so I don't think that it should be thought of as a direct replacement of SYB, though it does solve the same problems.

A good example of how those approaches differ from user's perspective can be extracted from the aeson library's functionality of serialization of a record to JSON:

Without generics

{-# LANGUAGE OverloadedStrings #-} import Data.Aeson  data Coord = Coord { x :: Double, y :: Double }  instance ToJSON Coord where    toJSON (Coord x y) = object ["x" .= x, "y" .= y] 

And use toJSON of ToJSON typeclass afterwards.

Using GHC.Generics

{-# LANGUAGE DeriveGeneric #-} import Data.Aeson     import GHC.Generics  data Coord = Coord { x :: Double, y :: Double } deriving Generic  instance ToJSON Coord 

And use the same toJSON of ToJSON typeclass afterwards.

Using SYB

{-# LANGUAGE DeriveDataTypeable #-} import Data.Data import Data.Aeson.Generic  data Coord = Coord { x :: Double, y :: Double } deriving (Data, Typeable) 

And use a specific toJSON from Data.Aeson.Generic with the following signature:

toJSON :: Data a => a -> Value