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