CODEWITHSUNDEEP
genericshaskell
bennofs
bennofs

Reputation: 11963

GHC.Generics or Data.Data?

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?

Upvotes: 35

Views: 3149

Answers (1)

Nikita Volkov
Nikita Volkov

Reputation: 43309

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

Upvotes: 27

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