examples/GenericSYB.hs · typeable/aeson

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1-- This example is basically the same as in Simplest.hs, only it uses

2-- SYB generics instead of explicit instances of ToJSON and FromJSON.

4-- This mechanism is much slower than the newer generics mechanism

5-- demonstrated in Generic.hs, but it works on versions of GHC older

6-- than 7.2.

8-- We enable the DeriveDataTypeable language extension so that GHC can

9-- automatically derive the Typeable and Data classes for us.

11{-# LANGUAGE DeriveDataTypeable #-}

13{-# LANGUAGE OverloadedStrings #-}

15import Data.Data (Typeable, Data)

16import Data.Aeson.Generic (decode, encode)

17import qualified Data.ByteString.Lazy.Char8 as BL

19-- To decode or encode a value using the generic machinery, we must

20-- make the type an instance of the Typeable and Data classes.

22data Coord = Coord { x :: Double, y :: Double }

23 deriving (Show, Typeable, Data)

25main :: IO ()

26main = do

27 let req = decode "{\"x\":3.0,\"y\":-1.0}" :: Maybe Coord

28 print req

29 let reply = Coord 123.4 20

30 BL.putStrLn (encode reply)