examples/Generic.hs · typeable/aeson

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

2-- GHC's builtin generics instead of explicit instances of ToJSON and

3-- FromJSON.

5-- We enable the DeriveGeneric language extension so that GHC can

6-- automatically derive the Generic class for us.

8{-# LANGUAGE DeriveGeneric #-}

10{-# LANGUAGE OverloadedStrings #-}

12import Data.Aeson (FromJSON, ToJSON, decode, encode)

13import qualified Data.ByteString.Lazy.Char8 as BL

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

16-- make the type an instance of the Generic class.

18import GHC.Generics (Generic)

20data Coord = Coord { x :: Double, y :: Double }

21 deriving (Show, Generic)

23-- While we still have to declare our type as instances of FromJSON

24-- and ToJSON, we do *not* need to provide bodies for the instances.

25-- Default versions will be supplied for us.

27instance FromJSON Coord

28instance ToJSON Coord

30main :: IO ()

31main = do

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

33 print req

34 let reply = Coord 123.4 20

35 BL.putStrLn (encode reply)