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 #-}

9{-# LANGUAGE OverloadedStrings #-}

11module Main (main) where

13import Prelude ()

14import Prelude.Compat

16import Data.Aeson (FromJSON, ToJSON, decode, encode)

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

18import GHC.Generics (Generic)

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

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

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

23 deriving (Show, Generic)

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

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

27-- Default versions will be supplied for us.

29instance FromJSON Coord

30instance ToJSON Coord

32main :: IO ()

33main = do

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

35 print req

36 let reply = Coord { x = 123.4, y = 20 }

37 BL.putStrLn (encode reply)