Haskell - Using Aeson for JSON (extended example from Aeson)

json.hs

-- JSON in Haskell using Aeson

{-# LANGUAGE OverloadedStrings #-}

import Control.Applicative ((<$>), (<*>), empty)
import Data.Aeson
import qualified Data.ByteString.Lazy.Char8 as BL

data Coord = Coord { 
    x :: Double, 
    y :: Double 
    } deriving (Show)

data Person = Person {
    firstname :: String,
    lastname :: String,
    email :: String,
    age :: Int
    } deriving (Show)

-- A ToJSON instance allows us to encode a value as JSON.

instance ToJSON Coord where
    toJSON (Coord xV yV) = object [ "x" .= xV, "y" .= yV ]

-- A FromJSON instance allows us to decode a value from JSON. This
-- should match the format used by the ToJSON instance.

instance ToJSON Person where
    toJSON (Person fn ln em age) = object [ "firstname" .= fn, "lastname" .= ln, "email" .= em, "age" .= age ]

instance FromJSON Coord where
  parseJSON (Object v) = Coord <$>
                         v .: "x" <*>
                         v .: "y"
  parseJSON _ = empty

main :: IO ()
main = do
  let req = decode "{\"x\":3.0,\"y\":-1.0}" :: Maybe Coord
  print req
  let reply = Coord 123.4 20
  BL.putStrLn (encode reply)
  let reply2 = Person "Johan" "Astborg" "johan@gecemmo.com" 29
  BL.putStrLn (encode reply2)