Skip to content
This repository
Bryan O'Sullivan
file 39 lines (27 sloc) 1.191 kb
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38
-- This example is basically the same as in Simplest.hs, only it uses
-- GHC's builtin generics instead of explicit instances of ToJSON and
-- FromJSON.

-- This example only works with GHC 7.2 or newer, as it uses the
-- datatype-generic programming machinery introduced in 7.2.

-- We enable the DeriveGeneric language extension so that GHC can
-- automatically derive the Generic class for us.

{-# LANGUAGE DeriveGeneric #-}

{-# LANGUAGE OverloadedStrings #-}

import Data.Aeson (FromJSON, ToJSON, decode, encode)
import qualified Data.ByteString.Lazy.Char8 as BL

-- To decode or encode a value using the generic machinery, we must
-- make the type an instance of the Generic class.

import GHC.Generics (Generic)

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

-- While we still have to declare our type as instances of FromJSON
-- and ToJSON, we do *not* need to provide bodies for the instances.
-- Default versions will be supplied for us.

instance FromJSON Coord
instance ToJSON Coord

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)
Something went wrong with that request. Please try again.