AesonTag.hs

{-# LANGUAGE DeriveGeneric #-}
module AesonTag where

import Data.Aeson

import GHC.Generics

data Foo
  = Bar Int
  | Quux { tag :: Bool }
  deriving (Generic, Show)

instance FromJSON Foo
instance ToJSON   Foo

test :: IO ()
test = do
  let x = Bar 1337
      y = Quux True

  print (encode x) -- "{\"tag\":\"Bar\",\"contents\":1337}"
  print (decode (encode x) :: Maybe Foo) -- Just (Bar 1337)
  print (encode y) -- "{\"tag\":true}"
  print (decode (encode y) :: Maybe Foo) -- Nothing