{-# LANGUAGE OverloadedStrings #-}

import Data.Aeson (FromJSON, eitherDecode)
import GHC.Generics (Generic)

data Bla = Bla
  { a :: Bool
  , b :: Bool
  }
  | Foo
  deriving (Show, Generic)
instance FromJSON Bla

main :: IO ()
main = do
  putStrLn $ show (eitherDecode "{\"tag\":\"Bla\",\"contents\":{\"a\":true,\"b\":false}}" :: Either String Bla)
  putStrLn $ show (eitherDecode "{\"tag\":\"Bla\",\"a\":true,\"b\":false}" :: Either String Bla)