{-# LANGUAGE OverloadedStrings #-}

import Data.Aeson as Aeson
import GHC.Generics

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

bs = "{\"tag\":\"Bla\",\"contents\":{\"a\":true,\"b\":false}}"

main :: IO ()
main =
  let message = eitherDecode bs :: Either String Bla
  in  putStrLn $ show message