Deriving show when using ExistentialQuantification extension?

Question

Why can't I derive Show here?

{-# LANGUAGE ExistentialQuantification #-}
data Obj = forall a. (Show a) => Item_Obj {get :: a, rest :: Obj} | No_Obj deriving Show

xs :: Obj
xs = Item_Obj 1 $ Item_Obj "foo" $ Item_Obj 'c' $ No_Obj

main :: IO ()
main = putStrLn . show $ xs

Answer 1

Such kind of context in not allowed in haskell-98 datatypes. Read this

Ofcourse you can write standalone instance by using StandaloneDeriving extension and let ghc do rest of the hardwork.

deriving instance Show Obj

Answer 2

Basically because GHC's head explodes when you try that. In other words, it simply hasn't been taught how to derive instances for existential types. Wait for a few version numbers to pass and then try again.