Haskell sorcery with Generics and Typeable for showing the structure of any value

generic-show.hs

import GHC.Generics
import Data.Typeable

deriving instance Show (f p) => Show (M1 i c f p)
deriving instance (Show (f p), Show (g p)) => Show ((f :*:g) p)
deriving instance (Show (f p), Show (g p)) => Show ((f :+:g) p)
deriving instance (Show c) => Show (K1 i c p)
deriving instance Show (V1 p)
deriving instance Show (U1 p)
deriving instance Show D
deriving instance Show C
deriving instance Show S

class ShowIO a where
  showIO :: a -> IO String

instance forall a . (Generic a, Show a, Show (Rep a ())) => ShowIO (IORef a) where
  showIO ref = readIORef ref >>= \x -> return $ "IORef " ++ show (from x :: Rep a ())

-- Alternatively, don't descend into IORefs, then we can do it purely.
-- With this, `show (x :: IORef Mytype)` just gives `"IORef Mytype".
instance forall a . (Typeable a) => Show (IORef a) where
  show x = "IORef " ++ show (typeOf x)