[Haskell-cafe] Deriving something else?

John Ky newhoggy at gmail.com
Mon Dec 8 07:03:42 EST 2008


I've defined a class and some instances, which I'm hoping would help me
"show" values of types that may include transactional elements.

class TShow a where
   tshow :: a -> IO String

instance Show (TVar a) where
   show = "%"

instance (Show a) => TShow a where
   tshow a = return $ show a

instance (Show a) => TShow (TVar a) where
   tshow ta = do
      a <- readTVar ta
      return $ show a

Having created a new class is it possible to do some magic so that it can be
put it into a deriving clause?

data Type = Type
   { field1 :: Int
   , field2 :: Int
   deriving Show

data AnotherType = AnotherType
   { field3 :: Int
   , field4 :: TVar Type
   deriving *TShow*


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