Heterogeneous list in Haskell

HList.hs

{-# LANGUAGE GADTs #-}
module HLists where
    data Hlist where
       HNil :: Hlist
       HCons :: a-> Hlist -> Hlist
       HConsShw :: Show a => a-> Hlist -> Hlist -- show here is horrible

    instance Show Hlist
        where show HNil =  ""
              show (HCons a l) =  "?" ++ ","++ show l
              show (HConsShw a l) =  show a ++","++ show l

    main = do
        putStrLn $ show $ HConsShw "123" (HConsShw 1 ( HConsShw 'a' HNil))
        --"123",1,'a',
        putStrLn $ show $ HConsShw "123" (HCons 1 ( HConsShw 'a' HNil))
        --"123",?,'a',