For the following code as an example: {-# LANGUAGE TypeOperators #-} infixr 5 :. data List a = Null | a :. List a deriving (Eq, Ord, Show, Read) The Show instance and the Read instance are inaware of the fixity of (:.): *Main> 2 :. 3 :. Null 2 :. (3 :. Null) *Main> read "2 :. 3 :. Null" :: List Int *** Exception: Prelude.read: no parse *Main> read "2 :. (3 :. Null)" :: List Int 2 :. (3 :. Null) The derived instances should be: instance Show a => Show (List a) where showsPrec p Null = showParen (11 <= p) (showString "Null") showsPrec p (x :. xs) = showParen (5 <= p) (go p (x :. xs)) where go _ Null = showString "Null" go p (x :. xs) = showsPrec p x . showString " :. " . go p xs instance Read a => Read (List a) where readPrec = parens $ do Ident "Null" <- lexP return Null +++ (do x <- readPrec Symbol ":." <- lexP xs <- readPrec return (x :. xs) )