I have recently heard that some people want to burn bridges (introducing Foldable and Traversable to Prelude) and I've been wondering if it was possible somehow allow Text and Bytestring like containers to make use of those functions. Something along the lines of import qualified Data.ByteString as BS newtype ByteString' a = ByteString' BS.ByteString type ByteString = ByteString' Word8 instance (ByteString' a ~ ByteString' Word8) => Functor (ByteString') where fmap f (ByteString' bs) = ByteString' $ BS.map f bs Or if DataContexts worked as you would expect. newtype (Word8 ~ a) => ByteString' a = ByteString' BS.ByteString However I couldn't find a solution and I was just wondering if it is possible. P.S. Using GADTS it does actually work for Foldable, but only because it doesn't have to output any ByteStrings. It doesn't work for Functor for instance. data ByteString' a where ByteString' :: BS.ByteString -> ByteString' Word8 type ByteString = ByteString' Word8 instance Foldable ByteString' where foldr f ini (ByteString' bs) = BS.foldr f ini bs Silvio