What I would do is hold the function to apply in the wrapper type.
import qualified Data.ByteString as BS
data ByteString' a = ByteString' (Word8 -> a) BS.ByteString
wrap :: BS.ByteString -> ByteString' Word8
wrap bs = ByteString' id bs
-- The type ensures you can only unwrap with a function Word8 -> Word8.
unwrap :: ByteString' Word8 -> ByteString
unwrap (ByteString' f bs) = BS.map f bs
-- Functor instance just concatenates the fmapped function.
instance Functor ByteString' where
fmap f (ByteString' g bs) = ByteString' (f . g) bs
-- Foldable instance just uses the fmapped function.
instance Foldable ByteString' where
foldr f z (ByteString' g bs) = BS.foldr (f . g) z bs
-- You could define foldr', foldl, etc. based on the ones in Data.ByteString.
-- Not strictly necessary, but nice to have.
As an added benefit, this doesn't require GADTs. It probably would if you wanted to implement Monad as well, though.
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
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