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.
On Feb 28, 2015 1:11 PM, "silvio"
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 _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe