On Wed, 2013-09-11 at 15:56 +0200, Nicolas Trangez wrote:
On Wed, 2013-09-11 at 14:35 +0100, Duncan Coutts wrote:
For mapM etc, personally I think a better solution would be if ByteString and Text and other specialised containers could be an instance of Foldable/Traversable.
Yes!
Those classes define mapM etc but currently they only work for containers that are polymorphic in their elements, so all specialised containers are excluded.
Indeed :-( Hence my question a couple of days ago.
I'm sure there must be a solution to that (I'd guess with type families) and that would be much nicer than adding mapM etc to bytestring itself. We would then just provide efficient instances for Foldable/Traversable.
I looked into this when this thread started, but bumped into at least one 'issue': when defining (OTOH) something like
import qualified Data.ByteString as BS import qualified Data.ByteString.Char8 as BS8
class Foldable t where type Elem c :: * foldr :: (Elem t -> b -> b) -> b -> t -> b
instance Foldable BS.ByteString where type Elem BS.ByteString = Word8 foldr = BS.foldr
instance Foldable BS8.ByteString where type Elem BS8.ByteString = Char foldr = BS8.foldr
which fails because BS.ByteString and BS8.ByteString are the same.
Right. I fear there's very little one could do about that. We do have one single type that is a specialised container for two types. That's fairly unusual. I'd probably just use Word8, especially since Char stuff is a bit iffy since it's really "Char8" not a full Char. Duncan