Re: [Haskell-cafe] is it possible to implement Functor for ByteString and Text

28 Feb 2015

      On Sun, Mar 1, 2015 at 8:11 AM, silvio  wrote:
...
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
If tweak the definition of Functor a bit, we can get that to work:

{-# LANGUAGE ConstraintKinds, TypeFamilies #-}

import qualified Data.ByteString as B
import Data.Word (Word8)
import GHC.Prim (Constraint)

newtype ByteString' a = ByteString' B.ByteString
    deriving (Eq, Ord, Show)

class Functor' f where
    type FunctorConstraint f a :: Constraint
    fmap' :: (FunctorConstraint f a, FunctorConstraint f b) => (a ->
b) -> f a -> f b

instance Functor' ByteString' where
    type FunctorConstraint ByteString' a = a ~ Word8
    fmap' f (ByteString' x) = ByteString' $ B.map f x

But I don't think it's possible with the original type class.

-- 
https://lambda.xyz

Re: [Haskell-cafe] is it possible to implement Functor for ByteString and Text

Chris Wong