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

[Christopher Done]

Ah, I'm talking nonsense. Ignore me.

On 28 February 2015 at 20:56, Christopher Done <chrisdone at gmail.com> wrote:
> You could have
> class IsWord8 a
> instance IsWord8 Word8
> instance (IsWord8 a) => Functor (ByteString a) where ...
>
> It would be a legitimate instance.
>
> On 28 February 2015 at 20:41, David Feuer <david.feuer at gmail.com> wrote:
>> This is impossible. The type of fmap is
>>
>> fmap :: Functor f => (a->b) -> f a -> f b
>>
>> You can use a GADT to effectively restrict what a is, because the
>> caller won't be able to provide a non-bottom f a if a has the wrong
>> type. But the caller can choose absolutely any type for b, and there's
>> nothing you can do about that.
>>
>> David
>>
>> On Sat, Feb 28, 2015 at 2:11 PM, silvio <silvio.frischi at gmail.com> 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
>>>
>>>
>>> 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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