Haskell Foldable Wats

[Chris Allen]

You can't not-include the instances because we'll just end up with 
orphans so that's not cricket I think.

On 02/24/2016 11:17 AM, amindfv at gmail.com wrote:
> It seems like very few people are changing their minds, after a lot of 
> discussion -- maybe it's time to agree to disagree, in the form of a 
> vote on whether to include the tuple instances.
>
>      If so, I'd think we'd want to come to the users with a discussion 
> of both sides and a poll, a la FTP.
>
> Tom
>
>
> El 24 feb 2016, a las 11:38, David Feuer <david.feuer at gmail.com 
> <mailto:david.feuer at gmail.com>> escribió:
>
>>
>>
>> On Wed, Feb 24, 2016 at 11:22 AM, Kosyrev Serge 
>> <_deepfire at feelingofgreen.ru <mailto:_deepfire at feelingofgreen.ru>> wrote:
>>
>>
>>     Are you also saying that this cannot be resolved by some kind of a
>>     type families-based type-level 'flip'?
>>
>>
>> It really can't, no. You can define
>>
>> type family FlipF f a b where
>>   FlipF f a b = f b a
>>
>> but FlipF, being a type family, is not first-class. You can't make 
>> any instances whatsoever for FlipF f a -- you'll probably get an 
>> error about a partially applied type family. What you *can* do is 
>> make a Flip newtype:
>>
>> newtype Flip f a b = Flip {unflip :: f b a}
>>
>> This behaves perfectly:
>>
>> instance Bifunctor f => Functor (Flip f a) where
>>   fmap f = Flip . first f . unflip
>> instance Profunctor f => Contravariant (Flip f a) where
>>   contramap f = Flip . lmap f . unflip
>> instance Bifunctor f => Bifunctor (Flip f) where
>>   bimap f g (Flip x) = Flip (bimap g f x)
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>
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