Haskell Foldable Wats

amindfv at gmail.com amindfv at gmail.com
Wed Feb 24 17:17:16 UTC 2016

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.


> El 24 feb 2016, a las 11:38, David Feuer <david.feuer at gmail.com> escribió:
>> On Wed, Feb 24, 2016 at 11:22 AM, Kosyrev Serge <_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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