Re: Haskell Foldable Wats

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@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 mailto:david.feuer@gmail.com> escribió:

...

On Wed, Feb 24, 2016 at 11:22 AM, Kosyrev Serge <_deepfire@feelingofgreen.ru mailto:_deepfire@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) _______________________________________________ Libraries mailing list Libraries@haskell.org mailto:Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

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