Haskell Foldable Wats

Kosyrev Serge _deepfire at feelingofgreen.ru
Wed Feb 24 21:56:29 UTC 2016

David Feuer <david.feuer at gmail.com> writes:
> 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}

How wrong would it be to say that the conceptually arbitrary choice of
the Functor instance is "saved" by a technical limitation, making it the
only choice?

How likely is a proper type-level 'flip' tomorrow -- for some value of
"proper" and "tomorrow"?

с уважениeм / respectfully,
Косырев Сергей

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