On Mon, 11 Jan 2016, Ryan Scott wrote:
What are the use cases where eq1 was not powerful enough and liftEq is needed?
The previous type signature of Data.Functor.Classes in transformers-0.4 resulted in some awkward instances, a prime example being found in Data.Functor.Compose:
import Data.Functor.Classes(Eq1(..))
newtype Compose f g a = Compose (f (g a)) instance (Functor f, Eq1 f, Eq1 g, Eq a) => Eq (Compose f g a) where Compose x == Compose y = eq1 (fmap Apply x) (fmap Apply y) instance (Functor f, Eq1 f, Eq1 g) => Eq1 (Compose f g) where eq1 = (==)
newtype Apply g a = Apply (g a) instance (Eq1 g, Eq a) => Eq (Apply g a) where Apply x == Apply y = eq1 x y
Btw. I already defined my own Apply type (I called it Wrap) because I found it useful. Would you mind to export it from 'transformers'?