On Fri, Jan 21, 2011 at 06:33:10PM -0500, Edward Kmett wrote:
I'd like to propose adding the following missing instances to Data.Foldable and Data.Traversable respectively, since there isn't a canonical location that they could be added outside of those packages without orphans, their definition is unambiguous, and they are quite useful (plus, I happen to need them for a monad transformer in my adjunctions package).
instance Foldable (Either a) where foldMap f (Left a) = mempty foldMap f (Right b) = f b
instance Foldable ((,)e) where foldMap f ea = f (snd ea)
instance Traversable (Either a) where traverse f (Left a) = pure (Left a) traverse f (Right a) = Right <$> f a
instance Traversable ((,)e) where traverse f ~(e,a) = (,) e <$> f a
+1 -Brent