On Sun, Nov 21, 2010 at 6:00 AM, Ross Paterson
On Sat, Nov 20, 2010 at 10:17:29PM -0500, David Menendez wrote:
The key code is,
instance Applicative f => Applicative (Backward f) where pure = Backward . pure Backward f <*> Backward a = Backward (a <**> f)
instance Traversable f => Traversable (Reverse f) where traverse f = fmap Reverse . runBackward . traverse (Backward . f) . getReverse
Yes, that's the same as Russell's code.
Aside from the distinction between Reverse and Backward, yes.
instance Alternative f => Alternative (Reverse f) where empty = Reverse empty Reverse x <|> Reverse y = Reverse (y <|> x)
Don't you need an Applicative instance for that?
Yes. For Reverse, the Functor, Applicative, and Monad instances just
lift the instances of the underlying type. (In fact, I'm not sure why
I wrote them out instead of using newtype deriving.)
--
Dave Menendez