Re: proposal #2461: add Traversable generalizations of mapAccumL?and mapAccumR

On Fri, Jul 25, 2008 at 10:12 AM, Michael Karcher wrote:

David Menendez wrote:

...

Incidentally, is there a Backward applicative functor transfomer defined anywhere?

newtype Backward f a = Backward { runBackward :: f a } deriving Functor

instance Applicative f => Applicative (Backward f) where pure = Backward . pure (Backward f) <*> (Backward a) = Backward (f <**> a)

My intuitive typechecker doesn't accept that. And GHCi 6.8.2 seems to agree (I had to enable GeneralizedNewtypeDeriving for obvious reasons). [..] Did I misunderstand something?

That's a typo on my part. I should have written "Backward (a <**> f)". -- Dave Menendez http://www.eyrie.org/~zednenem/