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

David Menendez wrote:

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

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David Menendez wrote:

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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)".

so you mean: Backward f <*> Backward a = Backward (a <**> f) ? According to the haddock of Control.Applicative, this line is semantically equivalent to Backward f <*> Backward a = Backward (f <*> a) So I don't see the point of this "transformer". It seems to do nothing, just as newtype Backward f a = Backward { runBackward :: f a } deriving (Functor, Applicative) would also do. Regards, Michael Karcher