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

[David Menendez]

On Fri, Jul 25, 2008 at 10:12 AM, Michael Karcher
<usenet at mkarcher.dialup.fu-berlin.de> wrote:
> David Menendez <dave at zednenem.com> 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 <dave at zednenem.com>
<http://www.eyrie.org/~zednenem/>