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

[Michael Karcher]

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).

/tmp/foo.hs:7:0:
    Occurs check: cannot construct the infinite type: a = (a -> b) -> b
    When trying to generalise the type inferred for `<*>'
      Signature type:     forall (f :: * -> *).
                          (Applicative f) =>
                          forall a1 b1.
                          Backward f (a1 -> b1) -> Backward f a1 -> Backward f b1
      Type to generalise: forall a1 b1.
                          Backward f (a1 -> b1) -> Backward f a1 -> Backward f b1
    In the instance declaration for `Applicative (Backward f)'

Did I misunderstand something?

Regards,
  Michael Karcher