[Haskell] An Alternative Data.List.Zipper

David Menendez dave at zednenem.com
Sat Jan 17 20:11:32 EST 2009

On Sat, Jan 17, 2009 at 7:49 PM, Jeff Wheeler <jeff at nokrev.com> wrote:
> On Sat, 2009-01-17 at 17:41 -0500, David Menendez wrote:
>> That's correct, but I think you'd be better off defining OpApplicative
>> (or Backward, as I call it) locally and avoiding the two reverses.
> I'll have to look into this more; I don't really understand applicatives
> right now, so I can't use them yet. :)

newtype Backwards f a = B { unB :: f a }

instance Functor f => Functor (Backwards f) where
    fmap f = B . fmap f . unB

instance Applicative f => Applicative (Backwards f) where
    pure = B . pure
    f <*> x = B (unB f <**> unB x)

traverseBackwards :: (Traversable t, Applicative f) = (a -> f b) -> t
a -> t (f b)
traverseBackwards f = unB . traverse (B . f)

>> If you look at a zipper as a list with a selected element, then it
>> doesn't make sense to talk about a zipper of an empty list.
> Apparently a zipper can be empty, as the focus is the rest of the list,
> not the current element. It seems that my file is not a Zipper, but
> rather a PointedList (thanks to roconner in #haskell).

This may be terminological confusion. I would have said that a
PointedList *is* a zipper.

Certainly, from the standpoint that zippers are comonads, you can't
have an empty zipper, because you always need to be able to retrieve
the selected value.

>> That being said, I'd prefer fromList to have the type [a] -> Maybe
>> (Zipper a), and similarly with next and previous. If people want to
>> live dangerously, they can use fromJust.
> I agree, and I've made this change. I found this annoying on
> next/previous though, so I've created a tryNext/tryPrevious that'll
> return an unchanged PointedList if it's already on the end.

I would define tryNext in terms of next, rather than the other way
around, but I guess it shouldn't make much difference. I'm also not
sure that returning the original pointed list is a good idea, since
trying to move beyond the boundaries of the zipper will usually be an

Dave Menendez <dave at zednenem.com>

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