This traverses the list three times (reverse, init and last are each linear time): fromListEnd xs = Zipper (reverse $ init xs) (last xs) [] But we only need to do it once: fromListEnd xs = let x:xs' = reverse xs in Zipper xs' x [] I don't *think* this has an effect on strictness/laziness, since both versions are strict in the spine of the list. --Max On Sat, Jan 17, 2009 at 10:33 AM, Jeff Wheeler <jeff@nokrev.com> wrote:

Hi,

(I also sent this to libraries@, but without response; I'm posting here for a wider audience.)

I'm somewhat of a beginner in Haskell, so take what I say with a grain of salt, please.

The ListZipper implementation seems very odd to me, and #haskell seemed to agree with me. The current package implements a Zipper with

...

data Zipper = Zipper ![a] ![a]

which allows for empty zippers, among other things. Very strange to me. It also seems unnecessarily strict. There are also no expected typeclasses implemented, like Foldable or Traversable.

I thought it would be interesting to try fixing these problems with a much cleaner design, and I'd like to share what I came up with:

http://hpaste.org/14030

I'd very much appreciate some criticism of the code so that I can improve it.

I'm not sure how best to provide this alternative on Hackage; should we make this merely a newer version of the old library, provide it in a conflicting package, or find a new namespace for it and provide it there?

Thanks,

Jeff Wheeler

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