On Mon, Mar 30, 2009 at 9:46 PM, Gü?nther Schmidt
Thanks Don,
I followed some examples but have not yet seen anything that would show me how, for instance, turn a nested Map like
Map Int (Map Int (Map String Double)
into a "zipped" version.
That is presuming of course that this use is feasible at all.
Hi Günther, a couple of weeks ago I was looking into Zippers my self as well. After reading all the documents mentioned in the other messages, I decided to go for my implementation as the proposed ones seemed to me unnecessarily complicated. You can see the discussion here: http://www.haskell.org/pipermail/haskell-cafe/2009-March/056942.html I have to thank Heinrich Apfelmus and Ryan Ingram because they pointed out a major flaw in my implementation and so I got Zippers and why they are implemented as such. What I've learned: Zippers are "structured collections[1] with a focus". Through a Zipper you can O(1) change the value of the focused element: that's the fundamental property. In addition, you can change the focus through a series of "moving" functions. Regarding their implementation, it's important to understand that the moving functions must be "aware" of the changes you made to the focused element. This is carried out by having the moving functions rebuild the context of the new focused element starting from the current focus' context. On the contrary, my implementation relied on laziness and partial application but lacked the "awareness" of the changes. If you can catch this difference, it's easy to grasp the Zipper/Delimited Continuation link and the statement "a zipper is a delimited continuation reified to data". Sorry for my explanation using elementary terms: I'm no computer science theorist ;) Hope this helped. Cristiano [1] By structured collection I mean lists, trees, graphs and so on.