[Haskell-cafe] Re: Looking for practical examples of Zippers

[Cristiano Paris]

On Mon, Mar 30, 2009 at 9:46 PM, Gü?nther Schmidt <gue.schmidt at web.de> wrote:
> 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.