> 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.
To clarify: there is no magic that turns a data structure into O(1)
access, just as a CPS transformation is not a magic bullet for speeding
up programs. Only the move functions (changing focus to some subset of
"adjacent" substructures) are O(1). Update functions need not be O(1).
And amortized random access time via a zipper is never less than
amortized random access of the optimal equivalent un-zippered data
structure.
Zippers are most effective when a structure is accessed by some
quasicontinuous path through it. Fortunately, this happens quite a lot,
although laziness does obviate the need for a zipper in many of these cases.
Dan
Cristiano Paris wrote:
On Mon, Mar 30, 2009 at 9:46 PM, Gü?nther Schmidt <[email protected]> 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.
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