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

Heinrich Apfelmus apfelmus at quantentunnel.de
Tue Mar 31 22:10:06 EDT 2009

Cristiano Paris wrote:
> On Tue, Mar 31, 2009 at 10:13 PM, Dan Weston <westondan at imageworks.com> wrote:
>>> 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.
> That's true. But, correct me if I'm wrong, updates on the focus site are O(1).

Yes. It's just that shifting the focus to the site from scratch does not
 take constant time, obviously.



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