I don't understand this: (def z (zip/zipper rest rest cons t)) Also, it didn't make sense why down arbitrarily brought you to the left child. Why wouldn't you have to say, "down-left"?
Thanks. On Sat, Jan 31, 2009 at 10:03 AM, James Reeves <weavejes...@googlemail.com>wrote: > > On Jan 31, 3:06 am, Jeffrey Straszheim <straszheimjeff...@gmail.com> > wrote: > > Does anyone know of a gentle introduction to the Zipper stuff, and how > > it is used in Clojure? > > My understanding of zippers is that they are a way of efficiently > navigating a tree data structure in a functional manner. So if we have > the tree: > > A > / \ > B C > | / \ > D E F > > Then it's quite easy to navigate down the tree by picking a subtree. > So if we want to go down to C: > > C > / \ > E F > > But what if we want to go back up? We've lost the reference back to A. > Zippers provide a solution to this; instead of taking a subtree, > zippers change the root of the tree: > > C > /|\ > E F A > | > B > | > D > > The links are still the same, but we've moved the root of the tree to > C. If we want to go to E, we can do that, too: > > E > | > C > / \ > F A > | > B > | > D > > Thus, zippers enable one to both quickly move up and down a tree in a > way that doesn't involve bi-directional nodes or a mutating "current > node" variable. > > In Clojure, using a tree is actually much simpler than the theory. > Let's represent our tree using lists: > > user=> (def t '(:a (:b :d) (:c :e :f))) > #'user/t > > In order to turn this into a zipper, we need to use the zipper > function. Because we've chosen to use lists to represent our tree, > this is pretty simple: > > user=> (require '[clojure.zip :as zip]) > nil > user=> (def z (zip/zipper rest rest cons t)) > #'user/z > > Now we've got our zipper, we can look at the original tree using zip/ > node: > > user=> (zip/node z) > (:a (:b :d) (:c :e :f)) > > And we can go down: > > => (def z (zip/down z)) > #'user/z > user=> (zip/node z) > (:b :d) > > Right: > > user=> (def z (zip/right z)) > #'user/z > user=> (zip/node z) > (:c :e :f) > > And back up again: > > user=> (def z (zip/up z)) > #'user/z > user=> (zip/node z) > (:a (:b :d) (:c :e :f)) > > All in a completely functional way. > > - James > > > --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en -~----------~----~----~----~------~----~------~--~---
