That seems simple enough. Thanks.
On Jan 31, 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
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