Cristiano Paris wrote:
> Ryan Ingram wrote:
>> ...
>> Here is the problem with your "update":
>>
>> tree = Fork (Leaf 1) (Leaf 2)
>> ztree = initZ tree
>>
>> test = fromJust $ do
>> z1 <- moveLeft ztree
>> let z2 = update z1 3
>> z3 <- moveUp z2
>> z4 <- moveLeft z3
>> this z4
>>
>> I'd expect "test" to equal 3, but I believe with your code that it
>> still equals 1. As apfelmus said, update needs to completely
>> re-construct the zipper structure with the tied knot, which defeats
>> the purpose of using a zipper in the first place.
>
> I got it. I dont't know what your expression "tied knot" is referring
> to but I got the point.
In doInitZ , you're basically using the s itself to define the
moveLeft and moveRight fields of s . You could as well write it as
initZ t = doInitZ Nothing t
where
doInitZ c (Leaf a) = ZContext c Nothing Nothing $ Just a
doInitZ c t@(Fork l r) = s
where s = ZContext c (Just $ doInitZ (Just s) l)
(Just $ doInitZ (Just s) r)
Nothing
Such self-reference is usually called "tying the knot", see also
http://www.haskell.org/haskellwiki/Tying_the_Knot
Regards,
apfelmus
--
http://apfelmus.nfshost.com
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