[Haskell-cafe] Question about zippers on trees

[Dominik Schrempf]

Thank you for the replies!

A specialized zipper suggested by Andrew Martin does the job quite well!

By the way, Control.Lens.Zipper, was factored out of lens into Control.Zipper in
newer versions.

Best,
Dominik

Jeffrey Brown <jeffbrown.the at gmail.com> writes:

> If you want an abstract solution, there's
> https://hackage.haskell.org/package/lens-3.2/docs/Control-Lens-Zipper.html.
>
>
>
> On Wed, Jul 22, 2020 at 9:20 AM Jeff Clites via Haskell-Cafe <
> haskell-cafe at haskell.org> wrote:
>
>> Shouldn’t that be:
>>
>>     _before    :: [TreePos a]
>>
>> etc.?
>>
>> Jeff
>>
>> On Jul 22, 2020, at 4:54 AM, Andrew Martin <andrew.thaddeus at gmail.com>
>> wrote:
>>
>> From containers, Tree is defined as:
>>
>>     data Tree a = Node
>>       { label :: a
>>       , children :: [Tree a]
>>       }
>>
>> (I've renamed the record labels.) What is a zipper into such a tree? I
>> think
>> that the [rosezipper](
>> https://hackage.haskell.org/package/rosezipper-0.2/docs/Data-Tree-Zipper.html
>> )
>> library gives a good definition. I'll specialized it to rose trees:
>>
>>     data TreePos a  = Loc
>>       { _content   :: Tree a -- ^ The currently selected tree.
>>       , _before    :: [Tree a] -- ^ Forest to the left
>>       , _after     :: [Tree a] -- ^ Forest to the right
>>       , _parents   :: [([Tree a], a, [Tree a])] -- ^ Finger to the
>> selected tree
>>       }
>>
>> I think that does it. I wouldn't recommend using a library for this kind
>> though. Just define `TreePos` in your code and then write the functions
>> that you happen to need.
>>
>> On Wed, Jul 22, 2020 at 7:41 AM Dominik Schrempf <
>> dominik.schrempf at gmail.com> wrote:
>>
>>> Hello Cafe!
>>>
>>> I am trying to modify a large 'Data.Tree.Tree'. I managed to modify node
>>> labels
>>> with specific indices in the form of @[Int]@ as they are defined in, for
>>> example, 'Control.Lens.At.Ixed' or 'Lens.Micro.GHC'.
>>>
>>> However, I also need to
>>> 1. modify the node label using information from nearby nodes (e.g., the
>>>    children);
>>> 2. modify the tree structure itself; for example, I may want to change the
>>>    sub-forest.
>>>
>>> Basically, I need a lens that focuses not on the node label, but on the
>>> node
>>> itself. I perceived that this is more difficult.
>>>
>>> I tried to use 'Control.Zipper'. I can use zippers to achieve point 1,
>>> albeit in
>>> a complicated way: (1) I need to go downwards to focus the specific node;
>>> (2) I
>>> need to traverse the children to collect data and save the data somewhere
>>> (how?
>>> in let bindings?); (3) I then go back upwards and change the node label
>>> using
>>> the collected data. Even so, I do not really manage to change the actual
>>> structure of the tree. I also briefly had a look at plates, but do not
>>> manage to
>>> use them in a proper way, maybe because the depth of my structures may be
>>> several hundred levels.
>>>
>>> Did you encounter similar problems in the past or could you point me to
>>> resources discussing these issues?
>>>
>>> Thank you!
>>> Dominik
>>>
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>>
>>
>>
>> --
>> -Andrew Thaddeus Martin
>>
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