Re: [Haskell-cafe] Question about zippers on trees

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 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@haskell.org> wrote:

...

Shouldn’t that be:

_before :: [TreePos a]

etc.?

Jeff

On Jul 22, 2020, at 4:54 AM, Andrew Martin 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.htm... ) 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@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

_______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

-- -Andrew Thaddeus Martin

_______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

_______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.