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

Nice timing, I was just reading Conal Elliott’s take on zippers [1][2] when you sent this question. It is interesting because it produces a concrete representation of the zippers via derivatives, like the manually constructed zippers, while still being completely generic*. I wonder if it is possible to combine the approach with lenses to get the same convenience while still being more concrete and flexible. Regards, Andreas [1]: http://conal.net/blog/posts/another-angle-on-zippers http://conal.net/blog/posts/another-angle-on-zippers [2]: http://hackage.haskell.org/package/functor-combo-0.3.6/docs/FunctorCombo-Zip... http://hackage.haskell.org/package/functor-combo-0.3.6/docs/FunctorCombo-Zip... * The library is sadly not implemented in terms of `GHC.Generics`, even though it uses the same combinators, since it predated it.

22 Jul 2020 kl. 13:40 skrev Dominik Schrempf :

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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