[Haskell-cafe] Question about zippers on trees

Andrew Martin andrew.thaddeus at gmail.com
Wed Jul 22 11:54:19 UTC 2020

>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](
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

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>

> 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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