[Haskell-cafe] Question about zippers on trees

[Andrew Martin]

It appears that you've already discovered this, but my experience with
zippers has been that abstractions that generalize them are cumbersome,
make type signatures more confusing, and don't really do that much work for
me. Maybe there are other users that don't have this experience, but I
prefer a more direct route with less generalization when I'm working with
zippers.

On Wed, Jul 22, 2020 at 8:02 AM Dominik Schrempf <dominik.schrempf at gmail.com>
wrote:

> Thank you for your fast answer.
>
> A direct implementation without using a library is interesting, thank you.
> I
> refrained from doing that, because I thought that Control.Zipper would
> actually
> do this for me. Actually, I was pretty successful with using
> Control.Zipper to
> change node labels, but failed doing more complicated stuff.
>
> Andrew Martin <andrew.thaddeus at gmail.com> writes:
>
> > 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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