# AbstractTrees - iterating without a tree

**URL:** <https://discourse.julialang.org/t/abstracttrees-iterating-without-a-tree/96339>\
**Category:** New to Julia\
**Created:** [March 20, 2023, 2:06pm UTC](https://discourse.julialang.org/t/abstracttrees-iterating-without-a-tree/96339 "2023-03-20T14:06:11Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![freeman](https://avatars.discourse-cdn.com/v4/letter/f/ec9cab/32.png) [@freeman](https://discourse.julialang.org/u/freeman)\
**Post date:** [March 20, 2023, 2:06pm UTC](https://discourse.julialang.org/t/abstracttrees-iterating-without-a-tree/96339/1 "2023-03-20T14:06:11Z")

</div>

If I have a tree, I can use AbstractTrees.jl to iterate it. All I needed was to define

```julia
import AbstractTrees as AT
AT.children(n::MyNode) = ...

```

Great!

But often times I don’t have a tree and I don’t want to build it in memory. Instead I have an object that we could consider the tree root, and I have operations that I can do to mutate the object which lead me to other nodes. For motivation, imagine that you want to walk the tree in a board game: you don’t want to have a copy of the board for every position that you’re considering, you just want to be able to iterate thru the positions (so you can hash them and score them). Question: can I get AbstractTrees.jl to mutate my object instead of moving from node to node?

Reading the docs I learned about an object called a TreeCursor. A cursor is an object that ATs.jl uses internally to navigate a tree. According to the docs:

> Tree iteration algorithms rely on `TreeCursor` objects. A `TreeCursor` is a wrapper for a tree node which contains all information needed to navigate to other positions within the tree. They themselves are nodes of a tree with the [`StoredParents`](https://juliacollections.github.io/AbstractTrees.jl/stable/#AbstractTrees.StoredParents) and [`StoredSiblings`](https://juliacollections.github.io/AbstractTrees.jl/stable/#AbstractTrees.StoredSiblings) traits.

So I thought, maybe instead of building a tree and passing it into ATs.jl I can build a cursor directly, and pass that? From the docs all I need to do is, in addition to `AT.children` also implement some traits plus the functions `AT.parent` and `AT.nextsibling`.

I’d like to hear from either someone who contributed to this library, or is experienced with it, whether this is a use-case (although not one covered by the documentation) or whether I’m missing something - and if the latter, _is there a way to use ATs.jls if I don’t want to build a tree_? The reason why I’m not super confident this idea might make sense is that, while the `AT.parent` case works great (mutate my object to go up one level and then return itself), it’s not clear how the `AT.children` would work - I could return an object with `getindex` defined on it, but it’s not clear that ATs won’t do something that will break this - for example call `AT.children`, make a copy, access the result to get one child node, call `AT.nextsibling` on it, then access a different of element of its `AT.children` copy.