jdomantas / internal-iterator Goto Github PK

It would be helpful to have an InternalIterator version of std::iter::from_fn

I need to iterate over all items in a composite tree and wanted to use this crate for that.
However the example in the readme only shows where the iterated item is an owned type. I would like to have an iterator over &T and &mut T types.

The immutable case is simply enough but for the mutable iterator i am struggeling with the borrow checker.
This was my attempt:

use std::ops::ControlFlow;

use internal_iterator::InternalIterator;

struct Node {
    nodes: Vec<Node>,
}
struct NodeIteratorMut<'a> {
    n: &'a mut Node,
}
impl<'a> InternalIterator for NodeIteratorMut<'a> {
    type Item = &'a mut Node;

    fn try_for_each<R, F>(self, mut f: F) -> std::ops::ControlFlow<R>
    where
        F: FnMut(Self::Item) -> std::ops::ControlFlow<R>,
    {
        iter_node(self.n, &mut f)
    }
}
fn iter_node<'a, T>(
    node: &'a mut Node,
    f: &mut impl FnMut(&'a mut Node) -> ControlFlow<T>,
) -> ControlFlow<T> {
    f(node)?;
    node.nodes.iter_mut().try_for_each(|n| iter_node(n, f))
}

fn main() {}

Which does not work :

error[E0499]: cannot borrow `node.nodes` as mutable more than once at a time
  --> crates\tree_view\examples\iter_test.rs:26:5
   |
21 | fn iter_node<'a, T>(
   |              -- lifetime `'a` defined here
...
25 |     f(node)?;
   |     -------
   |     | |
   |     | first mutable borrow occurs here
   |     argument requires that `*node` is borrowed for `'a`
26 |     node.nodes.iter_mut().try_for_each(|n| iter_node(n, f))
   |     ^^^^^^^^^^^^^^^^^^^^^ second mutable borrow occurs here

How could this be implemented?

Hello there!

I saw your post of reddit today, congratulations on your release 🥳 🎈. I was wondering if you were interested in trying to integrate this with other crates in the community. The slab-tree crate seems like a perfect use-case for a crate like this. While I don't own it, I do see the obvious potential between these two crates to create easily std-like iterable trees. Do you think a tree like API like slab-tree would be a good candidate for your crate to integrate with?

Thanks!