Internal Iterator

Internal iterator equivalent of std::iter::Iterator.

Featuring:

• std-like api
• #![forbid(unsafe)]
• zero dependencies
• optional dependency on std and alloc (enabled by default)

Motivation

The difference between external and internal iteration is:

• With external iteration, you call next() repeatedly to get elements out of the iterator. Iterators must store its iteration state to know where to pick up on each next call. External iteration gives more power to the consumer - for example, it is very easy to implement a for loop by transforming it to while let Some(item) = iter.next() { ... }.
• With internal iteration, you provide a closure and the iterator calls it with every element it wants to yield. This is similar to what Iterator::for_each would do. Iterators don't need to store the iteration state explicitly which simplifies iterator implementations for some data structures. Internal iteration gives more power for the iterator.

One example where internal iterators shine (and the original reason for this crate) is iteration over trees. Let's take a very simple tree of integers:

struct Tree(i32, Vec<Tree>);

To be able to implement next() we need to store the current position in the tree. We could represent this as a list of indices, each one indicating a child on the subtree in corresponding level:

struct TreeIter {
    tree: Tree,
    position: Vec<usize>,
}

The Iterator implementation would yield the value at the current position, and advance either to the subtree or the next sibling, climbing up

// returns None if walking down the tree went out of bounds in child vector in
// some level
fn find_subtree<'a>(tree: &'a Tree, pos: &[usize]) -> Option<&'a Tree> {
    match pos {
        [] => tree,
        [idx, rest @ ..] => {
            let child = &tree.1.get(*idx)?;
            find_subtree(child, rest)
        }
    }
}

impl Iterator for TreeIter {
    type Item = i32;

    fn next(&mut self) -> Option<i32> {
        loop {
            match self.position.as_slice() {
                [0, rest @ ..] => {
                    let current_tree = find_subtree(&self.tree, &self.position);
                    if let Some(tree) = current_tree {
                        let result = Some(tree.0);
                        if !tree.1.is_empty() {
                            // node has children, move position at first child
                            // of current position
                            self.position.push(0);
                        } else {
                            // node has no children, move position to next
                            // sibling
                            *self.position.last_mut().unwrap() += 1;
                        }
                        return Some(result);
                    } else {
                        // current position is out of bounds - move up by one
                        // and advance to next sibling, then loop over to try
                        // again
                        self.position.pop();
                        *self.position.last_mut().unwrap() += 1;
                    }
                }
                [1] => {
                    return None;
                }
                _ => unreachable!(),
            }
        }
    }
}

Whew, that was quite tricky, with all the index jugling and all.

Let's try the same with InternalIterator. The core method that drives the trait is try_for_each:

use std::ops::ControlFlow;

struct TreeInternalIter {
    tree: Tree,
}

// we need a helper because we need to use `f` multiple times, but an arbitrary
// FnMut cannot be copied or reborrowed
fn iter_helper<T>(tree: Tree, f: &mut impl FnMut(i32) -> ControlFlow<T>) -> ControlFlow<T> {
    f(tree.0)?;
    for child in tree.1 {
        iter_helper(child, f)?;
    }
    ControlFlow::Continue(())
}

impl InternalIterator for TreeInternalIter {
    type Item = i32;

    fn try_for_each<T, F>(self, mut f: F) -> ControlFlow<T>
    where
        F: FnMut(i32) -> ControlFlow<T>,
    {
        iter_helper(self.tree, &mut f)
    }
}

That was a lot more straightforward, less error prone, and does not even require any dynamic memory allocation! And thanks to std::ops::ControlFlow and ? operator the implementation is very easy to write.

Both of them allow constructing elaborate iterator pipelines:

let tree = Tree(4, vec![
    Tree(1, vec![]),
    Tree(3, vec![
        Tree(5, vec![]),
    ]),
    Tree(2, vec![]),
]);

let iterator_result = tree
    .into_iter()
    .map(|x| x * 2)
    .filter(|&x| x > 5)
    .flat_map(|x| [x, x * 10])
    .collect::<Vec<_>>();

assert_eq!(iterator_result, vec![8, 80, 6, 60, 10, 100]);

let internal_iterator_result = tree
    .into_internal_iter()
    .map(|x| x * 2)
    .filter(|&x| x > 5)
    .flat_map(|x| [x, x * 10])
    .collect::<Vec<_>>();

assert_eq!(internal_iterator_result, vec![8, 80, 6, 60, 10, 100]);

Internal iterators are not as expressive as external ones - they cannot be used with for loops or with some other adaptors that require external iteration. However, the loss of of expressiveness is not that big, and depending on your use-case might be a small price to pay for a far simpler iterator implementations.

Limitations

This crate aims to provide a straightforward api that is very similar to one in std, which means that some fancy features that could be provided by internal iteration are not available in this library.

About missing Iterator methods

Not all method equivalents from std::iter::Iterator are implemented. Some of those are impossible (zip is one of those), while most of the others are not implemented just because I didn't personally need them yet.

If you see value in this library but some of the methods you need are missing feel free to open an issue or submit a pull request.

License

Licensed under either of

• Apache License, Version 2.0 (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
• MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)

at your option.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.