Iter

A Rust Iterator is analogous to the union of a Dart Iterable and Iterator. Since Dart already has an Iterator class, to avoid confusion, the Dart implementation of the Rust iterator is Iter. Iter makes working with collections of rust types and regular Dart types a breeze. e.g.

List<int> list = [1, 2, 3, 4, 5];
Iter<int> filtered = list.iter().filterMap((e) {
  if (e % 2 == 0) {
    return e * 2;
  }
  return null;
});
expect(filtered, [4, 8]);
// or
filtered = list.iter().filterMapOpt((e) {
  if (e % 2 == 0) {
    return Some(e * 2);
  }
  return None;
});
expect(filtered, [4, 8]);

Iter can be retrieved by calling iter() on an Iterable or an Iterator. Iter can be iterated like an Iterable or Iterator, and is consumed like an Iterator.

List<int> list = [1, 2, 3, 4, 5, 6, 7, 8, 9];
Iter<int> iter = list.iter();
List<int> collect = [];
for (final e in iter.take(5).map((e) => e * e)) {
  if (e.isEven) {
    collect.add(e);
  }
}
expect(collect, [4, 16]);
int? next = iter.next();
expect(next, 6);
collect.add(next.unwrap());
next = iter.next();
collect.add(next.unwrap());
expect(next, 7);
while(iter.moveNext()){
  collect.add(iter.current * iter.current);
}
expect(collect, [4, 16, 6, 7, 64, 81]);
expect(iter,[]);

Iter contains many more useful methods than the base Dart Iterable class and works in all places you would reach for an Iterator - pub.dev

Dart vs Rust Example

Goal: Get the index of every "!" in a string not followed by a "?"

import 'package:rust/rust.dart';

void main() {
  List<int> answer = [];
  String string = "kl!sd!?!";
  Peekable<(int index, Arr<String> window)> iter = string
      .chars()
      .mapWindows(2, identity)
      .enumerate()
      .peekable();
  while (iter.moveNext()) {
    final (int index, Arr<String> window) = iter.current;
    switch (window) {
      case ["!", "?"]:
        break;
      case ["!", _]:
        answer.add(iter.current.$1);
      case [_, "!"] when iter.peek().isNone():
        answer.add(index + 1);
    }
  }
  expect(answer, [2, 7]);
}

Rust equivalent

use std::iter::Peekable;

fn main() {
  let mut answer: Vec<usize> = Vec::new();
  let string = "kl!sd!?!";
  let mut iter: Peekable<_> = string
      .chars()
      .map_windows(|w: &[char; 2]| *w)
      .enumerate()
      .peekable();

  while let Some((index, window)) = iter.next() {
      match window {
          ['!', '?'] => continue,
          ['!', _] => answer.push(index),
          [_, '!'] if iter.peek().is_none() => answer.push(index + 1),
          _ => continue,
      }
  }
  assert_eq!(answer, [2, 7]);
}

Additional Examples

    /// Extract strings that are 3 long inside brackets '{' '}' and are not apart of other strings
    String string = "jfsdjf{abcdefgh}sda;fj";
    Iter<String> strings = string.runes
        .iter()
        .skipWhile((e) => e != "{".codeUnitAt(0))
        .skip(1)
        .arrayChunks(3)
        .takeWhile((e) => e[2] != "}".codeUnitAt(0))
        .map((e) => String.fromCharCodes(e));
    expect(strings, ["abc", "def"]);

Misc

Clone

Another a big advantage of Iter over Iterable<T> and Iterator<T> is that Iter<T> is clonable. This means the iterator can be cloned without cloning the underlying data.

    var list = [1, 2, 3, 4, 5];
    var iter1 = list.iter();
    iter1.moveNext();
    var iter2 = iter1.clone();
    iter2.moveNext();
    var iter3 = iter2.clone();
    iter3.moveNext();
    var iter4 = iter1.clone();
    expect(iter1.collectList(), [2, 3, 4, 5]);
    expect(iter2.collectList(), [3, 4, 5]);
    expect(iter3.collectList(), [4, 5]);
    expect(iter4.collectList(), [2, 3, 4, 5]);

This allows for situations where you want to work ahead and not lose your iterator position, or pass the Iter to another function without needing to call e.g. collectList(), collectArr(), etc.