How do I use Rayon with an existing iterator?

Question

I turn a regex into a HashSet after doing some filtering. I am trying to use it with Rayon, but I can't figure out how to make Rayon work with an existing iterator without converting it to a vector first. Is this possible?

let re = Regex::new("url=\"(?P<url>.+?)\"").unwrap();
let urls: HashSet<String> = re.captures_iter(&contents)
    .map(|m| Url::parse(m.name("url").unwrap().as_str()))
    .filter(|parsed_url| parsed_url.is_ok())
    .map(|parsed_url| parsed_url.unwrap())
    .filter(|parsed_url| parsed_url.has_host())
    .map(|parsed_url| parsed_url.into_string())
    .collect();

Answer 1

This answer is outdated for the last version of rayon. See the other answer for a possible solution. It may or may not apply to your usecase.

---

Minimal reproduction:

extern crate rayon;

use rayon::prelude::*;

fn main() {
    let v = vec![1_i32, 2, 3, 4].into_iter();

    // no method named `par_iter` found for type `std::vec::IntoIter<i32>`
    let _ = v.par_iter().sum();
}

You cannot do that. Here are all the implementors of this feature, that are:

• BinaryHeap
• BTreeMap
• BTreeSet
• HashMap
• HashSet
• LinkedList
• VecDeque
• Option
• Range
• Result
• Slice/Array

I think that the reason why you cannot parallelize them is because iterators are lazy. An iterator is basically a current item Option<Item> and a next() method. You cannot split it in two parts to execute them in different threads.

Answer 2

This is possible now with ParallelBridge:

use rayon::iter::ParallelBridge;
use rayon::prelude::ParallelIterator;
use std::sync::mpsc::channel;

let rx = {
    let (tx, rx) = channel();

    tx.send("one!");
    tx.send("two!");
    tx.send("three!");

    rx
};

let mut output: Vec<&'static str> = rx.into_iter().par_bridge().collect();
output.sort_unstable();

assert_eq!(&*output, &["one!", "three!", "two!"]);