Does roc support asyncronous function calls into the host? · beginners

I had the idea of creating a web crawler platform 2y ago. At that time, there was no way for crawlers to call into the host in parallel. This can be useful while processing the response of a website. The crawler could ask the host to parse and fetch elements according to css or xpath selectors. This can be done using a list and map, but the call would happen synchronous on the host side. It would receive every map call one after the other.

There was discussion about Task.map2, but I've read the changelogs and couldn't find map2 nor Task in general. It seems that it has been removed.

Would be great if someone with a little bit more knowledge than me could quickly chime in and answer this. At least I couldn't find any references to async/await.

Anton (Sep 06 2025 at 09:44):

Richard Feldman (Sep 06 2025 at 11:42):

short answer is that this should be very ergonomic once the new compiler gets there! I have a WIP design doc for it. Can you remind me what some of the code would look like in a different language? I can translate it into what it'll look like in the upcoming design :smiley:

mainrs (Sep 07 2025 at 09:00):

I am looking for two types of parallel processing. One would be async/await for I/O-heavy operations like file reading, networking etc..

The other one would be parallel CPU intensive tasks, usually done on thread pools. In Rust, the first would be done using tokio, the second using rayon.

This is already possible in roc right now, but one has to sacrifice ergonomics for it. I can simply introduce platform-provided methods that take a list of arguments. And return a list of results. One can introduce custom types to bundle the arguments, but it makes working with the result types and arguments cumbersome in a functional way...

fn get_requests(urls: Vec<String>, bodies: Vec<Vec<u8>>,  headers: ....);
fn process_xpath(requests: Vec<Requests>, xpaths: Vec<String>, ....);

I think it would be nice to have the parallelism inside of roc itself instead of having to delegate it to the host.

Richard Feldman (Sep 07 2025 at 13:23):

some hosts can't support parallelism, but my goal is to make it easy to say "these can run in any order" so that it will run in parallel if the host supports it

mainrs (Sep 07 2025 at 14:15):

Thanks for letting me know! Are there any issues / resources I can keep an eye on to stay up-to-date? :)

Richard Feldman (Sep 07 2025 at 14:37):

I still need to finish writing it up, but I'll definitely post about it once it's done!

Krzysztof Skowronek (Oct 02 2025 at 08:46):

Meaning I would create a "runtime" (so some platforms would only offer a single threaded event loop) and use that runtime to run the standard set of async operations?

Drewry Pope (Mar 13 2026 at 04:09):

how is the parallel processing in roc design looking? would love to learn more about what that might end up looking like, even a rough draft. right now mostly interested in forEachPar or mapN type of thing, but also curious how one might handle async within roc someday.

I'm aware of the way you can send requests to platform and then loop over the list of tasks checking until they are done, which should actually work for most of what I'm doing. Eventually I hope to write some pure functions, or ones that call regular platform functions, using parallel without relying on the platform to provide special functions to enable it.

Stream: beginners

Topic: Does roc support asyncronous function calls into the host?

mainrs (Sep 06 2025 at 09:38):