Rust, how to perform basic recursive async? - rust

Question

I am just doing some quick experimenting in an attempt to learn the rust language, I have done a few successful async tests, this is my starting point:
use async_std::task;
use futures;
use std::time::SystemTime;
fn main() {
let now = SystemTime::now();
task::block_on(async {
let mut fs = Vec::new();
let sum = 100000000;
let chunks: u64 = 5; //only valid for factors of sum
let chunk_size: u64 = sum/chunks;
for n in 1..=chunks {
fs.push(task::spawn(async move {
add_range((n - 1) * chunk_size + 1, n * chunk_size + 1)
}));
}
let vals = futures::future::join_all(fs).await;
// 5000000050000000 for this configuration of inputs
println!("{}", vals.iter().sum::<u64>());
});
println!("{}ms", now.elapsed().unwrap().as_millis());
}
fn add_range(start: u64, end: u64) -> u64 {
println!("{}, {}", start, end);
let mut total: u64 = 0;
for n in start..end {
total += n;
}
return total;
}
by changing the value of chunks you can change how many task::spawns there are. Now rather than a flat set of workers, I want the add_range function to be recursive and to keep forking off workers based on the inputs, however following the compiler errors I have gotten myself quite tangled up:
use async_std::task;
use futures;
use std::future::Future;
use std::pin::Pin;
fn main() {
let pin_box_u64 = task::block_on(add_range(0, 10, 10, 1, 1001));
println!("{}", pin_box_u64/*how do i get u64 out of this*/)
}
// recursively calls itself in a branching tree structure
// forking off more worker threads
async fn add_range(
depth: u64,
chunk_split: u64,
chunk_size: u64,
start: u64,
end: u64,
) -> Pin<Box<dyn Future<Output = u64>>> {
println!("{}, {}, {}", depth, start, end);
// if the range of start to end is more than the allowed
// chunk_size then fork off more workers dividing
// the work up further.
if end - start > chunk_size {
let mut fs = Vec::new();
let next_chunk_size = (end - start) / chunk_split;
for n in 0..chunk_split {
let s = start + (next_chunk_size * n);
let mut e = start + (next_chunk_size * (n + 1));
if e > end {
e = end;
}
// spawn more workers
fs.push(task::spawn(add_range(depth + 1, chunk_split, chunk_size, s, e)));
}
return Box::pin(async move {
// join workers back up and do joining sum.
return futures::future::join_all(fs).await.iter().map(/*how do i get u64s out of here*/).sum::<u64>();
});
} else {
// else the work is less than the allowed chunk_size
// so lets now do the actual sum for my chunk
let mut total: u64 = 0;
for n in start..end {
total += n;
}
return Box::pin(async move { total });
}
}
I have played around with this for a while but I feel like Im just becoming more and more lost with the compiler errors.

Answer 1

You need to box the returned future, otherwise the compiler can't determine the size of the return type.
Additional context can be found here: https://rust-lang.github.io/async-book/07_workarounds/04_recursion.html
use std::pin::Pin;
use async_std::task;
use futures::Future;
use futures::FutureExt;
fn main() {
let pin_box_u64 = task::block_on(add_range(0, 10, 10, 1, 1001));
println!("{}", pin_box_u64)
}
// recursively calls itself in a branching tree structure
// forking off more worker threads
fn add_range(
depth: u64,
chunk_split: u64,
chunk_size: u64,
start: u64,
end: u64,
) -> Pin<Box<dyn Future<Output = u64> + Send + 'static>> {
println!("{}, {}, {}", depth, start, end);
// if the range of start to end is more than the allowed
// chunk_size then fork off more workers dividing
// the work up further.
if end - start > chunk_size {
let mut fs = Vec::new();
let next_chunk_size = (end - start) / chunk_split;
for n in 0..chunk_split {
let s = start + (next_chunk_size * n);
let mut e = start + (next_chunk_size * (n + 1));
if e > end {
e = end;
}
// spawn more workers
fs.push(task::spawn(add_range(
depth + 1,
chunk_split,
chunk_size,
s,
e,
)));
}
// join workers back up and do joining sum.
return futures::future::join_all(fs)
.map(|v| v.iter().sum::<u64>())
.boxed();
} else {
// else the work is less than the allowed chunk_size
// so lets now do the actual sum for my chunk
let mut total: u64 = 0;
for n in start..end {
total += n;
}
return futures::future::ready(total).boxed();
}
}