28 May 2016

Until recently, the concept of generators, or resumable functions, seemed to me like a cute idea with only niche use cases. Sure, I had heard that generators in python and javascript could make certain things much nicer, but how often does one really need to have what appears to be nothing more than a fancy iterator? It wasn’t until I followed this Rust RFC thread that the true potential of generators in Rust started to dawn on me. Today they are my personal number-one most-desired new feature for the language, mostly because I believe they are the biggest missing piece in Rust’s async story.

But how are generators relevant to async at all?

To understand that, let’s first imagine what generators might look like in Rust. We start with a trait definition (borrowed from eddyb’s comment in the above-linked thread):

pub enum Yield<T, R> {

pub trait Generator {
    type Value;
    type Return;
    fn next(&mut self) -> Yield<Self::Value, Self::Return>;

So a generator looks a lot like an iterator. You can ask it for the next value until you reach its end, signalled by Return. The interesting part is that there is a special way to construct values that implement Generator. For example:

fn fib_with_sum(n: usize) -<u64>-> u64 {
     let mut a = 0;
     let mut b = 1;
     let mut sum = 0;
     for _ in 0..n {
         yield a; // <--- new keyword "yield"
         sum += a;
         let next = a + b;
         a = b;
         b = next;
     return sum;

Here I’ve used some strawman syntax fn foo() -<T>-> R, which denotes that foo is a generator function that produces some number of T values and then finishes by producing an R. The yield keyword inside of a generator function means that a Yield::Value() should be produced and the generator should be paused. When you call fib_with_sum(), the value you get back is a generator, which can be used by calling next(), like this:

let mut g = fib_with_sum(5);
g.next(); // Yield::Value(0)
g.next(); // Yield::Value(1)
g.next(); // Yield::Value(1)
g.next(); // Yield::Value(2)
g.next(); // Yield::Value(3)
g.next(); // Yield::Return(7)

Another thing that we might want to do is to have one generator delegate to another generator. The yield from construction allows that:

fn sub_gen(n: u64) -<u64>-> bool {
    for i in 0..n {
        yield i
    return n == 3;

fn gen() -<u64>-> () {
   for j in 0.. {
       if yield from sub_gen(j) {
           return j;

Running this gives:

let mut g = gen();
g.next(); // Yield::Value(0)
g.next(); // Yield::Value(0)
g.next(); // Yield::Value(1)
g.next(); // Yield::Value(0)
g.next(); // Yield::Value(1)
g.next(); // Yield::Value(2)
g.next(); // Yield::Return(3)

Note that when the sub-generator is done, its return value gets plugged in at the yield from expression of the calling generator. So gen() continues until j == 3.

Using generators for async I/O

Now for the punchline. Using generators, we can define asynchronous reader and writer traits like this:

use std::io::{Error, ErrorKind, Result};

enum AsyncStatus {

pub trait AsyncWrite {
    /// Attempts to write all buf.len() bytes from buf into the
    /// stream. Returns once all of the bytes have been written.
    fn write(&mut self, bytes: &[u8]) -<AsyncStatus>-> Result<()>;

pub trait AsyncRead {
    /// Attempts to read buf.len() bytes from the stream,
    /// writing them into buf. Returns the number of bytes
    /// actually read. Returns as soon as min_bytes are
    /// read or EOF is encountered.
    fn try_read(&mut self,
                buf: &mut [u8],
                min_bytes: usize)
        -<AsyncStatus>-> Result<usize>;

    /// Like try_read(), but returns an error if EOF is
    /// encountered before min_bytes can be read.
    fn read(&mut self,
            buf: &mut [u8],
            min_bytes: usize)
        -<AsyncStatus>-> Result<usize>;
       let n = try!(yield from self.try_read(buf, min_bytes));
       if n < min_bytes {
                          format!("expected {} but got {} bytes",
                                  min_bytes, n)))
       } else {

Then, at the top level of our program, we have a task executor, where

type Task = Generator<Value=AsyncStatus, Return=()>;

The task executor owns a collection of tasks and is responsible for running them when they are ready. When a task runs and needs to pend on some I/O, it yields back what it’s currently waiting on, for example a file descriptor. When none of the tasks can make any progress, the executor calls a OS-specific API like kevent() to wait until more progress can be made.

Unlike with fibers/green threading, it remains very clear where switches-between-tasks can take place. Unlike with promises, we don’t have to be constantly allocating closures on the heap. Generator-based async-I/O seems like an all-around win!

(discussion on /r/rust)

-- posted by dwrensha

capnproto-rust on github
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