Access value of promise from async() function

[JosiahParry]

Using a modified example from {coro}, it is unclear how to access the value of the function. Here we return n after the async function has completed.

async_count_down <- coro::async(function(n) {
  while (n > 0) {
    cat("Down", n, "\n")
    coro::await(coro::async_sleep(1))
    n <- n - 1
  }
  n
})

n <- async_count_down()

Running this results in a promise (expected) however, trying to get the value via n$finally() results in an error.

At present it seems that using async() is intended only for functions with side effects. Is that correct?

[lionel-]

coro::async() inherits the properties of promises (https://rstudio.github.io/promises). By default they are scheduled by a top-level event loop that runs when R is idle so only that loop and the tasks it manages (promises including ones created by async functions) have access to values.

In practice this means you can only access values from an async function only from another async context by using await().

I think there are ways to create local event loops that allow you to block on a promise (like block_on() in Rust) but I haven't tried this: https://github.com/r-lib/later/blob/82d9609217fdddb601b1569891f9fcece62a78be/R/later.R#L28

[JosiahParry]

@lionel- picking this back up—thanks! i think what I'm after is essentially a way to either fully await the response akin to block_on() with tokio or std's .join() where we await the process from the main thread and get the result back.

Additionally, I'd quite like the ability to poll the future (to keep the rust analogies going std::future::Future::poll which says "pending" or "ready" with the value contained in it.

In my specific use case I have an async function created using coro::async() (defined here https://github.com/R-ArcGIS/arcgisutils/blob/gp/dev/geoprocessing-async.R) that starts an async job via rest api. Then the job is polled until it is completed. Then attempts to return the value of the async context.

However, the returned value is a promise and the only way I've been able to figure out how to extract that value is by digging into the R6 object itself with:

extract_promise_value <- function(promise) {
  promise_impl <- attr(promise, "promise_impl")
  if (promise_impl$.__enclos_env__$private$state == "fulfilled") {
    return(promise_impl$.__enclos_env__$private$value)
  } else {
    return(NULL)
  }
}

{coro} feels like the right place to get this type of behavior—but i may be entirely wrong as well

[lionel-]

Yep coro is a frontend to event loops, so you'll need to request this functionality in later/promises.

I've used coro with something akin to block_on() with https://github.com/gaborcsardi/async, but that's an entirely different backend than promises (and incompatible).