Replacement functions in R that don't take input

This seems very related to several other questions that have been asked (this one for example), but I can't quite figure out how to do exactly what I want. Maybe replacement functions are the wrong tool for the job, which would also be a perfectly acceptable answer. I am much more familiar with Python than R and I can easily think of how I want to do it in Python but I can't quite get my head around how to approach it in R.

The problem: I am trying to modify an object in place within a function, without having to return it, but I don't need to pass in the value that modifies it, because this value is the result of a function call that's already contained in the object.

More specifically, I have a list (technically it's an s3 class, but I don't think that's actually relevant to this issue) that contains some things relating to a process started with processx::process$new() call. For reproducibility, here's a toy shell script you can run, and the code to get my res object:

echo '
echo $1
sleep 1s
echo "naw 1"
sleep 1s
echo "naw 2"
sleep 1s
echo "naw 3"
sleep 1s
echo "naw 4"
sleep 1s
echo "naw 5"
echo "All done."'> naw.sh

Then my wrapper is something like this:

run_sh <- function(.args, ...) {
  p <- processx::process$new("sh", .args, ..., stdout = "|", stderr = "2>&1")
  return(list(process = p, orig_args = .args, output = NULL))
}

res <- run_sh(c("naw.sh", "hello"))

And res should look like

$process
PROCESS 'sh', running, pid 19882.

$output
NULL

$orig_args
[1] "naw.sh""hello"

So, the specific issue here is a bit peculiar to process$new but I think the general principle is relevant. I am trying to collect all the output from this process after it is finished, but you can only call process$new$read_all_output_lines() (or it's sibling functions) once because the first time it will return the result from the buffer and the subsequent times it returns nothing. Also, I am going to call a bunch of these and then come back to "check on them" so I can't just call res$process$read_all_output_lines() right away because then it will wait for the process to finish before the function returns, which is not what I want.

So I'm trying to store the output of that call in res$output and then just keep that and return it on subsequent calls. Soooo... I need to have a function to modify res in place with res$output <- res$process$read_all_output_lines().

Here's what I tried, based on guidance like this, but it didn't work.

get_output <- function(.res) {
  # check if process is still alive (as of now, can only get output from finished process)
  if (.res$process$is_alive()) {
    warning(paste0("Process ", .res$process$get_pid(), " is still running. You cannot read the output until it is finished."))
    invisible()
  } else {
    # if output has not been read from buffer, read it
    if (is.null(.res$output)) {
      output <- .res$process$read_all_output_lines()
      update_output(.res) <- output
    }
    # return output
    return(.res$output)
  }
}

`update_output<-` <- function(.res, ..., value) {
  .res$output <- value
  .res
}

Calling get_output(res) works the first time, but it does not store the output in res$output to be accessed later, so subsequent calls return nothing.

I also tried something like this:

`get_output2<-` <- function(.res, value) {
  # check if process is still alive (as of now, can only get output from finished process)
  if (.res$process$is_alive()) {
    warning(paste0("Process ", .res$process$get_pid(), " is still running. You cannot read the output until it is finished."))
    .res
  } else {
    # if output has not been read from buffer, read it
    if (is.null(.res$output)) {
      output <- .res$process$read_all_output_lines()
      update_output(.res) <- output
    }
    # return output
    print(value)
    .res
  }
}

Which just throws away the value but this feels silly because you have to call it with the assignment like get_output(res) <- "fake" which I hate.

Obviously I could also just return the modified res object, but I don't like that because then the user has to know to do res <- get_output(res) and if they forget to do that (the first time) then the output is lost to the ether and can never be recovered. Not good.

Any help is much appreciated!