Add `value` and `backtrace` methods for `promise_future` or a geter for the underlying future

[vspinu]

With pure futures one can use value and backtrace to inspect the failures. With promise_future only the brief error output is printed.

Could you plese provide on of the following:

• value and backtrace methods for promise_future
• way to retrieve the internal future object
• print full backtrace on errrors

Thanks!

[vspinu]

I actually see that future backtraces are useless. These are from the caller process only :(.

So this leaves only the last option among the 3 from OP.

[r2evans]

@vspinu, I asked something similar recently on SO (https://stackoverflow.com/q/74413807/3358272), and I'm deducing that the value portion of your request (if I'm reading this correctly) could be handled in the then, perhaps

ret <- NULL
future_promise({
  Sys.sleep(10)
  sample(3)
}) %>%
  then(~ {
    ret <<- .
  })

I'm not incredibly fond of it (and we can use arbitrary environments and/or better methods than <<-), but it appears to work clearly enough.