Nested with_progress()

[HenrikBengtsson]

Add support for nested with_progress() calls where it's only the outer layer (or?) that ends up presenting progress.

[HenrikBengtsson]

Clarification: The below example produces two rounds of progress bars. It would be neat if nested with_progress() could be detected and automatically "merged" such that all progress updates from an inner with_progress() are relayed up to the parent with_progress() environment. This way we could end up getting a single progress bar with 2 * 5 = 10 steps, instead of two with 5 steps each.

library(progressr)
handlers("progress")

xs <- list(a = 1:5, b = 1:5)

ys <- list()

with_progress({
  p <- progressor(along = xs, scale = 5.0)
  for (kk in seq_along(xs)) {
    x <- xs[[kk]]
    with_progress({
      ys[[kk]] <- slow_sum(x)
    })
  }
})