In some processes, intermediate results force recalculation of earlier ones. Can the progressr package support changes to the progression index during the calculation, so
I could report

step 1
step 2
step 3 oops, need to go back
step 2
step 3
step 4
done

Here’s some code that doesn’t work:

f <- function() {
  p <- progressr::progressor(4)
  i <- 0
  while (i < 4) {
    i <- i + 1
    p(message = sprintf("at step %d", i))
    if (i == 3 && runif(1) < 0.5) {
      p(message = "Oops, need to go back")
      i <- 2
    }
    Sys.sleep(1)
  }
}
handlers(global = TRUE)

set.seed(123)
f()

It steps through from 1 to 4, then gives nothing, and a lot of warnings at the end.