Nested progression info

[HenrikBengtsson]

In order to support refined progression info at the top level, aggregators should/could relay nested info that provide info on the current step and any nested steps, e.g. step = c(2, 23). Same for messages and max_steps.

[HenrikBengtsson]

Capture-gather-and-release progressions:

library(progressr)
with_progress({
p <- progressor(3L)
## Listen for progressions and if detected relay in 10 microsteps summing up to one.
## otherwise signal one step at the end
p(y1 <- slow_sum(1:10))
## Listen for progressions and if detected relay in 5 microsteps summing up to one,
## otherwise signal one step at the end
p(y2 <- slow_sum(5:1))
## Listen for progressions, but nothing signaled, so a one-step progression is signaled at the end
p(z <- y1 + y2)
})

But, it would be nice if we could write this as:

with_progress({
p <- progressor(3L)
## Listen for progressions and if detected relay in 10 microsteps summing up to one.
## otherwise signal one step at the end
p("listen")
y1 <- slow_sum(1:10)

## Listen for progressions and if detected relay in 5 microsteps summing up to one,
## otherwise signal one step at the end
p("listen")
y2 <- slow_sum(5:1)

## Listen for progressions, but nothing signaled, so a one-step progression is signaled at the end
p("listen")
z <- y1 + y2
})