Add purrr_continue()

[mgirlich]

Closes #1099.

The idea is to easily continue a call to map() and keep intermediate results.

devtools::load_all("~/GitHub/purrr/")
#> ℹ Loading purrr

slow_function <- function(x) {
  if (x == 5) {
    stop("something went wrong")
  }
  
  x - 1
}

slow_function2 <- function(x) {
  print(x)
  x
}

to_process <- 1:10
map(1:10, slow_function)
#> [1] 0
#> Error in `map()`:
#> ℹ In index: 5.
#> Caused by error in `.f()`:
#> ! something went wrong
#> Backtrace:
#>      ▆
#>   1. ├─purrr::map(1:10, slow_function)
#>   2. │ └─purrr:::map_("list", .x, .f, ..., .progress = .progress) at purrr/R/map.R:129:2
#>   3. │   ├─purrr:::with_indexed_errors(...) at purrr/R/map.R:182:2
#>   4. │   │ └─base::withCallingHandlers(...) at purrr/R/map.R:234:2
#>   5. │   ├─purrr:::call_with_cleanup(...)
#>   6. │   └─global .f(.x[[i]], ...)
#>   7. │     └─base::stop("something went wrong")
#>   8. └─base::.handleSimpleError(...)
#>   9.   └─purrr (local) h(simpleError(msg, call))
#>  10.     └─cli::cli_abort(...) at purrr/R/map.R:248:8
#>  11.       └─rlang::abort(...)
all <- purrr_continue(.f = slow_function2)
#> [1] 5
#> [1] 6
#> [1] 7
#> [1] 8
#> [1] 9
#> [1] 10
all
#> [[1]]
#> [1] 0
#> 
#> [[2]]
#> [1] 1
#> 
#> [[3]]
#> [1] 2
#> 
#> [[4]]
#> [1] 3
#> 
#> [[5]]
#> [1] 5
#> 
#> [[6]]
#> [1] 6
#> 
#> [[7]]
#> [1] 7
#> 
#> [[8]]
#> [1] 8
#> 
#> [[9]]
#> [1] 9
#> 
#> [[10]]
#> [1] 10

# for some reason this now starts at 5 :-(
run2 <- map(1:10, slow_function)
#> [1] 5

^{Created on 2023-09-05 with reprex v2.0.2}

[mgirlich]

For some reason I don't understand in the second run i is 5 at this point even though we just assigned 0 to it...

[mgirlich]

It would be even nicer if updating the original .f would work, i.e.

map(x, f)
# something went wrong

# update `f`
f <- function(x) ...
# no need to pass the new `f` to `purrr_continue()`
purrr_continue()

[hadley]

Closing in favour of #1196