user case of tar_skip / tar_change, deal with empty targets

[ropensci]

user case of tar_skip / tar_change, deal with empty targets

[wlandau]

My recommendations are to:

1. Use tar_files() instead of tar_change() and tar_file() individually (but please update tarchetypes to 6d72150 or later).
2. Write a custom function to that handles the case when the directory is empty. In the example below it returns an error via stopifnot(), but you can implement different behavior: for example, supply the path of an existing file containing default placeholder data. I believe these kinds of checks should be handled early in the pipeline.

library(targets)
dir.create("tmp")
writeLines("line1", "tmp/a")
writeLines("line2", "tmp/b")

tar_script({
  library(tarchetypes)
  options(crayon.enabled = FALSE)
  list_files <- function(dir) {
    files <- dir(dir, full.names = TRUE)
    stopifnot(length(files) > 0L)
    files
  }
  tar_pipeline(
    tar_files(files, list_files("tmp")),
    tar_target(lines, readLines(files), pattern = map(files))
  )
})

tar_make()
#> ● run target files_files
#> ● run branch files_98d8cd68
#> ● run branch files_33ec7836
#> ● run branch lines_3ac9849f
#> ● run branch lines_e2330cb9

tar_read(lines)
#> [1] "line1" "line2"

^{Created on 2020-10-26 by the reprex package (v0.3.0)}

There are workarounds that do not require tar_files():

library(targets)
dir.create("tmp")
writeLines("line1", "tmp/a")
writeLines("line2", "tmp/b")

tar_script({
  options(crayon.enabled = FALSE)
  list_files <- function(dir) {
    files <- dir(dir, full.names = TRUE)
    stopifnot(length(files) > 0L)
    files
  }
  tar_pipeline(
    tar_target(paths, list_files("tmp"), cue = tar_cue(mode = "always")),
    tar_target(files, paths, pattern = map(paths), format = "file"),
    tar_target(lines, readLines(files), pattern = map(files))
  )
})

tar_make()
#> ● run target paths
#> ● run branch files_fe460508
#> ● run branch files_dcad1233
#> ● run branch lines_ef9c4ed9
#> ● run branch lines_bc722500

^{Created on 2020-10-26 by the reprex package (v0.3.0)}

[ginolhac]

I ended using tar_files that remove all md5sum I was doing manually. And great idea with a placeholder, when I have no file, I catch this with a dummy file and listing further down is adapted. Thanks again! Much appreciated!

[ginolhac]

Thanks a lot William for your incredible reactivity and helpful comments. I will try both approaches tomorrow.
Thanks again for your precious time

[wlandau]

Glad to hear it's working for you.