Replicate the condition trigger from drake

[ropensci]

Replicate the condition trigger from drake

[wlandau]

Let's rename tar_suppress() to tar_skip().

tar_skip() is ostensibly the easier of the two logical directions. We just insert tar_cancel() inside the command. The following are equivalent:

tar_skip(x, run_command(), skip = should_skip())

tar_target(x, {
  targets::tar_cancel(should_skip())
  run_command()
})

Note: this allows the target to be skipped even if it does not already exist. This differs from tar_cue(mode = "never"), which still runs the target if it is missing from _targets/.

[wlandau]

tar_force() is a bit trickier. We should probably build it on top of tar_change() (#2) in a way that makes the auxiliary variable change at the right times. Sketch:

tar_force(x, run_command(), force = should_force())

which gets converted to

tar_change(x, run_command(), change = as_change(should_force()))

where

as_change <- function(condition) {
  previous <- trn(file.exists(tar_path()), readRDS(tar_path()), tempfile())
  trn(condition, tempfile(), previous)
}

We just need to update tar_change() so the auxiliary upstream target always has format = "rds". (We wouldn't want to go through tar_read() in this case anyway because it reads the metadata, which would be inefficient during tar_make().)