can we use cancel_if() with dynamic branching?

[ginolhac]

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Question

Hello,
your work is very much impressive and I'd to thank you for the hard work and dedication to this great package. I have a first success building a Rmd website with renv and gitlab-CI, super nice. The examples shared really helped a lot. Of course, now I want more complex things and I am wondering how to cancel a target with dynamic branching.
I know you are working getting sub-targets working for dynamic and it sounds complex (#1214), here I want to cancel a complete dynamic target based on a condition. I tried the
trigger = trigger(condition())

but got:

Error: Dynamic grouping variables are forbidden in the condition and change triggers. Found dynamic grouping variables for target downstream:
  empty

Then, I went to #1131 and thought this could be nice (thanks for #1212 BTW, that I discover testing the dev version of drake) with cancel_if()

library(drake)

f <- function(x) {
  cancel_if(length(x) < 1)
  "x"
}

plan <- drake_plan(
  empty = numeric(0), # with numeric(1) works
  downstream = target(f(empty), 
                     # trigger = trigger(condition = length(empty) > 0),
                      dynamic = map(empty))
)

make(plan)
#> ▶ target empty
#> ▶ dynamic downstream
#> > subtarget downstream_
#> Error: dynamic grouping variable empty needs more than 0 elements.

^{Created on 2020-03-13 by the reprex package (v0.3.0)}

so my question is:
how can I stop a dynamic target from being done based on a condition like no elements? In my case, I am building a teaching website, I have lecture / practicals / projects and they are all dynamic since appearing as the course goes on. With at least one of each, everything is fine.
I can also use keep_going = TRUE to survive one target failure but I don't like it. I'd prefer to invalidate right away a target (and dependencies) if basically nothing needs to be done, like no Rmd in practicals for example?
Many thanks in advance,

[wlandau]

Thanks for the support and encouragement. The best workaround I can think of is to flag emptiness with an NA and then cancel when you see it.

library(drake)

f <- function(x) {
  cancel_if(is.na(x)) # Cancel if you see the padded NA
  "x"
}

na_empty <- function(x) {
  if (!length(x)) {
    x <- NA
  }
  x
}

# Empty case
plan <- drake_plan(
  empty = na_empty(numeric(0)),
  downstream = target(
    f(empty), 
    dynamic = map(empty)
  )
)

make(plan)
#> ▶ target empty
#> ▶ dynamic downstream
#> > subtarget downstream_e6fd16da
#> ■ cancel downstream_e6fd16da
#> ■ finalize downstream

# Nonempty case
plan <- drake_plan(
  empty = na_empty(numeric(1)),
  downstream = target(
    f(empty), 
    dynamic = map(empty)
  )
)

make(plan)
#> ▶ target empty
#> ▶ dynamic downstream
#> > subtarget downstream_b3ddad2c
#> ■ finalize downstream

^{Created on 2020-03-13 by the reprex package (v0.3.0)}

[ginolhac]

Wow! thanks a lot, that's really nice, and easy enough.
I can safely drop the keep_going = TRUE and even the cancel gets its own orange color ;)
thanks again for such a fast help