object not found when creating targets list programmatically - targets-r-package

Question

I'm trying to generate a {targets} list programmatically, via a function in an R package.
get_pipeline <- function(which_countries) {
countries <- NULL # avoid R CMD CHECK warning
print(which_countries) # Shows that which_countries is available
list(
targets::tar_target(
name = countries,
command = which_countries # But here, which_countries is not found
)
)
}
The _targets.R file looks like this:
library(targets)
couns <- c("USA", "GBR")
TargetsQuestions::get_pipeline(couns)
I see the following error:
> tar_make()
[1] "USA" "GBR"
Error in enexpr(expr) : object 'which_countries' not found
Error in `tar_throw_run()`:
! callr subprocess failed: object 'which_countries' not found
Note that the which_countries variable is printable, but not found in the call to tar_target.
How can I get create the countries target successfully so that it contains the vector c("USA", "GBR")?
This code is in a GitHub repository at https://github.com/MatthewHeun/TargetsQuestions. To reproduce:
git clone https://github.com/MatthewHeun/TargetsQuestions
Build the package in RStudio.
targets::tar_make() at the Console.
Thanks in advance for any suggestions!

Answer 1

Thanks to #landau for pointing to https://wlandau.github.io/targetopia/contributing.html#target-factories which in turn points to the metaprogramming section of Advanced R at https://adv-r.hadley.nz/metaprogramming.html.
The solution turned out to be:
get_pipeline <- function(which_countries) {
list(
targets::tar_target_raw(
name = "countries",
# command = which_countries # which_countries must have length 1
# command = !!which_countries # invalid argument type
command = rlang::enexpr(which_countries) # Works
)
)
}
With _targets.R like this:
library(targets)
couns <- c("USA", "GBR")
TargetsQuestions::get_pipeline(couns)
the commands tar_make() and tar_read(countries), give
[1] "USA" "GBR"
as expected!