using glue::glue strings as targets

[jmbuhr]

Hi there, the tidyverse glue package (https://glue.tidyverse.org/reference/glue.html) allows neat string interpolation with variable substitution such as

x <- "hello"
glue::glue("{x} world")

however, due to the way theses strings are evaluated glue looks for theses variables in it's parent frame but is not able to find targets:

list(
  tar_target(input, "hello"),
  tar_target(test, glue::glue("{input} world"))
)

#> object 'input' not found

One way around that is assigning the upstream target to a temporary variable:

list(
  tar_target(input, "hello"),
  tar_target(test,{x <- input; glue::glue("{x} world")})
)

Is there a more elegant solution for this?

[wlandau]

In your first pipeline, the symbol input does not appear in the command of the target test, so targets does not know that test depends on input. Your workaround exposes the symbol and fixes the problem in that case. In theory it should be possible to write a function that automatically looks for glue strings and creates expressions with explicit symbols, but that's a lot of work. I do not know of other alternatives.

[nicki-dese]

Just wanted to add that glue actually makes it easy to work around this problem, because you can specify the variables you're calling within the function like so:

list(
  tar_target(input, "hello"),
  tar_target(test, glue::glue("{x} world", x = input))
)