[help] How to tidy-select target outputs within the command of a downstream target

[shiyw]

Help

• I understand and agree to https://books.ropensci.org/targets/help.html.

Description

Intention: in the second tar_map, I want to access the outputs of the first tar_map with name a statically expanded to a_1 and a_2.

list(
  tar_map(
    values = tibble::tibble(x = 1:3),
    tar_target(a, x)
  ),
  tar_map(
    values = tibble::tibble(y = 1:2),
    tar_target(b, a + y)
  )
)

However, when tar_make triggers the error: a does not exist.

I tried a few walkaround.

Those failed:

• get(paste0("a_", y)) : a_1 is not in the envir
• tar_load(paste0("a_", y)) or tar_read(paste0("a_", y)) : tar_load and tar_read are not allowed

Thus, I have to explicitly include these variables so that they can be captured into the enclosure. Like:

list(
  tar_map(
    values = tibble::tibble(x = 1:3),
    tar_target(a, x)
  ),
  tar_map(
    values = tibble::tibble(y = 1:2),
    tar_target(
      b,
      {
        list(a_1, a_2) |> invisible()
        get(paste0("a_", y)) + y
      }
    )
  )
)

Wrapping it up:

list(
  tar_map(
    values = tibble::tibble(x = 1:3),
    tar_target(a, x)
  ),
  tar_map(
    values = tibble::tibble(y = 1:2),
    tar_target(
      b,
      {
        tar_use(a_1, a_2)
        tar_get("a", y) + y
        # a <- tar_get2("a")  # factory version
        # a(y) + y  
      }
    )
  )
)

with some utils:

# should be tar_sourced
tar_use <- \(...) {}
tar_get <- function(basename, ...) {
  env <- parent.frame()
  get(paste(basename, ..., sep = "_"), envir = env)
}
# factory version
tar_get2 <- function(basename) {
  env <- parent.frame()
  function(...) {
    get(paste(basename, ..., sep = "_"), envir = env)
  }
}

My question: Is there any better (tidier) way to collect branching outputs within command of tar_target? For example, explicitly declare (with tidy_select) names by an parameter of tar_target.

PS: the required feature may also affect construction of dependency DAG. But currently I only care about the former.

[wlandau]

If I understand correctly, you want some branches of target a, and you want each branch of target a to have two branches of target b:

You can express this with nested tar_map()s. The inner tar_map() explains how b branches from a, and then the outer tar_map() replicates the whole thing over multiple branches of a.

library(targets)
library(tarchetypes)
list(
  tar_map(
    values = tibble::tibble(x = 1:2),
    tar_target(a, x),
    tar_map(
      values = tibble::tibble(y = 1:2),
      tar_target(b, a + y)
    )
  )
)

[shiyw]

I want some (but not all) branches of target a to have two branches of target b.

[wlandau]

In that case, you would need the full flexibility of something like tar_eval() (c.f. https://books.ropensci.org/targets/static.html#metaprogramming) or manually write out each target without static branching.

library(targets)
library(tarchetypes)
list(
  tar_eval(
    values = list(a = rlang::syms(c("a_1", "a_2"))),
    tar_target(a, x)
  ),
  tar_eval(
    values =  list(
      a = rlang::syms(c("a_1", "a_1", "a_2")),
      b = rlang::syms(c("b_1", "b_2", "b_3"))
    ),
    tar_target(b, a + y)
  )
)