tar_hook_before: Hook to prepend code

View source: R/tar_hook_before.R

tar_hook_beforeR Documentation

Hook to prepend code


Prepend R code to the commands of multiple targets.


tar_hook_before(targets, hook, names = NULL, set_deps = TRUE)



A list of target objects. The input target list can be arbitrarily nested, but it must consist entirely of target objects. In addition, the return value is a simple list where each element is a target object. All hook functions remove the nested structure of the input target list.


R code to insert. When you supply code to this argument, the code is quoted (not evaluated) so there is no need to wrap it in quote(), expression(), or similar.


Name of targets in the target list to apply the hook. Supplied with tidyselect helpers like starts_with(), as in names = starts_with("your_prefix_"). Targets not included in names still remain in the target list, but they are not modified because the hook does not apply to them.


Logical of length 1, whether to refresh the dependencies of each modified target by scanning the newly generated target commands for dependencies. If FALSE, then the target will keep the original set of dependencies it had before the hook. TRUE is recommended for nearly all situations. Only use FALSE if you have a specialized use case and you know what you are doing.


A flattened list of target objects with the hooks applied. Even if the input target list had a nested structure, the return value is a simple list where each element is a target object. All hook functions remove the nested structure of the input target list.

Target objects

Most tarchetypes functions are target factories, which means they return target objects or lists of target objects. Target objects represent skippable steps of the analysis pipeline as described at https://books.ropensci.org/targets/. Please read the walkthrough at https://books.ropensci.org/targets/walkthrough.html to understand the role of target objects in analysis pipelines.

For developers, https://wlandau.github.io/targetopia/contributing.html#target-factories explains target factories (functions like this one which generate targets) and the design specification at https://books.ropensci.org/targets-design/ details the structure and composition of target objects.

See Also

Other hooks: tar_hook_inner(), tar_hook_outer()


if (identical(Sys.getenv("TAR_LONG_EXAMPLES"), "true")) {
targets::tar_dir({ # tar_dir() runs code from a temporary directory.
  targets <- list(
    # Nested target lists work with hooks.
      targets::tar_target(x1, task1()),
      targets::tar_target(x2, task2(x1))
    targets::tar_target(x3, task3(x2)),
    targets::tar_target(y1, task4(x3))
    targets = targets,
    hook = print("Running hook."),
    names = starts_with("x")
targets::tar_manifest(fields = command)

tarchetypes documentation built on Oct. 4, 2023, 5:08 p.m.