R/tar_hook_outer.R

In tarchetypes: Archetypes for Targets

#' @title Hook to wrap commands
#' @export
#' @family hooks
#' @description Wrap the command of each target in an arbitrary R expression.
#' @details The expression you supply to `hook`
#'   must contain the special placeholder symbol `.x`
#'   so `tar_hook_outer()` knows where to insert the original command
#'   of the target.
#' @return 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.
#' @inheritSection tar_map Target objects
#' @inheritParams tar_hook_before
#' @param hook R code to wrap each target's command.
#'   The hook must contain the special placeholder symbol `.x`
#'   so `tar_hook_outer()` knows where to insert the original command
#'   of the target.
#'   The hook code is quoted (not evaluated) so there is no need
#'   to wrap it in `quote()`, `expression()`, or similar.
#' @examples
#' if (identical(Sys.getenv("TAR_LONG_EXAMPLES"), "true")) {
#' targets::tar_dir({ # tar_dir() runs code from a temporary directory.
#' targets::tar_script({
#'   targets <- list(
#'     # Nested target lists work with hooks.
#'     list(
#'       targets::tar_target(x1, task1()),
#'       targets::tar_target(x2, task2(x1))
#'     ),
#'     targets::tar_target(x3, task3(x2)),
#'     targets::tar_target(y1, task4(x3))
#'   )
#'   tarchetypes::tar_hook_outer(
#'     targets = targets,
#'     hook = postprocess(.x, arg = "value"),
#'     names = starts_with("x")
#'   )
#' })
#' targets::tar_manifest(fields = command)
#' })
#' }
tar_hook_outer <- function(targets, hook, names = NULL, set_deps = TRUE) {
  targets::tar_assert_scalar(set_deps)
  targets::tar_assert_lgl(set_deps)
  targets::tar_assert_nonmissing(set_deps)
  targets <- tar_copy_targets(targets)
  hook <- substitute(hook)
  targets::tar_assert_lang(hook)
  assert_hook_placeholder(hook)
  names_quosure <- rlang::enquo(names)
  walk_targets(
    targets = targets,
    names_quosure = names_quosure,
    fun = tar_hook_outer_insert,
    hook = hook,
    set_deps = set_deps
  )
  targets
}

tar_hook_outer_insert <- function(target, hook, set_deps) {
  assert_hook_expr(target)
  lang <- target$command$expr[[1]]
  expr <- tar_sub_expr(hook, values = list(.x = lang))
  tar_replace_command(target = target, expr = expr, set_deps = set_deps)
}