knitr language engine that runs {targets} code chunks in Target Markdown.

tar_engine_knitr(options)

Arguments

options

A named list of knitr chunk options.

Value

Character, output generated from knitr::engine_output().

Target Markdown interactive mode

Target Markdown has two modes:

  1. Non-interactive mode. This is the default when you run knitr::knit() or rmarkdown::render(). Here, the code in {targets} code chunks gets written to special script files in order to set up a targets pipeline to run later.

  2. Interactive mode: here, no scripts are written to set up a pipeline. Rather, the globals or targets in question are run in the current environment and the values are assigned to that environment.

The mode is interactive if !isTRUE(getOption("knitr.in.progress")), is TRUE. The knitr.in.progress option is TRUE when you run knitr::knit() or rmarkdown::render() and NULL if you are running one chunk at a time interactively in an integrated development environment, e.g. the notebook interface in RStudio: https://bookdown.org/yihui/rmarkdown/notebook.html. You can choose the mode with the tar_interactive chunk option. (In targets 0.6.0, tar_interactive defaults to interactive() instead of !isTRUE(getOption("knitr.in.progress")).)

Target Markdown chunk options

Target Markdown introduces the following knitr code chunk options. Most other standard knitr code chunk options should just work in non-interactive mode. In interactive mode, not all

  • tar_globals: Logical of length 1, whether to define globals or targets. If TRUE, the chunk code defines functions, objects, and options common to all the targets. If FALSE or NULL (default), then the chunk returns formal targets for the pipeline.

  • tar_interactive: Logical of length 1, whether to run in interactive mode or non-interactive mode. See the "Target Markdown interactive mode" section of this help file for details.

  • tar_name: name to use for writing helper script files (e.g. _targets_r/targets/target_script.R) and specifying target names if the tar_simple chunk option is TRUE. All helper scripts and target names must have unique names, so please do not set this option globally with knitr::opts_chunk$set().

  • tar_script: Character of length 1, where to write the target script file in non-interactive mode. Most users can skip this option and stick with the default _targets.R script path. Helper script files are always written next to the target script in a folder with an "_r" suffix. The tar_script path must either be absolute or be relative to the project root (where you call tar_make() or similar). If not specified, the target script path defaults to tar_config_get("script") (default: _targets.R; helpers default: _targets_r/). When you run tar_make() etc. with a non-default target script, you must select the correct target script file either with the script argument or with tar_config_set(script = ...). The function will source() the script file from the current working directory (i.e. with chdir = FALSE in source()).

  • tar_simple: Logical of length 1. Set to TRUE to define a single target with a simplified interface. In code chunks with tar_simple equal to TRUE, the chunk label (or the tar_name chunk option if you set it) becomes the name, and the chunk code becomes the command. In other words, a code chunk with label targetname and command mycommand() automatically gets converted to tar_target(name = targetname, command = mycommand()). All other arguments of tar_target() remain at their default values (configurable with tar_option_set() in a tar_globals = TRUE chunk).

Examples

if (identical(Sys.getenv("TAR_EXAMPLES"), "true")) {
# Register the engine.
if (requireNamespace("knitr", quietly = TRUE)) {
  knitr::knit_engines$set(targets = targets::tar_engine_knitr)
}
# Then, {targets} code chunks in a knitr report will run
# as described at https://books.ropensci.org/targets/markdown.html.
}