tar_sub {tarchetypes}

Create multiple expressions with symbol substitution.

Description

Loop over a grid of values and create an expression object from each one. Helps with general metaprogramming.

Usage

tar_sub(expr, values)

Arguments

expr

Starting expression. Values are iteratively substituted in place of symbols in expr to create each new expression.

values

List of values to substitute into expr to create the expressions. All elements of values must have the same length.

Value

A list of expression objects. Often, these expression objects evaluate to target objects (but not necessarily). See the "Target objects" section for background.

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 Metaprogramming utilities: tar_eval(), tar_eval_raw(), tar_sub_raw()

Examples

# tar_map() is incompatible with tar_render() because the latter
# operates on preexisting tar_target() objects. By contrast,
# tar_eval() and tar_sub() iterate over code farther upstream.
values <- list(
  name = lapply(c("name1", "name2"), as.symbol),
  file = list("file1.Rmd", "file2.Rmd")
)
tar_sub(tar_render(name, file), values = values)