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Create multiple expressions with symbol substitution.

Source: R/tar_sub.R, R/tar_sub_raw.R
tar_sub.Rd

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

tar_sub() expects an unevaluated expression for the expr object, whereas tar_sub_raw() expects an evaluated expression object.

Usage

tar_sub(expr, values)

tar_sub_raw(expr, values)

Arguments

expr

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

tar_sub() expects an unevaluated expression for the expr object, whereas tar_sub_raw() expects an evaluated expression object.

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 definition objects (but not necessarily). See the "Target definition objects" section for background.

Target definition objects

Most tarchetypes functions are target factories, which means they return target definition objects or lists of target definition objects. target definition 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 definition 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 definition objects.

See also

Other Metaprogramming utilities: tar_eval()

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)
#> [[1]]
#> expression(tar_render(name1, "file1.Rmd"))
#> 
#> [[2]]
#> expression(tar_render(name2, "file2.Rmd"))
#> 
tar_sub_raw(quote(tar_render(name, file)), values = values)
#> [[1]]
#> expression(tar_render(name1, "file1.Rmd"))
#> 
#> [[2]]
#> expression(tar_render(name2, "file2.Rmd"))
#>