targets-branching

name	targets-branching
description	Dynamic branching patterns and aggregation in targets pipelines. Use when creating branched targets with pattern = map(), setting up parameter targets, or handling branch aggregation.
user-invocable	false

Dynamic branching in targets

Use dynamic branching with pattern = map() inside tar_assign(). Do not use static branching (tar_map()) because tar_assign() injects the left-hand side of each assignment as the name argument into the right-hand side function call, and tar_map() does not have a name argument (its first argument is values).

Parameter targets

Define a parameter target that returns a one-row-per-group tibble, then branch over it:

tar_assign({
  params = make_params() |>
    tar_target()

  result = compute(data, params$group_var) |>
    tar_target(pattern = map(params))
})

Use NA (not NULL) for unused parameter values, since data frame rows cannot hold NULL.

How dynamic branch aggregation works

When a downstream target consumes a branched target without its own pattern, targets automatically aggregates all branches. The aggregation behavior depends on the object type:

• Data frames are row-bound. The aggregated result has no column identifying which branch produced each row. If downstream targets need to distinguish branches, carry a group identifier (e.g. age_label) through the data from the start.
• Non-data-frame objects (e.g. ggplot objects) cannot be row-bound and will error. Use iteration = "list" on the branched target so branches aggregate into a list:

  my_plot = make_plot(branch_data, params$label) |>
    tar_target(pattern = map(branch_data, params), iteration = "list")

  combined = combine_plots(my_plot) |>
    tar_target()

Writing functions for aggregated branches

Functions that consume aggregated branches should expect the aggregated form directly — a single data frame (not a list of data frames) or a list of non-df objects. Do not write functions that map() over a list of data frames when the input will be one row-bound data frame.