Before writing a function, **look for an existing packaged one that already
does the job** --- and prefer it over rolling your own:

- Check, roughly in this order: base R and the **tidyverse / r-lib** packages,
  then a focused, well-maintained CRAN package, then **our own lab packages**
  (e.g. `{bcs}`, `{ettbc}`, `{gha}`, and the shared workflows there). Packages
  can depend on each other, so reuse across our repos is fine.
- Reach for the packaged version unless it is genuinely unfit --- the wrong
  API, a heavy dependency for a one-liner, or it doesn't quite do what you
  need.

Packaged functions are tested, documented, and maintained by other people;
hand-rolling an equivalent duplicates that work, adds surface area to maintain,
and risks subtle bugs the package already fixed. For example, use
`withr::with_seed()` to set a seed and restore the RNG stream, rather than
hand-rolling a `.Random.seed` save/restore.

This is a default, not an absolute rule. A tiny, dependency-free helper can
beat pulling in a package, and sometimes nothing fits --- but look first, and
prefer the standard, well-known way over a bespoke one.

This is the R-function special case of the broader don't-reinvent-the-wheel
principle --- see [`dont-reinvent-wheel`](../principles/dont-reinvent-wheel.md)
for the general statement, which also covers whole features, the
fork-or-contribute preference for close-but-not-exact matches, and the
review-side application.