reprexes

When a technical problem is hard to pin down, don’t debug it in the full context of the motivating application. Extract a minimal, self-contained reproduction of the phenomenon, iterate candidate fixes on that (a fast, clean loop), then port the working fix back to the real code.

Reference: https://r4ds.hadley.nz/workflow-help.html#making-a-reprex. The payoff is real: often, the act of building a thorough reprex surfaces the cause on its own — the noise you strip away was hiding it.

When this fires

  • A bug whose cause isn’t obvious after a first read of the code.
  • A failure that only shows up deep inside a large pipeline, app, or a full quarto render — where each reproduction attempt is slow.
  • Iterating on a fix that’s expensive to test in the full context.
  • Before asking a human for help or filing an upstream/package issue — a reprex is what they’ll ask for anyway.

Don’t bother for a one-line obvious fix, or a problem you can already see and test cheaply in place.

The two requirements

A reprex is only useful if it is BOTH:

  1. Reproducible — it captures everything needed to trigger the phenomenon: every library() call, and code that creates every object it uses. Someone (or a fresh session) can run it cold and see the same thing.
  2. Minimal — everything unrelated to the phenomenon is stripped out: small or built-in data instead of the real dataset, only the lines that matter.

The tension between these two is the whole game: include enough to reproduce, but nothing more.

Procedure

  1. Hypothesize the minimal trigger. What is the smallest combination of data + operation you believe causes the phenomenon?

  2. Create a standalone scratch file outside the repo tree (e.g. /tmp/reprex.R, or a tiny /tmp/reprex.qmd for a render bug; on a non-Unix machine use tempfile(fileext = ".R") / tempdir() for a portable path). Put in it, in order:

    • the package loads (library(...)),
    • the minimal data (see tactics below),
    • the minimal code that triggers the phenomenon, with a comment marking where it goes wrong.

  3. Run it in a clean session and confirm it reproduces. For R, run with Rscript --vanilla /tmp/reprex.R (a fresh process; --vanilla skips .Rprofile/.Renviron so no stale globalenv() state or profile settings mask or fake the bug). For a Quarto page, render just that file: quarto render /tmp/reprex.qmd --to html, not the whole site.

  4. Minimize. Remove pieces until the phenomenon disappears — the last removal that “fixes” it implicates that piece. (Or build up from nothing until it appears.) Keep the data as small as it can be while still failing.

  5. Iterate fixes on the reprex, not the full app. This is the fast loop the whole technique exists to create.

  6. Port the fix back to the real code and verify it there.

  7. Clean up. Delete the scratch file (it lives in /tmp, so it never touches the repo). If the bug was subtle, consider promoting the reprex into a real regression test (testthat) instead of discarding it.

Minimizing the data

  • Prefer a built-in dataset (mtcars, mpg) or a hand-built tiny frame over the real data.
  • If you must use a slice of real data, serialize the minimal slice with dput() so the reprex recreates it inline — no external file dependency.
  • Shrink to the fewest rows/columns that still show the phenomenon.

R / Quarto specifics

  • In R packages and Quarto projects, reprexes are usually short R snippets or a single standalone .qmd page. Respect the repo’s lint config if the reprex code will be ported back.

  • The reprex package (tidyverse, https://reprex.tidyverse.org/) formats a reprex for sharing: it runs your code in a clean, separate R session (via callr) and emits code plus actual output. Copy the code and call reprex::reprex() (reads the clipboard by default), or point it at a file with reprex::reprex(input = "/tmp/reprex.R") (or a tempfile(fileext = ".R") path on non-Unix machines) — handy from a non-interactive CLI session where there’s no clipboard. Use it when the output is destined for a PR comment or an upstream issue. Useful arguments:

    • venue = — output format:

      • "gh" — GitHub-flavored Markdown (default)
      • "so" / "ds" — Stack Overflow / Discourse
      • "slack" — Slack message
      • "r" — runnable R script with commented output
      • "html" — HTML
      • "rtf" — rich text for presentations

    • session_info = TRUE — append sessionInfo() / sessioninfo::session_info(), so versions travel with the reprex (set this when the bug may be version-dependent).

    • std_out_err = TRUE — capture stdout/stderr too (e.g. system() / subprocess or C-level output that doesn’t come back as normal R results).

    • wd = — set the working directory when the code needs one.

    • Validation bonus: because reprex::reprex() runs in a fresh session, if it errors on a missing object or package, your example wasn’t actually self-contained — fix that before sharing.

    Companion helpers handle “wild-caught” reprexes (all exported in reprex 2.x): reprex::reprex_clean() (strip the #> output markers from a rendered/pasted reprex, leaving runnable code), reprex::reprex_rescue() (recover code from R-console output with >/+ prompts), and reprex::reprex_invert() (the inverse of reprex::reprex() — recover the input code from a rendered reprex).

  • When the bug might be version-dependent, capture sessionInfo() (or set session_info = TRUE above) in the reprex so versions are part of the record. If you suspect stale packages are the cause, tidyverse::tidyverse_update() outside the reprex can rule that out — but it updates packages, it doesn’t record versions, so don’t put it in the reprex itself.

  • Build artifacts (_site/, _freeze/, .quarto/) are common confounders for “it renders differently” bugs — a clean standalone render sidesteps stale freeze caches.

Before declaring the reprex good

  • A fresh session runs it top-to-bottom and shows the phenomenon (and nothing else breaks first).
  • It references no object, file, or option it didn’t create itself.
  • It’s as small as you can make it and still reproduce.

Don’t

  • Don’t paste the entire app/module — that’s the opposite of a reprex.
  • Don’t commit scratch reprex files; keep them in /tmp (or tempdir() on non-Unix machines) or a gitignored scratch path.
  • Don’t iterate fixes in the slow full-context loop once you have a reprex that reproduces.
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