feat: re-add simplify= to print.compare.loo (#366)

[LeonidasZhak]

Summary

This follow-up keeps the simplify restoration on print.compare.loo() and adds the missing focused coverage for the full-output path.

Thanks to maintainers

Thanks for the review and for pointing out the missing tests, scope cleanup, and AI disclosure requirement.

Issue or motivation

This PR follows up on #366 by restoring simplify = FALSE for print.compare.loo(), while keeping the default simplified output unchanged.

Root cause

The earlier branch restored the argument and print-path behavior, but it did not include focused test coverage for the full-output branch and it also carried an unrelated generated-file change.

Change

• restore simplify in print.compare.loo() with default TRUE
• document the restored full-output mode
• add focused tests for print(comp2, simplify = FALSE) and the full comparison columns
• keep the PR diff scoped by excluding the unrelated loo-package.Rd drift

Tests

• Rscript -e 'devtools::test(filter = "compare", stop_on_failure = TRUE)'
• R CMD build --no-build-vignettes .
• _R_CHECK_FORCE_SUGGESTS_=false R CMD check --no-manual --ignore-vignettes --no-build-vignettes loo_2.9.0.9000.tar.gz

I used AI assistance to help check for issues, refine the code, and run tests.

Scope

This follow-up does not change the default printed output, does not alter model-comparison calculations, and does not expand the feature beyond the existing print.compare.loo() path discussed here.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 12.50000% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.49%. Comparing base (286263d) to head (beec4b6).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
R/loo_compare.R	12.50%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #369      +/-   ##
==========================================
- Coverage   92.70%   92.49%   -0.22%     
==========================================
  Files          31       31              
  Lines        3029     3037       +8     
==========================================
+ Hits         2808     2809       +1     
- Misses        221      228       +7     

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[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if beec4b6 is merged into master:

• ✔️loo_function: 1.89s -> 1.89s [-0.6%, +0.33%]
• ✔️loo_matrix: 1.7s -> 1.7s [-0.58%, +0.67%]
  Further explanation regarding interpretation and methodology can be found in the documentation.

[florence-bockting]

Thank you @LeonidasZhak for opening this PR. We had some discussion about whether to include/exclude this argument from loo_compare in this PR.
Therefore, we (@avehtari, @jgabry) need probably to discuss first how we want to handle this aspect and then we will come back to your PR.

[florence-bockting]

Thanks again @LeonidasZhak for the PR.
Please read our AI contribution policy and disclose any use of AI.
Additionally, please add tests for any changes you make to functions; stating "manual verification" is not sufficient.
Finally, please avoid making changes to files unrelated to this PR, as was done in loo-package.Rd.

[LeonidasZhak]

Thanks for the guidance. I pushed a follow-up commit that adds focused test coverage for print(comp2, simplify = FALSE), keeps the assertions narrow, and removes the unrelated loo-package.Rd drift from the PR diff.

I used AI assistance to help check for issues, refine the code, and run tests.

Local checks I ran before pushing:

• Rscript -e 'devtools::test(filter = "compare", stop_on_failure = TRUE)'\n- R CMD build --no-manual --no-build-vignettes .\n- _R_CHECK_FORCE_SUGGESTS_=false R CMD check --ignore-vignettes --no-manual loo_2.9.0.9000.tar.gz\n- git diff --check