Refactor safely

A refactor promises that behaviour stays the same, so it needs evidence. Mixing behaviour changes into a move makes that impossible to review.

When to use

The goal is better structure with the same behaviour. If behaviour must change too, do it in a separate commit.

Steps

1. Record the baseline. Run the tests, type check and build, and save the results. Stop and fix the baseline first if it is failing in this area.

2. Check that tests protect this code. If the code has no tests, write tests that record its current behaviour first, using write-tests.

3. Move in small mechanical steps. Use the editor or language tools to rename and move where you can. Prefer git mv, so that history follows the file.

4. Commit structure apart from behaviour. A reviewer should be able to read a move commit as a move.

5. Update everything that names the old path. Search for it in imports, tests, docs, build config, CI, scripts, and strings built at runtime. Strings are easy to miss, because neither the compiler nor search-and-replace on imports finds them.

6. Check that tests still test something. A test that looks through a folder or matches a path can pass with nothing to check after a move. Confirm that test counts did not fall, and that each path-based check still finds its files.

7. Prove that behaviour is unchanged. Use the strongest check you can:

   • Compare the built output before and after, with paths normalized.
   • Compare the test count and results with the baseline.
   • Compare the public API or exported names.
   • Run the type checker with unused-name checks on.

8. Finish. Follow verify-and-report.

Stop and ask when

• You find a bug. Mention it, and do not fix it inside the refactor.
• A clean move needs a public API change.
• The change is too large to review, and it could be split.

Common mistakes

Tempting move	Why it fails	Do this instead
Fix a bug during the move	Reviewers cannot tell what changed	Use a separate commit
Reformat everything	Buries the real change	Keep formatting out
Say tests still pass	Fewer tests may be running	Compare the counts
Delete code that looks unused	It may be reached dynamically	Search for it, including strings
Add a new abstraction	That is a design change, not a refactor	Keep to the agreed goal

Report

Structure changed: <what moved>
Behaviour changed: none
Equivalence proof: <what you compared, and the result>
Test count: <before> -> <after>
References updated: <imports, docs, CI, scripts>