- Seal the hierarchy, then delete the `case _`. The wildcard you kept for safety is exactly what silences the exhaustivity warning the day someone adds a variant.
- Keep the effect type one layer deep. Two maps at the call site are cheaper than a transformer stack and the library it drags into the build, and they still read in six months.
- Put `require` straight in the case class body rather than hiding the constructor behind a smart apply. `copy` re-runs the body, so the invariant holds everywhere for one line.
- Spend implicits and givens on typeclass instances and nothing else. An implicit conversion or config value saves one argument and charges every later reader a grep.
- A local `var` that cannot escape its method is not shared state. Take the plain loop over the fold that needs a tuple accumulator and a paragraph explaining itself.