ch4570 Refactoring Catalog
A skill that improves structure through smell → prescription → small steps by applying
Martin Fowler's Refactoring (2nd ed.) catalog + a code-smell taxonomy + Clean Code heuristics.
For grounding facts see this skill's reference/kb/ (smell taxonomy, function/class refactorings,
conditionals/polymorphism, encapsulation moves, etc.); for higher-level principles see
reference/principles.md. The index is reference/kb/INDEX.md
(if you can't find the path, Glob for principles.md/INDEX.md from the bundle root).
Application order (proceed exactly like this)
- Confirm the safety net — grep for the target's regression tests. If absent, write a characterization test first (without one, cap a large change at MEDIUM).
- Smell scan — identify smells with the review hooks in
reference/kb/code-smells.mdand route throughreference/kb/INDEX.md. - Catalog mapping — prescribe a named refactoring for each smell (table below).
- Design small steps — split the prescription into behavior-preserving units and compile/test after each step.
- Report — Before/After + applied steps + verification commands. Prioritize by value÷risk.
Smell → refactoring quick table
| Smell | Prescription (catalog) |
|---|---|
| Duplicated code | Extract Function / Pull Up Method |
| Long function | Extract Function, Replace Temp with Query, Decompose Conditional |
| Long parameter list | Introduce Parameter Object, Preserve Whole Object |
| Large class (multiple responsibilities) | Extract Class |
| Feature Envy | Move Function / Move Field |
| Primitive obsession / data clumps | Replace Primitive with Object (Value Object) |
Repeated branching (when/if ladder) |
Replace Conditional with Polymorphism (sealed/strategy) |
| Nested conditionals | Replace Nested Conditional with Guard Clauses (early return) |
| Shotgun surgery / divergent change | Cohere via Move, Extract Class per responsibility |
Rules when prescribing
- Always cite the source (Fowler catalog name / Effective Kotlin item).
- Do not change behavior — if a feature change/bug fix is mixed in, separate it and report it apart.
- No over-engineering (KISS/YAGNI) — abstract only when duplication actually exists.
Example (Replace Conditional with Polymorphism)
// Before — type branching repeated across multiple methods
fun fee(type: PaymentType): Int = when (type) {
PaymentType.CARD -> 100; PaymentType.BANK -> 200
}
// After — sealed + polymorphism (adding a new type is the extension, OCP)
sealed interface Payment { fun fee(): Int }
data object Card : Payment { override fun fee() = 100 }
data object Bank : Payment { override fun fee() = 200 }
Application steps: ① model the sealed hierarchy → ② Move each branch body (test per case) → ③ replace call sites with the polymorphic call.