2nd/10_Wiki/Topics/Domain_Programming/Architecture/The Two Hats.md

id, title, category, status, canonical_id, aliases, duplicate_of, source_trust_level, confidence_score, verification_status, tags, raw_sources, last_reinforced, github_commit, tech_stack

id	title	category	status	canonical_id	aliases	duplicate_of	source_trust_level	confidence_score	verification_status	tags	raw_sources	last_reinforced	github_commit	tech_stack
wiki-2026-0508-the-two-hats	The Two Hats	10_Wiki/Topics	verified	self	Two Hats Rule Beck's Two Hats 두 모자	none	A	0.92	applied	tdd refactoring kent-beck discipline		2026-05-10	pending	language framework TypeScript Vitest

The Two Hats

매 한 줄

"매 한 번에 한 hat 만 — adding feature OR refactoring, never both". Kent Beck 의 TDD By Example (2003) 의 핵심 discipline. 매 mode-switching 의 명시적 awareness 가 매 code change 의 quality 를 결정. 매 Feathers 의 legacy work 에서도 동일 principle.

매 핵심

매 두 hat

• Adding Feature hat: 매 new behavior. 매 fail test 작성 → minimum code 로 pass. 매 design improvement 의 X.
• Refactoring hat: 매 behavior 의 preservation. 매 all test green. 매 new feature 의 X.

매 switching rule

1. 매 hat 의 명시적 awareness — 매 commit message 에 표시.
2. 매 transition 시 모든 test green 확인.
3. 매 hat 을 동시에 쓰면 — 매 step back, 매 separate commit.

매 응용

1. PR 분리 — 매 refactor PR + 매 feature PR (reviewer 부담 감소).
2. Commit hygiene — 매 "refactor: extract method" + 매 "feat: add discount logic".
3. Code review — 매 "이거 refactor 야 feature 야?" 질문 의 명시.

💻 패턴

Hat marker in commit

# Refactor hat
git commit -m "refactor: extract calculatePrice into PriceCalculator

No behavior change. All 142 tests green before and after."

# Feature hat
git commit -m "feat: add VIP tier discount

- Add VIPDiscount strategy
- Test: VIP gets 20% off
- Test: STANDARD unchanged"

TDD cycle with hats

// === FEATURE HAT ===
// 1. Write failing test
it('VIP customer gets 20% discount', () => {
  expect(calculateDiscount(100, 'VIP')).toBe(80);
});

// 2. Minimum code to pass (ugly OK)
function calculateDiscount(price: number, tier: string): number {
  if (tier === 'VIP') return price * 0.8;
  return price;
}

// === REFACTOR HAT === (all tests green)
// 3. Improve design without behavior change
type Tier = 'VIP' | 'STANDARD';
const DISCOUNT: Record<Tier, number> = { VIP: 0.2, STANDARD: 0 };

function calculateDiscount(price: number, tier: Tier): number {
  return price * (1 - DISCOUNT[tier]);
}
// Run tests → green. Commit as refactor.

// === FEATURE HAT === again for next behavior
it('GOLD customer gets 10% discount', () => { /* ... */ });

Mode-switch checklist

// Before switching to Refactor hat:
// [ ] All tests green?
// [ ] Last commit clean?
// [ ] No half-written feature?

// Before switching to Feature hat:
// [ ] Refactor commit complete?
// [ ] Tests still green?
// [ ] Clear what new behavior to add?

Sprout method (Feathers, refactor-friendly feature add)

// Legacy mess — don't refactor (Feature hat)
function processOrder(order: any) {
  // ... 300 lines, low test coverage ...
}

// Add new behavior in NEW pure function (Feature hat)
export function calculateLoyaltyPoints(order: Order): number {
  return Math.floor(order.total / 10);
}

// Wire it in — minimal touch
function processOrder(order: any) {
  const points = calculateLoyaltyPoints(order);  // sprout
  // ... existing mess unchanged ...
}
// Later, separate Refactor hat session can clean up `processOrder`.

Detecting hat conflation (smell)

// SMELL: PR titled "Add discount logic" but diff includes:
//   - new feature ✓
//   - rename 5 unrelated variables ✗
//   - extract 3 unrelated methods ✗
//   - reformat whole file ✗
//
// Reviewer cannot tell what changed semantically.
// Fix: split into 3-4 commits/PRs by hat.

Pre-commit hook (hat enforcement)

#!/bin/bash
# .git/hooks/pre-commit
msg=$(cat "$1")
if [[ ! "$msg" =~ ^(feat|fix|refactor|docs|test|chore): ]]; then
  echo "Commit message must start with type prefix indicating hat"
  exit 1
fi

매 결정 기준

상황	Hat
New behavior needed	Feature
Existing test green, code ugly	Refactor
Bug fix (changes behavior to correct)	Feature (failing test first)
Rename for clarity	Refactor
Extract method	Refactor
Add parameter	Feature (likely)
Mixed urge	Stop — split into two

기본값: 매 explicit hat awareness, 매 separate commit, 매 green test 의 transition gate.

🔗 Graph

• 부모: Test-Driven_Development · Refactoring_Best_Practices
• 변형: Red-Green-Refactor
• 응용: Sprout Method
• Adjacent: Testability_Architecture · Technical_Debt · Conventional Commits

🤖 LLM 활용

언제: PR splitting advice, commit message disciplining, refactor-vs-feature classification. 언제 X: 매 actual editor mode-switching automation — 매 human discipline 이 본질.

❌ 안티패턴

• Mixed PR: refactor + feature 의 단일 PR — 매 review 가 noise 에 묻힘.
• "While I'm here": 매 unrelated tweak — 매 feature 가 endless drift.
• Refactor in red: 매 broken test 에서 refactor — 매 behavior change 의 hidden source.
• Big-bang refactor: 매 multi-day refactor 의 isolation — 매 daily incremental 이 안전.

🧪 검증 / 중복

• Verified (Beck 2003 TDD By Example; Feathers 2004 WELC).
• 신뢰도 A.

🕓 Changelog

날짜	변경
2026-05-08	Phase 1
2026-05-10	Manual cleanup — Beck two hats + sprout + commit hygiene