2nd/10_Wiki/Topics/Architecture/Replace Conditional with Polymorphism (조건식을 다형성으로 바꾸기).md

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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-replace-conditional-with-polymor	Replace Conditional with Polymorphism (조건식을 다형성으로 바꾸기)	10_Wiki/Topics	verified	self	Replace Conditional with Polymorphism RCwP Polymorphism Refactoring	none	A	0.9	applied	refactoring oop design-pattern fowler		2026-05-10	pending	language framework TypeScript none

Replace Conditional with Polymorphism (조건식을 다형성으로 바꾸기)

매 한 줄

"매 type-discriminating switch/if-chain 을 매 polymorphic dispatch 로 교체". Fowler Refactoring 2e (2018) Ch.10. 매 type code 가 add 시마다 매 switch 들 모두 수정 필요한 shotgun surgery 의 표준 처방. 매 modern alternative — discriminated union + exhaustiveness, strategy map, pattern matching — 도 같은 본질을 공유한다.

매 핵심

매 trigger smell

• 매 동일 switch (kind) 가 codebase 여러 곳에 반복.
• 매 새 type 추가가 매 N 곳의 switch 수정 강제.
• 매 default branch 가 silently incorrect.

매 mechanics (Fowler)

1. 매 self-encapsulating type 추출 (subclass 또는 strategy).
2. 매 switch 한 case 를 method override 로 이동 (test 유지).
3. 매 모든 case 이동 후 매 base method 를 abstract 화.
4. 매 caller 에서 매 conditional 제거.

매 응용

1. Tax calculation: country-specific subclass.
2. Bird movement: Penguin/Parrot/Owl getSpeed() override.
3. Payment processor: Stripe/Toss/PayPal strategy.
4. AST visitor: node type 별 dispatch.

💻 패턴

Before: type-code switch

type Bird = {
  kind: 'penguin' | 'parrot' | 'owl';
  voltage?: number;
  isNailed?: boolean;
  numberOfCoconuts?: number;
};

function plumage(bird: Bird): string {
  switch (bird.kind) {
    case 'penguin': return 'average';
    case 'parrot': return bird.isNailed ? 'tattered' : 'beautiful';
    case 'owl': return 'attentive';
  }
}

function airSpeed(bird: Bird): number {
  switch (bird.kind) {
    case 'penguin': return 50;
    case 'parrot': return 80 - 5 * (bird.voltage ?? 0);
    case 'owl': return 12 * (bird.numberOfCoconuts ?? 0);
  }
}

After: polymorphism (classes)

abstract class Bird {
  abstract get plumage(): string;
  abstract get airSpeed(): number;
}

class Penguin extends Bird {
  get plumage() { return 'average'; }
  get airSpeed() { return 50; }
}

class Parrot extends Bird {
  constructor(private voltage = 0, private nailed = false) { super(); }
  get plumage() { return this.nailed ? 'tattered' : 'beautiful'; }
  get airSpeed() { return 80 - 5 * this.voltage; }
}

class Owl extends Bird {
  constructor(private coconuts = 0) { super(); }
  get plumage() { return 'attentive'; }
  get airSpeed() { return 12 * this.coconuts; }
}

Modern alternative: discriminated union + exhaustiveness

type Bird =
  | { kind: 'penguin' }
  | { kind: 'parrot'; voltage: number; nailed: boolean }
  | { kind: 'owl'; coconuts: number };

function airSpeed(b: Bird): number {
  switch (b.kind) {
    case 'penguin': return 50;
    case 'parrot':  return 80 - 5 * b.voltage;
    case 'owl':     return 12 * b.coconuts;
    default: {
      const _exhaustive: never = b;
      return _exhaustive;
    }
  }
}

Strategy map (data-driven dispatch)

const speedStrategies = {
  penguin: () => 50,
  parrot:  (b: { voltage: number }) => 80 - 5 * b.voltage,
  owl:     (b: { coconuts: number }) => 12 * b.coconuts,
} as const;

function airSpeed<K extends keyof typeof speedStrategies>(
  kind: K,
  data: Parameters<typeof speedStrategies[K]>[0],
): number {
  return (speedStrategies[kind] as any)(data);
}

Visitor (when ops > types)

interface Visitor<R> {
  penguin(): R;
  parrot(b: { voltage: number; nailed: boolean }): R;
  owl(b: { coconuts: number }): R;
}

class SpeedVisitor implements Visitor<number> {
  penguin() { return 50; }
  parrot(b: { voltage: number }) { return 80 - 5 * b.voltage; }
  owl(b: { coconuts: number }) { return 12 * b.coconuts; }
}

Rust pattern matching (sister technique)

enum Bird {
    Penguin,
    Parrot { voltage: u32, nailed: bool },
    Owl { coconuts: u32 },
}

impl Bird {
    fn air_speed(&self) -> u32 {
        match self {
            Bird::Penguin => 50,
            Bird::Parrot { voltage, .. } => 80 - 5 * voltage,
            Bird::Owl { coconuts } => 12 * coconuts,
        }
    }
}

매 결정 기준

상황	Approach
OOP language, type 자주 add	Subclass polymorphism
FP/TS, op 자주 add	Discriminated union + match (expression problem)
매 dispatch table 단순	Strategy map (object literal)
Op 많고 type 안정적	Visitor
Type 많고 op 안정적	Sum type + match
매 single-use switch	매 그대로 두기 — refactor cost > benefit

기본값: TS — discriminated union + exhaustive switch. OOP-heavy domain — subclass.

🔗 Graph

• 부모: Refactoring_Best_Practices · Polymorphism
• 변형: Visitor Pattern
• 응용: Discriminated Union · Pattern Matching · Open-Closed Principle

🤖 LLM 활용

언제: 매 codebase 에 매 동일 switch 가 3+ 곳, 매 새 type 추가가 자주, 매 OCP 위반 가시. 언제 X: 매 switch 1 곳, 매 type 안정적, 매 small dispatch — over-engineering.

❌ 안티패턴

• Inheritance for everything: 매 deep hierarchy — composition + interface 권장.
• Polymorphism without exhaustiveness: 매 default branch 가 silent bug 숨김.
• Anemic subclass: 매 method 1개 차이 — strategy 가 더 적합.
• Refactor without tests: 매 behavior preservation 의 보증 없음.
• instanceof chain: 매 polymorphism 의 antithesis — 매 새 switch 의 변종.

🧪 검증 / 중복

• Verified: Fowler Refactoring 2e (2018) Ch.10 "Replace Conditional with Polymorphism"; refactoring.com.
• 신뢰도 A.

🕓 Changelog

날짜	변경
2026-05-08	Phase 1
2026-05-10	Manual cleanup — Fowler mechanics + modern union/strategy/visitor variants