2nd/10_Wiki/Topics/Architecture/Discriminated_Unions.md

---
id: wiki-2026-0508-discriminated-unions
title: Discriminated Unions
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [Tagged Unions, Sum Types, Algebraic Data Types, ADT]
duplicate_of: none
source_trust_level: A
confidence_score: 0.95
verification_status: applied
tags: [typescript, types, functional, type-narrowing]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: typescript
  framework: type-system
---

# Discriminated Unions

## 매 한 줄
> **"매 한 literal field 의 매 type narrow"**. 매 TS / F# / Rust enum / Haskell ADT 의 same idea — 매 union 의 each variant 의 unique discriminant (tag) 의 carry, 매 compiler 의 매 switch 시 매 narrow. 매 2026 TS 5.7 의 매 dominant data modeling pattern.

## 매 핵심

### 매 anatomy
```typescript
type Result<T, E> =
  | { kind: "ok"; value: T }
  | { kind: "err"; error: E };
//    ^^^^^^^^^^^ 매 discriminant — 매 string / number / boolean literal.
```

### 매 narrowing rules
- 매 discriminant 의 literal 의 must.
- 매 `switch` / `if` 매 변수 의 narrow.
- 매 exhaustive check 의 `never` 의 leverage.

### 매 vs alternatives
- **vs class hierarchy**: 매 closed set, 매 pattern-match easy, 매 no `instanceof`.
- **vs enum**: 매 enum 의 variant 별 data 의 X — DU 의 carry.
- **vs untagged union**: 매 narrow 의 hard, 매 runtime check 의 brittle.

### 매 응용
1. Result / Option / Either monads.
2. State machine state.
3. Redux / xstate action / event.
4. API response variants.
5. AST node types.

## 💻 패턴

### Pattern 1: Result / Either
```typescript
type Result<T, E = Error> =
  | { ok: true; value: T }
  | { ok: false; error: E };

function parseJson<T>(s: string): Result<T> {
  try { return { ok: true, value: JSON.parse(s) }; }
  catch (e) { return { ok: false, error: e as Error }; }
}

const r = parseJson<User>(input);
if (r.ok) {
  r.value.name; // 매 narrowed 의 T
} else {
  r.error.message; // 매 narrowed 의 E
}
```

### Pattern 2: Exhaustive switch with `never`
```typescript
type Shape =
  | { kind: "circle"; radius: number }
  | { kind: "square"; side: number }
  | { kind: "rectangle"; w: number; h: number };

function area(s: Shape): number {
  switch (s.kind) {
    case "circle": return Math.PI * s.radius ** 2;
    case "square": return s.side ** 2;
    case "rectangle": return s.w * s.h;
    default:
      const _exhaustive: never = s;  // 매 새 variant 추가 시 compile error
      throw new Error(`unhandled: ${_exhaustive}`);
  }
}
```

### Pattern 3: State machine state
```typescript
type FetchState<T> =
  | { status: "idle" }
  | { status: "loading"; abortCtrl: AbortController }
  | { status: "success"; data: T; fetchedAt: Date }
  | { status: "error"; error: Error; retryCount: number };

function reducer<T>(s: FetchState<T>, ev: FetchEvent<T>): FetchState<T> {
  if (s.status === "loading" && ev.type === "abort") {
    s.abortCtrl.abort();
    return { status: "idle" };
  }
  // ...
}
```

### Pattern 4: Redux-style action
```typescript
type TodoAction =
  | { type: "ADD"; text: string }
  | { type: "TOGGLE"; id: string }
  | { type: "DELETE"; id: string }
  | { type: "EDIT"; id: string; text: string };

function reducer(state: Todo[], action: TodoAction): Todo[] {
  switch (action.type) {
    case "ADD": return [...state, { id: uid(), text: action.text, done: false }];
    case "TOGGLE": return state.map(t => t.id === action.id ? { ...t, done: !t.done } : t);
    case "DELETE": return state.filter(t => t.id !== action.id);
    case "EDIT": return state.map(t => t.id === action.id ? { ...t, text: action.text } : t);
  }
}
```

### Pattern 5: API response variants
```typescript
type ApiResponse<T> =
  | { status: 200; data: T }
  | { status: 401; reason: "expired" | "invalid" }
  | { status: 429; retryAfterSec: number }
  | { status: 500; traceId: string };

async function call<T>(url: string): Promise<ApiResponse<T>> { /* ... */ }
```

### Pattern 6: Pattern-match helper (ts-pattern)
```typescript
import { match, P } from "ts-pattern";

const message = match(response)
  .with({ status: 200 }, r => `Got ${r.data.length} items`)
  .with({ status: 401, reason: "expired" }, () => "Please refresh token")
  .with({ status: 429 }, r => `Wait ${r.retryAfterSec}s`)
  .with({ status: 500 }, r => `Error ${r.traceId}`)
  .exhaustive();
```

### Pattern 7: Zod parsing → DU
```typescript
import { z } from "zod";

const Event = z.discriminatedUnion("type", [
  z.object({ type: z.literal("click"), x: z.number(), y: z.number() }),
  z.object({ type: z.literal("key"), key: z.string() }),
  z.object({ type: z.literal("scroll"), dy: z.number() }),
]);
type Event = z.infer<typeof Event>;
```

### Pattern 8: assertNever helper
```typescript
export function assertNever(x: never): never {
  throw new Error(`unexpected: ${JSON.stringify(x)}`);
}
// 매 default case 의 use.
```

## 매 결정 기준
| 상황 | Approach |
|---|---|
| Closed set of variants | DU |
| Open extensible | class + interface |
| Boolean flag pair | DU (truthy combos 의 prevent) |
| Many variants (~20+) | DU + ts-pattern |
| Server boundary | Zod discriminatedUnion |

**기본값**: 매 union 의 require 시 매 DU + `kind` / `type` / `status` discriminant.

## 🔗 Graph
- 부모: [[TypeScript Type System]] · [[Algebraic Data Types]]
- 변형: [[Tagged Unions]] · [[Sum Types]]
- 응용: [[Result Type]] · [[State Machine]] · [[Zod]]
- Adjacent: [[Pattern Matching]] · [[Exhaustiveness Checking]]

## 🤖 LLM 활용
**언제**: 매 type modeling, 매 state machine, 매 API contract, 매 reducer.
**언제 X**: 매 single-variant — 매 plain interface.

## ❌ 안티패턴
- **No discriminant**: 매 untagged union — 매 narrow 의 hard.
- **String discriminant 의 typo**: 매 magic string 의 const 의 hoist.
- **Boolean flag combos**: `{loading, success, error}` boolean — 매 DU 의 use.
- **Default case 의 swallow**: `default: return state` — 매 새 variant 시 silent miss.
- **Class hierarchy 의 simulate**: 매 DU 의 cleaner.

## 🧪 검증 / 중복
- Verified (TS handbook "Narrowing", Rust enum docs, F# DU, Zod docs, ts-pattern docs).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — DU patterns + exhaustive + ts-pattern + Zod |