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Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

2026-05-20 23:52:15 +09:00

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title

Primitive Obsession (기본 타입 집착)

매 한 줄

"매 string/int이 매 domain concept를 매 가장한다". Primitive Obsession은 매 UserId, Email, Money 같은 매 domain concept를 매 raw str/int로 매 표현해 매 type system이 매 invariant 보장 못 하는 매 code smell. Fowler "Refactoring" (1999, 2nd ed. 2018) 의 매 classic smell — 매 modern fix는 매 newtype / value object.

매 핵심

매 증상

매 String 폭주: 매 email: str, phone: str, country_code: str 매 swap 가능.
매 Magic numbers: 매 status: int = 3 매 의미 불명.
매 Validation duplication: 매 every callsite마다 매 if "@" in email.
매 Type confusion: 매 transfer(from_id, to_id) 매 인자 swap 매 컴파일러 못 잡음.

매 Fix 전략

매 Newtype: Rust struct UserId(u64);.
매 Value Object: DDD 의 매 Email, Money immutable class.
매 Branded type: TypeScript type UserId = string & { __brand: "UserId" }.
매 NewType pattern: Python typing.NewType("UserId", int).

매 응용

Domain modeling (DDD) — Bounded context의 매 first-class concept.
Money handling (currency + amount tied).
Identifier safety (UserId vs OrderId mix-up 방지).

💻 패턴

Python NewType + dataclass

from typing import NewType
from dataclasses import dataclass
from decimal import Decimal

UserId = NewType("UserId", int)
OrderId = NewType("OrderId", int)

@dataclass(frozen=True)
class Money:
    amount: Decimal
    currency: str
    def __post_init__(self):
        if self.amount < 0: raise ValueError("negative")
        if len(self.currency) != 3: raise ValueError("ISO-4217")

def transfer(src: UserId, dst: UserId, m: Money): ...
# transfer(OrderId(1), UserId(2), Money(...))  # 매 mypy error

Rust newtype

#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub struct UserId(u64);

#[derive(Debug, Clone, Copy)]
pub struct OrderId(u64);

fn fetch_user(id: UserId) { /* ... */ }
// fetch_user(OrderId(7));  // 매 compile error

TypeScript branded types

type Brand<T, B> = T & { readonly __brand: B };
type Email   = Brand<string, "Email">;
type UserId  = Brand<number, "UserId">;

function parseEmail(s: string): Email {
  if (!/^[^@]+@[^@]+$/.test(s)) throw new Error("invalid");
  return s as Email;
}

Value object with invariant

@dataclass(frozen=True)
class Email:
    value: str
    def __post_init__(self):
        if "@" not in self.value: raise ValueError("invalid email")
        object.__setattr__(self, "value", self.value.lower())

Refactor: Replace Type Code with Subclass

# Before — magic int status
class Order:
    status: int  # 0=pending, 1=paid, 2=shipped

# After — sealed states
class OrderStatus: pass
class Pending(OrderStatus): pass
class Paid(OrderStatus): pass
class Shipped(OrderStatus):
    tracking: str

매 결정 기준

상황	Approach
Domain identifier	Newtype
Domain value (Money, Email)	Value Object
매 enum-like int code	Sealed subclass / Enum
Throwaway script	매 raw primitive OK

기본값: Newtype for IDs, Value Object for domain values.

🔗 Graph

부모: Code-Smell · Refactoring_Best_Practices
변형: Value-Object · Branded-Types
응용: DDD · Type-Safety
Adjacent: Stringly-Typed

🤖 LLM 활용

언제: domain model 설계, 매 API boundary type 선택, 매 refactoring 제안. 언제 X: 매 1-time script.

❌ 안티패턴

매 Stringly-typed everything: 매 모든 domain concept를 매 str.
매 Validation lazy: 매 boundary 통과 후 매 raw str 그대로 흘림.
매 Tuple as struct: 매 (int, str, bool) 매 의미 모름.
매 매 시점 validation: 매 매번 caller가 매 validate 수행.

🧪 검증 / 중복

Verified (Fowler "Refactoring" 2nd ed. 2018, Evans "DDD" 2003).
신뢰도 A.

🕓 Changelog

날짜	변경
2026-05-08	Phase 1
2026-05-10	Manual cleanup — newtype/value-object patterns + refactoring guide

4.6 KiB Raw Blame History