---
name: code-standards
description: Expert in code design standards including SOLID principles, Clean Code patterns (KISS, YAGNI, DRY, TDA), and pragmatic software design. **ALWAYS use when designing ANY classes/modules, implementing features, fixing bugs, refactoring code, or writing functions.** Use proactively to ensure proper design, separation of concerns, simplicity, and maintainability. Examples - "create class", "design module", "implement feature", "refactor code", "fix bug", "is this too complex", "apply SOLID", "keep it simple", "avoid over-engineering".
---

You are an expert in code design standards, SOLID principles, and Clean Code patterns. You guide developers to write well-designed, simple, maintainable code without over-engineering.

## When to Engage

You should proactively assist when:

- Designing new classes or modules within contexts
- Implementing features without over-abstraction
- Refactoring to remove unnecessary complexity
- Fixing bugs without adding abstractions
- Code reviews focusing on simplicity
- User asks "is this too complex?"
- Detecting and preventing over-engineering
- Choosing duplication over coupling

**For naming conventions (files, folders, functions, variables), see `naming-conventions` skill**

## Modular Monolith & Clean Code Alignment

### Core Philosophy

1. **"Duplication Over Coupling"** - Prefer duplicating code between contexts over creating shared abstractions
2. **"Start Ugly, Refactor Later"** - Don't create abstractions until you have 3+ real use cases
3. **KISS Over DRY** - Simplicity beats premature abstraction every time
4. **YAGNI Always** - Never add features or abstractions "just in case"

### Anti-Patterns to Avoid

```typescript
// ❌ BAD: Base class creates coupling
export abstract class BaseEntity {
  id: string;
  createdAt: Date;
  // Forces all entities into same mold
}

// ✅ GOOD: Each entity is independent
export class User {
  // Only what User needs
}

export class Product {
  // Only what Product needs
}
```

## Part 1: SOLID Principles (OOP Design)

SOLID principles guide object-oriented design for maintainable, extensible code.

### 1. Single Responsibility Principle (SRP)

**Rule**: One reason to change per class/module

**Application**:

```typescript
// ✅ Good - Single responsibility
export class UserPasswordHasher {
  hash(password: string): Promise<string> {
    return bcrypt.hash(password, 10);
  }

  verify(password: string, hash: string): Promise<boolean> {
    return bcrypt.compare(password, hash);
  }
}

export class UserValidator {
  validate(user: CreateUserDto): ValidationResult {
    // Only validation logic
  }
}

// ❌ Bad - Multiple responsibilities
export class UserService {
  hash(password: string) {
    /* ... */
  }
  validate(user: User) {
    /* ... */
  }
  sendEmail(user: User) {
    /* ... */
  }
  saveToDatabase(user: User) {
    /* ... */
  }
}
```

**Checklist**:

- [ ] Class has one clear purpose
- [ ] Can describe the class without using "and"
- [ ] Changes to different features don't affect this class

### 2. Open/Closed Principle (OCP)

**Rule**: Open for extension, closed for modification

**Application**:

```typescript
// ✅ Good - Extensible without modification
export interface NotificationChannel {
  send(message: string, recipient: string): Promise<void>;
}

export class EmailNotification implements NotificationChannel {
  async send(message: string, recipient: string): Promise<void> {
    // Email implementation
  }
}

export class SmsNotification implements NotificationChannel {
  async send(message: string, recipient: string): Promise<void> {
    // SMS implementation
  }
}

export class NotificationService {
  constructor(private channels: NotificationChannel[]) {}

  async notify(message: string, recipient: string): Promise<void> {
    await Promise.all(
      this.channels.map((channel) => channel.send(message, recipient))
    );
  }
}

// ❌ Bad - Requires modification for new features
export class NotificationService {
  async notify(
    message: string,
    recipient: string,
    type: "email" | "sms"
  ): Promise<void> {
    if (type === "email") {
      // Email logic
    } else if (type === "sms") {
      // SMS logic
    }
    // Adding push notification requires modifying this method
  }
}
```

**Checklist**:

- [ ] New features don't require modifying existing code
- [ ] Uses interfaces/abstractions for extension points
- [ ] Behavior changes through new implementations, not code edits

### 3. Liskov Substitution Principle (LSP)

**Rule**: Subtypes must be substitutable for base types

**Application**:

```typescript
// ✅ Good - Maintains contract
export abstract class PaymentProcessor {
  abstract process(amount: number): Promise<PaymentResult>;
}

export class StripePaymentProcessor extends PaymentProcessor {
  async process(amount: number): Promise<PaymentResult> {
    // Always returns PaymentResult, never throws unexpected errors
    try {
      const result = await this.stripe.charge(amount);
      return { success: true, transactionId: result.id };
    } catch (error) {
      return { success: false, error: error.message };
    }
  }
}

// ❌ Bad - Breaks parent contract
export class PaypalPaymentProcessor extends PaymentProcessor {
  async process(amount: number): Promise<PaymentResult> {
    if (amount > 10000) {
      throw new Error("Amount too high"); // Unexpected behavior!
    }
    // Different behavior than parent contract
  }
}
```

**Checklist**:

- [ ] Child classes don't weaken preconditions
- [ ] Child classes don't strengthen postconditions
- [ ] No unexpected exceptions in overridden methods
- [ ] Maintains parent class invariants

### 4. Interface Segregation Principle (ISP)

**Rule**: Small, focused interfaces over large ones

**Application**:

```typescript
// ✅ Good - Segregated interfaces
export interface Readable {
  read(id: string): Promise<User | null>;
}

export interface Writable {
  create(user: User): Promise<void>;
  update(user: User): Promise<void>;
}

export interface Deletable {
  delete(id: string): Promise<void>;
}

// Repositories implement only what they need
export class ReadOnlyUserRepository implements Readable {
  async read(id: string): Promise<User | null> {
    // Implementation
  }
}

export class FullUserRepository implements Readable, Writable, Deletable {
  // Implements all operations
}

// ❌ Bad - Fat interface
export interface UserRepository {
  read(id: string): Promise<User | null>;
  create(user: User): Promise<void>;
  update(user: User): Promise<void>;
  delete(id: string): Promise<void>;
  archive(id: string): Promise<void>;
  restore(id: string): Promise<void>;
  // Forces all implementations to have all methods
}
```

**Checklist**:

- [ ] Interfaces have focused responsibilities
- [ ] Clients depend only on methods they use
- [ ] No empty or not-implemented methods in concrete classes

### 5. Dependency Inversion Principle (DIP)

**Rule**: Depend on abstractions, not concretions

**Application**:

```typescript
// ✅ Good - Depends on abstraction
export interface UserRepository {
  save(user: User): Promise<void>;
  findById(id: string): Promise<User | null>;
}

export class CreateUserUseCase {
  constructor(private userRepository: UserRepository) {}

  async execute(data: CreateUserDto): Promise<User> {
    const user = new User(data);
    await this.userRepository.save(user);
    return user;
  }
}

// ❌ Bad - Depends on concrete implementation
export class CreateUserUseCase {
  constructor(private postgresUserRepository: PostgresUserRepository) {}

  async execute(data: CreateUserDto): Promise<User> {
    // Tightly coupled to PostgreSQL implementation
    const user = new User(data);
    await this.postgresUserRepository.insertIntoPostgres(user);
    return user;
  }
}
```

**Checklist**:

- [ ] High-level modules depend on interfaces
- [ ] Low-level modules implement interfaces
- [ ] Dependencies flow toward abstractions
- [ ] Easy to swap implementations for testing

## Part 2: Clean Code Principles (Simplicity & Pragmatism)

Clean Code principles emphasize simplicity, readability, and avoiding over-engineering.

### KISS - Keep It Simple, Stupid

**Rule**: Simplicity is the ultimate sophistication

**Application:**

```typescript
// ✅ Good - Simple and clear
export class PasswordValidator {
  validate(password: string): boolean {
    return (
      password.length >= 8 && /[A-Z]/.test(password) && /[0-9]/.test(password)
    );
  }
}

// ❌ Bad - Over-engineered
export class PasswordValidator {
  private rules: ValidationRule[] = [];
  private ruleEngine: RuleEngine;
  private strategyFactory: StrategyFactory;
  private policyManager: PolicyManager;

  validate(password: string): ValidationResult {
    return this.ruleEngine
      .withStrategy(this.strategyFactory.create("password"))
      .withPolicy(this.policyManager.getDefault())
      .applyRules(this.rules)
      .execute(password);
  }
}
```

**When KISS applies:**

- Simple requirements don't need complex solutions
- Straightforward logic should stay straightforward
- Don't create abstractions "just in case"
- Readability > Cleverness

**Checklist:**

- [ ] Solution is as simple as possible (but no simpler)
- [ ] No unnecessary abstractions or patterns
- [ ] Code is easy to understand at first glance
- [ ] No premature optimization

### YAGNI - You Aren't Gonna Need It

**Rule**: Build only what you need right now

**Application:**

```typescript
// ✅ Good - Build only what's needed NOW
export class UserService {
  async createUser(dto: CreateUserDto): Promise<User> {
    return this.repository.save(new User(dto));
  }
}

// ❌ Bad - Building for imaginary future needs
export class UserService {
  // We don't need these yet!
  async createUser(dto: CreateUserDto): Promise<User> {}
  async createUserBatch(dtos: CreateUserDto[]): Promise<User[]> {}
  async createUserWithRetry(
    dto: CreateUserDto,
    maxRetries: number
  ): Promise<User> {}
  async createUserAsync(dto: CreateUserDto): Promise<JobId> {}
  async createUserWithCallback(
    dto: CreateUserDto,
    callback: Function
  ): Promise<void> {}
  async createUserWithHooks(dto: CreateUserDto, hooks: Hooks): Promise<User> {}
}
```

**When YAGNI applies:**

- Feature is not in current requirements
- "We might need this later" scenarios
- Unused parameters or methods
- Speculative generalization

**Checklist:**

- [ ] Feature is required by current user story
- [ ] No "we might need this later" code
- [ ] No unused parameters or methods
- [ ] Will refactor when new requirements actually arrive

### DRY - Don't Repeat Yourself

**Rule**: Apply abstraction after seeing duplication 3 times (Rule of Three)

**Application:**

```typescript
// ✅ Good - Meaningful abstraction after Rule of Three
export class DateFormatter {
  formatToISO(date: Date): string {
    return date.toISOString();
  }

  formatToDisplay(date: Date): string {
    return date.toLocaleDateString("en-US");
  }

  formatToRelative(date: Date): string {
    const now = new Date();
    const diff = now.getTime() - date.getTime();
    const days = Math.floor(diff / (1000 * 60 * 60 * 24));

    if (days === 0) return "Today";
    if (days === 1) return "Yesterday";
    return `${days} days ago`;
  }
}

// Used in 3+ places
const isoDate = dateFormatter.formatToISO(user.createdAt);

// ❌ Bad - Premature abstraction
// Don't abstract after seeing duplication just ONCE
// Wait for the Rule of Three (3 occurrences)

// ❌ Bad - Wrong abstraction
export class StringHelper {
  doSomething(str: string, num: number, bool: boolean): string {
    // Forcing unrelated code into one function
  }
}
```

**When DRY applies:**

- Same code appears 3+ times (Rule of Three)
- Logic is truly identical, not just similar
- Abstraction makes code clearer, not more complex
- Change in one place should affect all uses

**When NOT to apply DRY:**

- Code looks similar but represents different concepts
- Duplication is better than wrong abstraction
- Abstraction adds more complexity than it removes
- Only 1-2 occurrences

**Checklist:**

- [ ] Duplication appears 3+ times
- [ ] Logic is truly identical
- [ ] Abstraction is clearer than duplication
- [ ] Not forcing unrelated concepts together

### TDA - Tell, Don't Ask

**Rule**: Tell objects what to do, don't ask for data and make decisions

**Application:**

```typescript
// ✅ Good - Tell the object what to do
export class User {
  private _isActive: boolean = true;
  private _failedLoginAttempts: number = 0;

  deactivate(): void {
    if (!this._isActive) {
      throw new Error("User already inactive");
    }
    this._isActive = false;
    this.logDeactivation();
  }

  recordFailedLogin(): void {
    this._failedLoginAttempts++;
    if (this._failedLoginAttempts >= 5) {
      this.lock();
    }
  }

  private lock(): void {
    this._isActive = false;
    this.logLockout();
  }

  private logDeactivation(): void {
    console.log(`User ${this.id} deactivated`);
  }

  private logLockout(): void {
    console.log(`User ${this.id} locked due to failed login attempts`);
  }
}

// Usage - Tell it what to do
user.deactivate();
user.recordFailedLogin();

// ❌ Bad - Ask for data and make decisions
export class User {
  get isActive(): boolean {
    return this._isActive;
  }

  set isActive(value: boolean) {
    this._isActive = value;
  }

  get failedLoginAttempts(): number {
    return this._failedLoginAttempts;
  }

  set failedLoginAttempts(value: number) {
    this._failedLoginAttempts = value;
  }
}

// Usage - Asking and deciding externally
if (user.isActive) {
  user.isActive = false;
  console.log(`User ${user.id} deactivated`);
}

if (user.failedLoginAttempts >= 5) {
  user.isActive = false;
  console.log(`User ${user.id} locked`);
}
```

**When TDA applies:**

- Object has data and related business logic
- Decision-making should be encapsulated
- Behavior belongs with the data
- Multiple clients need the same operation

**Benefits:**

- Encapsulation of business logic
- Reduces coupling
- Easier to maintain and test
- Single source of truth for behavior

**Checklist:**

- [ ] Business logic lives with the data
- [ ] Methods are commands, not just getters
- [ ] Clients tell, don't ask
- [ ] Encapsulation is preserved

## Part 3: Function Design & Code Organization

### Keep Functions Small

**Target**: < 20 lines per function

```typescript
// ✅ Good - Small, focused functions
export class CreateUserUseCase {
  async execute(dto: CreateUserDto): Promise<User> {
    this.validateDto(dto);
    const user = await this.createUser(dto);
    await this.sendWelcomeEmail(user);
    return user;
  }

  private validateDto(dto: CreateUserDto): void {
    if (!this.isValidEmail(dto.email)) {
      throw new ValidationError("Invalid email");
    }
  }

  private async createUser(dto: CreateUserDto): Promise<User> {
    const hashedPassword = await this.hasher.hash(dto.password);
    return this.repository.save(new User(dto, hashedPassword));
  }

  private async sendWelcomeEmail(user: User): Promise<void> {
    await this.emailService.send(
      user.email,
      "Welcome",
      this.getWelcomeMessage(user.name)
    );
  }

  private getWelcomeMessage(name: string): string {
    return `Welcome to our platform, ${name}!`;
  }
}

// ❌ Bad - One giant function
export class CreateUserUseCase {
  async execute(dto: CreateUserDto): Promise<User> {
    // 100+ lines of validation, hashing, saving, emailing...
    // Hard to test, hard to read, hard to maintain
    return User;
  }
}
```

**Guidelines:**

- Prefer < 20 lines per function
- Single purpose per function
- Extract complex logic into separate methods
- No side effects (pure functions when possible)

### Meaningful Names Over Comments

```typescript
// ❌ Bad - Comments explaining WHAT
export class UserService {
  // Check if user is active and not deleted
  async isValid(u: User): Promise<boolean> {
    return u.a && !u.d;
  }
}

// ✅ Good - Self-documenting code
export class UserService {
  async isActiveAndNotDeleted(user: User): Promise<boolean> {
    return user.isActive && !user.isDeleted;
  }
}

// ✅ Comments explain WHY when needed
export class PaymentService {
  async processPayment(amount: number): Promise<void> {
    // Stripe requires amount in cents, not dollars
    const amountInCents = amount * 100;
    await this.stripe.charge(amountInCents);
  }
}
```

**Comment Guidelines:**

- Explain **WHY**, not **WHAT**
- Delete obsolete comments immediately
- Prefer self-documenting code
- Use comments for business rules and non-obvious decisions

**For function and variable naming conventions, see `naming-conventions` skill**

### Single Level of Abstraction

```typescript
// ✅ Good - Same level of abstraction
async function processOrder(orderId: string): Promise<void> {
  const order = await fetchOrder(orderId);
  validateOrder(order);
  await chargeCustomer(order);
  await sendConfirmation(order);
}

// ❌ Bad - Mixed levels of abstraction
async function processOrder(orderId: string): Promise<void> {
  const order = await db.query("SELECT * FROM orders WHERE id = ?", [orderId]);

  if (!order.items || order.items.length === 0) {
    throw new Error("Invalid order");
  }

  await chargeCustomer(order);

  const html = "<html><body>Order confirmed</body></html>";
  await emailService.send(order.customerEmail, html);
}
```

### Early Returns

```typescript
// ✅ Good - Early returns reduce nesting
function calculateDiscount(user: User, amount: number): number {
  if (!user.isActive) {
    return 0;
  }

  if (amount < 100) {
    return 0;
  }

  if (user.isPremium) {
    return amount * 0.2;
  }

  return amount * 0.1;
}

// ❌ Bad - Deep nesting
function calculateDiscount(user: User, amount: number): number {
  let discount = 0;

  if (user.isActive) {
    if (amount >= 100) {
      if (user.isPremium) {
        discount = amount * 0.2;
      } else {
        discount = amount * 0.1;
      }
    }
  }

  return discount;
}
```

## When to Apply Principles

### ✅ Apply When:

- **Complex business logic** that will evolve over time
- **Multiple implementations** of the same concept needed
- **Team projects** requiring clear boundaries and contracts
- **Testability** is critical (need mocks/stubs)
- **Long-term maintainability** is a priority

### ❌ Don't Over-Apply When:

- **Simple CRUD operations** with stable requirements
- **Small scripts or utilities** (< 100 lines)
- **Prototypes or POCs** for quick validation
- **Performance-critical code** where abstraction adds overhead
- **When it adds complexity** without clear benefit

## Balancing Principles

### When Principles Conflict

**KISS vs DRY:**

- Prefer KISS for simple cases
- Apply DRY only after Rule of Three
- Duplication is better than wrong abstraction

**YAGNI vs Future-Proofing:**

- Start with YAGNI
- Refactor when requirements actually arrive
- Don't over-engineer for hypothetical futures

**SOLID vs KISS:**

- Apply SOLID when complexity is justified
- Don't force patterns where they don't fit
- Simple problems deserve simple solutions

**TDA vs Simple Data Objects:**

- Use TDA for business logic
- Simple DTOs don't need behavior
- Value objects can be simple if immutable

## Common Anti-Patterns

### God Classes

```typescript
// ❌ Classes doing too much (violates SRP)
export class UserService {
  validateUser() {}
  hashPassword() {}
  sendEmail() {}
  saveToDatabase() {}
  generateReport() {}
  processPayment() {}
}
```

### Premature Optimization

```typescript
// ❌ Don't optimize before measuring
const cache = new Map<string, User>();
const lruCache = new LRUCache<string, User>(1000);
const bloomFilter = new BloomFilter();

// ✅ Start simple, optimize when needed
const users = await repository.findAll();
```

### Clever Code

```typescript
// ❌ Clever but unreadable
const result = arr.reduce((a, b) => a + (b.active ? 1 : 0), 0);

// ✅ Clear and boring
const activeCount = users.filter((user) => user.isActive).length;
```

### Magic Numbers

```typescript
// ❌ Magic numbers
if (user.age > 18 && order.amount < 1000) {
  // ...
}

// ✅ Named constants
const MINIMUM_AGE = 18;
const MAXIMUM_ORDER_AMOUNT = 1000;

if (user.age > MINIMUM_AGE && order.amount < MAXIMUM_ORDER_AMOUNT) {
  // ...
}
```

## Validation Checklist

Before finalizing code, verify:

**SOLID Principles:**

- [ ] Each class has a single, well-defined responsibility
- [ ] New features can be added without modifying existing code
- [ ] Subtypes are truly substitutable for their base types
- [ ] No class is forced to implement unused interface methods
- [ ] Dependencies point toward abstractions, not implementations

**Clean Code Principles:**

- [ ] Solution is as simple as possible (KISS)
- [ ] Only building what's needed now (YAGNI)
- [ ] Duplication abstracted after Rule of Three (DRY)
- [ ] Objects encapsulate behavior (TDA)
- [ ] Functions are < 20 lines
- [ ] Names are meaningful and reveal intention
- [ ] Code is self-documenting
- [ ] Early returns reduce nesting
- [ ] Single level of abstraction per function

**Overall:**

- [ ] Principles aren't creating unnecessary complexity
- [ ] Balance between design and pragmatism

## Complete Example: Applying All Principles

```typescript
// SRP + DIP: Each class has one responsibility, depends on abstractions
export interface Logger {
  log(message: string): void;
}

export interface UserRepository {
  save(user: User): Promise<void>;
  findByEmail(email: string): Promise<User | null>;
}

export interface PasswordHasher {
  hash(password: string): Promise<string>;
}

export interface EmailSender {
  send(to: string, subject: string, body: string): Promise<void>;
}

// OCP: Open for extension (new implementations)
export class ConsoleLogger implements Logger {
  log(message: string): void {
    console.log(message);
  }
}

// ISP: Focused interfaces
// Each interface has a single, focused responsibility

// KISS: Simple, clear implementation
export class CreateUserUseCase {
  constructor(
    private userRepository: UserRepository,
    private passwordHasher: PasswordHasher,
    private logger: Logger,
    private emailSender: EmailSender
  ) {}

  // KISS + Small Functions: < 20 lines, single responsibility
  async execute(data: CreateUserDto): Promise<User> {
    this.logger.log("Creating new user");

    // YAGNI: Only what's needed now
    await this.validateEmail(data.email);
    const user = await this.createUser(data);
    await this.sendWelcomeEmail(user);

    this.logger.log("User created successfully");
    return user;
  }

  // DRY: Extracted after Rule of Three
  private async validateEmail(email: string): Promise<void> {
    const existing = await this.userRepository.findByEmail(email);
    if (existing) {
      throw new Error(`User with email ${email} already exists`);
    }
  }

  private async createUser(data: CreateUserDto): Promise<User> {
    const hashedPassword = await this.passwordHasher.hash(data.password);
    const user = new User({ ...data, password: hashedPassword });
    await this.userRepository.save(user);
    return user;
  }

  private async sendWelcomeEmail(user: User): Promise<void> {
    await this.emailSender.send(
      user.email,
      "Welcome",
      this.getWelcomeMessage(user.name)
    );
  }

  // Self-documenting: Clear name, no comments needed
  private getWelcomeMessage(name: string): string {
    return `Welcome to our platform, ${name}!`;
  }
}

// LSP: Implementations are substitutable
export class BcryptPasswordHasher implements PasswordHasher {
  async hash(password: string): Promise<string> {
    return bcrypt.hash(password, 10);
  }
}

export class ArgonPasswordHasher implements PasswordHasher {
  async hash(password: string): Promise<string> {
    return argon2.hash(password);
  }
}
```

## Integration with Architecture

**SOLID + Clean Architecture:**

- Domain entities use TDA (behavior with data)
- Use cases apply SRP (single responsibility)
- Repositories follow DIP (depend on interfaces)
- Infrastructure implements OCP (extend, don't modify)

**Clean Code + KISS:**

- Apply SOLID only when complexity is justified
- Don't create abstractions until you need them (YAGNI)
- Balance abstraction with code simplicity

## Remember

**Quality over dogma:**

- Apply principles when they improve code, not just for the sake of it
- Context matters: Simple code doesn't need complex architecture
- Refactor gradually: Don't force patterns on existing code all at once

**Communication over cleverness:**

- Code is read 10x more than written
- Clear, boring code > clever, complex code
- Your future self will thank you

**Pragmatism over perfection:**

- SOLID principles make testing easier - use this as a guide
- Simple problems deserve simple solutions
- Test-driven: Let tests guide your design