SOLID Principles

Deep Knowledge: Use mcp__documentation__fetch_docs with technology: solid-principles for comprehensive documentation.

When NOT to Use This Skill

This skill focuses on OOP design principles. Do NOT use for:

• Functional programming - SOLID is primarily for OOP, use FP-specific patterns instead
• Simple scripts/utilities - SOLID adds complexity; use for larger systems
• Performance-critical code - Abstraction overhead may impact performance
• General code quality - Use clean-code skill for naming, functions, readability

Anti-Patterns

Quick Troubleshooting

S - Single Responsibility

// ❌ Bad - Multiple responsibilities
class User {
  save() { /* database logic */ }
  sendEmail() { /* email logic */ }
  generateReport() { /* report logic */ }
}

// ✅ Good - Single responsibility each
class User { /* user data only */ }
class UserRepository { save(user: User) { } }
class EmailService { send(to: string, message: string) { } }
class ReportGenerator { generate(user: User) { } }

O - Open/Closed

// ❌ Bad - Modify class to add new payment
class PaymentProcessor {
  process(payment: Payment) {
    if (payment.type === 'credit') { /* ... */ }
    else if (payment.type === 'paypal') { /* ... */ }
    // Must modify to add new type
  }
}

// ✅ Good - Extend without modification
interface PaymentMethod {
  process(amount: number): Promise<void>;
}

class CreditCardPayment implements PaymentMethod { }
class PayPalPayment implements PaymentMethod { }
class CryptoPayment implements PaymentMethod { } // New, no changes needed

L - Liskov Substitution

// ❌ Bad - Square can't substitute Rectangle
class Rectangle {
  setWidth(w: number) { this.width = w; }
  setHeight(h: number) { this.height = h; }
}
class Square extends Rectangle {
  setWidth(w: number) { this.width = this.height = w; } // Breaks expectation
}

// ✅ Good - Use composition or separate abstractions
interface Shape {
  getArea(): number;
}
class Rectangle implements Shape { }
class Square implements Shape { }

I - Interface Segregation

// ❌ Bad - Fat interface
interface Worker {
  work(): void;
  eat(): void;
  sleep(): void;
}
class Robot implements Worker {
  eat() { throw new Error('Robots dont eat'); } // Forced to implement
}

// ✅ Good - Segregated interfaces
interface Workable { work(): void; }
interface Eatable { eat(): void; }
interface Sleepable { sleep(): void; }

class Human implements Workable, Eatable, Sleepable { }
class Robot implements Workable { }

D - Dependency Inversion

// ❌ Bad - High-level depends on low-level
class UserService {
  private db = new MySQLDatabase(); // Concrete dependency
}

// ✅ Good - Depend on abstractions
interface Database {
  query(sql: string): Promise<any>;
}

class UserService {
  constructor(private db: Database) { } // Injected abstraction
}

// Can use any implementation
new UserService(new MySQLDatabase());
new UserService(new PostgresDatabase());
new UserService(new MockDatabase()); // For testing

Authoritative Sources

• SOLID Principles by Robert C. Martin - https://blog.cleancoder.com/uncle-bob/2020/10/18/Solid-Relevance.html
• Agile Software Development by Robert C. Martin - https://www.oreilly.com/library/view/agile-software-development/0135974445/

Reference Documentation

• Clean Code
• Design Patterns
• Quality Principles (Consolidated)