design-patterns-skill

Overview

Structured guidance on programming principles and design patterns from foundational software engineering books. Ensures code follows industry-standard practices for readability, maintainability, simplicity, and architectural soundness.

When to Apply

• Code Generation: Writing new functions, classes, or modules
• Code Review: Evaluating pull requests or existing codebases
• Refactoring: Improving code structure and clarity
• Architecture Design: Choosing appropriate patterns and abstractions

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Core Philosophy

1. Readability over cleverness — Code is read more than written
2. Simplicity over complexity — Use the simplest solution that works
3. Testability by design — Write code that's easy to test
4. Incremental improvement — Leave code better than you found it
5. Patterns as tools — Apply patterns when they clarify, not by default

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Principle Categories

1. Readability & Clarity

• Descriptive naming, consistent formatting, self-documenting code, small focused functions
• Reference: references/patterns/readability.md

2. Simplicity & Efficiency

• KISS, DRY, YAGNI
• Reference: references/patterns/simplicity.md

3. Design & Architecture

• SRP, composition over inheritance, program to interfaces
• Patterns: Factory, Strategy, Observer, Decorator, Adapter, Command, Singleton
• Reference: references/patterns/design-architecture.md

4. Testing & Quality

• Automated testing, focused assertions, edge case coverage
• Reference: references/patterns/testing.md

5. Error Handling

• Clear error messages, early validation, proper exception usage
• Reference: references/patterns/error-handling.md

6. Maintainability

• Boy Scout Rule, continuous refactoring, atomic commits, automation
• Reference: references/patterns/maintainability.md

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AI-Specific Guidance

When generating or reviewing code, always:

1. Check for AI pitfalls listed in each principle
2. Avoid pattern prediction bias — don't use patterns just because they're common
3. Question generic naming — resist data, temp, result without context
4. Validate edge cases — don't skip error handling
5. Keep functions focused — resist combining unrelated operations
6. Match project conventions — maintain consistency with existing codebase

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Quick Reference

| Situation | Apply | |-----------|-------| | Function > 20 lines | Split into smaller functions (SRP) | | Repeated code blocks | Extract to function/constant (DRY) | | Complex conditionals | Strategy or State pattern | | Object creation logic | Factory pattern | | Cross-cutting concerns | Decorator or Observer pattern | | Incompatible interfaces | Adapter pattern | | Need undo/logging | Command pattern | | Global access point | Singleton (use sparingly) |

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Sources

• Clean Code — Robert C. Martin
• The Pragmatic Programmer — Andrew Hunt & David Thomas
• Code Complete — Steve McConnell
• Refactoring — Martin Fowler
• Design Patterns — Gang of Four