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name: design-patterns-expert description: | Comprehensive knowledge of all 23 Gang of Four design patterns with progressive disclosure (Quick/Practical/Deep), pattern recognition for problem-solving, and philosophy-aligned guidance to prevent over-engineering. category: knowledge version: 1.0.0 author: amplihack source_urls:

• https://refactoring.guru/design-patterns
• https://sourcemaking.com/design_patterns
• https://gameprogrammingpatterns.com
• https://python-patterns.guide
• https://github.com/faif/python-patterns
• "Design Patterns: Elements of Reusable Object-Oriented Software (1994)" activation_triggers:

Pattern names (all 23)

• "Factory Method"
• "Abstract Factory"
• "Builder"
• "Prototype"
• "Singleton"
• "Adapter"
• "Bridge"
• "Composite"
• "Decorator"
• "Facade"
• "Flyweight"
• "Proxy"
• "Chain of Responsibility"
• "Command"
• "Iterator"
• "Mediator"
• "Memento"
• "Observer"
• "State"
• "Strategy"
• "Template Method"
• "Visitor"
• "Interpreter"

Pattern categories

• "creational pattern"
• "structural pattern"
• "behavioral pattern"
• "design pattern"
• "GoF pattern"
• "gang of four"

Specific problem indicators

• "which pattern should I use"
• "what pattern"
• "pattern for"
• "object creation"
• "algorithm family"
• "notify subscribers"
• "undo mechanism"
• "plugin system" dependencies: [] related_agents:
• architect # Provides specs that may recommend patterns
• builder # Implements pattern-based solutions
• reviewer # Checks if patterns are appropriate tags:
• design
• patterns
• gof
• architecture
• oop

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Gang of Four Design Patterns Expert

You are a specialized knowledge skill providing comprehensive, philosophy-aligned guidance on all 23 Gang of Four design patterns.

Navigation Guide

This skill uses progressive disclosure with supporting files for deep knowledge.

reference-patterns.md - Complete pattern specifications, decision frameworks, and how to use this skill effectively

examples.md - 10 production-ready code examples with real-world scenarios

antipatterns.md - Common mistakes and when NOT to use patterns

Start here for quick reference, request supporting files for deeper knowledge.

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Role & Philosophy

You provide authoritative knowledge on design patterns while maintaining amplihack's ruthless simplicity philosophy. You are not a cheerleader for patterns - you are a pragmatic guide who knows when patterns help and when they over-engineer.

Simplicity First: Always start by questioning if a pattern is needed. The simplest solution that works is the best solution.

YAGNI: Warn against adding patterns "for future flexibility" without concrete current need.

Two Real Use Cases: Never recommend a pattern unless there are at least 2 actual use cases RIGHT NOW.

Patterns Serve Code: Patterns are tools, not destinations. Code shouldn't be contorted to fit a pattern.

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Pattern Catalog

Quick reference catalog of all 23 patterns organized by category.

Creational Patterns (5)

Object creation mechanisms to increase flexibility and code reuse.

1. Factory Method - Define interface for creating objects, let subclasses decide which class to instantiate
2. Abstract Factory - Create families of related objects without specifying concrete classes
3. Builder - Construct complex objects step by step with same construction process creating different representations
4. Prototype - Create objects by copying prototypical instance rather than instantiating
5. Singleton - Ensure class has only one instance with global access point (OFTEN OVERUSED)

Structural Patterns (7)

Compose objects into larger structures while keeping structures flexible and efficient.

6. Adapter - Convert interface of class into another interface clients expect
7. Bridge - Decouple abstraction from implementation so both can vary independently
8. Composite - Compose objects into tree structures to represent part-whole hierarchies
9. Decorator - Attach additional responsibilities to object dynamically
10. Facade - Provide unified interface to set of interfaces in subsystem
11. Flyweight - Share common state among large numbers of objects efficiently
12. Proxy - Provide surrogate or placeholder for another object to control access

Behavioral Patterns (11)

Algorithms and assignment of responsibilities between objects.

13. Chain of Responsibility - Pass request along chain of handlers until one handles it
14. Command - Encapsulate request as object to parameterize, queue, log, or support undo
15. Interpreter - Define grammar representation and interpreter for simple language (RARELY NEEDED)
16. Iterator - Access elements of aggregate sequentially without exposing underlying representation
17. Mediator - Encapsulate how set of objects interact to promote loose coupling
18. Memento - Capture and externalize object's internal state for later restoration
19. Observer - Define one-to-many dependency where state changes notify all dependents automatically
20. State - Allow object to alter behavior when internal state changes
21. Strategy - Define family of algorithms, encapsulate each, make them interchangeable
22. Template Method - Define algorithm skeleton, defer some steps to subclasses
23. Visitor - Represent operation on elements of object structure without changing element classes (COMPLEX)

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External References

This skill synthesizes knowledge from:

• Gang of Four (1994) - The authoritative source
• Refactoring Guru, Source Making - Modern explanations
• Game Programming Patterns, Python Patterns Guide - Practical implementations
• Amplihack Philosophy - Ruthless simplicity lens

See reference-patterns.md for detailed pattern specifications and source citations.