GoF Design Patterns

Goal

• Choose the simplest design that solves the real change pressure.
• Select one primary pattern (two only if responsibilities are clearly split).
• Keep recommendations language-agnostic and explicit about tradeoffs.
• Support coding tasks with pattern-aligned implementation guidance.

Simple Workflow

1. Capture context with three questions: what changes often, what hurts now, and which constraint dominates.
2. Shortlist up to three candidates using

   references/DECISION_FRAMEWORK.md

   (canonical source). Use

   references/CATALOG.md

   only for quick intent lookup.
3. Select one pattern (or

   do not use a pattern

   ) based on future change-cost reduction and lower indirection.
4. If the task includes code changes, always read both:

• references/pattern-<selected-pattern>.md

• scripts/<selected-pattern>.ts

  (illustrative reference only)

1. Apply the design incrementally, adapted to the project language and constraints.
2. Validate with

   references/PITFALLS.md

   , then run a final quality/activation self-check with

   references/TESTS.md

   , and format the answer with

   references/OUTPUT_TEMPLATES.md

   .

Core Rules

• Language first: choose by problem forces and context, not by language syntax.
• Simplicity first: if recurring change pressure is weak, explicitly recommend

  do not use a pattern

  .
• Script policy: scripts are examples to read and adapt, not mandatory to execute.

Output Contract

• Always provide an explicit decision.
• Always include one simpler alternative and why it was rejected.
• Always include tradeoffs (

  Pros

  and

  Cons

  ).
• If code is shown, include a short adaptation note for other languages.

Navigation

Core References

• Decision Framework
• Catalog
• Pitfalls and Anti-Patterns
• Output Templates
• Trigger Tests

Pattern Cards

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