Design patterns

Design Patterns Skill

You are applying design patterns from the Refactoring Guru / Gang of Four catalog to help the user improve their codebase's architecture. This skill works standalone or as part of Phase 3 (Transform) of the Refactor Pilot Framework.

Quick Decision Tree

What does the user need?
├── IDENTIFY which patterns to apply
│   └── Go to "Pattern Identification Workflow"
├── APPLY a specific pattern they already chose
│   └── Go to "Pattern Application Workflow"
├── LEARN about a pattern before deciding
│   └── Go to "Pattern Reference" (see references/)
└── REFACTOR existing code toward a pattern
    └── Go to "Pattern Refactoring Workflow"

Does a codebase analysis exist (refactor-notes/04-project-summary.md)?
├── YES → Use it to inform pattern recommendations
└── NO → Analyze the target code area before recommending patterns

Are safety net tests in place?
├── YES → Proceed with transformation
└── NO → Recommend running generate-tests first

Pattern Identification Workflow

When the user wants to know which patterns fit their code:

Step 1: Identify Code Smells That Patterns Solve

Scan the target code for these smell → pattern mappings:

Code Smell	Suggested Pattern(s)
Giant if/else or switch on type	Strategy, State, Command
Copy-pasted logic with minor variations	Template Method, Strategy
Constructor with 5+ parameters	Builder
new keyword scattered everywhere	Factory Method, Abstract Factory
God class doing too many things	Facade, Mediator, Command
Tight coupling between modules	Observer, Mediator, Bridge
Adding features requires modifying existing code	Decorator, Strategy, Visitor
Complex object creation logic	Builder, Factory Method, Prototype
Need to support undo/history	Memento, Command
Traversing complex structures	Iterator, Composite, Visitor
Different behavior for different states	State
Adapting incompatible interfaces	Adapter, Facade
Controlling access to expensive resources	Proxy, Flyweight
Many similar objects consuming memory	Flyweight, Prototype

Step 2: Evaluate Fit

For each candidate pattern, assess:

1. Problem match — Does the pattern solve the actual problem, or just look similar?
2. Complexity trade-off — Is the pattern simpler than the current code, or does it add unnecessary indirection?
3. Team familiarity — Will the team understand and maintain this pattern?
4. Language fit — Does the language support this pattern idiomatically? (e.g., Strategy is trivial with first-class functions)
5. Scale appropriateness — Is the codebase large enough to justify the pattern's overhead?
6. Module depth — Will the pattern make modules deeper (simple interface, more hidden complexity) or shallower (more files/classes that each do little)? A shallow result disqualifies the pattern. See skills/refactor-code/references/deep-modules.md for the Depth Test.

Rule of thumb: If the pattern adds more classes/files than the problem warrants, it's over-engineering. Patterns should reduce complexity, not add it. In AI-maintained codebases the cost is concrete: every extra layer is another file in the context window, more tokens, and another hop where an agent can lose the thread.

Step 3: Recommend with Rationale

Present recommendations as:

## Pattern Recommendation: [Pattern Name]

**Problem:** [What code smell or structural issue this solves]
**Current code:** [Brief description of what exists now]
**After applying:** [What the code will look like]
**Trade-offs:** [What you gain vs. what complexity you add]
**Confidence:** HIGH / MEDIUM / LOW
**Files affected:** [List of files that would change]

Pattern Application Workflow

When the user wants to apply a specific pattern:

Step 1: Verify Prerequisites

• Confirm safety net tests exist (or accept the risk)
• Read the pattern reference (see references/ docs)
• Identify all code locations that will change

Step 2: Apply Incrementally

Follow the three-pass approach from the refactor-code skill:

1. Extract — Pull out the varying behavior or creation logic into separate units
2. Introduce the pattern — Create the pattern's structural elements (interfaces, base classes, factories, etc.)
3. Wire up — Replace the original code with the pattern-based implementation

Step 3: Verify Behavior Preservation

• Run all tests after each step
• Confirm no public API changes (unless intentional)
• Add // PATTERN: [PatternName] comments at key structural points for documentation

Step 4: Explain the Pattern Application

After applying, document:

• Why this pattern was chosen over alternatives
• How to extend the pattern (e.g., "to add a new strategy, create a class implementing X")
• Any simplifications made for the specific language/framework

Pattern Refactoring Workflow

When refactoring existing code toward a pattern:

Step 1: Map Current Structure to Pattern Roles

Identify which existing classes/functions/modules map to each role in the target pattern. For example, for Strategy:

• Context → the class that currently contains the if/else
• Strategy interface → the abstraction to extract
• Concrete strategies → each branch of the conditional

Step 2: Incremental Transformation

Transform in small, testable steps:

1. Extract the interface/protocol/trait
2. Implement one concrete variant
3. Verify tests pass
4. Migrate remaining variants one at a time
5. Remove the original conditional logic
6. Clean up and document

Critical: Never change more than one pattern role at a time. Each step should be independently committable and testable.

Pattern Catalog Quick Reference

Creational Patterns (object creation)

Pattern	When to Use	Key Benefit
Factory Method	Subclasses decide which class to instantiate	Open/Closed principle for creation
Abstract Factory	Families of related objects	Consistent object families
Builder	Complex objects with many optional parts	Readable construction, immutable objects
Prototype	Copying existing objects is cheaper than creating new ones	Avoids complex initialization
Singleton	Exactly one instance needed globally	Controlled global access

See references/creational-patterns.md for detailed guides with code examples.

Structural Patterns (composition)

Pattern	When to Use	Key Benefit
Adapter	Incompatible interfaces need to work together	Integration without modifying source
Bridge	Separate abstraction from implementation	Independent evolution of both
Composite	Tree structures with uniform treatment	Simplifies client code
Decorator	Add behavior without subclassing	Flexible runtime composition
Facade	Simplify a complex subsystem	Reduced coupling to internals
Flyweight	Many similar objects consuming memory	Shared state, lower memory
Proxy	Control access to another object	Lazy loading, access control, logging

See references/structural-patterns.md for detailed guides with code examples.

Behavioral Patterns (communication)

Pattern	When to Use	Key Benefit
Chain of Responsibility	Multiple handlers for a request	Decoupled sender/receiver
Command	Encapsulate actions as objects	Undo, queue, log operations
Iterator	Traverse collections without exposing internals	Uniform traversal interface
Mediator	Reduce chaotic dependencies between objects	Centralized communication
Memento	Capture and restore object state	Undo without violating encapsulation
Observer	Notify dependents of state changes	Loose coupling, event-driven
State	Behavior changes based on internal state	Eliminates state conditionals
Strategy	Interchangeable algorithms	Open/Closed principle for behavior
Template Method	Algorithm skeleton with customizable steps	Code reuse with variation points
Visitor	Add operations to objects without modifying them	Separation of concerns

See references/behavioral-patterns.md for detailed guides with code examples.

Integration with Refactor Pilot Phases

This skill integrates with the broader framework:

• Phase 1 (Analyze): During codebase analysis, flag code smells that indicate pattern opportunities
• Phase 2 (Test): Ensure safety nets cover the behavior that patterns will restructure
• Phase 3 (Transform): Apply patterns using the three-pass approach
• Phase 4 (Verify): Confirm pattern application didn't change behavior, check extensibility

Output

After pattern work, present:

1. Summary of patterns identified or applied
2. Before/after comparison of key code sections
3. Extensibility guide (how to add new variants/behaviors using the pattern)
4. Any // REVIEW flags where the pattern application needs human judgment

Critical Rules

• Patterns are tools, not goals. Don't apply a pattern just because you can.
• Deep modules beat patterns. When a pattern and a single deeper module both solve the problem, choose the deeper module. The default answer to "which pattern fits?" is often "none — collapse the layers instead."
• Simpler is better. If a function solves the problem, don't create a class hierarchy.
• Language idioms first. Use language-native features (closures, protocols, traits) before reaching for classical OOP patterns.
• One pattern at a time. Apply and verify one pattern before starting the next.
• Preserve behavior. Pattern refactoring must not change what the code does.
• Document the "why." Every pattern application should explain the problem it solves.