Decorator Pattern

Overview

The Decorator pattern attaches additional responsibilities to objects dynamically, providing a flexible alternative to subclassing for extending functionality. In NERV, this pattern enables composition of behaviors, enhancing objects with additional capabilities without modifying their core implementation.

Key Concepts

• Component Interface: Common interface for both decorators and decorated objects
• Concrete Component: Base object that can be decorated
• Decorator: Maintains a reference to a component object and conforms to its interface
• Concrete Decorator: Adds responsibilities to the component

Benefits

• Dynamic Extension: Add behaviors to objects at runtime
• Composition Over Inheritance: Avoid class explosion from inheritance hierarchies
• Single Responsibility: Keep classes focused on core functionality
• Open/Closed Principle: Extend behavior without modifying existing code
• Combinatorial Flexibility: Mix and match decorators in any order

Implementation Considerations

• Interface Conformance: Ensure decorators preserve the component interface
• Decoration Order: Consider how order affects behavior when multiple decorators are used
• Performance: Be aware of potential overhead from multiple decorator layers
• Transparency: Make decorator presence transparent to clients
• State Sharing: Handle state consistently across decorator layers

Core Interface

Core Decorator Interface

The Effect Monad demonstrates the decorator pattern with its extension method:

// TypeScript-like pseudo-code
interface Effectful<T> {
  withEffect(effect: Effect): Effectful<T>;
}

Pattern Variations

Class-Based Decorator

Traditional implementation using object composition with the same interface.

Example

// TypeScript-like pseudo-code
interface Component {
  operation(): string;
}

class ConcreteComponent implements Component {
  operation(): string {
    return "ConcreteComponent";
  }
}

abstract class Decorator implements Component {
  constructor(protected component: Component) {}
  abstract operation(): string;
}

class ConcreteDecoratorA extends Decorator {
  operation(): string {
    return `DecoratorA(${this.component.operation()})`;
  }
}

// Usage
const component = new ConcreteComponent();
const decorated = new ConcreteDecoratorA(component);
console.log(decorated.operation()); // Output: DecoratorA(ConcreteComponent)

Function-Based Decorator

Uses higher-order functions to wrap and enhance function behavior.

Common Pitfall

When using function decorators, be careful to preserve function metadata like name and docstrings, as they can be lost during decoration.

Parameterized Decorator

Decorators that accept parameters to customize their behavior.

Mixin Decorator

Uses multiple inheritance to add functionality to a class.

Integration in NERV

The Decorator pattern is used throughout NERV in these contexts:

• Effect System: Decorating operations with side effects
• Stream Processing: Adding behaviors to stream handlers
• Middleware: Enhancing event propagation in the event bus
• Telemetry: Adding observability to components

EffectMonad Implementation

The EffectMonad uses the decorator pattern to attach side effects to operations. Each effect decorator wraps the original value while adding a new effect to the effect collection. The pattern allows for composing multiple effects while maintaining the original value’s identity and providing a clean way to handle effects separately from core logic.

Usage in NERV Components

• EffectMonad: Uses decorator pattern to add effects to operations
• StreamHandler: Decorators add functionality like buffering or throttling
• EventBus: Middleware acts as decorators for event processing
• ResourceManager: Decorators add monitoring to resource operations

Related NERV Patterns

• Effect System: Uses decorator pattern extensively
• Reactive Event Mesh: Uses decorators for event middleware
• Temporal Versioning: Can use decorators for versioning

Related Design Patterns

• Composite Pattern
• Strategy Pattern
• Chain of Responsibility
• Proxy Pattern