Erlang Design Pattern Reference Cheat Sheet: A Quick Guide to Functional and Concurrent Programming

30.3 Design Pattern Reference Cheat Sheet

Introduction

Welcome to the Erlang Design Pattern Reference Cheat Sheet, your go-to resource for quickly recalling and comparing the design patterns discussed throughout this guide. This section provides concise summaries of each pattern, highlighting their intent, applicability, and unique features in Erlang. Whether you’re a seasoned Erlang developer or new to the language, this cheat sheet will serve as a handy reference to enhance your understanding and application of design patterns in functional and concurrent programming.

Creational Patterns

Factory Pattern

• Category: Creational
• Intent: Provide an interface for creating objects in a superclass, but allow subclasses to alter the type of objects that will be created.
• Structure Diagram:

    classDiagram
	    class Factory {
	        +createProduct()
	    }
	    class Product
	    Factory --> Product

• Key Participants: Factory, Product
• Applicability: Use when a class can’t anticipate the class of objects it must create.
• Erlang Unique Features: Leverage Erlang’s module system to create factory functions that return different module implementations.

Builder Pattern

• Category: Creational
• Intent: Separate the construction of a complex object from its representation, allowing the same construction process to create different representations.
• Structure Diagram:

    classDiagram
	    class Builder {
	        +buildPart()
	    }
	    class Product
	    Builder --> Product

• Key Participants: Builder, Product
• Applicability: Use when the algorithm for creating a complex object should be independent of the parts that make up the object.
• Erlang Unique Features: Use functional composition to build complex data structures incrementally.

Singleton Pattern

• Category: Creational
• Intent: Ensure a class has only one instance and provide a global point of access to it.
• Structure Diagram:

    classDiagram
	    class Singleton {
	        -instance
	        +getInstance()
	    }

• Key Participants: Singleton
• Applicability: Use when exactly one instance of a class is needed.
• Erlang Unique Features: Utilize Erlang’s process dictionary or application environment for singleton-like behavior.

Structural Patterns

Adapter Pattern

• Category: Structural
• Intent: Convert the interface of a class into another interface clients expect, allowing classes to work together that couldn’t otherwise due to incompatible interfaces.
• Structure Diagram:

    classDiagram
	    class Adapter {
	        +request()
	    }
	    class Adaptee {
	        +specificRequest()
	    }
	    Adapter --> Adaptee

• Key Participants: Adapter, Adaptee
• Applicability: Use when you want to use an existing class, and its interface does not match the one you need.
• Erlang Unique Features: Implement adapters using Erlang’s behavior modules to provide a uniform interface.

Proxy Pattern

• Category: Structural
• Intent: Provide a surrogate or placeholder for another object to control access to it.
• Structure Diagram:

    classDiagram
	    class Proxy {
	        +request()
	    }
	    class RealSubject {
	        +request()
	    }
	    Proxy --> RealSubject

• Key Participants: Proxy, RealSubject
• Applicability: Use when you need to control access to an object.
• Erlang Unique Features: Use processes as proxies to manage access and control over resources.

Decorator Pattern

• Category: Structural
• Intent: Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.
• Structure Diagram:

    classDiagram
	    class Decorator {
	        +operation()
	    }
	    class Component {
	        +operation()
	    }
	    Decorator --> Component

• Key Participants: Decorator, Component
• Applicability: Use to add responsibilities to individual objects dynamically and transparently.
• Erlang Unique Features: Use higher-order functions to wrap and extend functionality.

Behavioral Patterns

Strategy Pattern

• Category: Behavioral
• Intent: Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it.
• Structure Diagram:

    classDiagram
	    class Strategy {
	        +algorithmInterface()
	    }
	    class Context {
	        +setStrategy(Strategy)
	        +executeStrategy()
	    }
	    Context --> Strategy

• Key Participants: Strategy, Context
• Applicability: Use when you need to use different variants of an algorithm.
• Erlang Unique Features: Implement strategies using higher-order functions and passing them as parameters.

Observer Pattern

• Category: Behavioral
• Intent: Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.
• Structure Diagram:

    classDiagram
	    class Observer {
	        +update()
	    }
	    class Subject {
	        +attach(Observer)
	        +detach(Observer)
	        +notify()
	    }
	    Subject --> Observer

• Key Participants: Observer, Subject
• Applicability: Use when an object should notify other objects without making assumptions about who these objects are.
• Erlang Unique Features: Use Erlang’s message passing to implement observer-like behavior.

Command Pattern

• Category: Behavioral
• Intent: Encapsulate a request as an object, thereby allowing for parameterization of clients with queues, requests, and operations.
• Structure Diagram:

    classDiagram
	    class Command {
	        +execute()
	    }
	    class Invoker {
	        +setCommand(Command)
	        +executeCommand()
	    }
	    Invoker --> Command

• Key Participants: Command, Invoker
• Applicability: Use to parameterize objects with operations.
• Erlang Unique Features: Use message passing to encapsulate commands and send them to processes.

Design Considerations

• When to Use Patterns: Consider the problem context and the specific requirements of your application. Patterns are not one-size-fits-all solutions and should be applied judiciously.
• Erlang-Specific Features: Leverage Erlang’s strengths in concurrency, fault tolerance, and functional programming when implementing patterns.
• Common Pitfalls: Avoid overusing patterns or applying them inappropriately. Ensure that the pattern chosen aligns with the problem being solved.

Differences and Similarities

• Factory vs. Builder: Both are creational patterns, but Factory is used for creating objects without specifying the exact class, while Builder is used for constructing complex objects step by step.
• Adapter vs. Proxy: Adapter is used to change the interface of an existing object, while Proxy controls access to an object.
• Strategy vs. Command: Strategy is about choosing an algorithm at runtime, while Command encapsulates a request as an object.

Quiz: Design Pattern Reference Cheat Sheet

Remember, this cheat sheet is just a starting point. As you delve deeper into Erlang’s design patterns, you’ll discover more nuances and applications. Keep experimenting, stay curious, and enjoy the journey!