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- Herausgeber: Packt Publishing
- Kategorie: Fachliteratur
- Sprache: Englisch
Design patterns make developers’ lives easier by helping them write great software that is easy to maintain, runs efficiently, and is valuable to the company or people concerned. You’ll learn about the various features of Scala and will be able to apply well-known, industry-proven design patterns in your work.
The book starts off by focusing on some of the most interesting and latest features of Scala while using practical real-world examples. We will be learning about IDE’s and Aspect Oriented Programming. We will be looking into different components in Scala. We will also cover the popular "Gang of Four" design patterns and show you how to incorporate functional patterns effectively. The book ends with a practical example that demonstrates how the presented material can be combined in real-life applications. You’ll learn the necessary concepts to build enterprise-grade applications.
By the end of this book, you’ll have enough knowledge and understanding to quickly assess problems and come up with elegant solutions.
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Veröffentlichungsjahr: 2018
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Scala Design Patterns Second Edition
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Contributors
About the author
Ivan Nikolov is a technical architect based in London. He works in the ad tech industry and uses Scala in combination with libraries and technologies such as Spark, Hadoop, RabbitMQ, Kafka, SQL and NoSQL stores, and Akka. He also uses other JVM and scripting languages. Some of the projects Ivan has worked on include a large-scale real-time machine learning platform, batch processing solutions, and high load APIs. Ivan also likes getting involved with open source projects, whether it be to contribute or get inspiration and good ideas.
About the reviewer
Vasilis Nicolaou is a software engineer, Linux and FOSS hobbyist and enthusiast. He began his career at CERN as a Linux developer and now works for BookingGo as a senior software engineer developing microservices and distributed system solutions with Scala and Akka.
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Table of Contents
Title Page
Copyright and Credits
Scala Design Patterns Second Edition
Packt Upsell
Why subscribe?
PacktPub.com
Contributors
About the author
About the reviewer
Packt is searching for authors like you
Preface
Who this book is for
What this book covers
To get the most out of this book
Download the example code files
Conventions used
Get in touch
Reviews
The Design Patterns Out There and Setting Up Your Environment
Design patterns
Scala and design patterns
The need for design patterns and their benefits
Design pattern categories
Creational design patterns
The abstract factory design pattern
The factory method design pattern
The lazy initialization design pattern
The singleton design pattern
The object pool design pattern
The builder design pattern
The prototype design pattern
Structural design patterns
The adapter design pattern
The decorator design pattern
The bridge design pattern
The composite design pattern
The facade design pattern
The flyweight design pattern
The proxy design pattern
Behavioral design patterns
The value object design pattern
The null object design pattern
The strategy design pattern
The command design pattern
The chain of responsibility design pattern
The interpreter design pattern
The iterator design pattern
The mediator design pattern
The memento design pattern
The observer design pattern
The state design pattern
The template method design pattern
The visitor design pattern
Functional design patterns
Monoids
Monads
Functors
Scala-specific design patterns
The lens design pattern
The cake design pattern
Pimp my library
Stackable traits
The type class design pattern
Lazy evaluation
Partial functions
Implicit injection
Duck typing
Memoization
Choosing a design pattern
Setting up the development environment
Installing Scala
Tips for installing Scala manually
Tips for installing Scala using SBT
Scala IDEs
Dependency management
SBT
Maven
SBT versus Maven
Summary
Traits and Mixin Compositions
Traits
Traits as interfaces
Mixing in traits with variables
Traits as classes
Extending classes
Extending traits
Mixin compositions
Mixing traits in
Composing
Composing simple traits
Composing complex traits
Composing with self-types
Clashing traits
Same signatures and return types
Same signatures and different return types traits
Same signatures and return types mixins
Same signatures and different return types mixins
Multiple inheritance
The diamond problem
The limitations
Linearization
Rules of inheritance hierarchies
Linearization rules
How linearization works
Initialization
Method overriding
Testing traits
Using a class
Mixing the trait in
Mixing into the test class
Mixing into the test cases
Running the tests
Traits versus classes
Summary
Unification
Functions and classes
Functions as classes
Function literals
Functions without syntactic sugar
Increased expressivity
Algebraic data types and class hierarchies
ADTs
Sum ADTs
Product ADTs
Hybrid ADTs
The unification
Pattern matching
Pattern matching with values
Pattern matching for product ADTs
Modules and objects
Using modules
Summary
Abstract and Self Types
Abstract types
Generics
Abstract types
Generics versus abstract types
Usage advice
Polymorphism
Subtype polymorphism
Parametric polymorphism
Ad hoc polymorphism
Adding functions for multiple types
Self types
Using self types
Requiring multiple components
Conflicting components
Self types and the cake design pattern
Self types versus inheritance
Inheritance leaking functionality
Summary
Aspect-Oriented Programming and Components
Aspect-oriented programming
Understanding application efficiency
Timing our application without AOP
Timing our application with AOP
Components in Scala
Using Scala's expressive power to build components
Implementing components
Self types for components
Summary
Creational Design Patterns
What are creational design patterns?
The factory method design pattern
An example class diagram
A code example
Scala alternatives
What it is good for?
What it is not so good for?
The abstract factory
An example class diagram
A code example
Scala alternatives
What it is good for?
What it is not so good for?
Other factory design patterns
The static factory
The simple factory
Factory combinations
Lazy initialization
An example class diagram
A code example
What it is good for?
What it is not so good for?
The singleton design pattern
An example class diagram
A code example
What it is good for?
What it is not so good for?
The builder design pattern
An example class diagram
A code example
A Java-like implementation
Implementation with a case class
Using generalized type constraints
Changing the Person class
Adding generalized type constraints to the required methods
Using the type-safe builder
Using require statements
What it is good for?
What it is not so good for?
The prototype design pattern
An example class diagram
A code example
What it is good for?
What it is not so good for?
Summary
Structural Design Patterns
Defining structural design patterns
The adapter design pattern
Example class diagram
Code example
The adapter design pattern with final classes
The adapter design pattern the Scala way
What it is good for
What it is not so good for
The decorator design pattern
Example class diagram
Code example
The decorator design pattern the Scala way
What it is good for
What it is not so good for
The bridge design pattern
Example class diagram
Code example
The bridge design pattern the Scala way
What it is good for
What it is not so good for
The composite design pattern
Example class diagram
Code example
What it is good for
What it is not so good for
The facade design pattern
Example class diagram
Code example
What it is good for
What it is not so good for
The flyweight design pattern
Example class diagram
Code example
What it is good for
What it is not so good for
The proxy design pattern
Example class diagram
Code example
What it is good for
What it is not so good for
Summary
Behavioral Design Patterns – Part One
Defining behavioral design patterns
The value object design pattern
An example class diagram
A code example
Alternative implementation
What it is good for
What it is not so good for
The null object design pattern
An example class diagram
A code example
What it is good for
What it is not so good for
The strategy design pattern
An example class diagram
A code example
The strategy design pattern the Scala way
What it is good for
What it is not so good for
The command design pattern
An example class diagram
A code example
The command design pattern the Scala way
What it is good for
What it is not so good for
The chain of responsibility design pattern
An example class diagram
A code example
The chain of responsibility design pattern the Scala way
What it is good for
What it is not so good for
The interpreter design pattern
An example class diagram
A code example
What it is good for
What it is not so good for
Summary
Behavioral Design Patterns – Part Two
The iterator design pattern
Example class diagram
Code example
What it is good for
What it is not so good for
The mediator design pattern
Example class diagram
Code example
What it is good for
What it is not so good for
The memento design pattern
Example class diagram
Code example
What it is good for
What it is not so good for
The observer design pattern
Example class diagram
Code example
What it is good for
What it is not so good for
The state design pattern
Example class diagram
Code example
What it is good for
What it is not so good for
The template method design pattern
Example class diagram
Code example
What it is good for
What it is not so good for
The visitor design pattern
Example class diagram
Code example
The visitor design pattern the Scala way
What it is good for
What it is not so good for
Summary
Functional Design Patterns – the Deep Theory
Abstraction and vocabulary
Monoids
What are monoids?
Monoids in real life
Using monoids
Monoids and foldable collections
Monoids and parallel computations
Monoids and composition
When to use monoids
Functors
Functors in real life
Using our functors
Monads
What is a monad?
The flatMap method
The unit method
The connection between map, flatMap, and unit
The names of the methods
The monad laws
Monads in real life
Using monads
The Option monad
A more advanced monad example
Monad intuition
Summary
Applying What We Have Learned
The lens design pattern
Lens example
Without the lens design pattern
Immutable and verbose
Using mutable properties
With the lens design pattern
Minimizing the boilerplate
The cake design pattern
Dependency injection
Dependency injection libraries and Scala
Dependency injection in Scala
Writing our code
Wiring it all up
Unit testing our application
Other dependency injection alternatives
Implicits for dependency injection
Reader monad for dependency injection
The pimp my library design pattern
Using pimp my library
Pimp my library in real life
The stackable traits design pattern
Using stackable traits
The type class design pattern
Type class example
Type class design pattern alternatives
Lazy evaluation
Evaluating by-name parameters only once
Alternative lazy evaluation
Partial functions
Partial functions are not partially applied functions
Partially defined functions
Implicit injection
Implicit conversions
Dependency injection using implicits
Testing with implicit dependency injection
Duck typing
Duck typing example
Duck typing alternatives
When to use duck typing
Memoization
Memoization example
Memoization alternatives
Summary
Real-Life Applications
Reasons to use libraries
The Scalaz library
Monoids in Scalaz
Using monoids
Testing monoids
Monads in Scalaz
Using monads
Testing monads
The possibilities of Scalaz
Writing a complete application
Application specifications
Implementation
The libraries to use
Reading the application configuration
Reading the scheduler configuration
Scheduling tasks
Accessing a database
Executing console commands
Writing some code
Wiring it all up
The end result
Testing our application
Unit testing
Application testing
The future of our application
Summary
Other Books You May Enjoy
Leave a review - let other readers know what you think
Preface
Software engineering and design has existed for many years now. We use software almost everywhere in our lives, and this makes programs distinct in terms of the problems they solve.
Regardless of the number of things that can be done with programming, there are still some specific features that repeat over and over again. Over time, people have come up with some best practices that help to tackle specific patterns that emerge in programs. These are called design patterns.
Design patterns solve not only commonly occurring problems, but also deal with language limitations. No matter what the specific design patterns are and what single issue they solve, all of them in the end aim at producing better software. This includes improved readability, simplicity, easier maintainability, testability, extendibility, and efficiency. Today, design patterns are an important part of every good software engineer's arsenal.
Together with the large number of problems that we tackle with programming, there are also many languages that we can use. Every language is different and has its strengths and weaknesses, so we also need to take this into consideration when doing something. In this book, we will look at design patterns from the point of view of Scala.
Scala has become extremely popular in the last couple of years, and the numbers using it keep growing. Many companies use it in production for various purposes—big data processing, writing APIs, machine learning, and so on. Switching to Scala from popular languages, such as Java, turns out to be quite simple because it is a hybrid of an object-oriented language and a functional programming language. Using Scala to its full potential, however, requires us to be familiar with not only the object-oriented features, but also with the functional ones. The use of Scala could improve performance and the time it takes to implement the features. One of the reasons is the really high expressivity of Scala.
The fact that Scala is close to object-oriented languages means that many of the design patterns for object-oriented programming are still applicable here. The fact that it is also functional means that some other design patterns are also applicable, and some of the object-oriented ones could be modified to better fit the paradigm of Scala. In this book, we will be focusing on all of them—we will go through some specific features of Scala and then look at the popular Gang of Four design patterns viewed from the Scala perspective. We will also become familiar with design patterns that are exclusive to Scala and understand different functional programming concepts, including monoids and monads. Having meaningful examples always makes learning and understanding easier. We will try to provide examples that you can easily map to real problems that you would potentially be solving. We will also introduce some libraries that will be useful for anyone who writes real-world applications.
Who this book is for
This book is aimed for people who already have some knowledge of Scala, but want to get a more practical understanding of how to apply it in real-world application development. This book is also useful as a reference to consult while designing applications. Having an understanding of the importance of using best practices and writing nice code is good; however, even if you don't, hopefully, you will be convinced by the time you finish reading this book. Prior knowledge of design patterns is not required, but if you are familiar with some, this book will be useful, as we will take a look at them from the point of view of Scala.
What this book covers
Chapter 1, The Design Patterns Out There and Setting Up Your Environment, is a brief introduction to design patterns, why they exist, and their different types. This chapter also provides you with tips on how you can set up your environment in order to easily run the examples in the book.
Chapter 2, Traits and Mixin Compositions, talks about traits and mixin compositions in Scala, multiple inheritance, and the rules of linearization that the Scala programming language uses when extending multiple traits.
Chapter 3, Unification, covers the various unifications that exist in Scala, which makes it as expressive as it is.
Chapter 4, Abstract and Self Types, covers the different types of polymorphism that exist in Scala, which help to make generic and extendible software.
Chapter 5, Aspect-Oriented Programming and Components, shows the concept of aspect-oriented programming and how it can be applied to Scala. This chapter also explains what components are and how to build applications using multiple small and simple components.
Chapter 6, Creational Design Patterns, covers the most popular creational design patterns from the Gang of Four. All patterns are viewed from the point of view of Scala, and alternatives are shown where applicable.
Chapter 7, Structural Design Patterns, goes through the most popular structural design patterns from the Gang of Four from the Scala point of view. This chapter also shows Scala alternatives where this is applicable and gives usage advice.
Chapter 8, Behavioral Design Patterns - Part One, covers part one of the behavioral design patterns from the Gang of Four viewed from the Scala perspective. This chapter also provides examples and usage advice.
Chapter 9, Behavioral Design Patterns - Part Two, covers part two of the behavioral design patterns from the Gang of Four viewed from the Scala perspective. This chapter also provides examples and usage advice.
Chapter 10, Functional Design Patterns - the Deep Theory, delves into pure functional programming concepts, such as monoids, functors, and monads. This chapter also explains these concepts in an understandable way, along with some examples.
Chapter 11, Applying What We Have Learned, presents design patterns that are specific to Scala. It is loaded with examples along with theory and usage advice.
Chapter 12, Real-Life Applications, introduces you to the Scalaz library. You will write a complete application that applies many of the concepts learned in the book, and this chapter will finish off with a summary.
To get the most out of this book
This book assumes that the reader is already familiar with Scala. We have provided examples for each chapter in projects using Maven and SBT. You should have some knowledge of either one of these tools and have it installed on your machine. You are also recommended to have a modern and up-to-date IDE installed on your computer, for example, IntelliJ. You are encouraged to open the actual projects, as the examples in this book focus on the design patterns and, in some cases, imports are omitted in favor of space.
The examples in the book were written and tested on a Unix-based operating system; however, they should also successfully compile and run on Windows.
Download the example code files
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The Design Patterns Out There and Setting Up Your Environment
In the world of computer programming, there are multiple ways to create a solution to a given problem. However, some might wonder whether there is a correct way of achieving a specific task. The answer is yes; there is always a right way, but in software development, there are usually multiple right ways to achieve a task. Some factors exist that guide the programmer to the right solution and, depending on them, people tend to get the expected result. These factors could define many things—the actual language being used, the algorithm, the type of executable produced, the output format, and the code structure. In this book, the language is already chosen for us—Scala. There are, however, a number of ways to use Scala, and we will be focusing on them—the design patterns.
In this chapter, we will explain what design patterns are and why they exist. We will go through the different types of design patterns that are out there. This book aims to provide useful examples to aid you in the learning process, and being able to run them easily is key. Hence, some points on how to set up a development environment properly will be given here. The top-level topics we will go through are as follows:
What is a design pattern and why do they exist?
The main types of design patterns and their features
Choosing the right design pattern
Setting up a development environment in real life
The last point doesn't have much to do with design patterns. However, it is always a good idea to build projects properly, as this makes it much easier to work in the future.
Design patterns
Before delving into the Scala design patterns, we have to explain what they actually are, why they exist, and why it is worth being familiar with them.
Software is a broad subject, and there are innumerable examples of things people can do with it. At first glance, most of these things are completely different—games, websites, mobile phone applications, and specialized systems for different industries. There are, however, many similarities in how software is built. Many times, people have to deal with similar issues, no matter the type of software they create. For example, computer games, as well as websites, might need to access a database. And throughout time, by experience, developers learn how structuring their code differs for the various tasks that they perform.
Design patterns are best practices at which the software community has arrived over a period of time. They are supposed to help you write efficient, readable, testable, and easily extendable code. In some cases, they can be the result of a programming language not being expressive enough to elegantly achieve a goal. This means that more feature-rich languages might not even need a design pattern, while others still do. Scala is one of those rich languages, and in some cases, it makes the use of some design patterns obsolete or simpler. We will see how exactly it does that in this book.
The lack or existence of a certain functionality within a programming language also makes it able to implement additional design patterns that others cannot. The opposite is also valid—it might not be able to implement things that others can.
Scala and design patterns
Scala is a hybrid language that combines features from object-oriented and functional languages. This not only allows it to keep some of the well-known object-oriented design patterns relevant, but also provides various other ways of exploiting its features to write code that is clean, efficient, testable, and extendable all at the same time. The hybrid nature of the language also makes some of the traditional object-oriented design patterns obsolete, or possible, using other cleaner techniques.
The need for design patterns and their benefits
Writing code without the conscious use of a design pattern is something many software engineers do. In the end, however, they either end up using one without realizing it, or they end up with code that can be improved in some way. As we mentioned earlier, design patterns help to write efficient, readable, extendable, and testable code. All these features are really important to companies in the industry.
Even though in some cases it is preferable to quickly write a prototype and get it out, it is more usually the case that a piece of software is supposed to evolve. Maybe you will have experience of extending some badly written code, but regardless, it is a challenging task and takes a really long time, and sometimes it feels that rewriting it would be easier. Moreover, this makes introducing bugs into the system much more likely.
Code readability is also something that should be appreciated. Of course, one could use a design pattern and still have their code hard to read, but generally, design patterns help. Big systems are usually worked on by many people, and everyone should be able to understand what exactly is going on. Also, people who join a team are able to integrate much more easily and quickly if they are working on a well-written piece of software.
Testability is something that prevents developers from introducing bugs when writing or extending code. In some cases, code could be created so badly that it is not even testable. Design patterns are supposed to eliminate these problems as well.
While efficiency is often connected with algorithms, design patterns could also affect it. A simple example could be an object that takes a long time to instantiate, and instances are used in many places in an application, but could be made a singleton instead. You will see more concrete examples in the later chapters of this book.
Design pattern categories
The fact that software development is an extremely broad topic leads to a number of things that can be done with programming. Requirements can vary greatly between different industries and engineering teams. These facts have caused many different design patterns to be invented. This is further contributed to by the existence of various programming languages with different features and levels of expressiveness.
This book focuses on the design patterns from the point of view of Scala. As we mentioned previously, Scala is a hybrid language. This leads us to a few famous design patterns that are not needed anymore—one example is the null object design pattern, which can simply be replaced by Scala's Option. Other design patterns become possible using different approaches—the decorator design pattern can be implemented using stackable traits. Finally, some new design patterns become available that are applicable specifically to the Scala programming language—the cake design pattern, pimp my library, and so on. We will focus on all of these and make it clear where the richness of Scala helps us to make our code even cleaner and simpler.
Even if there are many different design patterns, they can all be grouped in the following:
Creational
Structural
Behavioral
Functional
Scala-specific design patterns
Some of the design patterns that are specific to Scala can be assigned to the previous groups. They can either be additions or replacements of the already existing ones. They are typical to Scala and take advantage of some advanced language features or simply features not available in other languages.
The first three groups contain the famous Gang of Four design patterns. Every design pattern book covers them and so will we. The rest, even if they can be assigned to one of the first three groups, will be specific to Scala and functional programming languages. In the next few subsections, we will explain the main characteristics of the listed groups and briefly present the actual design patterns that fall under them.
Creational design patterns
The creational design patterns deal with object creation mechanisms. Their purpose is to create objects in a way that is suitable to the current situation, which could lead to unnecessary complexity and the need for extra knowledge if they were not there. The main ideas behind the creational design patterns are as follows:
Knowledge encapsulation about the concrete classes
Hiding details about the actual creation and how objects are combined
We will be focusing on the following creational design patterns in this book:
The abstract factory design pattern
The factory method design pattern
The lazy initialization design pattern
The singleton design pattern
The object pool design pattern
The builder design pattern
The prototype design pattern
The following few sections give a brief definition of what these patterns are. They will be looked at in depth individually later in this book.
The abstract factory design pattern
This is used to encapsulate a group of individual factories that have a common theme. When used, the developer creates a specific implementation of the abstract factory and uses its methods in the same way as in the factory design pattern to create objects. It can be thought of as another layer of abstraction that helps to instantiate classes.
The factory method design pattern
This design pattern deals with the creation of objects without explicitly specifying the actual class that the instance will have—it could be something that is decided at runtime based on many factors. Some of these factors can include operating systems, different data types, or input parameters. It gives developers the peace of mind of just calling a method rather than invoking a concrete constructor.
The lazy initialization design pattern
This design pattern is an approach to delay the creation of an object or the evaluation of a value until the first time it is needed. It is much more simplified in Scala than it is in an object-oriented language such as Java.
The singleton design pattern
This design pattern restricts the creation of a specific class to just one object. If more than one class in the application tries to use such an instance, then this same instance is returned for everyone. This is another design pattern that can be easily achieved with the use of basic Scala features.
The object pool design pattern
This design pattern uses a pool of objects that are already instantiated and ready for use. Whenever someone requires an object from the pool, it is returned, and after the user is finished with it, it puts it back into the pool manually or automatically. A common use for pools are database connections, which generally are expensive to create; hence, they are created once and then served to the application on request.
The builder design pattern
The builder design pattern is extremely useful for objects with many possible constructor parameters that would otherwise require developers to create many overrides for the different scenarios an object could be created in. This is different to the factory design pattern, which aims to enable polymorphism. Many of the modern libraries today employ this design pattern. As we will see later, Scala can achieve this pattern really easily.
The prototype design pattern
This design pattern allows object creation using a clone() method from an already created instance. It can be used in cases when a specific resource is expensive to create or when the abstract factory pattern is not desired.
Structural design patterns
Structural design patterns exist in order to help establish the relationships between different entities in order to form larger structures. They define how each component should be structured so that it has very flexible interconnecting modules that can work together in a larger system. The main features of structural design patterns include the following:
The use of composition to combine the implementations of multiple objects
Help build a large system made of various components by maintaining a high level of flexibility
In this book, we will focus on the following structural design patterns:
The adapter design pattern
The decorator design pattern
The bridge design pattern
The composite design pattern
The facade design pattern
The flyweight design pattern
The proxy design pattern
The next subsections will put some light on what these patterns are about before we delve into them later in this book.
The adapter design pattern
The adapter design pattern allows the interface of an existing class to be used from another interface. Imagine that there is a client who expects your class to expose a doWork() method. You might have the implementation ready in another class, but the method is called differently and is incompatible. It might require extra parameters too. This could also be a library that the developer doesn't have access to for modifications. This is where the adapter can help by wrapping the functionality and exposing the required methods. The adapter is useful for integrating the existing components. In Scala, the adapter design pattern can be easily achieved using implicit classes.
The decorator design pattern
Decorators are a flexible alternative to sub classing. They allow developers to extend the functionality of an object without affecting other instances of the same class. This is achieved by wrapping an object of the extended class into one that extends the same class and overrides the methods whose functionality is supposed to be changed. Decorators in Scala can be built much more easily using another design pattern called stackable traits.
The bridge design pattern
The purpose of the bridge design pattern is to decouple an abstraction from its implementation so that the two can vary independently. It is useful when the class and its functionality vary a lot. The bridge reminds us of the adapter pattern, but the difference is that the adapter pattern is used when something is already there and you cannot change it, while the bridge design pattern is used when things are being built. It helps us to avoid ending up with multiple concrete classes that will be exposed to the client. You will get a clearer understanding when we delve deeper in the topic, but for now, let's imagine that we want to have a FileReader class that supports multiple different platforms. The bridge will help us end up with FileReader, which will use a different implementation, depending on the platform. In Scala, we can use self-types in order to implement a bridge design pattern.
The composite design pattern
The composite is a partitioning design pattern that represents a group of objects that are to be treated as only one object. It allows developers to treat individual objects and compositions uniformly and to build complex hierarchies without complicating the source code. An example of composite could be a tree structure where a node can contain other nodes, and so on.
The facade design pattern
The purpose of the facade design pattern is to hide the complexity of a system and its implementation details by providing the client with a simpler interface to use. This also helps to make the code more readable and to reduce the dependencies of the outside code. It works as a wrapper around the system that is being simplified and, of course, it can be used in conjunction with some of the other design patterns mentioned previously.
The flyweight design pattern
The flyweight design pattern provides an object that is used to minimize memory usage by sharing it throughout the application. This object should contain as much data as possible. A common example given is a word processor, where each character's graphical representation is shared with the other same characters. The local information then is only the position of the character, which is stored internally.
The proxy design pattern
The proxy design pattern allows developers to provide an interface to other objects by wrapping them. They can also provide additional functionality, for example, security or thread-safety. Proxies can be used together with the flyweight pattern, where the references to shared objects are wrapped inside proxy objects.
Behavioral design patterns
Behavioral design patterns increase communication flexibility between objects based on the specific ways they interact with each other. Here, creational patterns mostly describe a moment in time during creation, structural patterns describe a more or less static structure, and behavioral patterns describe a process or flow. They simplify this flow and make it more understandable.
The main features of behavioral design patterns are as follows:
What is being described is a process or flow
The flows are simplified and made understandable
They accomplish tasks that would be difficult or impossible to achieve with objects
In this book, we will focus our attention on the following behavioral design patterns:
The value object design pattern
The null object design pattern
The strategy design pattern
The command design pattern
The chain of responsibility design pattern
The interpreter design pattern
The iterator design pattern
The mediator design pattern
The memento design pattern
The observer design pattern
The state design pattern
The template method design pattern
The visitor design pattern
The following subsections will give brief definitions of the aforementioned behavioral design patterns.
The value object design pattern
Value objects are immutable and their equality is based not on their identity, but on their fields being equal. They can be used as data transfer objects, and they can represent dates, colors, money amounts, numbers, and more. Their immutability makes them really useful in multithreaded programming. The Scala programming language promotes immutability, and value objects are something that naturally occur there.
The null object design pattern
Null objects represent the absence of a value and they define a neutral behavior. This approach removes the need to check for null references and makes the code much more concise. Scala adds the concept of optional values, which can replace this pattern completely.
The strategy design pattern
The strategy design pattern allows algorithms to be selected at runtime. It defines a family of interchangeable encapsulated algorithms and exposes a common interface to the client. Which algorithm is chosen could depend on various factors that are determined while the application runs. In Scala, we can simply pass a function as a parameter to a method, and depending on the function, a different action will be performed.
The command design pattern
This design pattern represents an object that is used to store information about an action that needs to be triggered at a later time. The information includes the following:
The method name
The owner of the method
Parameter values
The client then decides which commands need to be executed and when by the invoker. This design pattern can easily be implemented in Scala using the by-name parameters feature of the language.
The chain of responsibility design pattern
The chain of responsibility is a design pattern where the sender of a request is decoupled from its receiver. This way, it makes it possible for multiple objects to handle the request and to keep logic nicely separated. The receivers form a chain where they pass the request and, if possible, they process it, and if not, they pass it to the next receiver. There are variations where a handler might dispatch the request to multiple other handlers at the same time. This somehow reminds us of function composition, which in Scala can be achieved using the stackable traits design pattern.
The interpreter design pattern
The interpreter design pattern is based on the ability to characterize a well-known domain with a language with a strict grammar. It defines classes for each grammar rule in order to interpret sentences in the given language. These classes are likely to represent hierarchies as grammar is usually hierarchical as well. Interpreters can be used in different parsers, for example, SQL or other languages.
The iterator design pattern
The iterator design pattern is when an iterator is used to traverse a container and access its elements. It helps to decouple containers from the algorithms performed on them. What an iterator should provide is sequential access to the elements of an aggregate object without exposing the internal representation of the iterated collection.
The mediator design pattern
This pattern encapsulates the communication between different classes in an application. Instead of interacting directly with each other, objects communicate through the mediator, which reduces the dependencies between them, lowers the coupling, and makes the overall application easier to read and maintain.
The memento design pattern
This pattern provides the ability to roll back an object to its previous state. It is implemented with three objects—originator, caretaker, and memento. The originator is the object with the internal state; the caretaker will modify the originator, and a memento is an object that contains the state that the originator returns. The originator knows how to handle a memento in order to restore its previous state.
The observer design pattern
This design pattern allows the creation of publish/subscribe systems. There is a special object called subject that automatically notifies all the observers when there are any changes in the state. This design pattern is popular in various GUI toolkits and generally where event handling is needed. It is also related to reactive programming, which is enabled by libraries such as Akka. We will see an example of this towards the end of this book.
The state design pattern
This design pattern is similar to the strategy design pattern, and it uses a state object to encapsulate different behavior for the same object. It improves the code's readability and maintainability by avoiding the use of large conditional statements.
The template method design pattern
This design pattern defines the skeleton of an algorithm in a method and then passes some of the actual steps to the subclasses. It allows developers to alter some of the steps of an algorithm without having to modify its structure. An example of this could be a method in an abstract class that calls other abstract methods, which will be defined in the children.
The visitor design pattern
The visitor design pattern represents an operation to be performed on the elements of an object structure. It allows developers to define a new operation without changing the original classes. Scala can minimize the verbosity of this pattern compared to the pure object-oriented way of implementing it by passing functions to methods.
Functional design patterns
We will be looking into all of the preceding design patterns from the point of view of Scala. This means that they will look different than in other languages, but they still haven't been designed specifically for functional programming. Functional programming is much more expressive than object-oriented programming. It has its own design patterns that help to make the life of a programmer easier. We will focus on:
Monoids
Monads
Functors
After we've looked at some Scala functional programming concepts, and we've been through these, we will mention some interesting design patterns from the Scala world.
A brief explanation of the preceding listed patterns will follow in the next few subsections.
Monads
In functional programming, monads are structures that represent computations as sequences of steps. Monads are useful for building pipelines, adding operations with side effects cleanly to a language where everything is immutable, and implementing compositions. This definition might sound vague and unclear, but explaining monads in a few sentences seems to be something hard to achieve. Later in this book, we will focus on them and try and clear things up without the use of a complex mathematical theory. We will try to show why monads are useful and what they can help with, as long as developers understand them well.
Functors
Functors come from category theory, and as for monads, it takes time to explain them properly. We will look at functors later in this book. For now, you could remember that functors are things that can allow us to lift a function of the type A => B to a function of the type F[A] => F[B].
Scala-specific design patterns
The design patterns in this group could be assigned to some of the previous groups. However, they are specific to Scala and exploit some of the language features that we will focus on in this book, and so we've decided to place them in their own group.
We will focus our attention on the following:
The lens design pattern
The cake design pattern
Pimp my library
Stackable traits
The type class design pattern
Lazy evaluation
Partial functions
Implicit injection
Duck typing
Memoization
The next subsections will give you some brief information about these patterns before we properly study them later in this book.
The lens design pattern
The Scala programming language promotes immutability. Having objects immutable makes it harder to make mistakes. However, sometimes mutability is required and the lens design pattern helps us to achieve this nicely.
The cake design pattern
The cake design pattern is the Scala way to implement dependency injection. It is something that is used quite a lot in real-life applications, and there are numerous libraries that help developers achieve it. Scala has a way of doing this using language features, and this is what the cake design pattern is all about.
Pimp my library
Many times, engineers need to work with libraries, which are made to be as generic as possible. Sometimes, we need to do something more specific to our use case, though. The pimp my library design pattern provides a way to write extension methods for libraries, which we cannot modify. We can also use it for our own libraries as well. This design pattern also helps to achieve better code readability.
Stackable traits
Stackable traits is the Scala way to implement the decorator design pattern. It can also be used to compose functions, and it's based on a few advanced Scala features.
The type class design pattern
This design pattern allows us to write generic code by defining a behavior that must be supported by all members of a specific type class. For example, all numbers must support the addition and subtraction operations.
Lazy evaluation
Often, engineers have to deal with operations that are slow and/or expensive. Sometimes, the result of these operations might not even be needed. Lazy evaluation is a technique that postpones the operation execution until it is actually needed. It could be used for application optimization.
Partial functions
Mathematics and functional programming are really close together. As a consequence, some functions exist that are only defined for a subset of all the possible input values they can get. A popular example is the square root function, which only works for non-negative numbers. In Scala, such functions can be used to efficiently perform multiple operations at the same time or to compose functions.
Implicit injection
Implicit injection is based on the implicit functionality of the Scala programming language. It automatically injects objects whenever they are needed, as long as they exist in a specific scope. It can be used for many things, including dependency injection.
