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Scala Design Patterns
Scala Design Patterns

Scala Design Patterns: Design modular, clean, and scalable applications by applying proven design patterns in Scala , Second Edition

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Scala Design Patterns

Traits and Mixin Compositions

Before digging into some actual design patterns, we have to make sure that many of the Scala language concepts are clear to the reader. Many of those concepts will later be used in implementing the actual design patterns, and being aware of the possibilities, limitations, and pitfalls are key factors that enable us to correctly and efficiently write code. Even though those concepts are not considered official design patterns, they could still be used to write good software. In some cases, due to the richness of Scala, some concepts could replace a design pattern by just using language features. After all, as we have already said before, design patterns are there because a programming language lacks features and is not rich enough to complete certain tasks.

The first topic that we will look into is about traits and mixin compositions. They provide...

Traits

Many of you might have different perspectives of traits in Scala. They can be viewed not only as interfaces in other languages, but also as classes with only parameter-less constructors.

Trait parameters

The Scala programming language is quite dynamic and has evolved quickly throughout the years. According to the language creators, the Dotty project is the future of Scala and it is a place where passing parameters to traits and many other features are being tested and implemented. The main idea behind it is language simplification, and more information can be found at http://dotty.epfl.ch and https://scala-lang.org/blog/2017/05/31/first-dotty-milestone-release.html.

In the following few sections, we will see the traits from different points of view and try to give you some ideas about how they can be used.

...

Mixin compositions

Scala allows developers to extend many traits in a single class. This adds the possibility of achieving multiple inheritance and saves a lot of effort in code writing, which has to be performed in languages where extending many classes is not allowed. In this subtopic, we will show how traits can be mixed in a specific class or used to create anonymous classes with some specific functionality while writing our code.

Mixing traits in

First of all, let's modify the code from the previous example. It is a really simple change and it will also show exactly how traits can be mixed in:

object MixinRunner extends Ping with Pong {
def main(args: Array[String]): Unit = {
ping()
pong()
}
}

As can be seen...

Multiple inheritance

We inevitably had to mention multiple inheritance in the previous sections due to the fact that we can mix multiple traits and they can all have their own implementations of the methods. Multiple inheritance is not only a powerful technique, but also a dangerous one, and some languages such as Java have decided to not even allow it. As we already saw, Scala allows this, but with some limitations. In this subsection, we will present the problems of multiple inheritance and show how Scala deals with them.

The diamond problem

Multiple inheritance suffers from the diamond problem.

Let's have a look at the following diagram:

Here, both B and C extend A, and then D extends B and C. Some ambiguities might...

Linearization

As we already saw, traits offer a form of multiple inheritance. In such cases, the hierarchy is not necessarily linear, but forms an acyclic graph that needs to be flattened upon compilation. What linearization does is this—it specifies a single linear order for all of the ancestors of a class, including both the regular superclass chain and the parent chains of all of the traits.

We will not have to deal with linearization in traits that contain no code. However, if we use mixins, we will have to consider it. The following will be affected by linearization:

  • Method definitions
  • Variables (both mutable—var and immutable—val)

We already saw a simple example of linearization previously. Things, however, can get much more complicated and unexpected if the rules of linearization are not clear.

...

Testing traits

Testing is a really important part of software development. It ensures that changes to a certain piece of code do not end up producing errors either in the methods that were changed, or somewhere else.

There are different testing frameworks that one can use, and it really is a matter of personal preference. In this book, we have used ScalaTest (http://www.scalatest.org), as this is the one I use in my projects; it is understandable, readable, and easy to use.

In some cases, if a trait is mixed into a class, we could end up testing the class. However, we might want to test only a specific trait. It does not make much sense to test a trait that doesn't have all its methods implemented, so here we will look into the ones that have their code written (mixins). Also, the unit tests that we will show here are quite simple, but they are just for illustration purposes...

Traits versus classes

Traits could be similar, but also very different to classes. It could be hard for a developer to choose which one to use in various cases, but here we will try to provide some general guidelines that should help.

Use classes:

  • When a behavior is not going to be reused at all or in multiple places
  • When you plan to use your Scala code from another language, for example, if you are building a library that could be used in Java

Use traits:

  • When a behavior is going to be reused in multiple unrelated classes.
  • When you want to define interfaces and want to use them outside Scala, for example, Java. The reason is that the traits that do not have any implementations are compiled similar to interfaces.

Summary

In this chapter, we went through traits and mixin compositions in Scala. By now, you should have a good understanding of what these are and what can be achieved by using them. We also went through the examples of the different uses of traits and what to watch out for when using them. We presented the limitations of using traits for multiple inheritance. Traits are an extremely powerful concept, but they have their pitfalls as we saw with multiple inheritance, so you should use them carefully. Linearization was covered in depth and you should be familiar with what to expect when using traits for multiple inheritance and why things work exactly the way they currently do.

Testing is an essential part of every good software project, and we also presented how it should be done for traits. Last, but not least, we prepared a few guidelines that should help developers choose between...

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Key benefits

  • •Unleash the power of Scala and apply it in the real world to build scalable and robust applications.
  • •Learn about using and implementing Creational, Structural, Behavioral, and Functional design patterns in Scala
  • •Learn how to build scalable and extendable applications efficiently

Description

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.

Who is this book for?

If you want to increase your understanding of Scala and apply design patterns to real-life application development, then this book is for you.Prior knowledge of Scala language is assumed/ expected.

What you will learn

  • Immerse yourself in industry-standard design patterns—structural, creational, and behavioral—to create extraordinary applications
  • See the power of traits and their application in Scala
  • Implement abstract and self types and build clean design patterns
  • Build complex entity relationships using structural design patterns
  • Create applications faster by applying functional design patterns
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Length: 396 pages
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Language : English
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Table of Contents

13 Chapters
The Design Patterns Out There and Setting Up Your Environment Chevron down icon Chevron up icon
Traits and Mixin Compositions Chevron down icon Chevron up icon
Unification Chevron down icon Chevron up icon
Abstract and Self Types Chevron down icon Chevron up icon
Aspect-Oriented Programming and Components Chevron down icon Chevron up icon
Creational Design Patterns Chevron down icon Chevron up icon
Structural Design Patterns Chevron down icon Chevron up icon
Behavioral Design Patterns – Part One Chevron down icon Chevron up icon
Behavioral Design Patterns – Part Two Chevron down icon Chevron up icon
Functional Design Patterns – the Deep Theory Chevron down icon Chevron up icon
Applying What We Have Learned Chevron down icon Chevron up icon
Real-Life Applications Chevron down icon Chevron up icon
Other Books You May Enjoy Chevron down icon Chevron up icon

Customer reviews

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Full star icon Full star icon Full star icon Full star icon Half star icon 4.1
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5 star 57.1%
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Akram Ahmad Aug 22, 2018
Full star icon Full star icon Full star icon Full star icon Full star icon 5
- I read the first edition of this book (Scala Design Patterns, by Ivan Nikolov) cover to cover when it first came out, and was blown away by the high quality of the content, impressed by the clear presentation.- Well, in the second edition of the book, all that is still there, plus several improvements. Frankly, this is one of my all-time favorite (technical) books that has gotten even better :)- Scala Design Patterns is simply a pleasure to read, with copious code illustrating the ideas, remarkable both for its quality and quantity, and one in which Ivan has shared some of the highest quality Scala (production level) that I have seen to date. Marvelous coverage of design patterns can be found therein.- I thought I knew a thing or two about Scala, and therefore was pleasantly surprised to learn a bunch of great ideas to elevate my coding skills and design repertoire!
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Amazon Customer Dec 29, 2020
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I am loving it! Very clear code samples and plenty of them. Very easy to read.
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Meysam Nov 16, 2019
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This book puts all your language knowledge into practice. The examples are short and clear. I highly recommend it.
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Larry Nov 19, 2020
Full star icon Full star icon Full star icon Full star icon Full star icon 5
Very good book! Some code was over my head, but that is probably due to my own insufficient knowledge of Scala...I've been studying Scala on and off for a few years as a hobby but never did much of a project yet. After finishing this book, I think I will feel more comfortable with Scala so I can finally do a decent project.I definitely would like to encourage Ivan to write additional Scala books.
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Amazon Customer Aug 21, 2019
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Overall that is good book.
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