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Scala Design Patterns
Scala Design Patterns

Scala Design Patterns: Write efficient, clean, and reusable code with Scala

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Scala Design Patterns

Chapter 2. 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 the key factors that enable to correctly and efficiently write code. Even though those concepts are not considered as "official" design patterns, they could still be used to write good software. Even 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 the developer with a possibility to share already implemented functionality...

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.

Tip

Trait parameters

The Scala programming language is quite dynamic and it has evolved quickly. One of the directions that will be investigated for the 2.12 version of the language are trait parameters. More information can be found at http://www.scala-lang.org/news/roadmap-next/.

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

Traits as interfaces

Traits can be viewed as interfaces in other languages, for example, Java. They, however, allow the developers to implement some or all of their methods. Whenever there is some code in a trait, the trait is called a mixin. Let's have a look at the following example:

trait Alarm { 
  def trigger(): String 
}

Here Alarm is an interface. Its only method...

Mixin compositions

Scala allows developers to extend many traits in a single class. This adds the possibility of achieving multiple inheritance and save a lot of efforts in code writing, which has to be performed in languages where extending many classes is not allowed. In this sub topic, 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 last 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 it can be seen from the preceding code, we can add multiple traits to a class. We've used object in the example just because of the main method. This would be similar to creating a class with no constructor parameters.

Tip

How to mix traits in?

Mixing traits into a class...

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:

The diamond problem

Here, both B and C extend A and then D extends B and C. Some ambiguities might arise from this. Let's say that there was a method that was originally defined in A, but both B and C override it. What would happen if D calls this method? Which one will it exactly call?

All the preceding questions make things ambiguous and this could lead to mistakes...

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.

Rules of inheritance hierarchies

Before looking into linearization rules, we need to be clear on some inheritance rules in Scala:

  • In Java, even if a class does not explicitly extend...

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. We will be looking into more complex and meaningful tests in the following chapters of this book.

Using a class...

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.

Tip

Trait parameters

The Scala programming language is quite dynamic and it has evolved quickly. One of the directions that will be investigated for the 2.12 version of the language are trait parameters. More information can be found at http://www.scala-lang.org/news/roadmap-next/.

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

Traits as interfaces

Traits can be viewed as interfaces in other languages, for example, Java. They, however, allow the developers to implement some or all of their methods. Whenever there is some code in a trait, the trait is called a mixin. Let's have a look at the following example:

trait Alarm { 
  def trigger(): String 
}

Here Alarm is an interface. Its only method, trigger...

Mixin compositions


Scala allows developers to extend many traits in a single class. This adds the possibility of achieving multiple inheritance and save a lot of efforts in code writing, which has to be performed in languages where extending many classes is not allowed. In this sub topic, 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 last 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 it can be seen from the preceding code, we can add multiple traits to a class. We've used object in the example just because of the main method. This would be similar to creating a class with no constructor parameters.

Tip

How to mix traits in?

Mixing traits into a class is done with...

Left arrow icon Right arrow icon

Key benefits

  • • Unleash the power of Scala and apply it in the real world.
  • • Increase your efficiency by leveraging the power of Creational, Structural, Behavioural, and Functional design patterns.
  • • Build object oriented and functional applications quickly and effectively.

Description

Scala has become increasingly popular in many different IT sectors. The language is exceptionally feature-rich which helps developers write less code and get faster results. Design patterns make developer’s 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 will learn about the various features of Scala and 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 features of Scala while using practical real-world examples. We will also cover the popular "Gang of Four" design patterns and show you how to incorporate functional patterns effectively. By the end of this book, you will 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 it to real-life application development, then this book is for you. We’ve also designed the book to be used as a quick reference guide while creating applications. Previous Scala programming knowledge is expected.

What you will learn

  • • Immerse yourself in industry-standard design patterns—structural, creational, and behavioral—to create extraordinary applications.
  • • Feel 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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Table of Contents

13 Chapters
1. The Design Patterns Out There and Setting Up Your Environment Chevron down icon Chevron up icon
2. Traits and Mixin Compositions Chevron down icon Chevron up icon
3. Unification Chevron down icon Chevron up icon
4. Abstract and Self Types Chevron down icon Chevron up icon
5. Aspect-Oriented Programming and Components Chevron down icon Chevron up icon
6. Creational Design Patterns Chevron down icon Chevron up icon
7. Structural Design Patterns Chevron down icon Chevron up icon
8. Behavioral Design Patterns – Part 1 Chevron down icon Chevron up icon
9. Behavioral Design Patterns – Part 2 Chevron down icon Chevron up icon
10. Functional Design Patterns – The Deep Theory Chevron down icon Chevron up icon
11. Functional Design Patterns – Applying What We Learned Chevron down icon Chevron up icon
12. Real-Life Applications Chevron down icon Chevron up icon
Index Chevron down icon Chevron up icon

Customer reviews

Rating distribution
Full star icon Full star icon Full star icon Full star icon Half star icon 4.2
(5 Ratings)
5 star 60%
4 star 20%
3 star 0%
2 star 20%
1 star 0%
Tomer Ben David Apr 01, 2017
Full star icon Full star icon Full star icon Full star icon Full star icon 5
OMG forget about all the other scala books read this one! What a lovely surprise! Enjoying reading this books, it's explaining things so concise and well. At first I thought this was only about design patterns but then I found it to be a great book about scala. I find his explanations so well- "What is important about monoids is that they give us the possibility to work with many different types of values in a common way"- "monads are structures that represent computations as sequence of steps"- "functors are things that can allow us to lift a function of type A => B to a function of the type F[A] => F[B]"- "the cake design pattern is the scala way to implement dependency injection"- "stackable traits is the scala way to implement the decorator design pattern"- "type class design pattern allows us to write generic code by defining a behaviour that must be supported by all members of a specific type class"and it goes on and on! just perils of clear explanations! what a lovely book.
Amazon Verified review Amazon
Abhishek Srivastava Jun 09, 2016
Full star icon Full star icon Full star icon Full star icon Full star icon 5
I loved this book. I loved the initial part of the book which has a great coverage of traits. The book then moves to the GOF patterns (which were kind of known to me) but I loved the coverage of cake pattern and Scalaz.I highly recommend this book.
Amazon Verified review Amazon
Akram Ahmad Jan 16, 2017
Full star icon Full star icon Full star icon Full star icon Full star icon 5
Wow, I must confess that I'm thoroughly impressed by the uniformness of excellence of fine book offers in its helpful coverage of diverse Scala topics. This is an excellent second or third book for somebody has spent time getting comfortable with learning and using Scala. As the author correctly notes, "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". And he does a really good job of covering both features (OO and FP). Plus, having meaningful examples makes learning and understanding a more pleasant experience. To that end, the author has provided plenty of examples that you can map to real problems that you would potentially be solving. All in all, "Scala Design Patterns" will be a superb addition to your programming library!
Amazon Verified review Amazon
Paolo R. Jul 21, 2016
Full star icon Full star icon Full star icon Full star icon Empty star icon 4
I found the book a useful collection of best practises and techniques you normally gain over time and after months of study on various resources.Here you have all in one place, ready to consult if you're a novice or an experienced dev.Definitely recommended if you are an experienced Java dev and you're looking for how to do things in the right way, without wasting days in researching!
Amazon Verified review Amazon
Amazon Customer Jun 16, 2017
Full star icon Full star icon Empty star icon Empty star icon Empty star icon 2
I put 2 stars because the Kindle version is impossible to read. There is no format at all.
Amazon Verified review Amazon
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