Learning Scala Programming: Object-oriented programming meets functional reactive to create Scalable and Concurrent programs

A paradigm is simply a way of doing something. So a programming paradigm means a way of programming or a certain pattern of writing programs. There are a number of programming paradigms in existence, but four of them have gained popularity:

With its roots in the mathematics discipline, the functional programming paradigm is simple. It works on the theory of functions which produce values that are immutable. Immutable values mean they can't be modified later on directly. In the functional paradigm, all computations are performed by calling self/other functions. Functions are first-class citizens in the functional world. This opens up a new world of possibilities where all computations are driven by a certain need.

The object-oriented planet revolves around encapsulation and abstractions. The logical grouping of components makes maintenance of larger and complex programs easier. Data and models are encapsulated in objects. Information hiding is effective for containing an object's properties. Inheritance hierarchies, the concept of classes, and messaging between objects makes the whole model/pattern of object-oriented programming a partial success.

Scala, being a multi-paradigm language, supports both paradigms. As we're learning Scala, we have the power of both of these paradigms. We can create functions as we need them, and also have objects talking to other objects. We can have class hierarchies and abstractions. With this, dominance over a particular paradigm will not affect another.

Today the need for concurrency, immutability, heterogeneity, reactiveness, and fault tolerant architectures with ever-shrinking development life cycles has drastically increased. In this era, languages such as Scala do more than they need to with their support for functional as well as object-oriented programming.

For a programmer like us, a language is a tool to create something meaningful. We tend to reuse and manipulate other tools as well, in our case let's say other libraries. Now, we would like to work with a language which provides us extensibility and flexibility in terms of its use. Scala does this. This powerful language lets you mix in newly created traits (you may not have heard about this, but you can compare it to Java's interfaces). There are a number of ways we can make our code more meaningful and of course concise. If used smartly, you can create your own custom constructs with native language features. So this language is as exciting as you are!

This is one of the reasons to learn it. There are other reasons behind why we would choose Scala over any other languages, and there's quite a few. Let's take them one by one. But first let's get confused:

Great! Now you know three main characteristics of this language. But it's hard to swallow. It's a functional language, and every function is an object. Really?

The following is an example of a trait defined in Scala, called Function1:

package scala
trait Function1[A, B] {
        def apply(x: A) : B
}

There are more of these, from Function0 to Function22. There's a certain way of using these. We'll be using them many times in this book. We also refer to these as A => B (we call it, A to B). It means this function takes a parameter of type A, does some operation as defined, and returns a value of type B:

val answer = new Functiona1[Int, Int] {
        def apply(x: Int): Int = x * 2
}

This feels a bit too much to start with but getting familiar with these constructs is a good idea. val is a keyword used to declare a value type. It means, once declared and instantiated, you can't change it further. This answer = (x: Int) => x * 2 becomes a function literal that can be passed to another function. We get to this point because we were able to instantiate an object of our Function1 trait (we'll see how this works in Chapter 7, Next Steps in Object-Oriented Scala).

Think of any two lucky numbers, now represent how you can add them. Suppose your numbers were 42 + 61. Here, your numbers 42 and 61 are objects of type Int and + is a method on type Int. This is the way you and Scala are going to treat entities. We'll treat entities as objects and operations performed on them as methods. And this is what makes this language scalable.

We can perform functional operations where inputs are transformed to outputs rather than changing data/state of them. With this in mind, most of our operations (almost all) will not depend on state change; means functions are not going to have side effects. One example could be a function which takes your date of birth and returns your age in terms of the number of years and months:

class YearsAndMonths(years: Int, months: Int)
def age(birthdate: Date): YearsAndMonths = //Some Logic

This is a pure function because it does not manipulate the input. It takes input, transforms, and gives output. Case class is just to help us here define the age in a certain manner. With this, we can introduce more terminology called referentially transparent methods. Our age method can be called referentially transparent. These method calls can be replaced by the result without changing any meaning/semantics of your program.

Pure functions, the concept of immutability, and referential transparency are here only to make this language more powerful. There are more reasons to choose this language as a tool for your next application.

Consider efficiency and optimization as factors for a language to be well performant. Scala utilizes JVM for this. JVM uses Just in Time (JIT) compilation, adaptive optimization techniques for improved performance. Running on JVM makes Scala interoperable with Java. You've multitudinous libraries available as tools for reuse.

If anywhere in your mind you're comparing Java and Scala's performance, let's get it clear. Both Java and Scala programs are compiled into bytecode. JVM understands bytecode and runs it for you. So it mostly depends on the way you write a program. Scala blends in some syntax sugar, compiler logic that can cause your program to be more/less performant than Java. Mix-ins using traits can be an asset to your program architecture but may affect your program's performance. But alternatives in Java may cost the same or more. So it is more about your core understanding of constructs and how your code is going to compile and perform. It takes some time and effort to understand so the choice is yours; as a smart programmer, you may go for a syntactically powerful language.

You are going to write succinct code with Scala. There are a lot of examples we can look at to see Scala's syntax conciseness. Let's take an example from Scala's rich collections and create a Map:

val words = Map ("Wisdom" -> "state of being wise")
println(words("Wisdom"))

> state of being wise

The preceding code is creating a map of words and their meaning. Only Map ("Wisdom" -> "state of being wise") is the amount of code we have to write to make it possible. No need to add semicolons. We did not even mention the type of our value and the Scala compiler was able to infer it. Type inference is a characteristic of this language. Because of Type inference, a lot of times we omit type declaration and use a value directly. This way, using only a minimal set of words/tokens you can express the logic to implement them. Constructs like case classes and pattern matching take away the extra effort one might have to make and makes writing code joyful. It also helps you reduce written code by a good margin.

Scala is a mixture of functional and object-oriented worlds. It gives two benefits. First, you can leverage the power of functional constructs: higher-order functions, nested functions, pure functions, and closures. You get to work with more available (and recommended) immutable data structures. Working with immutable code helps in eliminating code that can introduce side effects or state change. This also makes this language suitable for concurrent programming. This is just another advantage Scala provides. Second, you've all the object-oriented goodies available.

You can define traits, mix them in with classes or objects, and achieve inheritance. The creation of objects, defining abstracts, and sub-classing is also possible in Scala.

In the early days (great, if even in the present) you may have come across this:

f : R -> N

This is the mathematical representation of a function. This is how we denote any function f's domain and co-domains. In this case a function, f maps values from a set of real numbers to a set of natural numbers. With this deep abstraction level, you can think of Scala's rich type system. Some of the numerous types available are parameterized, structural, compound, existential, path-dependent, higher-kinded, and yes, we are discussing abstract types. An explanation of all these is beyond the scope of this book. But if you're curious, you may refer to Scala documentation at https://www.scala-lang.org/documentation/. Knowledge of these helps a lot when designing frameworks or libraries.

Scala recommends the use of immutable data structures, immutable collections, use of value types, functional compositions, and transformations. Along with these, the use of actors and other concurrent constructs have made it so easy to write concurrent programs. Mostly, programmers do not have to deal with the complication of thread life cycle management, because of modern constructs such as actors and reactors available in the form of native support and through libraries. Akka is one of these toolkits available, written in Scala. Also, the use of futures and promises enables writing asynchronous code.

Simply defined, asynchronous code is where your program control returns immediately after calling a block of instruction (that is a function), having started some parallel/ background effort to complete your request. This means your program flow will not stop because of a certain function taking time to complete.

Scala.js is specifically designed for the frontend and helps you avoid type-based mistakes as Scala.js is able to infer to types. You can leverage performance optimization and interoperability with some already available JavaScript frameworks such as Angular and React. Then added to that, you have macros available that help you extend the language.

There are many options available to make your programming journey easier. Scala IDE provides numerous editing and debugging options for development of Scala-based applications. The Scala IDE is built on top of a known Eclipse IDE. There are also plugins available to write Scala applications. We'll take a look at how to install and use IDE for Scala development in the coming sections.

Scala is very deep. Rich type abstractions, reflection, and macros all help you build some really powerful libraries and frameworks. Scala documentation explains everything to you: from parameterized types to reflection components. Understanding compile-time reflection (macros) and runtime reflection are essential for writing frameworks using Scala. And it's fun.

One of the biggest reasons for the growth of Scala as a programming language and its success is the vast online support available. The Scala team has put in a good amount of work and have come up with rich documentation. You can find documentation at http://docs.scala-lang.org

Learning Scala is challenging but fun. It brings out the best in you as a programmer. Isn't it fun to think and write shorter and smarter syntax with almost the same performance capabilities?

Do the needful. If Java is not already installed on your machine, you may refer to Oracle's website at https://docs.oracle.com/javase/8/docs/technotes/guides/install/install_overview.html for instructions on how to install Java for your operating system.

SBT, as the name suggests, is a Simple Build Tool. From managing all source files to their compiled target versions to downloading all dependencies, SBT helps you create a Scala application with ease. You can configure how your test cases run. SBT comes with various commands for such tasks.

To install SBT on your machine, perform the following:

After installation, you may check the version, so open a command prompt/terminal and type this:

sbt sbt-version
[info] 0.13.11

You should get the corresponding version number.

There is more than one way of interacting with Scala. One of them is using Scala Interpreter (REPL). To run Scala REPL using SBT, just give the following command in the command prompt/terminal:

sbt console

This command will run Scala REPL.

To run Scala REPL using Scala binary, perform the following:

If so, congrats. You've done it. Now it's asking you to type any expression. You may try typing any expression. Try anything, like 1 + 2 or 1 + "2". REPL is your playground to learn Scala.

After getting familiar with Scala REPL, now is the time to install IDE (Integrated Development Environment). There are options available to work with Scala in IDE. Choose what fits the best for you. Eclipse lovers can go for Scala IDE. To download:

If you're accustomed to IntelliJ IDE, you may go for the plugin download for SBT. This will enable you to create Scala applications. To get started with Scala development on IntelliJ IDE:

With this, you're ready to work with Scala on IntelliJ IDE. If you're IDE neutral, you may choose whichever suits the best. We'll use IntelliJ IDE (Community Edition) version 2017.1 with SBT version 0.13.15 and Scala 2.12.2 version.