Functional Programming in Scala

Paul Chiusano , Runar Bjarnason

• Publisher: Manning
• Year: 2014
• ISBN:
  9781638353959
• Topic: Informatica
• Subtopic: Linguaggi di programmazione

Summary Functional Programming in Scala is a serious tutorial for programmers looking to learn FP and apply it to the everyday business of coding. The book guides readers from basic techniques to advanced topics in a logical, concise, and clear progression. In it, you'll find concrete examples and exercises that open up the world of functional programming.Purchase of the print book includes a free eBook in PDF, Kindle, and ePub formats from Manning Publications. About the Technology Functional programming (FP) is a style of software development emphasizing functions that don't depend on program state. Functional code is easier to test and reuse, simpler to parallelize, and less prone to bugs than other code. Scala is an emerging JVM language that offers strong support for FP. Its familiar syntax and transparent interoperability with Java make Scala a great place to start learning FP. About the Book Functional Programming in Scala is a serious tutorial for programmers looking to learn FP and apply it to their everyday work. The book guides readers from basic techniques to advanced topics in a logical, concise, and clear progression. In it, you'll find concrete examples and exercises that open up the world of functional programming.This book assumes no prior experience with functional programming. Some prior exposure to Scala or Java is helpful. What's Inside

• Functional programming concepts
• The whys and hows of FP
• How to write multicore programs
• Exercises and checks for understanding

About the Authors Paul Chiusano and Rúnar Bjarnason are recognized experts in functional programming with Scala and are core contributors to the Scalaz library. Table of Contents
PART 1 INTRODUCTION TO FUNCTIONAL PROGRAMMING

• What is functional programming?
• Getting started with functional programming in Scala
• Functional data structures
• Handling errors without exceptions
• Strictness and laziness
• Purely functional state
• PART 2 FUNCTIONAL DESIGN AND COMBINATOR LIBRARIES
• Purely functional parallelism
• Property-based testing
• Parser combinators
• PART 3 COMMON STRUCTURES IN FUNCTIONAL DESIGN
• Monoids
• Monads
• Applicative and traversable functors
• PART 4 EFFECTS AND I/O
• External effects and I/O
• Local effects and mutable state
• Stream processing and incremental I/O

Part 1. Introduction to functional programming

We begin this book with a radical premise—that we will restrict ourselves to constructing programs using only pure functions with no side effects such as reading from files or mutating memory. This idea, of functional programming, leads to a very different way of writing programs than you may be used to. We therefore start from the very beginning, relearning how to write the simplest of programs in a functional way.

In the first chapter, we’ll explain exactly what functional programming means and give you some idea of its benefits. The rest of the chapters in part 1 introduce the basic techniques for functional programming in Scala. Chapter 2 introduces Scala the language and covers fundamentals like how to write loops functionally and manipulate functions as ordinary values. Chapter 3 deals with in-memory data structures that may change over time. Chapter 4 talks about handling errors in pure functions, and chapter 5 introduces the notion of non-strictness, which can be used to improve the efficiency and modularity of functional code. Finally, chapter 6 introduces modeling stateful programs using pure functions.

The intent of this first part of the book is to get you thinking about programs purely in terms of functions from inputs to outputs, and to teach you the techniques you’ll need in part 2, when we start writing some practical code.

Chapter 1. What is functional programming?

Functional programming (FP) is based on a simple premise with far-reaching implications: we construct our programs using only pure functions—in other words, functions that have no side effects. What are side effects? A function has a side effect if it does something other than simply return a result, for example:

• Modifying a variable
• Modifying a data structure in place
• Setting a field on an object
• Throwing an exception or halting with an error
• Printing to the console or reading user input
• Reading from or writing to a file
• Drawing on the screen

We’ll provide a more precise definition of side effects later in this chapter, but consider what programming would be like without the ability to do these things, or with significant restrictions on when and how these actions can occur. It may be difficult to imagine. How is it even possible to write useful programs at all? If we can’t reassign variables, how do we write simple programs like loops? What about working with data that changes, or handling errors without throwing exceptions? How can we write programs that must perform I/O, like drawing to the screen or reading from a file?

The answer is that functional programming is a restriction on how we write programs, but not on what programs we can express. Over the course of this book, we’ll learn how to express all of our programs without side effects, and that includes programs that perform I/O, handle errors, and modify data. We’ll learn how following the discipline of FP is tremendously beneficial because of the increase in modularity that we gain from programming with pure functions. Because of their modularity, pure functions are easier to test, reuse, parallelize, generalize, and reason about. Furthermore, pure functions are much less prone to bugs.

In this chapter, we’ll look at a simple program with side effects and demonstrate some of the benefits of FP by removing these side effects. We’ll also discuss the benefits of FP more generally and define two important concepts—referential transparency and the substitution model.

1.1. The benefits of FP: a simple example

Let’s look at an example that demonstrates some of the benefits of programming with pure functions. The point here is just to illustrate some basic ideas that we’ll return to throughout this book. This will also be your first exposure to Scala’s syntax. We’ll talk through Scala’s syntax much more in the next chapter, so don’t worry too much about following every detail. As long as you have a basic idea of what the code is doing, that’s what’s important.

1.1.1. A program with side effects

Suppose we’re implementing a program to handle purchases at a coffee shop. We’ll begin with a Scala program that uses side effects in its implementation (also called an impure program).

Listing 1.1. A Scala program with side effects

The line cc.charge(cup.price) is an example of a side effect. Charging a credit card involves some interaction with the outside world—suppose it requires contacting the credit card company via some web service, authorizing the transaction, charging the card, and (if successful) persisting some record of the transaction for later reference. But our function merely returns a Coffee and these other actions are happening on the side, hence the term “side effect.” (Again, we’ll define side effects more formally later in this chapter.)

As a result of this side effect, the code is difficult to test. We don’t want our tests to actually contact the credit card company and charge the card! This lack of testability is suggesting a design change: arguably, CreditCard shouldn’t have any knowledge baked into it about how to contact the credit card company to actually execute a charge, nor should it have knowledge of how to persist a record of this charg...