In the last few years, we have seen a widespread tendency to create new languages for JVM. There are all sorts of new languages with different paradigms and different ways of working.
Clojure is one of those languages, one that we believe is worth learning.
Over the course of this book, you will learn about Clojure and how opinionated it is. You will learn why immutable objects are not only possible, but it is a good idea to use them.
You will learn about functional programming and see how it fits the concept of immutable programs.
You will understand the very powerful idea of representing your code as a data structure of the same language.
It is important to note that we will build all this knowledge on top of what you already know; this book assumes that you understand the Java language and a bit of how it works. It assumes that you understand how to create classes and objects, how to call methods, and also a bit about the JVM. We will find similarities and differences from the Java language that you already know, so you can understand how the Clojure world works.
It is said that a great programmer is not the one who knows a lot of different programming languages, it is someone who knows different programming paradigms.
Clojure brings ideas from Lisp and functional programming, which are different to what you are used to. This book will help you understand the power behind these ideas and why a language so old still exists and has a place.
Learning Lisp is one of the great pleasures of life, I hope you enjoy it!
Chapter 1, Getting Started with Clojure, is your first step with Clojure, from how to install an interpreter, how to use the IntelliJ plugin Cursive Clojure, and how to start writing your first lines of code in Clojure.
Chapter 2, Namespaces, Packages, and Tests, deals with how every other language needs a way to organize our code. We do it with namespaces, and we start learning by doing little experiments and by comparing with the Java packages we already know.
Chapter 3, Interacting with Java, discusses the fact that one of the most useful features of Clojure is that it can be hosted on top of other platforms. Java is probably one of the most common platforms, and here we learn how to interact with Java code and libraries and how to expose our Clojure code to Java.
Chapter 4, Collections and Functional Programming, tells us that functional programming and immutable data structures (or collections) are fundamental to programming in the Clojure language; here we understand how we can write meaningful programs using immutable data structures and how functional programming is ideal for that.
Chapter 5, Multimethods and Protocols, introduces new tools and ideas of Clojure that help you write much more flexible and simple codes. Destructuring allows you to gain instant access to the data you need from a data structure. Multimethods and protocols are similar to Java's polymorphism but give you a whole new level of flexibility that you could only dream about.
Chapter 6, Concurrency, tells us about how in the modern world concurrency plays an extremely important part. This chapter also tells us about Clojure's native primitives for concurrency and again you will learn why immutable data structures are a great ally when writing concurrent programs.
Chapter 7, Macros in Clojure, deals with the fact that these are not the Excel macros you are used to, they are a way in which you can modify your program's source code at compile time. This is one of Lisp's big ideas and one that gives you immense flexibility. Here we understand a little of how to use this feature.
You need the Java 8 SDK.
You should be able to run samples on any OS; our samples should be easier to follow in environments where there is a shell available. (We focus mainly on Mac OS X.)
This book is for developers who are familiar with Java and the JVM.
Ideally, you know how the classloader works, how to generate JAR files and consume them; you are familiar with Java's most common libraries and classes.
If you are not familiar with this, you will be able to follow this book but you won't get the most from the comparisons and samples.
In this book, you will find a number of text styles that distinguish between different kinds of information. Here are some examples of these styles and an explanation of their meaning.
Code words in text, database table names, folder names, filenames, file extensions, pathnames, dummy URLs, user input, and Twitter handles are shown as follows: "We are using :require to include functions from the clojure.test and the ns-playground.core packages."
A block of code is set as follows:
curl -O https://raw.githubusercontent.com/technomancy/leiningen/stable/bin/lein # The next step just set up the lein script in your path, you can do it any way you wish mv lein ~/bin echo "export PATH=$PATH:~/bin/">> ~/.bashrc source ~/.bashrc # Everything should be running now, let's test it lein help
When we wish to draw your attention to a particular part of a code block, the relevant lines or items are set in bold:
lein new app getting-started
cd getting-started
lein run
# Hello, world!Any command-line input or output is written as follows:
lein uberjar
New terms and important words are shown in bold. Words that you see on the screen, for example, in menus or dialog boxes, appear in the text like this: "After that you can run any tests, just open your test file and go to Tools | Run Tests in the current NS in REPL."
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Errata
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