Why Clojure?

There are a number of reasons to learn Clojure. The discipline of staying immutable by default, the simplicity of functions as first-class entities, the practicality of the JVM, and… well, I could go on and on, but this book’s goal isn’t to sell you on Clojure. There are already several fantastic books out there for that. My first Clojure book was Programming Clojure [Hal09], and my most recent intro-to-Clojure recommendation has been Clojure Programming [ECG12] (trust me, these are actually two different books, despite their naming similarities). The Joy of Clojure [FH11] is excellent as well, but it can be a bit advanced for Clojure newcomers. This book probably falls into the same category: you should already know a bit of Clojure in order to get the most out of this book.

So in the interest of focus and brevity, I’m going to be assuming throughout this book that you already know some Clojure. That said, if you haven’t written a single line of Clojure before, never fear! You should set this book aside briefly and go through the Clojure Koans,^[1] and I’ll meet you back here in a couple of hours. We’ll be tackling some advanced topics in this book, so if you haven’t worked through the koans, a book, or a Clojure project, you may need to spend some extra time with some of the chapters in this book to get more Clojure under your belt.

You’ll also want to make sure you have Leiningen^[2] installed and know how to do things like launch a REPL (Read-Eval-Print Loop, lein repl from a command line), read the in-REPL docstrings, and peek here and there at the source code.

From this point forward, you should know your way around a Clojure REPL. I’ll do my best to keep the progression steady through the tutorial part of the book, with each step building upon the one before. But my realistic, software-estimating side says I won’t think of everything, and here and there I’ll get too excited and skip a step. I hope that when that happens, you’ll reach for your nearest REPL, recover quickly, and forgive my exuberance.