Published in June 2010, Practical Clojure by Luke VanderHart and Stuart Sierra is the latest Clojure book to hit stores. Despite the Clojure 1.0 jar shown at the beginning of the book, this title tries to cover the current version of the language, including references to concepts that will be introduced by the upcoming 1.2 version.
The target audience of this book is programmers who are absolutely new to Clojure. It didn't strike me as being particularly aimed at developers who are coming from the Java camp, or the Lisp camp; in this regard, the book is rather "background agnostic", even though Lisp programmers will feel much more at home than Java programmers will, due to the nature of the language itself.
The authors of the book are clearly well versed in this new language (Sierra is part of Clojure/core, the equivalent of the A-Team in Clojureland) and their confidence with the concepts presented is demonstrated throughout the book. Their explanations tend to be clear and to the point. Longer discussions are occasionally included when required to introduce concepts that are novel to most programmers, like the Software Transactional Memory (STM), refs, atoms and agents.
The book starts out by presenting a short but well-argued case for why Clojure is a worthwhile language, and then focuses almost exclusively on the core of the language. I'm afraid they do so to the detriment of the ecosystem surrounding Clojure. The authors don't talk about how to install Clojure, recommend editors and IDEs (albeit a few are casually mentioned), or how to use build tools like Ant, Maven or Leiningen.
clojure.contrib, a fundamental extension library, is barely mentioned and there is no coverage of other important libraries or emerging frameworks. For instance, perhaps expectedly, Compojure (a web framework) and Incanter (a statistical and graphical environment) are only mentioned as examples of DSLs, however examples of their usage are not provided. (I believe the authors mistakenly refer to Compojure as Enclojure, which is a different project).
Despite the narrow focus, Practical Clojure doesn't shy away from complex subjects and manages to include a chapter on Java interoperability, parallel programming, metaprogramming, and performance considerations. It does so briefly however, favoring a cursory presentation of the fundamental concepts rather than in-depth coverage, which would provide the reader with the degree of confidence required to approach real world problems.
The core language is covered in a manner that acts as both a tutorial and a reference. Major concepts, data structures, and common functions are presented to the reader with an endless supply of tiny examples. It's easy to fly through them, but typing along in the REPL will be a far more valuable exercise for readers who hope to retain the information presented.
This leads us to another shortcoming of this book, which is the lack of more structured and complex examples. When I define their examples as "tiny", with very few exceptions, I really mean it. For the first few chapters of the book the examples don't get much larger than calculating the square root of a number through Newton's method or adding contacts to an address book. Most of the other examples do a good job of illustrating the point they are trying to make with one, two, or just a handful of lines of code.
This is an actual sample of the kind of examples you'll find throughout the book to illustrate many core API functions:
user=> (reduce + [1 2 3 4 5])
Note that this approach is didactically valid, because it isolates the function to show exactly how it works. After dozens of these functions though, you may expect larger examples to show how to integrate the use of some of these functions and data structures you've learned about. Such examples are seldom included. Furthermore, the book lacks any exercise for the reader. Foundational books that fail to offer many articulated examples and that lack exercises, tend to make it hard for the reader to retain the information and get some hands-on practice.
I have lots of respect for short books that get to the point and avoid wasting the reader's time. C Programming Language (2nd Edition) (aka K&R) is notoriously acclaimed thanks to its clear and concise nature. However, Clojure is not C, and I feel that the 198 pages fall a little short when it comes to introducing this wonderful language to new readers. There is more to Clojure than simply surveying the language itself, even though I suspect that certain readers may appreciate this extremely narrow focus.
Overall the book is well-edited, despite the presence of minor issues. Aside from a few typos (e.g., "becauseall" on page 79), readers may find the formatting to be slightly inconsistent at times. For example in chapter 5 when presenting sequences, after the map function has been introduced, the font for the subsequent functions is substantially decreased for no apparent reason. Readers may be misled into thinking that the functions presented afterward are somehow different from the previous ones, when in fact they're all defined in clojure.core. In Listing 6-3, at page 103, the authors present their first "complex" example (the address book) and they do so by using, among others, doseq. This macro was not introduced before that page nor is it really explained within the example.
From a physical standpoint, this book is a rather thin and wide paperback. A small font, coupled with small margins and a wide layout, imply that the readability of the book suffers a little. The paper itself is off-white, fairly thick and slightly textured, not as pleasant to the touch as other books by Apress or most other technical publishers, even though I recognize that this is a matter of taste (some people may actually love it because of these characteristics).
With two introductory Clojure books on the market, drawing comparisons is unavoidable. Stuart Halloway's Programming Clojure (Pragmatic Programmers) is a slightly older book (published in May 2009), which grants Practical Clojure a distinct advantage. This is not to say that Programming Clojure is obsolete, on the contrary it's still a valid choice, but it doesn't illustrate some of the new features that are available today. For example, in chapter 13 Practical Clojure introduces protocols and datatypes that will be available in Clojure 1.2 for the first time. Given that Halloway's book was published more than a year ago, there was no possible way he could have included such powerful abstractions at the time.
Despite being older and less methodical than Practical Clojure, Programming Clojure tends to offer more complex examples. In the introduction of Programming Clojure you'll see examples which Practical Clojure fails to include until much later in the book. Practical Clojure, the subject of this review, may leave you wanting for more practical examples of how all the language features fit together. Whereas Programming Clojure may leave you longing for more consistent explanations of how each part of the language works on its own.
Practical Clojure and Programming Clojure are competitors in the marketplace, but it wouldn't be a bad idea to get a hold of both, because they complement each other quite well, in my opinion. Having to pick just one, I would probably recommend Practical Clojure, given its more consistent and up to date presentation. The sizzle offered by Programming Clojure, can be found to a much greater degree in upcoming and less introductory books, such as The Joy of Clojure: Thinking the Clojure Way. In this sense, reading Practical Clojure first followed by The Joy of Clojure, would be a solid learning path (Clojure in Action is another worthy addition, but it doesn't replace The Joy of Clojure, which is a real gem).
In conclusion, Practical Clojure is not the Clojure equivalent of the highly praised Practical Common Lisp, from the same publisher. Reading it cover to cover and typing all the snippets included within, will not give you enough knowledge to start writing complex, idiomatic Clojure programs out of the gate.
However, if you are learning Clojure today, I do recommend this book. It's a clear, well thought-out, concise introduction to the language that will give you a solid foundation as you go on to learn more about Clojure and Lisp in general.