Clojure is a modern Lisp dialect that runs on the JVM. Since its release in 2007, it's become quite popular with elite hackers of a certain persuasion. Michael Fogus and Chris Houser's Joy of Clojure is a thoroughly enjoyable bit of summer reading explaining the philosophy behind the language, showing how Clojure promotes simplicity and flexibility, deals with time and state, and generally brings fun back into programming.
Concurrency is one of the motivating factors behind Clojure. Hickey's approach to sane management of state is anchored on the concept of immutability. The language encourages "immutability by default" with persistent collections - efficient implementations of list, vector, map and set that preserve historical versions of their state. When concurrency becomes necessary, Clojure provides some very modern constructs including STM (software transactional memory), agents similar to the actor model of Erlang, and atomic values.
Clojure's interop with it's host platform gracefully bridges the conceptual gap between Java and Lisp, enabling Clojure to call into Java code making available the wealth of Java libraries. It's equally possible to expose Clojure APIs to Java code. Dynamically generating Java classes and proxies as well as creating and implementing interfaces on the fly leads to the belief in the Clojure community that "Clojure does Java better than Java." In spite of the differences in semantics, Clojure feels like a natural layer on top of Java, raising the levels of abstraction and dynamism while easing many pain-points that every Java programmer stumbles over.
Aside from persistent data structures and Java interop, Clojure comes with a bunch of functional goodness built in. Clojure's multimethods put control of method dispatch into the programmer's hands. Macros let you construct your own flow-of-control structures, as demonstrated by do-until and unless examples in the book. Illustrating how Lisp and it's macros can be used to construct little languages or DSLs, the book implements a mini-SQL interpreter.
One especially nice aspect of the book is the putting it all together sections, which cover examples like: lazy quicksort, A*, and a builder for chess moves. These longer examples are still bite sized, making them more easily digested than the extended case studies found in some books.
The Joy of Clojure is not an introductory book nor is it a language reference. It will appeal to the reader who already has some programming experience. It's a good idea to spend some time with the online tutorials first. Where the book is strongest is answering the why questions, getting you started learning how to think about programming in Clojure and showing you how Clojure changes the way you think.
Companies using Clojure:
- Prismatic: a highly addictive social content recommendation engine who's backend pipeline and APIs are written mostly in Clojure. Prismatic's architecture is a great example of how to use machine learning and social graphs in a real application.
- Climate corporation: data driven weather insurance
- Relevance: a consultancy and home of Clojure/core.
Popular Clojure projects and libraries:
- Leiningen a build tool and dependency management system
- Incanter a Clojure-based, R-like platform for statistical computing and graphics
- Chris Granger's Light Table IDE
- Noir web framework
- Datomic: a distributed rethinking of the database
- core.logic: Prolog-like logic programming for Clojure
Luckily for those wanting to learn more without leaving their hammock, there are lots of videos about Clojure. A lot of clear thinking about software, whether you do it in Clojure or not, can be found in Rich Hickey's talks.
- Are we there yet?
- Hammock driven development
- Simplicity made easy
- Rich Hickey's talks on InfoQ
- Clojure on Blip.TV
- More Clojure Videos