Clojure

Clojure

File:Clojure-glyph.svg
Paradigm	functional, multiparadigm
Appeared in	2007
Designed by	Rich Hickey
Stable release	1.1.0 (2009-12-31)
Typing discipline	dynamic, strong
Influenced by	Lisp, ML, Haskell, Erlang
OS	Cross-platform
License	Eclipse Public License
Website	http://clojure.org

Clojure (pronounced like closure) is a modern dialect of the Lisp programming language. It is a general-purpose language supporting interactive development that encourages a functional programming style, and simplifies multithreaded programming. Clojure runs on the Java Virtual Machine and the Common Language Runtime. Clojure honors the code-as-data philosophy and has a sophisticated Lisp macro system.

Philosophy

Rich Hickey developed Clojure because he wanted a modern Lisp for functional programming, symbiotic with the established Java Platform, and designed for concurrency.

Syntax

Like any other Lisp, Clojure's syntax is built on S-expressions that are first parsed into data structures by a reader before being compiled. Clojure's reader supports literal syntax for maps, sets and vectors in addition to lists, and these are given to the compiler as they are. In other words, the Clojure compiler does not compile only list data structures, but supports all of the mentioned types directly. Clojure is a Lisp-1, and is not intended to be code-compatible with other dialects of Lisp.

Macros

Clojure's macro system is very similar to that in Common Lisp with the exception that Clojure's version of the backquote (called "syntax quote") qualifies symbols with their namespace. This helps prevent unintended name capture as binding to namespace-qualified names is forbidden. It is possible to force a capturing macro expansion, but this must be done explicitly. Clojure also disallows rebinding global names in other namespaces that have been imported into the current namespace.

Language features

• Dynamic development with a read-eval-print loop
• Functions as first-class objects with an emphasis on recursion instead of side-effect-based looping
• Lazy sequences
• Provides a rich set of immutable, persistent data structures
• Concurrent programming through software transactional memory, an agent system, and a dynamic var system
• Clojure is a compiled language producing JVM bytecode
• Tight Java integration: By compiling into JVM Byte code Clojure applications can be easily packaged and deployed to JVM's and app servers without additional complexity. The language also provides macros which make it simple to use existing Java API's. Clojure's data structures all implement standard Java Interfaces, making it easy to run code implemented in Clojure from Java.

Examples

Hello world:

(println "Hello, world!")

A thread-safe generator of unique serial numbers:

(let [i (atom 0)]
  (defn generate-unique-id
    "Returns a distinct numeric ID for each call."
    []
    (swap! i inc)))

An anonymous subclass of java.io.Writer that doesn't write to anything, and a macro using that to silence all prints within it:

(def bit-bucket-writer
  (proxy [java.io.Writer] []
    (write [buf] nil)
    (close []    nil)
    (flush []    nil)))

(defmacro noprint
  "Evaluates the given expressions with all printing to *out* silenced."
  [& forms]
  `(binding [*out* bit-bucket-writer]
     ~@forms))

(noprint
 (println "Hello, nobody!"))

10 Threads manipulating one shared data structure, which consists of 100 vectors each one containing 10 (initially sequential) unique numbers. Each thread then repeatedly selects two random positions in two random vectors and swaps them. All changes to the vectors occur in transactions by making use of clojure's software transactional memory system. That's why even after 1000 iterations of each thread no number got lost.

(defn run [nvecs nitems nthreads niters]
  (let [vec-refs (vec (map (comp ref vec)
                           (partition nitems (range (* nvecs nitems)))))
        swap #(let [v1 (rand-int nvecs)
                    v2 (rand-int nvecs)
                    i1 (rand-int nitems)
                    i2 (rand-int nitems)]
                (dosync
                 (let [temp (nth @(vec-refs v1) i1)]
                   (alter (vec-refs v1) assoc i1 (nth @(vec-refs v2) i2))
                   (alter (vec-refs v2) assoc i2 temp))))
        report #(do
                 (prn (map deref vec-refs))
                 (println "Distinct:"
                          (count (distinct (apply concat (map deref vec-refs))))))]
    (report)
    (dorun (apply pcalls (repeat nthreads #(dotimes [_ niters] (swap)))))
    (report)))

(run 100 10 10 100000)

Output of previous example:

([0 1 2 3 4 5 6 7 8 9] [10 11 12 13 14 15 16 17 18 19] ...
 [990 991 992 993 994 995 996 997 998 999])
Distinct: 1000
 
([382 318 466 963 619 22 21 273 45 596] [808 639 804 471 394 904 952 75 289 778] ...
 [484 216 622 139 651 592 379 228 242 355])
Distinct: 1000

Bibliography

• Halloway, Stuart (May 28, 2009), Programming Clojure (1st ed.), Pragmatic Bookshelf, pp. 304, ISBN 1934356336, http://pragprog.com/titles/shcloj/programming-clojure 
• VanderHart, Luke (December 31, 2009), Practical Clojure (1st ed.), Apress, pp. 350, ISBN 1430272317, http://www.apress.com/book/view/9781430272311 

External links

• Clojure home page
• GitHub code repository for Clojure
• A comprehensive overview of Clojure
• Full Disclojure - Screencast