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Clojure

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Clojure
File:Clojure-glyph.svg
Paradigmfunctional, multi-paradigm
Designed byRich Hickey
First appeared2007
Stable release
1.2.0 / August 19, 2010; 14 years ago (2010-08-19)
Typing disciplinedynamic, strong
PlatformJVM, CLR
LicenseEclipse Public License
Websiteclojure.org
Influenced by
Common Lisp, Scheme, Prolog, ML, Haskell, Erlang[1]

Clojure (pronounced "closure"[2]) is a recent 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. Like Lisps, Clojure treats code as data and has a sophisticated 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.[3][4]

Clojure's approach to concurrency is characterized by the concept of identities,[5] which represent a series of immutable states over time. Since states are immutable values, any number of workers can operate on them in parallel, and concurrency becomes a question of managing changes from one state to another. For this purpose, Clojure provides several mutable reference types, each having well-defined semantics for the transition between states.

Syntax

Like most other Lisps, 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

Examples

Hello world: 

(println "Hello, world!")

GUI Hello World: 

(javax.swing.JOptionPane/showMessageDialog nil "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 100 000 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

References

  1. ^ Rich Hickey (30 June 2009). "Books that influenced Clojure". Retrieved 11 September 2009.
  2. ^ http://groups.google.com/group/clojure/msg/766b75baa7987850?
  3. ^ "Rationale". Rich Hickey. clojure.org. Retrieved 2008-10-17.
  4. ^ http://channel9.msdn.com/shows/Going+Deep/Expert-to-Expert-Rich-Hickey-and-Brian-Beckman-Inside-Clojure/
  5. ^ "On State and Identity". Rich Hickey. clojure.org. Retrieved 2010-03-01.

Further reading

Wikibooks has a book on the topic of: Clojure Programming
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