The Clojure Programming Language

The Clojure Programming Language

Clojure is a dynamic, general-purpose programming language, combining the approachability and interactive development of a scripting language with an efficient and robust infrastructure for multithreaded programming. Clojure is a compiled language, yet remains completely dynamic – every feature supported by Clojure is supported at runtime. Clojure provides easy access to the Java frameworks, with optional type hints and type inference, to ensure that calls to Java can avoid reflection.

Clojure is a dialect of Lisp, and shares with Lisp the code-as-data philosophy and a powerful macro system. Clojure is predominantly a functional programming language, and features a rich set of immutable, persistent data structures. When mutable state is needed, Clojure offers a software transactional memory system and reactive Agent system that ensure clean, correct, multithreaded designs.

History & Development Process:

Rich Hickey is the creator of the Clojure language. Before Clojure, he developed dotLisp, a similar project based on the .NET Framework, and three earlier attempts to provide interoperability between Lisp and Java: a Java foreign language interface for Common Lisp (jfli), A Foreign Object Interface for Lisp (FOIL), and a Lisp-friendly interface to Java Servlets (Lisplets).

Hickey spent about 2½ years working on Clojure before releasing it publicly, much of that time working exclusively on Clojure with no outside funding. At the end of this time, Hickey sent an email announcing the language to some friends in the Common Lisp community.

The development process is community-driven and is managed at the Clojure Community website. The website contains planning documents and an issue tracker where bugs may be filed. General development discussion occurs at the Clojure Dev Google Group.While anyone can submit bug reports and ideas, to contribute patches one must sign the Clojure Contributor agreement, JIRA tickets are processed by a team of screeners and finally Rich Hickey approves the changes

 Features:

Clojure has a set of useful features that together form a simple, coherent, and powerful tool.

 Dynamic Development

Clojure is a dynamic environment you can interact with. Almost all of the language constructs are reified, and thus can be examined and changed. You can grow your program, with data loaded, adding features, fixing bugs, testing, in an unbroken stream.

Functional Programming

Clojure provides the tools to avoid mutable state, provides functions as first-class objects, and emphasizes recursive iteration instead of side-effect based looping. Clojure is impure, yet stands behind the philosophy that programs that are more functional are more robust.

LISP

Clojure is a member of the Lisp family of languages. Many of the features of Lisp have made it into other languages, but Lisp's approach to code-as-data and its macro system still set it apart. Additionally, Clojure’s maps, sets, and vectors are as first class in Clojure as lists are in Lisp.

Runtime Polymorphism

Systems that utilize runtime polymorphism are easier to change and extend. Clojure offers simple, powerful and flexible mechanisms for runtime polymorphism. Clojure’s protocols and datatypes features add mechanisms for abstraction and data structure definition with no compromises vs the facilities of the host platform.

Concurrent Programming

Clojure simplifies multi-threaded programming in several ways. Because the core data structures are immutable, they can be shared readily between threads. Clojure, being a practical language, allows state to change but provides mechanism to ensure that, when it does so, it remains consistent, while alleviating developers from having to avoid conflicts manually using locks etc.

Hosted on the JVM

Clojure is designed to be a hosted language, sharing the JVM type system, GC, threads etc. All functions are compiled to JVM bytecode. Clojure is a great Java library consumer, offering the dot-target-member notation for calls to Java. Clojure supports the dynamic implementation of Java interfaces and classes.

Example:

Hello world:

(println "Hello world!")

Defining a function:

(defn square [x]

  (* x x))

GUI "Hello world" by calling the Java Swing library:

(javax.swing.JOptionPane/showMessageDialog nil "Hello World" )

Using Unicode (Hello 世 ("World") using the CJK code point for that word):

(println (str "Hello, " \u4e16)) ; to the console

(javax.swing.JOptionPane/showMessageDialog nil (str "Hello, " \u4e16 "!")); using Java GUI

A thread-safe generator of unique serial numbers (though, like many other Lisp dialects, Clojure has a built-in gensym function that it uses internally):

(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 it 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.

(defn run [nvecs nitems nthreads niters]

  (let [vec-refs (->> (range (* nvecs nitems)) (partition nitems) (map (comp ref vec)) vec)

        swap #(let [v1 (rand-int nvecs)

                    v2 (rand-int nvecs)

                    i1 (rand-int nitems)

                    i2 (rand-int nitems)]

                (dosync

                 (let [tmp (nth @(vec-refs v1) i1)]

                   (alter (vec-refs v1) assoc i1 (nth @(vec-refs v2) i2))

                   (alter (vec-refs v2) assoc i2 tmp))))

        report #(let [derefed (map deref vec-refs)]

                  (prn derefed)

                  (println "Distinct:" (->> derefed (apply concat) distinct count)))]

    (report)

    (dorun (apply pcalls (repeat nthreads #(dotimes [_ niters] (swap)))))

    (report)))

(run 100 10 10 100000)

Output of prior 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

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