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AssumptionsSome experience with Java development Little or no experience with ClojureYou want to try something different!Haskell, Erlang andScala seem hairy!

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Why Clojure ?

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Why get functional ?4 cores in a typical developers Mac book ProHow long until 128 coresin a laptop is common?

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Parallelism made simplerProvides tools to create massively parallelapplications Not a panacea

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Think differentlyFunctional concepts Changing state

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What is Clojure

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One of many JVM languages Java Groovy Jruby Jython Scala Clojure

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A little more ubiquitous.Net framework – DLR/CLR In browsers > ClojureScriptNative compilers

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Very small syntax ( ) [ ]def, defn, defprojectlist, map, vector, set let # _ atom

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It uses mathematical concepts Functions have a value For a given set of arguments,you should always get the same result (Referential transparency)

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A better Lisp ! Sensible () usage Sensible macro names JVM InteroperabilityLisp is the 2nd oldest programming language still in use

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Which LISP is your wingman ?Common Lisp Clojure

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Clojure is modular

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The dark side of Clojure (x)

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The dark side of Clojure ((x))

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The dark side of Clojure (((x)))

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The dark side of Clojure((((x))))

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The dark side of Clojure(((((x)))))

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...verses non-lisp languages () verses { () };

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Well nearly....([] ((()))) verses{ ({([])}) };

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Comparing Clojure with Java

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Clojure conceptsEncourages Pure Functional approach- use STM to change stateFunctions as first class citizens - functions as arguments as they return a valueMake JVM interoperation simple - easy to use your existing Java applications

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Its all byte code in the end..Any object in Clojure is just a regular javaobjectTypes inherit from: java.lang.object

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Prefix notation3 + 4 + 5 – 6 / 3(+ 3 4 5 (- (/ 6 3)))

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Prefix notation everywhere(defn square-the-number[x] (* x x))

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Defining a data structure( def my-data-structure [ data ] )( def days-of-the-week [“Monday” “Tuesday” “Wednesday”])

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Really simple data structure(def jr0cket {:first-name "John", :last-name "Stevenson"})

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Defining simple data(def name data)

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Calling Behaviour(function-name data data)

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ImmutableData structures

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List – Ordered collection(list 1 3 5 7)(1 3 5 7)(quote (1 3 5 7)) (1 2 3) ; 1 is not a function

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Vectors – hashed ordered list[:matrix-characters [:neo :morpheus :trinity :smith]](first [:neo :morpheus :trinity :smith])(nth [:matrix :babylon5 :firefly] 2)(concat [:neo] [:trinity])

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Maps – unordered key/values{:a 1 :b 2} {:a {:a 1}} {:a 1, :b 2} {:a {:a 1}}{ :a 1 :b } {{:a 1} :a}java.lang.ArrayIndexOutOfB {{:a 1} :a}oundsException: 3 ; idiom - put :a on the left{ :a 1 :b 2} {:a 1, :b 2}

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Lists are for code and dataVectors are for data and arguments Its not that black and white, but its a useful starting point when learning

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A few interestingClojure examples

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RatioUnique data type (/ 2 4) (/ 2.0 4)Allow lazy evaluation (/ 1 3)Avoid loss of (/ 1.0 3)precision (class (/ 1 3)

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Java made simpler// Javaimport java.util.Date;{Date currentDate = new Date();}; Clojure(import java.util.Date)(new Date)

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Joda Time(use clj-time.core)(date-time 1986 10 14)(hour (date-time 1986 10 14 22))(from-time-zone (date-time 2012 10 17) (time-zone-for-offset -8))(after? (date-time 1986 10) (date-time 1986 9))(show-formatters)

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Calling Java GUI(javax.swing.JOptionPane/showMessageDialog nil "Hello JAX London" )

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Working with JavaJava Classes● fullstop after class name (JFrame. ) (Math/cos 3) ; static method callJava methods● fullstop before method name (.getContentPane frame) ;;method name first (. frame getContentPane) ;;object first

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What class is that...(class (str "Jr0cket"))java.lang.String(class (defn hello-world [name] (str "Hellocruel world")))clojure.lang.Var

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Get coding !

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clojure.orgdocs.clojure.org

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All hail the REPLAn interactive shell forclojureFast feedback loopfor clojure

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Managing a Clojure project

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MavenJust like any other Java projectStep 1)Add Clojure library dependency to yourPOMStep 2)Download the Internet !!!

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Leiningen leiningen.orglein new ● Create a new Clojure projectlein deps ● Download all dependencieslein repl ● Start the interactive shell (repl)lein jack-in ● Start a REPL server

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Emacs

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Light tableKickstarter project to build a development environment, focused on the developer experience

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Functional Web

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webnoir.org

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Clojure calling Java web stuff(let [conn] (doto (HttpUrlConnection. Url) (.setRequestMethod “POST”) (.setDoOutput true) (.setInstaneFollowRedirects true))])

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Getting a little more functional

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Recursive functions● Functions that call ● Tail recursion themselves ● Avoids blowing the stack● Fractal coding ● A trick as the JVM does not support tail recursion directly :-(

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Tail recursion(defn recursive-counter (print answer) (if (< answer 1000) (recur (+ answer 4))))

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Where to find out more... clojure.org/ cheatsheet

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Hacking Clojure challenges

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Mutable State

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Software Transactional MemoryProvides safe,concurrent access tomemoryAgents allowencapsulated accessto mutable resources http://www.studiotonne.com/illustration/software-transactional-memory/

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Coding state changes● Atoms● Try / catch ; Clojure atom code (def mouseposition (atom [0 0])) [mx my] @mouseposition (reset! mouseposition [x y])

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Database accessClojure-contrib has sqllibraryKorma - "Tasty SQL forClojure"CongoMongo forMongoDBhttps://devcenter.heroku.com/articles/clojure-web-application

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Getting Creative with Clojure

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Overtone: Music to your ears

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Visuals to your ears● Quil – clojure library for processing

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Thank you London Clojurian Google Group@jr0cket