Clojure for Java developers

Assumptions Some experience with Java development Little or no experience with Clojure You want to try something different! Haskell, Erlang and Scala seem hairy!

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

Parallelism made simpler Provides tools to create massively parallel applications Not a panacea

Think differently Functional concepts Changing state

One of many JVM languages Java Groovy JRuby Jython Scala Clojure Jaskell Erjalang

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

Very small syntax ( ) [ ], { } def, defn, defproject list, map, vector, set let . _ atom

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

Clojure concepts Encourages Pure Functional approach - use STM to change state Functions as first class citizens - functions as arguments as they return a value Make JVM interoperation simple - easy to use your existing Java applications

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

Which LISP is your wingman ? Common Lisp Clojure

The dark side of Clojure ( x )( x )

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

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

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

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

() verses { () }; Clojure verses Java syntax

([] (([]))) verses { ({([])}) }; Well actually more like....

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

What class is that... (class "Jr0cket") => java.lang.String (class (defn hello-world [name] (str "Hello cruel world"))) => clojure.lang.Var Using (type …) you can discover either the metadata or class of something

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

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

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

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

Defining a data structure ( def my-data-structure [ data ] ) ( def days-of-the-week [“Monday” “Tuesday” “Wednesday”]) You can dynamically redefine what the name binds to, but the vector is immutable

Really simple data structure (def jr0cket {:first-name "John", :last-name "Stevenson"}) Using a map to hold key-value pairs. A good way to group data into a meaningful concept

Defining simple data (def name data)

Calling Behaviour (function-name data data)

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

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

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

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

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)

Importing Java into Clojure (ns drawing-demo (:import [javax.swing Jpanel JFrame] [java.awt Dimension]))

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

Simplifying with doto doto evaluates the first expression in a chain of expressions and saves it in a temporary variable. It then inserts that variable as the first argument in each of the following expressions. Finally, doto returns the value of the temporary variable.

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

All hail the REPL An interactive shell for clojure Fast feedback loop for clojure

Maven Just like any other Java project Step 1) Add Clojure library dependency to your POM Step 2) Download the Internet !!!

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

Eclipse & Clojure = Counter Clockwise

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

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

Deploy Clojure to the Cloud lein new heroku my-web-app

Getting a little more functional

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

Recursion – managing memory (defn recursive-counter [value] (print value) (if (< value 1000) (recur (+ value 4)))) (recursive-counter 100)

Software Transactional Memory Provides safe, concurrent access to memory Agents allow encapsulated access to mutable resources http://www.studiotonne.com/illustration/software-transactional-memory/

Atoms - Coding state changes ; Clojure atom code (def mouseposition (atom [0 0])) (let [[mx my] @mouseposition]) (reset! mouseposition [x y])

Database access Clojure-contrib has sql library Korma - "Tasty SQL for Clojure" CongoMongo for MongoDB https://devcenter.heroku.com/articles/clojure-web-application

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

Thank you @jr0cket London Clojurians Google Group

Clojure for Java developers

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