This document provides an overview of Clojure and why one may want to try it. Some key points include: - Clojure is a functional programming language that runs on the JVM and allows easy interoperability with Java. - It has a very small and elegant syntax based on Lisp with sensible macro names and prefix notation. - Clojure encourages pure functional programming and the use of immutable data structures, while providing tools like Software Transactional Memory to allow safe mutable state changes. - Its focus on functions as first-class citizens and referential transparency can provide benefits for writing parallel and concurrent code more easily compared to other languages.

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

3. Why Clojure ?

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

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

6. Think differently
Functional concepts
Changing state

7. What is Clojure

8. One of many JVM languages
Java
Groovy
Jruby
Jython
Scala
Clojure

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

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

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

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

14. Which LISP is your wingman ?
Common Lisp Clojure

15. Clojure is modular

17. The dark side of Clojure
(x)

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

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

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

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

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

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

24. Comparing Clojure
with Java

25. 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

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

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

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

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

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

32. Defining simple data
(def name data)

33. Calling Behaviour
(function-name data data)

34. Immutable
Data structures

35. 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

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

37. 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}

38. 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

39. A few interesting
Clojure examples

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

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

42. 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)

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

45. 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

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

47. Get coding !

48. clojure.org
docs.clojure.org

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

52. Managing a Clojure
project

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

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

56. Emacs

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

62. Functional Web

63. webnoir.org

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

65. Getting a little more functional

66. 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 :-(

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

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

69. Hacking Clojure challenges

70. Mutable State

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

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

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

74. Getting Creative
with Clojure

75. Overtone: Music to your ears

76. Visuals to your ears
● Quil – clojure library for processing

78. Thank you
London Clojurian
Google Group
@jr0cket