A lightning talk on Clojure given in 7 minutes and 20 seconds at one of the London Java Community events.

1. ma de
r e a lly r e a lly
Ta l k
s im p le

2. W h y C lo ju r e ?

3. Why get functional ?
4 cores in a Mac book Pro for developers

4. W h a t is
C lo ju r e

7. Clojure is small and flexible

8. 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

9. A better Lisp !
Sensible () usage
Sensible macro names
JVM Interoperability

10. Which LISP is your wingman ?
Common Lisp Clojure

11. The dark side of Clojure
( x )

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

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

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

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

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

17. Well nearly....
([] ((())))
==
{ ( {( []) }) };

18. C o m p a r in g
J a va
w it h
C lo ju r e

19. It s a ll b y t e c o d e in t h e
e nd ..
Any object in clojure is just a regular java object
A reference type inheriting from:
j ava. l ang. obj ec t

21. Prefix notation
( def n s quar e- t he- number
[ x]
( * x x) )

22. Im m u t a b le
D a ta
s truc ture s

23. List – Ordered collection
( l i s t 1 3 5 7)
' ( 1 3 5 7)
( 1 2 3) ; 1 i s n o t a
f unct i on

24. Vectors – hashed ordered list
[ : m r i x- c har ac t er s [ : neo
at
: m pheus : t r i ni t y : s m t h] ]
or i
( f i r s t [ : n e o : mo r p h e u s : t r i n i t y
: s mi t h ] )
( nt h [ : mat r i x : b ab yl o n 5 : f i r e f l y
: s t ar g at e ] 2 )
( c onc at [ : n e o ] [ : t r i n i t y] )

25. Maps – unordered key/values
{ : a 1 : b 2} { : a { : a 1} }
{ : a 1 , : b 2} {: a {: a 1}}
{ :a 1 :b } { { : a 1} : a}
j ava. l an g . Ar r ayI n d e x Ou t Of Bo { { : a 1 } : a}
u n d s Ex c e p t i o n : 3
; i d i om - put : a on t he
left
{ : a 1 : b 2}
{ : a 1 , : b 2}

26. L is t s a r e f o r
c ode
Ve c t o r s a r e
fo r d a ta

27. Defining a data structure
( def m dat a- s t r uc t ur e
y-
[ dat a ] )
( def days - of - t he- week
[ “Monday” “ Tues day”
“W ednes day” ] )

28. Example data structure
( def j r 0c ket
{ : f i r s t - nam " J ohn" ,
e
: l as t - name
" St evens on" } )

29. G e t c o d in g !

30. c lo ju r e .
org
d o c s . c lo ju r
e .o rg

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

34. M a n a g in g a
c lo ju r e
p r o je c t

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

36. le in in g e n
Leiningen
.o rg
lein new Create a new clojure project
lein deps Download all dependencies
lein repl Start the interactive shell (repl)
lein swank Start repl server

38. Ema c s

41. A fe w
in t e r e s t in g
C lo ju r e
e x a m p le s

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

43. Calling Java... ooooo!!
( j avax . s wi n g . JOp t i o n Pan e /
s h o wMe s s ag e D i al o g n i l
" He l l o W r l d " )
o

44. Importing Java into Clojure
( ns dr aw ng- dem
i o
( : i m t [ j avax. s w ng J panel
por i
J Fr am e]
[ j ava. awt
Di m i on] ) )
ens

45. Working with Java
Java Classes
fullstop after class name
( J Fr am )
e.
(Math/cos 3) ; static method call
Java methods
fullstop before method name
( . get Cont ent Pane f r am ;;method name first
e)
( . f r am get Cont ent Pane) ;;object first
e

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

47. Clojure calling Java web stuff
( l et [ c onn]
( dot o ( Ht t pUr l Connec t i on.
Ur l )
( . s et Reques t M hod
et
“ POST” )
( . s et DoOut put t r ue)
( . s et I ns t aneFol l ow Redi r ec t
s t r ue) ) ] )

48. 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 :-(

49. Tail recursion
( def n r ec ur s i ve- c ount er
( pr i nt ans w )er
( i f ( < ans w er 1000)
( r ec ur ( + ans wer 4) ) ) )

50. Where to find out more...
c l oj ur e. or g/ c heat s h
eet

51. M u t a b le S t a t e

52. Software Transactional Memory
Provides safe, concurrent access to memory
Agents allow encapsulated access to mutable
resources

53. F u n c t io n a l
We b

54. Noir w e b n o ir . o r g

55. Th a n k yo u
London Cl oj ur i ans
c l oj ur e. or g
@ r 0c ket
j