Find out why Clojure delighted Uncle Bob and why is used by huge corporations like Netflix, Wallmart, Daily Mail, (Allegro is joining this list) and why, regarding Greenspun's tenth rule, you already wrote your software in lisp.

1. Clojure

2. About me
• humble software engineer @allegro
• kung fu master in spare time
• Almost none of those thoughts are mine.
Presumably there are no new thoughts after
Aristotle. For sure after 1970…

3. What we think we are
doing?
Computer science

5. What do we do?

6. What do we really do?

7. What we really do

8. Meet Rich

11. Who
• Object Oriented?
• Functional Programming?
• What does it mean?

13. stolen
from
Uncle
Bob

15. Async is everywhere

16. OO vs functional

18. Quiz
)}; )}; )}; )}; )}; )};
Most frequent code in JS
Most frequent code in Clojure
) ) ) ) ) ) ) )

19. First impressions

21. Quick intro to clojure

22. John McCarty
• Lisp
• interpreters
• high-level metaprogramming
• garbage collection
• dynamic programming
• functional programming
• programming with recursive
functions
• …

23. Quick intro to clojure
• f(x,y) -> (f x y)
• deleteOffer(2, :force) -> (delete-offer 2 :force)
function params

24. (def x 1) int x = 1;
(fn [x] (* x x)(def f ))
(f 5)
(defn f [x] (* x x))

25. Quick intro to clojure
• List
(1 2 3 4 5) (adrian krzychu wojtek) (list 1 2 3)
• Vectors
[1 2 3 4 5] [fred ethel lucy]
• Map
{:a 1, :b 2, :c 3} {1 “ethel” 2 “fred}
• Sets
#{fred ethel lucy}
• Everything nested

26. Persistent data structure

27. Tail recursion

30. show some code!

31. (defn neighbours [[x y]]
(for [dx [-1 0 1] dy (if (zero? dx) [-1 1] [-1 0 1])]
[(+ dx x) (+ dy y)]))
(defn step [cells]
(set (for [[loc n] (frequencies (mapcat neighbours cells))
:when (or (= n 3) (and (= n 2) (cells loc)))]
loc)))
code from Christophe Grand

32. stolen from Rich Hickey

33. stolen from Rich Hickey

37. stolen from Rich Hickey

38. stolen from Rich Hickey

39. The same? Json
def builder = new groovy.json.JsonBuilder()
def root = builder.people {
person {
firstName 'Krzysztof'
lastName 'Kaczmarek'
}
}
(json {:person {
:firstName “Krzysztof”
:lastName “Kaczmarek”}})
Groovy Clojure

40. The same? Config
offerCoreStockService:
serviceAddress: service://offercore-stock-service
offerCoreService:
serviceAddress: service://offercore-service-server
{:offerCoreStockService
:serviceAddress “service://offercore-stock-service”
:offerCoreService
:serviceAddress “service://offercore-service-server”}
YML Clojure (edn)

41. The same?

42. – Greenspun's tenth rule
“Any sufficiently complicated C or Fortran
program contains an ad hoc, informally-
specified, bug-ridden, slow implementation of
half of Common Lisp.”

44. @RestController @EnableAutoConfiguration
public class GreetingController {
private static final String template = "Hello, %s!";
private final AtomicLong counter = new AtomicLong();
@RequestMapping("/greeting") public Greeting greeting(
@RequestParam(value="name", defaultValue="World") String name) {
return new Greeting(counter.getAndIncrement(),
} }
String.format(template, name));
example provided by P. Kapała

45. @RestController @EnableAutoConfiguration
public class GreetingController {
private static final String template = "Hello, %s!";
private final AtomicLong counter = new AtomicLong();
@RequestMapping("/greeting") public Greeting greeting(
@RequestParam(value="name", defaultValue="World") String name) {
return new Greeting(counter.getAndIncrement(),
} }
String.format(template, name));
example provided by P. Kapała

46. (def counter (atom 0))
(defn greet [target]
{:id (swap! counter inc)
:name (str "Hello, " (or target "World") "!")})
(defroutes app-routes
(GET "/greeting" {params :params}
(response (greet (:name params))))
(route/not-found "Not Found"))
example provided by P. Kapała

47. (def counter (atom 0))
(defn greet [target]
{:id (swap! counter inc)
:name (str "Hello, " (or target "World") "!")})
(defroutes app-routes
(GET "/greeting" {params :params}
(response (greet (:name params))))
(route/not-found "Not Found"))
example provided by P. Kapała

48. –-- Dr. Alan Kay, 2003.
“OOP to me means only messaging, local
retention and protection and hiding of state-
process, and extreme late-binding of all things. It
can be done in Smalltalk and in LISP. There are
possibly other systems in which this is possible,
but I'm not aware of them.”

50. –Rich Hickey
“Not everything is awesome.”

51. Fear - dynamic typing
Report createReport(Data data) {…}
vs
(create-report data)

52. Fear: Refactoring

53. Clojure w allegro

55. Success story?
• Simple rest service
• 2000 lines of code in java
• 150 lines of code in clojure (one 27” screen)
• most people exp: 1/5 of codebase

56. Java interop
(. Array new)
(. MyAwesomeService doJava “foo” “bar”)

57. Adoption

58. Last language?

59. ClojureScript
• Hosted
• Hello world => 2 LOC in JS
• Is it slow?
• Debugging
• > 2300 watchers
• 68 contributors

60. Clojure.async
• queues in app
• making app simpler to reason about

61. Clojure.async
First go
block
>! my queue <!
second go
block

62. How to learn

63. 4clojure.org

65. Hype is coming

66. –Adrian Cockroft
“A lot of the best programmers and the most
productive programmers I know are writing
everything in Clojure and swearing by it, and
then just producing ridiculously sophisticated
things in a very short time. And that programmer
productivity matters. “
http://thenewstack.io/the-new-stack-makers-adrian-cockcroft-on-sun-netflix-clojure-
go-docker-and-more/

71. Q & A?
krzysztof.kaczmarek@me.com