The fact that Clojure is a dialect of Lisp makes it feel completely alien to Java developers, and they miss the opportunity to learn this dynamic and functional programming language for the JVM. Clojure’s focus on immutability makes it very useful for concurrency. This presentation introduces Clojure in a way that feels natural to Java developers. By seeing how well Clojure interoperates with Java, you will learn how to take advantage of this wonderful language and still use all the frameworks and features of the JVM.

1. for Java developers
Clojure
jan.kronquist@jayway.com
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2. About me
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3. About me
1986 - Basic
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4. About me
1990 - 68K assembly
1986 - Basic
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5. About me
1990 - 68K assembly1986 - Basic
1995 - OOP
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6. 1997 - University
About me
1990 - 68K assembly1986 - Basic
1995 - OOP
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7. 1997 - University
About me
1990 - 68K assembly1986 - Basic
2005 - IoC,Aspects, Mixins
1995 - OOP
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8. 2008 - Functional 1997 - University
About me
1990 - 68K assembly1986 - Basic
2005 - IoC,Aspects, Mixins
1995 - OOP
onsdag 25 september 13

9. 2012 - Lisp & dynamic typing
2008 - Functional
1997 - University
About me
1990 - 68K assembly1986 - Basic
2005 - IoC,Aspects, Mixins
1995 - OOP
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10. This talk is not
A comprehensive introduction to Clojure
About idiomatic Clojure
Another talk about functional programming
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11. Killer apps
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12. Killer apps
Prismatic
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13. Killer apps
Prismatic
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14. Killer apps
Prismatic
Datomic
Storm
ClojureScript
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15. What is Clojure?
Created 2007 by Rich Hickey
Lisp
Runs on JVM, CLR & JavaScript
Design for concurrency
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16. Clojure example
(defn divisible? [n d]
(== 0 (mod n d)))
(defn divides [n]
(partial divisible? n))
(declare primes)
(defn prime? [n]
(not (some (divides n) (take-while #(< % n) primes))))
(def primes (lazy-cat
[2 3 5]
(filter prime? (drop 7 (range)))))
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17. Why Clojure?
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18. Why Clojure?
Lisp - Code as data & Syntactic abstraction
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19. Why Clojure?
Lisp - Code as data & Syntactic abstraction
Functional - Declarative, Immutable
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20. Why Clojure?
Lisp - Code as data & Syntactic abstraction
Functional - Declarative, Immutable
Interactive development environment
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21. Why Clojure?
Lisp - Code as data & Syntactic abstraction
Functional - Declarative, Immutable
Interactive development environment
Great eco-system - dynamic helps compatibility
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22. Why Clojure?
Lisp - Code as data & Syntactic abstraction
Functional - Declarative, Immutable
Interactive development environment
Great eco-system - dynamic helps compatibility
Wrapper-free Java access
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23. Why Clojure?
Lisp - Code as data & Syntactic abstraction
Functional - Declarative, Immutable
Interactive development environment
Great eco-system - dynamic helps compatibility
Wrapper-free Java access
Less complexity
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24. Common complaints
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25. Common complaints
Lisp looks weird
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26. Common complaints
Dynamic typing is scary
Lisp looks weird
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27. Common complaints
Dynamic typing is scary
Lisp looks weird
Just a toy language
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28. Common complaints
Dynamic typing is scary
Lisp looks weird
Macros? Are you crazy?
Just a toy language
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29. Lispness
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30. Clojure Atomic Data Types
Arbitrary precision integers: 12345678987654
Doubles: 1.234
BigDecimals: 1.234M
Ratios: 22/7
Strings: "fred" , Characters: a b c
Symbols: fred ethel , Keywords: :fred :ethel
Booleans: true false , Null: - nil
Regex patterns #"a*b"
Rich Hickey - Clojure for Java Programmers - http://www.youtube.com/watch?v=P76Vbsk_3J0
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31. JavaScript Data Structures
Arrays
[1, 2, 3] ["fred", "ethel", "lucy"]
Objects
{name: "Jan Kronquist", age: 37}
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32. Clojure Data Structures
Vectors
[1, 2, 3] ["fred", "ethel", "lucy"]
Maps
{:name "Jan Kronquist", :age 37}
move colon
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33. Clojure Data Structures
Vectors
[1 2 3] ["fred" "ethel" "lucy"]
Maps
{:name "Jan Kronquist" :age 37}
commas are whitespace!
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34. Clojure Data Structures
Vectors - indexed access
[1 2 3] ["fred" "ethel" "lucy"]
Maps
{:name "Jan Kronquist" :age 37}
Lists - singly linked
(1 2 3 4 5) (fred ethel lucy) (list 1 2 3)
Sets
#{fred ethel lucy}
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35. Demo in REPL
user=> "hello"
"hello"
user=> 5
5
user=> [1 2 3]
[1 2 3]
user=> qwe
java.lang.RuntimeException: Unable to resolve symbol: qwe
user=> (def qwe "hello world")
#'user/qwe
user=> qwe
"hello world"
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36. "Hello world")
The strangeness of Lisp
println(
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37. "Hello world")
The strangeness of Lisp
println(
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38. "Hello world")
The strangeness of Lisp
println(
(operator operand1 operand2 ...)
Determines how the list is evaluated
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39. (= (.toString (+ 1 2)) "3")
Example evaluation
(= (.toString (+ 1 2)) "3")
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40. (= (.toString (+ 1 2)) "3")
Example evaluation
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41. (= (.toString (+ 1 2)) "3")
Example evaluation
(= (.toString 3) "3")
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42. (= (.toString (+ 1 2)) "3")
Example evaluation
(= (.toString 3) "3")
(= "3" "3")
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43. (= (.toString (+ 1 2)) "3")
Example evaluation
(= (.toString 3) "3")
(= "3" "3")
true
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44. onsdag 25 september 13

45. int i = 5; (def i 5) OR (let [i 5] ...)
if (x > 5) {
return y;
} else {
return z;
}
(if (> x 5)
y
z)
x * y * z (* x y z)
foo(x, y, z) (foo x y z)
object.method(x, y) (.method object x y)
public String sayHello(String x) {
return "Hello " + x;
}
(defn sayHello [x]
(str "Hello " x))
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46. int i = 5; (def i 5) OR (let [i 5] ...)
if (x > 5) {
return y;
} else {
return z;
}
(if (> x 5)
y
z)
x * y * z (* x y z)
foo(x, y, z) (foo x y z)
object.method(x, y) (.method object x y)
public String sayHello(String x) {
return "Hello " + x;
}
(defn sayHello [x]
(str "Hello " x))
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47. int i = 5; (def i 5) OR (let [i 5] ...)
if (x > 5) {
return y;
} else {
return z;
}
(if (> x 5)
y
z)
x * y * z (* x y z)
foo(x, y, z) (foo x y z)
object.method(x, y) (.method object x y)
public String sayHello(String x) {
return "Hello " + x;
}
(defn sayHello [x]
(str "Hello " x))
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48. int i = 5; (def i 5) OR (let [i 5] ...)
if (x > 5) {
return y;
} else {
return z;
}
(if (> x 5)
y
z)
x * y * z (* x y z)
foo(x, y, z) (foo x y z)
object.method(x, y) (.method object x y)
public String sayHello(String x) {
return "Hello " + x;
}
(defn sayHello [x]
(str "Hello " x))
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49. int i = 5; (def i 5) OR (let [i 5] ...)
if (x > 5) {
return y;
} else {
return z;
}
(if (> x 5)
y
z)
x * y * z (* x y z)
foo(x, y, z) (foo x y z)
object.method(x, y) (.method object x y)
public String sayHello(String x) {
return "Hello " + x;
}
(defn sayHello [x]
(str "Hello " x))
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50. int i = 5; (def i 5) OR (let [i 5] ...)
if (x > 5) {
return y;
} else {
return z;
}
(if (> x 5)
y
z)
x * y * z (* x y z)
foo(x, y, z) (foo x y z)
object.method(x, y) (.method object x y)
public String sayHello(String x) {
return "Hello " + x;
}
(defn sayHello [x]
(str "Hello " x))
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51. Java Quiz
public static void main(String[] args) {
int bang = 1;
do while (bang>=1)
System.out.print(" bang is "+ bang);
while (bang>1);
}
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52. Macros
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53. Working with Java classes
(defn display [text]
(let [frame (new JFrame "MyFrame")]
(.add frame (new JLabel text))
(.setSize frame 300 200)
(.setVisible frame true)))
static void display(String text) {
JFrame frame = new JFrame("MyFrame");
frame.add(new JLabel(text));
frame.setSize(300, 200);
frame.setVisible(true);
}
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54. Working with Java classes
(defn display [text]
(let [frame (new JFrame "MyFrame")]
(.add frame (new JLabel text))
(.setSize frame 300 200)
(.setVisible frame true)))
static void display(String text) {
JFrame frame = new JFrame("MyFrame");
frame.add(new JLabel(text));
frame.setSize(300, 200);
frame.setVisible(true);
}
(display "Hello World")
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55. Working with Java classes
(defn display [text]
(let [frame (new JFrame "MyFrame")]
(.add frame (new JLabel text))
(.setSize frame 300 200)
(.setVisible frame true)))
static void display(String text) {
JFrame frame = new JFrame("MyFrame");
frame.add(new JLabel(text));
frame.setSize(300, 200);
frame.setVisible(true);
}
a pattern!
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56. static void display(String text) {
JFrame frame = new JFrame("MyFrame");
with (frame) {
.add(new JLabel(text));
.setSize(300, 200);
.setVisible(true);
}
}
Working with Java classes
(defn display [text]
(let [frame (new JFrame "MyFrame")]
(.add frame (new JLabel text))
(.setSize frame 300 200)
(.setVisible frame true)))
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57. (defn display [text]
(let [frame (new JFrame "MyFrame")]
(doto frame
(.add (new JLabel text))
(.setSize 300 200)
(.setVisible true))))
Working with Java classes
static void display(String text) {
JFrame frame = new JFrame("MyFrame");
with (frame) {
.add(new JLabel(text));
.setSize(300, 200);
.setVisible(true);
}
}
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58. (defn display [text]
(doto (new JFrame "MyFrame")
(.add (new JLabel text))
(.setSize 300 200)
(.setVisible true)))
Working with Java classes
static void display(String text) {
JFrame frame = new JFrame("MyFrame");
frame.add(new JLabel(text));
frame.setSize(300, 200);
frame.setVisible(true);
}
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59. (defn display [text]
(doto (new JFrame "MyFrame")
(.add (new JLabel text))
(.setSize 300 200)
(.setVisible true)))
Working with Java classes
static void display(String text) {
JFrame frame = new JFrame("MyFrame");
frame.add(new JLabel(text));
frame.setSize(300, 200);
frame.setVisible(true);
}
Ok, nice,
but in practice you would
never want something like this?
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60. static void write(String fileName, String text) throws IOException {
try (Writer writer = new FileWriter(fileName)) {
writer.write(text);
}
}
Trust me, you want macros
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61. static void write(String fileName, String text) throws IOException {
try (Writer writer = new FileWriter(fileName)) {
writer.write(text);
}
}
Trust me, you want macros
(defn write [fileName text]
(with-open [writer (new FileWriter fileName)]
(.write writer text)))
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62. Toy?
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63. Structure in
Modules - package
Abstraction - interface
Implementation - class
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64. Structure in
Modules - package
Abstraction - interface
Implementation - class
Problems
mutable state?
static methods?
inheritance?
constants?
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65. Structure in
Modules
namespace - first-class, dynamic, import, aliasing
Abstraction
defprotocol - can be added later
Implementation
defrecord - immutable, equals, hashcode, etc
deftype - may mutate, only user functionilty
reify - singleton
gen-class & proxy - for Java interop
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66. Records - creating
(ns my.namespace)
(defrecord Person [firstName lastName])
(new Person "Jan" "Kronquist")
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67. Records - creating
; #my.namespace.Person{:firstName "Jan", :lastName "Kronquist"}
(ns my.namespace)
(defrecord Person [firstName lastName])
(new Person "Jan" "Kronquist")
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68. Records - field access
; #my.namespace.Person{:firstName "Jan", :lastName "Kronquist"}
(ns my.namespace)
(defrecord Person [firstName lastName])
(new Person "Jan" "Kronquist")
(def person (new Person "Jan" "Kronquist"))
(.firstName person)
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69. Records - field access
; #my.namespace.Person{:firstName "Jan", :lastName "Kronquist"}
(ns my.namespace)
(defrecord Person [firstName lastName])
(new Person "Jan" "Kronquist")
(def person (new Person "Jan" "Kronquist"))
(.firstName person)
; "Jan"
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70. Records - named parameters
; #my.namespace.Person{:firstName "Jan", :lastName "Kronquist"}
(ns my.namespace)
(defrecord Person [firstName lastName])
(new Person "Jan" "Kronquist")
(def person (new Person "Jan" "Kronquist"))
(.firstName person)
; "Jan"
(map->Person {:lastName "Kronquist" :firstName "Jan"})
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71. Records - named parameters
; #my.namespace.Person{:firstName "Jan", :lastName "Kronquist"}
(ns my.namespace)
(defrecord Person [firstName lastName])
(new Person "Jan" "Kronquist")
(def person (new Person "Jan" "Kronquist"))
(.firstName person)
; "Jan"
(map->Person {:lastName "Kronquist" :firstName "Jan"})
; #my.namespace.Person{:firstName "Jan", :lastName "Kronquist"}
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72. Records and protocols
(defprotocol Nameable
(getName [this]))
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73. Records and protocols
(defprotocol Nameable
(getName [this]))
(defrecord Person [firstName lastName]
Nameable
(getName [this] (str firstName " " lastName)))
(getName (new Person "Jan" "Kronquist"))
; "Jan Kronquist"
Person class implements
Nameable interface
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74. Records and protocols
(defprotocol Nameable
(getName [this]))
(defrecord Person [firstName lastName])
(extend-protocol Nameable
Person
(getName [this] (str (.firstName this) " " (.lastName this))))
(getName (new Person "Jan" "Kronquist"))
; "Jan Kronquist"
(defrecord Person [firstName lastName]
Nameable
(getName [this] (str firstName " " lastName)))
(getName (new Person "Jan" "Kronquist"))
; "Jan Kronquist"
Person class implements
Nameable interface
Person extended by
Nameable after definition!
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75. Records and protocols
(defprotocol Nameable
(getName [this]))
(defrecord Person [firstName lastName])
(extend-protocol Nameable
Person
(getName [this] (str (.firstName this) " " (.lastName this))))
(getName (new Person "Jan" "Kronquist"))
; "Jan Kronquist"
(defrecord Person [firstName lastName]
Nameable
(getName [this] (str firstName " " lastName)))
(.getName (new Person "Jan" "Kronquist"))
; "Jan Kronquist"
Method getName
exists on Person class
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76. Records and protocols
(defprotocol Nameable
(getName [this]))
(defrecord Person [firstName lastName])
(extend-protocol Nameable
Person
(getName [this] (str (.firstName this) " " (.lastName this))))
(.getName (new Person "Jan" "Kronquist"))
; IllegalArgumentException No matching field found: getName
(defrecord Person [firstName lastName]
Nameable
(getName [this] (str firstName " " lastName)))
(.getName (new Person "Jan" "Kronquist"))
; "Jan Kronquist"
Method getName
exists on Person class
But not in this case!
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77. Editors and IDEs?
Eclipse - Counterclockwise
IntelliJ - La Clojure
Light Table
Sublime
Textmate
Emacs
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78. Eclipse Counterclockwise
✓Restrictive formatting
✓Paren coloring
✓Typing suggestions
✓Being Eclipse
https://code.google.com/p/counterclockwise/
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79. Light table
✓Cool
✓Insta-REPL
http://www.lighttable.com/
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80. Sublime
✓General purpose editor
✓Typing suggestions
http://www.sublimetext.com/
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81. Repositories
Maven style
Maven central
http://clojars.org/repo
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82. Build tools
Maven plugin
Gradle plugin
Leiningen
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83. Leiningen
(defproject myproject "0.5.0-SNAPSHOT"
:description "A project for doing things."
:url "http://github.com/foo/bar"
:dependencies [[org.clojure/clojure "1.5.1"]
[ring/ring-core "1.2.0 "]]
:plugins [[lein-ring "0.4.5"]])
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84. Dynamic typing
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85. Dynamic typing?
Robert Smallshire - The Unreasonable Effectiveness of Dynamic Typing for Practical Programs
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86. Dynamic typing is scary
Compiler won’t find errors
Limited tool support
Performance?
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87. What made me think twice
Web server
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88. What made me think twice
Web server
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89. Clojure makes dynamic typing ok
REPL
Immutable data structure
Pure functions
Automated tests
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90. Dynamic typing and performance?
Clojure is compiled to bytecode
Generally good enough!
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91. Dynamic typing and performance?
Clojure is compiled to bytecode
Generally good enough!
(defn len [^String x]
(.length x))
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92. Studies
Stefan Hanenberg & Lutz Prechelt
Dynamic is more productive
No difference in reliability
Robert Smallshire - 2 % defects are type errors (GitHub)
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93. Conclusion
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94. Macros? Are you crazy?
Dynamic typing is scary
Convinced?
Lisp looks weird
Just a toy language
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95. Macros? Are you crazy?
Dynamic typing is usable
Convinced?
Lisp looks weird
Just a toy language
REPL
Immutable data structure
Pure functions
Automated tests
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96. Macros? Are you crazy?
Lisp is consistent
Dynamic typing is usable
Convinced?
Just a toy language
REPL
Immutable data structure
Pure functions
Automated tests
(operation operand1 operand2 ...)
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97. Macros? Are you crazy?
Lisp is consistent
Dynamic typing is usable
Convinced? REPL
Immutable data structure
Pure functions
Automated tests
Interesting language
(operation operand1 operand2 ...)
Namespaces
Prototcols
Records
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98. Macros? Are you crazy?
Lisp is consistent
Dynamic typing is usable
Convinced? REPL
Immutable data structure
Pure functions
Automated tests
Interesting language
(operation operand1 operand2 ...)
Lazy seqs STM Functional
Namespaces
Prototcols
Records
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99. Macros? Maybe.......
Lisp is consistent
Dynamic typing is usable
Convinced? REPL
Immutable data structure
Pure functions
Automated tests
Interesting language
(operation operand1 operand2 ...)
Lazy seqs STM Functional
Namespaces
Prototcols
Records
try (Writer writer = new FileWriter(fileName))
writer.write(text);
}
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100. Macros? Maybe.......
Lisp is consistent
Dynamic typing is usable
Convinced? REPL
Immutable data structure
Pure functions
Automated tests
Interesting language
(operation operand1 operand2 ...)
Lazy seqs STM Functional
Namespaces
Prototcols
Records
try (Writer writer = new FileWriter(fileName))
writer.write(text);
}
DSL Reuse Structure
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101. Further resources
http://clojure.org/
http://tryclj.com/
http://www.4clojure.com
http://clojure-doc.org/
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102. Questions?
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