Clojure: Your New Favorite Language

An overview of what Clojure is, and what makes it special.

Updated as presented March 18th at Portland JUG.

1. Clojure: Your New Favorite Language Howard M. Lewis Ship
 hlship@gmail.com @hlship
2. “A language that doesn't affect the way you think about programming is not worth knowing.”
   ! – Alan Perlis
 First recipient of the Turing Award
3. Power Focus Simplicity
4. “In mathematics, a function is a relation between a set of inputs and a set of permissible outputs with the property that each input is related to exactly one output.”
5. First Name String Last Name String SSN String Donate to Campaign Boolean
6. public class TaxPayer { private String firstName, lastName, SSN; ! private boolean donate = false; ! public String getFirstName() { return firstName; } ! public void setFirstName(String firstName) { this.firstName = firstName; } ! public String getLastName() { return lastName; } ! public void setLastName(String lastName) { this.lastName = lastName; … } Noun Verb Verb Verb Verb
8. manny.triggerConversation(0);
9. (def tax-payer {
 :first-name "Howard" :last-name "Lewis Ship" :ssn "…" :donate true })
10. O.M.G.
    Lisp!
12. (def tax-payer {
 :first-name "Howard" :last-name "Lewis Ship" :ssn "…" :donate true }) (assoc tax-payer :tax-schedule "C")
 ➠ {:first-name "Howard"
 :last-name "Lewis Ship"
 :ssn "…"
 :donate true
 :tax-schedule "C"
 } Generic Verb
13. “It is better to have 100 functions operate on one data structure than 10 functions on 10 data structures.”
 
 — Alan Perlis
15. “All non-trivial abstractions, to some degree, are leaky.” ! — Joel Spolsky
16. What makes Clojure different?
17. What makes Clojure better?
18. The full language … all the time
19. javac java
 (JVM) Java source .class
20. (…) Read Eval Print Loop
 REPL Clojure 
 Source
21. Clojure Basics
22. Test In Production Proper Integration Testing Unit Testing REPL
23. Pop Quiz Can any of these parenthesis be removed? if ((user != null && ((user.perms & WRITE_PERM) != 0)) || (record.version == snapshot.version)) { … } Don't Know / Don't Care
24. if ((user != null && ((user.perms & WRITE_PERM) != 0)) || (record.version == snapshot.version)) { … } (if (or (and user (-> user :perms (bit-and WRITE_PERM) pos?) (= (:version record) (:version snapshot))) … )
25. “Has anybody noticed that as languages grow in complexity, the best programmers switch to simpler languages?”
 —“Uncle” Bob Martin
26. Solving Problems in Clojure
27. Call a function Define a function Define and pass a function How do I do ______ in Clojure?
28. (defn largest [p1 p2] (if (> p1 p2) p1 p2)) symbol vector of parameters
29. Boring!
32. “Fifteen for two” “A run for four”
33. “A long double run for ten” “A run of four” “A different run of four” “A pair for two” “Fifteen for two”
34. Five or Four Card Flush Fifteens Pairs Runs of three, four, five Right Jack — “His Knees”
35. public enum Suit { SPADES, HEARTS, DIAMONDS, CLUBS }
36. public enum Rank { ACE(1), TWO(2), THREE(3),
 … JACK(10), QUEEN(10), KING(10); ! public final int points; ! Rank(int points) { this.points = points; } }
37. public class Card { public final Suit suit; ! public final Rank rank; ! public Card(Suit suit, Rank rank) { this.suit = suit; this.rank = rank; } ! @Override public String toString() { return String.format("%s of %s", StringUtils.capitalize(rank.name().toLowerCase()), StringUtils.capitalize(suit.name().toLowerCase())); } }
38. Clojure
39. { :rank :ace :suit :spades }
40. (defn points [rank] (case rank :ace 1 :jack 10 :queen 10 :king 10 rank))
41. points ) ( ( ) :rank flip-card
42. points ) ( ( ) :rank flip-card
43. points :rank flip-card (-> ) ) ( ( ) flipCard.getRank().getPoints() vs.
45. Scoring — Java
46. public interface Scorer { void score(List<Card> hand, List<Score> scores); } ! public class Score { public final int points; ! public final String description; ! public final Set<Card> cards = new HashSet<>(); ! public Score(int points, String description) { this.points = points; this.description = description; } ! public Score(int points, String description, Collection<Card> cards) { this(points, description); ! this.cards.addAll(cards); } }
47. public class FlushScorer implements Scorer { @Override public void score(List<Card> hand, List<Score> scores) { Suit flipSuit = hand.get(0).suit; ! boolean fiveFlush = true; ! for (int i = 1; i < 5; i++) { if (hand.get(i).suit != flipSuit) { fiveFlush = false; break; } } ! if (fiveFlush) { scores.add(new Score(5, "five card flush", hand)); ! return; } ! Suit firstSuit = hand.get(1).suit; ! for (int i = 2; i < 5; i++) { if (hand.get(i).suit != firstSuit) { return; } } ! scores.add(new Score(4, "four card flush", hand.subList(1, 5))); } }
48. (defn flush? "Returns true if the set of cards all have the same suit." [hand] (let [suits (map :suit hand) first-suit (first suits)] (every? #(= first-suit %) (rest suits)))) !
49. (defn flush? "Returns true if the set of cards all have the same suit." [hand] (let [suits (map :suit hand) first-suit (first suits)] (every? #(= first-suit %) (rest suits)))) !
50. (defn flush? "Returns true if the set of cards all have the same suit." [hand] (let [suits (map :suit hand) first-suit (first suits)] (every? #(= first-suit %) (rest suits)))) !
51. (map inc [3 4 5]) (4 5 6) ➠ (list (inc 3) (inc 4) (inc 5))
52. [{:suit :spades :rank 3} {:suit :hearts :rank 4} {:suit :hearts :rank 9} {:suit :hearts :rank 10} {:suit :hearts :rank :queen}] (:spades :hearts :hearts :hearts :hearts) (map :suit hand)
53. (defn flush? "Returns true if the set of cards all have the same suit." [hand] (let [suits (map :suit hand) first-suit (first suits)] (every? #(= first-suit %) (rest suits)))) !
54. Clojure seqs first - first element in collection rest - remaining elements in collection next - remaining elements or nil • java.util.Collection • String • Java primitive array • java.util.Map
55. (defn flush? "Returns true if the set of cards all have the same suit." [hand] (let [suits (map :suit hand) first-suit (first suits)] (every? #(= first-suit %) (rest suits)))) !
56. (defn flush? "Returns true if the set of cards all have the same suit." [hand] (let [suits (map :suit hand) first-suit (first suits)] (every? #(= first-suit %) (rest suits)))) !
58. (defn score-flush "Returns a score map or nil depending on finding a four or five card flush." [hand] (cond (flush? hand) {:points 5 :text "five card flush" :cards hand} (flush? (rest hand)) {:points 4 :text "four card flush" :cards (rest hand)}))
59. (defn score "Constructs a score map." [points text cards] {:points points :text text :cards cards}) ! (defn score-flush "Returns a score map…" [hand] (cond (flush? hand) (score 5 "five card flush" hand) (flush? (rest hand)) (score 4 "four card flush" (rest hand))))
60. Scoring Pairs
61. public class PairsScorer implements Scorer { @Override public void score(List<Card> hand, List<Score> scores) { for (int i = 0; i < 4; i++) { Card left = hand.get(i); ! for (int j = i + 1; j < 5; j++) { Card right = hand.get(j); ! if (left.rank == right.rank) { Score score = new Score(2, "pair"); score.cards.add(left); score.cards.add(right); ! scores.add(score); } } } } } Match Cards Identify Pairs Score
62. Pop Quiz (rest [1 2 3])➠ ____ (rest '(2 3))➠ ____ (rest '(3))➠ ____ (rest ())➠ ____ (2 3) (3) () ()
63. (take 5 (iterate rest [1 2 3])) ([1 2 3] (2 3) (3) () ()) ➠
64. (defn score-pairs "Finds all pairs in the hand, returning a scoring map for each one." [hand] (for [sub-hand (take (-> hand count dec) (iterate rest hand)) :let [left (first sub-hand)] right (rest sub-hand) :when (= (:rank left) (:rank right))] (score 2 "a pair" [left right])))
65. (defn score-pairs "Finds all pairs in the hand, returning a scoring map for each one." [hand] (for [sub-hand (take (-> hand count dec) (iterate rest hand)) :let [left (first sub-hand)] right (rest sub-hand) :when (= (:rank left) (:rank right))] (score 2 "a pair" [left right])))
66. (for [suit [:hearts :diamonds :spades :clubs] rank [:ace 2 3 4 5 6 7 8 9 10 :jack :queen :king]] {:suit suit :rank rank}) ({:suit :hearts, :rank :ace} {:suit :hearts, :rank 2} … {:suit :clubs, :rank :queen} {:suit :clubs, :rank :king}) ➠
67. (defn score-pairs "Finds all pairs in the hand, returning a scoring map for each one." [hand] (for [sub-hand (take (-> hand count dec) (iterate rest hand)) :let [left (first sub-hand)] right (rest sub-hand) :when (= (:rank left) (:rank right))] (score 2 "a pair" [left right])))
68. (defn score-pairs "Finds all pairs in the hand, returning a scoring map for each one." [hand] (for [sub-hand (take (-> hand count dec) (iterate rest hand)) :let [left (first sub-hand)] right (rest sub-hand) :when (= (:rank left) (:rank right))] (score 2 "a pair" [left right])))
69. (defn score-pairs "Finds all pairs in the hand, returning a scoring map for each one." [hand] (for [sub-hand (take (-> hand count dec) (iterate rest hand)) :let [left (first sub-hand)] right (rest sub-hand) :when (= (:rank left) (:rank right))] (score 2 "a pair" [left right])))
70. (defn score-pairs "Finds all pairs in the hand, returning a scoring map for each one." [hand] (for [sub-hand (take (-> hand count dec) (iterate rest hand)) :let [left (first sub-hand)] right (rest sub-hand) :when (= (:rank left) (:rank right))] (score 2 "a pair" [left right])))
71. (defn score-pairs "Finds all pairs in the hand, returning a scoring map for each one." [hand] (for [sub-hand (take (-> hand count dec) (iterate rest hand)) :let [left (first sub-hand)] right (rest sub-hand) :when (= (:rank left) (:rank right))] (score 2 "a pair" [left right])))
72. (defn score-pairs "Finds all pairs in the hand, returning a scoring map for each one." [hand] (for [sub-hand (take (-> hand count dec) (iterate rest hand)) :let [left (first sub-hand)] right (rest sub-hand) :when (= (:rank left) (:rank right))] (score 2 "a pair" [left right]))) Match Cards Identify Pairs Score
74. Extending Clojure in Clojure
76. private final Lock lock = new ReentrantReadWriteLock(); ! public String getCachedValue() { try { lock.readLock().lock(); ! … } finally { lock.readLock().unlock(); } }
77. private final Lock lock = new ReentrantReadWriteLock(); ! public String getCachedValue() { withReadLock(lock) { … } }
78. Who owns the Java language?
79. Brian Goetz
80. Not You
81. Caution: Head Exploding Zone
82. Literals
 
 "Hello" 2.5 nil Vectors
 
 [ … ] Lists 
 '(1 2 3) Maps
 ! { … } Function Calls, Special Forms, Macros (a b c) Sets
 ! #{ … } Forms
83. Bytecode Generation Code Forms Macro Expansion def if let fn … .
84. (defn check-cache [key] (with-read-lock ) (read-from-cache! cache key) cache-lock )
85. (defn check-cache [key] ) (read-from-cache! cache key) (let [… ] (try … (finally …)) cache-lock )
86. (defmacro with-read-lock [lock & body] `(let [read-lock# (.readLock ~lock)] (try (.lock read-lock#) ~@body (finally (.unlock read-lock#)))))
87. (defmacro with-read-lock [lock & body] `(let [read-lock# (.readLock ~lock)] (try (.lock read-lock#) ~@body (finally (.unlock read-lock#)))))
88. (defmacro with-read-lock [lock & body] `(let [read-lock# (.readLock ~lock)] (try (.lock read-lock#) ~@body (finally (.unlock read-lock#))))) Syntax Quote
89. (defmacro with-read-lock [lock & body] `(let [read-lock# (.readLock ~lock)] (try (.lock read-lock#) ~@body (finally (.unlock read-lock#))))) read-lock__1234__auto__
90. (defmacro with-read-lock [lock & body] `(let [read-lock# (.readLock ~lock)] (try (.lock read-lock#) ~@body (finally (.unlock read-lock#)))))
91. (defmacro with-read-lock [lock & body] `(let [read-lock# (.readLock ~lock)] (try (.lock read-lock#) ~@body (finally (.unlock read-lock#)))))
92. (defmacro with-read-lock [lock & body] `(let [read-lock# (.readLock ~lock)] (try (.lock read-lock#) ~@body (finally (.unlock read-lock#)))))
93. “Lisp isn't a language, it's a building material” ! —Alan Kay
 Co-creator of Smalltalk
94. 1st Rule Of Macros Don't Write 
 Macros
95. (defn with-read-lock [lock f] (let [read-lock (.readLock lock)] (try (.lock read-lock) (f) (finally (.unlock read-lock))))) (with-read-lock my-lock #(println my-lock))
96. Beyond the language
115. Wrap up
116. Not Covered Here Concurrency — Vars, Atoms, and Refs ClojureScript — Clojure in the Browser Datomic — The Database as a Value Ring, Compojure — Web and API Servers Pedestal, Reagent, Om, Hoplon — Client Frameworks core.test, midje, speclj — testing frameworks 100's of built-in functions and macros
117. Howard's Workflow 1. Terrific Idea! 2. Implement 3. Test 4. Gloat/Tweet 5. Discover it's already in clojure.core 6. Repeat
118. Power Focus Simplicity ⊙ Macros ⊙ Java Interop ⊙ Syntax ⊙ Immutability ⊙ REPL ⊙ clojure.core
119. “Simplicity and elegance are unpopular because they require hard work and discipline to achieve and education to be appreciated.” ! —Edsger Dijkstra
120. http://www.flickr.com/photos/39997856@N03/10865983036/ http://www.flickr.com/photos/ericparker/5555616243/ http://thenounproject.com/term/gears/2174/ http://www.flickr.com/photos/64958688@N00/3604756480/ http://www.flickr.com/photos/manugomi/2884678938/ http://www.flickr.com/photos/7955505@N05/489992128/ http://www.flickr.com/photos/44718043@N06/4153625805/ http://www.flickr.com/photos/skrb/5107280055/ http://www.flickr.com/photos/subconscience/297682093/ http://xkcd.com/297/ http://thenounproject.com/term/keyboard/19457
121. Links http://clojure.org http://clojure.org/cheatsheet http://kapeli.com/dash https://github.com/ technomancy/leiningen http://clojars.org http://www.lighttable.com/ https://nightcode.info/
 http://cursiveclojure.com/ https://code.google.com/p/ counterclockwise/ https://www.4clojure.com/ http://clojurekoans.com/ http://www.cognitect.com/ podcast http://howardlewisship.com
122. Q & A Howard M. Lewis Ship
 hlship@gmail.com @hlship