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  • 1. Clojure: Functional Concurrency for the JVM Howard M. Lewis Ship TWD Consulting hlship@gmail.com 1 © 2010 Howard Lewis Ship
  • 2. Clojure: Why Functional Programming Matters Howard M. Lewis Ship TWD Consulting hlship@gmail.com 2 © 2010 Howard Lewis Ship
  • 3. Clojure: Towards the Essence of Programming Howard M. Lewis Ship TWD Consulting hlship@gmail.com 3 © 2010 Howard Lewis Ship
  • 4. essence noun the intrinsic nature or indispensable quality of something, esp. something abstract, that determines its character : conflict is the essence of drama. 4 © 2010 Howard Lewis Ship
  • 5. Mainstream Programming Applications Frameworks Libraries Language Operating System 5 © 2010 Howard Lewis Ship
  • 6. Ceremony vs. Essence 6 © 2010 Howard Lewis Ship
  • 7. Is your language ... 7 © 2010 Howard Lewis Ship
  • 8. … holding you back? 8 © 2010 Howard Lewis Ship
  • 9. Java: Data Encapsulated in Objects Stock Stock Stock ticker: AAPL ticker: MSFT ticker: ORCL lastTrade: 203.25 lastTrade: 29.12 lastTrade: 21.90 open: 204.50 open: 29.08 open: 21.83 shares: 100 shares: 50 shares: 200 public static void sortByLastTrade(List<Stock> portfolio) { Comparator<Stock> c = new Comparator<Stock>() { public int compare(Stock o1, Stock o2) { public static void sortByOpen(List<Stock> portfolio) return o1.getLastTrade() - o2.getLastTrade(); { } Comparator<Stock> c = new Comparator<Stock>() }; { public int compare(Stock o1, Stock o2) Collections.sort(portfolio, c); { } return o1.getOpen() - o2.getOpen(); } }; Collections.sort(portfolio, c); } 9 © 2010 Howard Lewis Ship
  • 10. Clojure: Data in Maps and Lists :ticker AAPL :ticker MSFT :ticker ORCL :last-trade 203.25 :last-trade 29.12 :last-trade 21.90 { :open 204.50 } { :open 29.08 }{ :open 21.83 } :shares 100 :shares 50 :shares 200 user=> portfolio [{:ticker "AAPL", :last-trade 203.25M, :open 204.50M, :shares 100} {:ticker "MSFT", :last-trade 29.12M, :open 29.08M, :shares 50} {:ticker "ORCL", :last-trade 21.90M, :open 21.83M, :shares 200}] user=> (sort-by :last-trade portfolio) ({:ticker "ORCL", :last-trade 21.90M, :open 21.83M, :shares 200} {:ticker "MSFT", :last-trade 29.12M, :open 29.08M, :shares 50} {:ticker "AAPL", :last-trade 203.25M, :open 204.50M, :shares 100}) user=> (sort-by :shares portfolio) ({:ticker "MSFT", :last-trade 29.12M, :open 29.08M, :shares 50} {:ticker "AAPL", :last-trade 203.25M, :open 204.50M, :shares 100} {:ticker "ORCL", :last-trade 21.90M, :open 21.83M, :shares 200}) user=> 10 © 2010 Howard Lewis Ship
  • 11. Functional JavaScript var portfolio = [ { ticker: "AAPL", lastTrade: 203.25, open: 204.50}, { ticker: "MSFT", lastTrade: 29.12, open: 29.08 }, { ticker: "ORCL", lastTrade: 21.90, open: 21.83 } ] portfolio.sortBy(function (stock) { return stock.lastTrade }).toJSON() [{"ticker": "ORCL", "lastTrade": 21.9, "open": 21.83}, {"ticker": "MSFT", "lastTrade": 29.12, "open": 29.08}, {"ticker": "AAPL", "lastTrade": 203.25, "open": 204.5}] 11 © 2010 Howard Lewis Ship
  • 12. Clojure: The Language 12 © 2010 Howard Lewis Ship
  • 13. Reading Lisp (defn render-json "Renders JSON content (typically, a map or a seq) as the response. The response content type is set to "application/json". Returns true." [env json-value] (let [response (-> env :servlet-api :response)] (.setContentType response "application/json") (with-open [writer (.getWriter Aargh! response)] (binding [*out* writer] (print-json json-value)))) true) 13 © 2010 Howard Lewis Ship
  • 14. Code is Data Quoted list of '(1 2 3) numbers (biggest 5 42) Function call Function definition (defn biggest "Find the maximum of two numbers" [x y] (if (> x y) x y)) 14 © 2010 Howard Lewis Ship
  • 15. Java: Complex Structure int result = a * b - start + offset; Higher precendence, then left to right: ((a * b) - start) + offset + (+ - offset (- (* a b) * start start)) offset) a b 15 © 2010 Howard Lewis Ship
  • 16. Clojure: Form Is Structure 16 © 2010 Howard Lewis Ship
  • 17. Read Eval Print Loop user=> (defn biggest "Find the maximum of two numbers" [x y] (if (> x y) x y)) #=(var user/biggest) user=> (biggest 5 42) 42 user=> (doc biggest) ------------------------- user/biggest ([x y]) Find the maximum of two numbers nil user=> '(1 2 3) (1 2 3) user=> '(biggest 5 42) (biggest 5 42) user=> (first '(biggest 5 42)) biggest user=> (eval '(biggest 5 42)) 42 17 © 2010 Howard Lewis Ship
  • 18. REPL and Compilation Source Code Repl Input Clojure User Classes Java Evaluator Compiler Clojure Source Files Java Libraries JVM Operating System 18 © 2010 Howard Lewis Ship
  • 19. Clojure Literals user=> 42 42 user=> "A Clojure String" "A Clojure String" user=> space space user=> A A user=> nil nil user=> :balance :balance user=> true true user=> false false 19 © 2010 Howard Lewis Ship
  • 20. Clojure Literals user=> 5 5 user=> 5.001 5.001 user=> 22/7 22/7 user=> (* 2 22/7) 44/7 user=> (* 100000 100000 100000) 1000000000000000 user=> (+ 5. 0.000000000000000001) 5.0 user=> (+ 5.0M 0.000000000000000001M) 5.000000000000000001M 20 © 2010 Howard Lewis Ship
  • 21. Java Interop: Function Calls (.method-name receiver arguments…) (.setNamespaceAware factory true) (.. factory newSAXParser (parse src handler)) (.parse (.newSAXParser factory) src handler) factory.newSAXParser().parse(src, handler); 21 © 2010 Howard Lewis Ship
  • 22. Clojure Collections: Lists 4 lst user=> (def lst '(1 2 3)) #=(var user/lst) user=> lst (1 2 3) user=> (first lst) 1 1 user=> (rest lst) (2 3) user=> (conj lst 4) 2 (4 1 2 3) user=> (cons 4 lst) (4 1 2 3) 3 22 © 2010 Howard Lewis Ship
  • 23. Clojure Collections: Vectors user=> (def v [:moe :larry :curly]) #=(var user/v) user=> v [:moe :larry :curly] user=> (first v) :moe user=> (rest v) (:larry :curly) user=> (conj v :shemp) [:moe :larry :curly :shemp] user=> (cons :shemp v) (:shemp :moe :larry :curly) user=> v [:moe :larry :curly] user=> (v 1) :larry vector is a function of its indexes 23 © 2010 Howard Lewis Ship
  • 24. Clojure Collections: Maps user=> (def m {:first-name "Howard" :last-name "Lewis Ship"}) #=(var user/m) user=> m {:last-name "Lewis Ship", :first-name "Howard"} user=> (get m :last-name) "Lewis Ship" map is a user=> (m :last-name) function of "Lewis Ship" user=> (assoc m :company "TWD") its keys {:company "TWD", :last-name "Lewis Ship", :first-name "Howard"} user=> m {:last-name "Lewis Ship", :first-name "Howard"} user=> (:first-name m) "Howard" user=> (:ssn m) nil Keywords are functions, too! 24 © 2010 Howard Lewis Ship
  • 25. Clojure Collections: Sets user=> (def s #{"Howard" "Suzanne" "Molly" "Jim"}) #=(var user/s) user=> s #{"Howard" "Jim" "Molly" "Suzanne"} user=> (contains? s "Howard") true user=> (contains? s "howard") false set is a user=> (s "Howard") function of "Howard" its elements user=> (s "Rhys") nil user=> (conj s "Howard") #{"Howard" "Jim" "Molly" "Suzanne"} user=> (conj s "Scott") #{"Howard" "Jim" "Molly" "Suzanne" "Scott"} 25 © 2010 Howard Lewis Ship
  • 26. Functional Programming 26 © 2010 Howard Lewis Ship
  • 27. My First Program No it 10 X = 1 doesn't! 20 PRINT X 30 X = X + 1 40 GOTO 20 27 © 2010 Howard Lewis Ship
  • 28. No Mutable State 28 © 2010 Howard Lewis Ship
  • 29. No Side Effects 29 © 2010 Howard Lewis Ship
  • 30. Purity 30 © 2010 Howard Lewis Ship
  • 31. Simplicity Predictability Testability 31 © 2010 Howard Lewis Ship
  • 32. Functional Programming in Java public void saveOrUpdate(final Employee employee) { HibernateCallback callback = new HibernateCallback() { public Object doInHibernate(Session session) throws HibernateException,SQLException { session.saveOrUpdate(employee); return null; } }; hibernateTemplate.execute(callback); } Outer function controls the SwingUtilities.invokeLater(new Runnable() context: { public void run() • Thread { • Exception handling progressBar.setValue(progressBar.getValue() + 1); } • Parameters }); 32 © 2010 Howard Lewis Ship
  • 33. Functional Java Collections public interface Predicate<T> { boolean accept(T value); } public static <T> Collection<T> filter(Predicate<T> pred, Collection<T> coll) { Collection<T> out = new ArrayList<T>(); for (T item : coll) { if (pred.accept(item)) out.add(item); } return out; } return CollectionUtils.filter(new Predicate<String>() { public boolean accept(String value) { return !value.startsWith("."); Essence } }, names); 33 © 2010 Howard Lewis Ship
  • 34. Functional Clojure Collections Function Anonymous parameter function (filter #(not (.startsWith % ".")) names) Member access form user=> (def names ["fred" "barney" ".hidden" "wilma"]) #=(var user/names) user=> (filter #(not (.startsWith % ".")) names) ("fred" "barney" "wilma") user=> (remove #(.startsWith % ".") names) ("fred" "barney" "wilma") user=> 34 © 2010 Howard Lewis Ship
  • 35. Composing Functions (filter #(not (.startsWith % ".")) names) function as parameter to function (defn require-extension [ext] !Closure Oriented (fn [file-name] (= ext (last (split-string file-name "."))))) Programming" function as return value (filter (require-extension "gz") names) composing functions 35 © 2010 Howard Lewis Ship
  • 36. Java: Iterative Steps 36 © 2010 Howard Lewis Ship
  • 37. Java: Data Encapsulated in Objects Stock Stock Stock ticker: AAPL ticker: MSFT ticker: ORCL lastTrade: 203.25 lastTrade: 29.12 lastTrade: 21.90 open: 204.50 open: 29.08 open: 21.83 shares: 100 shares: 50 shares: 200 public static List<Double> getOpens(List<Stock> portfolio) { List<Double> result = new ArrayList<Double>(); for (Stock stock : portfolio) { result.add(stock.getOpen()); } return result; } 37 © 2010 Howard Lewis Ship
  • 38. Clojure: Flow of Transformations 38 © 2010 Howard Lewis Ship
  • 39. Clojure: Data in Transformable Collections :ticker AAPL :ticker MSFT :ticker ORCL :last-trade 203.25 :last-trade 29.12 :last-trade 21.90 { :open 204.50 } { :open 29.08 }{ :open 21.83 } :shares 100 :shares 50 :shares 200 user=> portfolio [{:ticker "AAPL", :last-trade 203.25M, :open 204.50M, :shares 100} {:ticker "MSFT", :last-trade 29.12M, :open 29.08M, :shares 50} {:ticker "ORCL", :last-trade 21.90M, :open 21.83M, :shares 200}] user=> (map :open portfolio) (204.50M 29.08M 21.83M) user=> (apply + (map #(* (:last-trade %) (:shares %)) portfolio)) 26161.00M user=> (map #(assoc % :delta (- (% :last-trade) (% :open))) portfolio) ({:delta -1.25M, :ticker "AAPL", :last-trade 203.25M, :open 204.50M, :shares 100} {:delta 0.04M, :ticker "MSFT", :last-trade 29.12M, :open 29.08M, :shares 50} {:delta 0.07M, :ticker "ORCL", :last-trade 21.90M, :open 21.83M, :shares 200}) user=> 39 © 2010 Howard Lewis Ship
  • 40. map La z y f :open acts as a function user=> (map :open portfolio) (204.5 29.08 21.83) user=> (defn last-trade-value [stock] (* (:last-trade stock) (:shares stock))) #'user/last-trade-value user=> (map last-trade-value portfolio) (20325.00M 1456.00M 4380.00M) user=> 40 © 2010 Howard Lewis Ship
  • 41. Laziness is a Virtue user=> (take 20 (iterate inc 1)) (1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20) user=> (take 20 (map * (iterate inc 1) (iterate inc 1))) (1 4 9 16 25 36 49 64 81 100 121 144 169 196 225 256 289 324 361 400) 41 © 2010 Howard Lewis Ship
  • 42. 42 © 2010 Howard Lewis Ship
  • 43. map La z y f N seqs # N parameters user=> (map #(assoc %1 :sort-index %2) (reverse (sort-by :last-trade portfolio)) (iterate inc 0)) ({:sort-index 0, :ticker "AAPL", :last-trade 203.25M, :open 204.50M, :shares 100} {:sort-index 1, :ticker "MSFT", :last-trade 29.12M, :open 29.08M, :shares 50} {:sort-index 2, :ticker "ORCL", :last-trade 21.90M, :open 21.83M, :shares 200}) user=> 43 © 2010 Howard Lewis Ship
  • 44. reduce f user=> (map #(* (% :last-trade) (% :shares)) portfolio) (20325.00M 1456.00M 4380.00M) user=> (reduce + (map #(* (% :last-trade) (% :shares)) portfolio)) 26161.00M user=> (reduce + (map #(* (% :last-trade) (% :shares)) portfolio)) (reduce + '(20325.00M 1456.00M 4380.00M)) (+ (+ 20325.00M 1456.00M) 4380.00M) (+ 21781.00M 4380.00M) 26161.00M 44 © 2010 Howard Lewis Ship
  • 45. reduce f user=> (def input-string "Clojure is a fascinating language with unique capabilities and total integration with Java.") #'user/input-string user=> (seq input-string) (C l o j u r e space i s space a space f a s c i n a t i n g space l a n g u a g e space w i t h space u n i q u e space c a p a b i l i t i e s space a n d space t o t a l space i n t e g r a t i o n space w i t h space J a v a .) user=> (reduce (fn [m k] (update-in m [k] #(inc (or % 0)))) Optional initial value {} (seq input-string)) {space 12, a 12, b 1, C 1, c 2, d 1, e 5, f 1, g 4, h 2, i 11, J 1, j 1, l 4, . 1, n 7, o 3, p 1, q 1, r 2, s 3, t 8, u 4, v 1, w 2} user=> 45 © 2010 Howard Lewis Ship
  • 46. filter / remove La z y f ? user=> (remove #(< 100 (% :shares)) portfolio) ({:ticker "AAPL", :last-trade 203.25, :open 204.5, :shares 100} {:ticker "ORCL", :last-trade 21.9, :open 21.83, :shares 57}) user=> (filter #(< 100 (% :shares)) portfolio) ({:ticker "MSFT", :last-trade 29.12, :open 29.08, :shares 125}) user=> 46 © 2010 Howard Lewis Ship
  • 47. for: list comprehension La z y user=> (for [suit [:hearts :clubs :spades :diamonds] value (range 1 4)] [suit value]) ([:hearts 1] [:hearts 2] [:hearts 3] [:clubs 1] [:clubs 2] [:clubs 3] [:spades 1] [:spades 2] [:spades 3] [:diamonds 1] [:diamonds 2] [:diamonds 3]) user=> (for [x (range 0 4) y (range 0 (inc x))] [x y]) ([0 0] [1 0] [1 1] [2 0] [2 1] [2 2] [3 0] [3 1] [3 2] [3 3]) user=> (for [x (range 0 9) :when (odd? x) y (range 1 (inc x))] [x y]) ([1 1] [3 1] [3 2] [3 3] [5 1] [5 2] [5 3] [5 4] [5 5] [7 1] [7 2] [7 3] [7 4] [7 5] [7 6] [7 7]) user=> 47 © 2010 Howard Lewis Ship
  • 48. !Somehow the idea of reusability got attached to object-oriented programming in the 1980s, and no amount of evidence to the contrary seems to be able to shake it free." Paul Graham © 2010 Howard Lewis Ship
  • 49. Language Ownership 49 © 2010 Howard Lewis Ship
  • 50. Who Owns the Java Language? James Gosling Mark Reinhold 50 © 2010 Howard Lewis Ship
  • 51. Not You 51 © 2010 Howard Lewis Ship
  • 52. Control of the Compiler Source Code Repl Input Clojure User Classes Java Evaluator Compiler Clojure Source Files Java Libraries JVM Operating System 52 © 2010 Howard Lewis Ship
  • 53. Who Owns Clojure? Rich Hickey 53 © 2010 Howard Lewis Ship
  • 54. Clojure: written in Clojure 54 © 2010 Howard Lewis Ship
  • 55. Clojure Macros short circuit at first false/nil Reader (and a b c d) Evaluator macro expansion and macro Bytecode (let [and__4422__auto__ a] (if and__4422__auto__ Generation (and b c d) and__4422__auto__)) Recursively expanded 55 © 2010 Howard Lewis Ship
  • 56. Macros # Special Forms Code Forms Macro Expansion def if let fn . … Bytecode Generation 56 © 2010 Howard Lewis Ship
  • 57. Macros are Special Functions (defmacro and "Evaluates exprs one at a time, from left to right. If a form returns logical false (nil or false), and returns that value and doesn't evaluate any of the other expressions, otherwise it returns the value of the last expr. (and) returns true." ([] true) ([x] x) ([x & next] `(let [and# ~x] (if and# (and ~@next) and#)))) • `(…) — Syntax Quote (allowing replacements) • and# — Generate a new unique symbol • ~x — Unquote x • ~@next — Unquote next and splice in multiple values 57 © 2010 Howard Lewis Ship
  • 58. Macros are Special Functions (defmacro and ([] true) ([x] x) (and a b c d) ([x & next] `(let [and# ~x] (if and# (and ~@next) and#)))) •x#a • next # '(b c d) • Inside syntax quote: • and# # and__4422__auto__ • ~x # a • ~next # '(b c d) • (and ~@next) # (and b c d) 58 © 2010 Howard Lewis Ship
  • 59. Simplifying Boilerplate Code public void testLink() { IMocksControl control = EasyMock.createControl(); HttpServletRequest request = control.newMock(HttpServletRequest.class); HttpServletResponse response = control.newMock(HttpServletResponse.class); EasyMock.expect(request.getContextPath()).andReturn("/ctx"); EasyMock.expect(response.encodeURL("/ctx/accounts/list")).andReturn("*encoded*"); control.replay(); assertEquals(…, "*encoded*"); control.verify(); } (deftest test-link (with-mocks [request HttpServletRequest response HttpServletResponse] (:train (expect .getContextPath request "/ctx") (expect .encodeURL response "/ctx/accounts/list" "*encoded*")) (:test (is (= (link request response list-accounts-with-loop) "*encoded*"))))) 59 © 2010 Howard Lewis Ship
  • 60. Embedded DSLs (defview root-index [env] :html [ :head [ :title [ "Cascade Blog" ] ] :body [ :h1 [ "Cascade Blog" ] :html :ul { :class "recent-postings" } [ (template-for [posting (recent-postings env)] :li [ (render-link env show-posting (posting :id) (posting :title)) ]) :head :body ] ] ]) :title :h1 :ul "Cascade Blog" "Cascade Blog" (template-for …) 60 © 2010 Howard Lewis Ship
  • 61. Macro Expansions (defn list-items [coll] (template (format "%d items" (count coll)) :ul {:class :item-list} [ (template-for [item coll] :li [item]))) Expand simple Extend Clojure placeholder to language from executable code within Clojure (defn list-items [coll] (cascade.internal.viewbuilder/combine (format "%d items" (count coll)) (cascade.dom/element-node :ul {:class :item-list} (cascade.internal.viewbuilder/combine (for [item coll] (cascade.dom/element-node :li nil (cascade.internal.viewbuilder/combine item))))))) 61 © 2010 Howard Lewis Ship
  • 62. !More than anything else, I think it is the ability of Lisp programs to manipulate Lisp expressions that sets Lisp apart … when I hear people complain about Lisp's parentheses, it sounds to my ears like someone saying: "I tried one of those bananas, which you say are so delicious. The white part was ok, but the yellow part was very tough and tasted awful."" Paul Graham © 2010 Howard Lewis Ship
  • 63. Wrap Up 63 © 2010 Howard Lewis Ship
  • 64. Clojure • 1.1 release: Dec 31 2009 • Simple, regular syntax • Improves on Lisp: vectors, maps, sets http://www.clojure.org • Fully integrates with Java • Impressive functional & concurrency support • Many features not covered here 64 © 2010 Howard Lewis Ship
  • 65. Stuart Halloway Pragmatic Bookshelf http://pragprog.com/titles/shcloj/programming-clojure 65 © 2010 Howard Lewis Ship
  • 66. http://java.ociweb.com/mark/clojure/article.html 66 © 2010 Howard Lewis Ship
  • 67. http://tapestryjava.blogspot.com 67 © 2010 Howard Lewis Ship
  • 68. Object Oriented 68 © 2010 Howard Lewis Ship
  • 69. Functional 69 © 2010 Howard Lewis Ship
  • 70. Image Credits © 2007 Jon Fife http://flickr.com/photos/good-karma/577632972/ © 2007 Casey Marshall http://www.flickr.com/photos/rsdio/497112391/ © 2009 Howard M. Lewis Ship http://www.flickr.com/photos/hlship/3603090614/ © 2009 Andrew Baird http://www.flickr.com/photos/scruffy/3708615414/ © 2008 Miles Sabin http://www.flickr.com/photos/montpelier/2915114545/ © 2003 A. Lipson http://www.andrewlipson.com/escher/relativity.html © 2007 Alan Chia http://flickr.com/photos/seven13avenue/2080281038/ © 2007 Woodley Wonderworks http://flickr.com/photos/wwworks/2222523486/ © Randall Munroe http://xkcd.com/297/ © 2008 Manu Gómez http://www.flickr.com/photos/manugomi/2884678938/ © 2008 Marcin Wichary http://www.flickr.com/photos/mwichary/2827326852/ © 2006 Marvin (PA) http://www.flickr.com/photos/mscolly/145052885/ 70 © 2010 Howard Lewis Ship
  • 71. Image Credits © 2007 John Kannenberg http://www.flickr.com/photos/jkannenberg/541057337/ © 2006 scott ogilvie http://www.flickr.com/photos/scottog/100582274/ © 2008 Ariel H. http://www.flickr.com/photos/fotosrotas/2730733412/ 71 © 2010 Howard Lewis Ship