Clojure:
Functional
Concurrency for
the JVM
Howard M. Lewis Ship
TWD Consulting
hlship@gmail.com
                1      © ...
Clojure: Why
Functional
Programming
Matters
Howard M. Lewis Ship
TWD Consulting
hlship@gmail.com
                2      © ...
Clojure: Towards
the Essence of
Programming
Howard M. Lewis Ship
TWD Consulting
hlship@gmail.com
                3      © ...
essence
noun
the intrinsic nature or indispensable quality of
something, esp. something abstract, that
determines its char...
Mainstream Programming

         Applications

         Frameworks

           Libraries

          Language

       Opera...
Ceremony
   vs.
 Essence
   6   © 2010 Howard Lewis Ship
Is your language ...




                7      © 2010 Howard Lewis Ship
… holding
    you back?
8        © 2010 Howard Lewis Ship
Java: Data Encapsulated in Objects
           Stock                  Stock                    Stock
        ticker: AAPL  ...
Clojure: Data in Maps and Lists
       :ticker     AAPL             :ticker        MSFT           :ticker     ORCL


     ...
Functional JavaScript




var portfolio = [ { ticker: "AAPL", lastTrade: 203.25, open: 204.50},
                  { ticker...
Clojure: The Language

            12          © 2010 Howard Lewis Ship
Reading Lisp

(defn render-json
  "Renders JSON content (typically, a map or a seq) as the
   response. The response conte...
Structural View
  defn render-json env json-value
    let
      response -> env :servlet-api :response

     .setContentTy...
Java: Complex Structure
int f = 9 * c / 5 + 32;



      Higher precendence, then left to right: ((9   * c) / 5) + 32




...
Clojure:
 Form Is
Structure
    16   © 2010 Howard Lewis Ship
Code is Data

                 Quoted list of
'(1 2 3)         numbers




(biggest 5 42)                Function call

  ...
Read Eval Print Loop


       user=> (defn biggest
         "Find the maximum of two numbers"
         [x y]
         (if ...
REPL and Compilation


                                                  Source Code

Repl Input                 Clojure
 ...
Clojure Literals


        user=> "A Clojure String"
        "A Clojure String"
        user=> space
        space
       ...
Numeric Literals


          user=> 5
          5
          user=> 5.001
          5.001
  Ratio   user=> 22/7
          2...
Java Interop: Function Calls
    (.method-name receiver arguments…)


    factory.setNamespaceAware(true)




    factory....
Clojure Collections: Lists




                            4
                                     lst
user=> (def lst '(1 ...
Clojure Collections: Vectors

       user=> (def v [:moe :larry :curly])
       #=(var user/v)
       user=> v
       [:mo...
Clojure Collections: Maps



user=> (def m {:first-name "Howard" :last-name "Lewis Ship"})
#=(var user/m)
user=> m
{:last-...
Clojure Collections: Sets


user=> (def s #{"Howard" "Suzanne" "Molly" "Jim"})
#=(var user/s)
user=> s
#{"Howard" "Jim" "M...
Functional Programming

            27      © 2010 Howard Lewis Ship
My First Program



                            No it
         10   X = 1       doesn't!
         20   PRINT X
         30...
No
Mutable
 State
   29   © 2010 Howard Lewis Ship
No Side
Effects
   30     © 2010 Howard Lewis Ship
Purity
  31     © 2010 Howard Lewis Ship
Simplicity
Predictability
 Testability
      32    © 2010 Howard Lewis Ship
Functional Java Collections

public interface Predicate<T>
{
  boolean accept(T value);
}

public static <T> Collection<T>...
Functional Clojure Collections
                          Function
          Anonymous       parameter
          function

...
Composing Functions
(filter #(not (.startsWith % ".")) names)




                     function as
                     pa...
Java: Iterative Steps




                   36   © 2010 Howard Lewis Ship
Java: Data Encapsulated in
Objects
                Stock                Stock                Stock
             ticker: AA...
Clojure: Flow of Transformations




                 38           © 2010 Howard Lewis Ship
Clojure: Data in Transformable
Collections
       :ticker     AAPL             :ticker          MSFT           :ticker    ...
Clojure: Data in Transformable
Collections
       :ticker     AAPL             :ticker          MSFT           :ticker    ...
map




                                                               La
                                                ...
Laziness is
                                             a Virtue




user=>   (take   20 (iterate inc 1))
(1 2 3   4 5 6 ...
43   © 2010 Howard Lewis Ship
map




                                                                             La
                                  ...
reduce

                                               f
user=> (map #(* (% :last-trade) (% :shares)) portfolio)
(20325.00...
reduce


                                                         f
                      user=> (def input-string "Clojur...
filter / remove




                                                                      La
                              ...
for: list comprehension




                                                           La
                                ...
❝Somehow the idea of reusability
got attached to object-oriented
programming in the 1980s, and
no amount of evidence to th...
Language Ownership

           50        © 2010 Howard Lewis Ship
Who Owns the Java Language?




    James Gosling        Mark Reinhold




                    51              © 2010 Howa...
Not You
   52     © 2010 Howard Lewis Ship
Control of the Compiler


                                                  Source Code

Repl Input                 Clojur...
Who Owns Clojure?




            Rich Hickey




                54        © 2010 Howard Lewis Ship
Clojure: written in Clojure




                 55           © 2010 Howard Lewis Ship
Short Circuits
                 && stops with first false


if (person.isPharaoh() &&                                   (if...
Caution:
Head Exploding Zone
            57        © 2010 Howard Lewis Ship
Clojure Macros
                                       short circuit at
                                       first false/n...
Macros ➠ Special Forms
                   Code Forms




                     Macro
                   Expansion




     ...
Macros are Special Functions
(defmacro and
  "Evaluates exprs one at a time, from left to right. If a form
  returns logic...
Macros are Special Functions
(defmacro and
  ([] true)
  ([x] x)
                                            (and a b c d)...
Simplifying Boilerplate Code
public void testLink()
{
  IMocksControl control = EasyMock.createControl();
  HttpServletReq...
Embedded DSLs


 (defview root-index
   [env]
   :html [
     :head [
       :title [ "Cascade Blog" ]
     ]
     :body [...
Macro Expansions
           (defn list-items [coll]
             (template
               (format "%d items" (count coll))...
❝More than anything else, I think it is the
ability of Lisp programs to manipulate
Lisp expressions that sets Lisp apart …...
Wrap Up

          66   © 2010 Howard Lewis Ship
essence
noun
the intrinsic nature or indispensable quality of
something, esp. something abstract, that
determines its char...
Control is the
  Essence of
Programming
      68    © 2010 Howard Lewis Ship
Evaluation
    69   © 2010 Howard Lewis Ship
If
When
How
 70    © 2010 Howard Lewis Ship
Clojure
• 1.1 release: Dec 31 2009

• Simple, regular syntax

• Improves on Lisp: vectors, maps, sets
                    ...
Stuart Halloway

                               Pragmatic Bookshelf




http://pragprog.com/titles/shcloj/programming-cloj...
http://java.ociweb.com/mark/clojure/article.html




                      73                  © 2010 Howard Lewis Ship
http://tapestryjava.blogspot.com




                        74         © 2010 Howard Lewis Ship
Object Oriented




                  75   © 2010 Howard Lewis Ship
Functional




             76   © 2010 Howard Lewis Ship
Image Credits
  © 2007 Jon Fife
  http://flickr.com/photos/good-karma/577632972/
                                          ...
Image Credits
  © 2007 John Kannenberg
  http://www.flickr.com/photos/jkannenberg/541057337/
                              ...
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Clojure: Towards The Essence of Programming

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Clojure is wonderful new language for the JVM that's really making waves: a functional Lisp dialect with full interoperation with Java, and a suite of excellent concurrency utilities ... but it's more than that.

Clojure is a highly expressive and highly adaptable language that allows you to focus on your craft in a way traditional languages such as Java and C# do not. In this session, we'll introduce the language and its core utilities and then introduce the fundamental concepts of functional programming as they apply to Clojure. Clojure has been described as a "high level language beamed back from the near future" ... come see what your future may hold, and why you're really going to like it!

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  • Looks like a great talk indeed.

    But the last two slides, the fuzzy Lego 'World' comparision, destroyed it a bit. In fact Clojure, with it's pure functions, is much more like the solid lego pieces who are combinable in many imaginative ways.

    I would compare Clojure with the lego and OO with building a sandcastle. Even though you might have sandboxes to form your sandinstances in OO, it takes a continious effort to keep the whole thing somewow stable. And even then one is still not able to ensure stability and a predicable beheaviour...
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  • hi, great slides - very inspiring !
    But the download doesn't work - the file seems to be broken !?!
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Clojure: Towards The Essence of Programming

  1. 1. Clojure: Functional Concurrency for the JVM Howard M. Lewis Ship TWD Consulting hlship@gmail.com 1 © 2010 Howard Lewis Ship
  2. 2. Clojure: Why Functional Programming Matters Howard M. Lewis Ship TWD Consulting hlship@gmail.com 2 © 2010 Howard Lewis Ship
  3. 3. Clojure: Towards the Essence of Programming Howard M. Lewis Ship TWD Consulting hlship@gmail.com 3 © 2010 Howard Lewis Ship
  4. 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. 5. Mainstream Programming Applications Frameworks Libraries Language Operating System 5 © 2010 Howard Lewis Ship
  6. 6. Ceremony vs. Essence 6 © 2010 Howard Lewis Ship
  7. 7. Is your language ... 7 © 2010 Howard Lewis Ship
  8. 8. … holding you back? 8 © 2010 Howard Lewis Ship
  9. 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. 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. 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. 12. Clojure: The Language 12 © 2010 Howard Lewis Ship
  13. 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. 14. Structural View defn render-json env json-value let response -> env :servlet-api :response .setContentType response "application/json" with-open writer .getWriter response binding *out* writer print-json json-value 14 © 2010 Howard Lewis Ship
  15. 15. Java: Complex Structure int f = 9 * c / 5 + 32; Higher precendence, then left to right: ((9 * c) / 5) + 32 + (+ / 32 (/ (* 9 c) * 5 5) 32) 9 c 15 © 2010 Howard Lewis Ship
  16. 16. Clojure: Form Is Structure 16 © 2010 Howard Lewis Ship
  17. 17. 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)) 17 © 2010 Howard Lewis Ship
  18. 18. 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 18 © 2010 Howard Lewis Ship
  19. 19. REPL and Compilation Source Code Repl Input Clojure User Classes Java Evaluator Compiler Clojure Source Files Java Libraries JVM Operating System 19 © 2010 Howard Lewis Ship
  20. 20. Clojure Literals user=> "A Clojure String" "A Clojure String" user=> space space user=> A A user=> nil Java null nil user=> :balance :balance user=> true true user=> false false 20 © 2010 Howard Lewis Ship
  21. 21. Numeric Literals user=> 5 5 user=> 5.001 5.001 Ratio user=> 22/7 22/7 user=> (* 2 22/7) 44/7 user=> (* 100000 100000 100000) BigInteger 1000000000000000 user=> (+ 5. 0.000000000000000001) 5.0 user=> (+ 5.0M 0.000000000000000001M) 5.000000000000000001M BigDecimal 21 © 2010 Howard Lewis Ship
  22. 22. Java Interop: Function Calls (.method-name receiver arguments…) factory.setNamespaceAware(true) factory.newSaxParser().parse(src, handler) ( .. factory newSaxParser ( (.. (parse src handler)) )) 22 © 2010 Howard Lewis Ship
  23. 23. 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 23 © 2010 Howard Lewis Ship
  24. 24. 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] 24 © 2010 Howard Lewis Ship
  25. 25. 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" user=> (assoc m :company "TWD") {:company "TWD", :last-name "Lewis Ship", :first-name "Howard"} user=> m {:last-name "Lewis Ship", :first-name "Howard"} user=> (get m:ssn) nil 25 © 2010 Howard Lewis Ship
  26. 26. 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 user=> (conj s "Howard") #{"Howard" "Jim" "Molly" "Suzanne"} user=> (conj s "Scott") #{"Howard" "Jim" "Molly" "Suzanne" "Scott"} 26 © 2010 Howard Lewis Ship
  27. 27. Functional Programming 27 © 2010 Howard Lewis Ship
  28. 28. My First Program No it 10 X = 1 doesn't! 20 PRINT X 30 X = X + 1 40 GOTO 20 28 © 2010 Howard Lewis Ship
  29. 29. No Mutable State 29 © 2010 Howard Lewis Ship
  30. 30. No Side Effects 30 © 2010 Howard Lewis Ship
  31. 31. Purity 31 © 2010 Howard Lewis Ship
  32. 32. Simplicity Predictability Testability 32 © 2010 Howard Lewis Ship
  33. 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. 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. 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. 36. Java: Iterative Steps 36 © 2010 Howard Lewis Ship
  37. 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. 38. Clojure: Flow of Transformations 38 © 2010 Howard Lewis Ship
  39. 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 #(get % :open) portfolio) (204.50M 29.08M 21.83M) user=> 39 © 2010 Howard Lewis Ship
  40. 40. 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=> (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=> 40 © 2010 Howard Lewis Ship
  41. 41. map La zy f keywords acts as functions (:keyword map) == (get map :keyword) 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=> 41 © 2010 Howard Lewis Ship
  42. 42. 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) 42 © 2010 Howard Lewis Ship
  43. 43. 43 © 2010 Howard Lewis Ship
  44. 44. map La zy 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=> 44 © 2010 Howard Lewis Ship
  45. 45. 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 45 © 2010 Howard Lewis Ship
  46. 46. 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=> 46 © 2010 Howard Lewis Ship
  47. 47. filter / remove La zy 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=> 47 © 2010 Howard Lewis Ship
  48. 48. for: list comprehension La zy 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=> 48 © 2010 Howard Lewis Ship
  49. 49. ❝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
  50. 50. Language Ownership 50 © 2010 Howard Lewis Ship
  51. 51. Who Owns the Java Language? James Gosling Mark Reinhold 51 © 2010 Howard Lewis Ship
  52. 52. Not You 52 © 2010 Howard Lewis Ship
  53. 53. Control of the Compiler Source Code Repl Input Clojure User Classes Java Evaluator Compiler Clojure Source Files Java Libraries JVM Operating System 53 © 2010 Howard Lewis Ship
  54. 54. Who Owns Clojure? Rich Hickey 54 © 2010 Howard Lewis Ship
  55. 55. Clojure: written in Clojure 55 © 2010 Howard Lewis Ship
  56. 56. Short Circuits && stops with first false if (person.isPharaoh() && (if person.isDead() && (all-true constructPyramid()) (.isPharaoh person) { (.isDead person) person.bury(); (construct-pyramid)) } (.bury person)) all parameters to function all-true evaluated first public static boolean allTrue(boolean... inputs) { for (boolean input : inputs) { if (!input) return false; } return true; } 56 © 2010 Howard Lewis Ship
  57. 57. Caution: Head Exploding Zone 57 © 2010 Howard Lewis Ship
  58. 58. 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 58 © 2010 Howard Lewis Ship
  59. 59. Macros ➠ Special Forms Code Forms Macro Expansion def if let fn . … Bytecode Generation 59 © 2010 Howard Lewis Ship
  60. 60. 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 60 © 2010 Howard Lewis Ship
  61. 61. 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) 61 © 2010 Howard Lewis Ship
  62. 62. 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*"))))) 62 © 2010 Howard Lewis Ship
  63. 63. 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 …) 63 © 2010 Howard Lewis Ship
  64. 64. 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))))))) 64 © 2010 Howard Lewis Ship
  65. 65. ❝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
  66. 66. Wrap Up 66 © 2010 Howard Lewis Ship
  67. 67. essence noun the intrinsic nature or indispensable quality of something, esp. something abstract, that determines its character : conflict is the essence of drama. 67 © 2010 Howard Lewis Ship
  68. 68. Control is the Essence of Programming 68 © 2010 Howard Lewis Ship
  69. 69. Evaluation 69 © 2010 Howard Lewis Ship
  70. 70. If When How 70 © 2010 Howard Lewis Ship
  71. 71. 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 71 © 2010 Howard Lewis Ship
  72. 72. Stuart Halloway Pragmatic Bookshelf http://pragprog.com/titles/shcloj/programming-clojure 72 © 2010 Howard Lewis Ship
  73. 73. http://java.ociweb.com/mark/clojure/article.html 73 © 2010 Howard Lewis Ship
  74. 74. http://tapestryjava.blogspot.com 74 © 2010 Howard Lewis Ship
  75. 75. Object Oriented 75 © 2010 Howard Lewis Ship
  76. 76. Functional 76 © 2010 Howard Lewis Ship
  77. 77. 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/ 77 © 2010 Howard Lewis Ship
  78. 78. 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/ © 2006 John Ryan Brubaker http://www.flickr.com/photos/subconscience/297682093/ 78 © 2010 Howard Lewis Ship
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