Codemash-Clojure.pdf

© 2010 Howard Lewis Ship

Clojure: Functional Concurrency for the JVM

Howard M. Lewis ShipTWD [email protected]

1


Clojure: Why Functional Programming Matters


2


Clojure: Towards the Essence of Programming


3


essence |ˈesəns|nounthe intrinsic nature or indispensable quality of something, esp. something abstract, that determines its character : conflict is the essence of drama.

4


Mainstream Programming

5

Operating System

Language

Libraries

Frameworks

Applications


Ceremony vs.

Essence6


Is your language ...

7

© 2010 Howard Lewis Ship8

… holding you back?


Java: Data Encapsulated in Objects

9

Stockticker: AAPL

lastTrade: 203.25open: 204.50shares: 100

Stockticker: MSFT


Stockticker: ORCL


public static void sortByLastTrade(List<Stock> portfolio) { Comparator<Stock> c = new Comparator<Stock>() { public int compare(Stock o1, Stock o2) { return o1.getLastTrade() - o2.getLastTrade(); } };

Collections.sort(portfolio, c); }

public static void sortByOpen(List<Stock> portfolio) { Comparator<Stock> c = new Comparator<Stock>() { public int compare(Stock o1, Stock o2) { return o1.getOpen() - o2.getOpen(); } };

Collections.sort(portfolio, c); }


Clojure: Data in Maps and Lists

10

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=>

{ }:ticker AAPL

:last-trade 203.25

:open 204.50 { }:ticker MSFT

:last-trade 29.12

:open 29.08 { }:ticker ORCL

:last-trade 21.90

:open 21.83

:shares 100 :shares 50 :shares 200


Functional JavaScript

11

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}]


Clojure: The Language

12

(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 response)] (binding [*out* writer] (print-json json-value)))) true)


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 response)] (binding [*out* writer] (print-json json-value)))) true)

Aargh!


Code is Data

'(1 2 3)

(biggest 5 42)

(defn biggest "Find the maximum of two numbers" [x y] (if (> x y) x y))

Quoted list of numbers

Function call

Function definition

14


Java: Complex Structure

15

int result = a * b - start + offset;

Higher precendence, then left to right: ((a * b) - start) + offset

*

a b

-

+

start

offset(+ (- (* a b) start)) offset)


Clojure: Form Is

Structure16


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)42user=> (doc biggest)-------------------------user/biggest([x y]) Find the maximum of two numbersniluser=> '(1 2 3)(1 2 3)user=> '(biggest 5 42)(biggest 5 42)user=> (first '(biggest 5 42))biggestuser=> (eval '(biggest 5 42))42

17

Operating System

JVM

Java Libraries

User Classes

ClojureRepl Input

Clojure Source Files

Java Compiler

Source Code

Evaluator


REPL and Compilation

18


Clojure Literals

user=> 4242user=> "A Clojure String""A Clojure String"user=> \space\spaceuser=> \A\Auser=> nilniluser=> :balance:balanceuser=> truetrueuser=> falsefalse

19


Clojure Literals

user=> 55user=> 5.0015.001user=> 22/722/7user=> (* 2 22/7)44/7user=> (* 100000 100000 100000)1000000000000000user=> (+ 5. 0.000000000000000001)5.0user=> (+ 5.0M 0.000000000000000001M)5.000000000000000001M

20


Java Interop: Function Calls

21

(.setNamespaceAware factory true)

(.method-name receiver arguments…)

(.parse (.newSAXParser factory) src handler)

(.. factory newSAXParser (parse src handler))

factory.newSAXParser().parse(src, handler);

1

2

3

lst


Clojure Collections: Lists

user=> (def lst '(1 2 3))#=(var user/lst)user=> lst(1 2 3)user=> (first lst)1user=> (rest lst)(2 3)user=> (conj lst 4)(4 1 2 3)user=> (cons 4 lst)(4 1 2 3)

4

22


Clojure Collections: Vectors

user=> (def v [:moe :larry :curly])#=(var user/v)user=> v[:moe :larry :curly]user=> (first v):moeuser=> (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


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=> (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=> (:first-name m)"Howard"user=> (:ssn m)nil

map is a function of its keys

Keywords are functions, too!

24


Clojure Collections: Sets

user=> (def s #{"Howard" "Suzanne" "Molly" "Jim"})#=(var user/s)user=> s#{"Howard" "Jim" "Molly" "Suzanne"}user=> (contains? s "Howard")trueuser=> (contains? s "howard")falseuser=> (s "Howard")"Howard"user=> (s "Rhys")niluser=> (conj s "Howard")#{"Howard" "Jim" "Molly" "Suzanne"}user=> (conj s "Scott")#{"Howard" "Jim" "Molly" "Suzanne" "Scott"}

set is a function of its elements

25


Functional Programming

26


10 X = 120 PRINT X30 X = X + 140 GOTO 20

My First Program

No it doesn't!


No Mutable

State28


No Side Effects

29


Purity30


Simplicity Predictability

Testability31


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);}

SwingUtilities.invokeLater(new Runnable() { public void run() { progressBar.setValue(progressBar.getValue() + 1); } });

Outer function controls the context:• Thread• Exception handling• Parameters

32


Functional Java Collections

return CollectionUtils.filter(new Predicate<String>(){ public boolean accept(String value) { return !value.startsWith("."); }}, names);

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;}

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

33

Essence


Functional Clojure Collections

(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=>

Anonymous function

Function parameter

34

(defn require-extension [ext] (fn [file-name] (= ext (last (split-string file-name ".")))))


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

function as parameter to function

(filter (require-extension "gz") names)

function as return value

composing functions

35

!Closure Oriented Programming"


Java: Iterative Steps

36


Java: Data Encapsulated in Objects

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

Stockticker: AAPL


Stockticker: MSFT


Stockticker: ORCL



Clojure: Flow of Transformations

38


Clojure: Data in Transformable Collections

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))26161.00Muser=> (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

{ }:ticker AAPL

:last-trade 203.25

:open 204.50 { }:ticker MSFT

:last-trade 29.12

:open 29.08 { }:ticker ORCL

:last-trade 21.90

:open 21.83

:shares 100 :shares 50 :shares 200


Lazymap

40

user=> (map :open portfolio)(204.5 29.08 21.83)user=> (defn last-trade-value [stock] (* (:last-trade stock) (:shares stock)))#'user/last-trade-valueuser=> (map last-trade-value portfolio)(20325.00M 1456.00M 4380.00M)user=>

:open acts as a function

f


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



Lazymap

43

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=>

N seqs # N parameters

f

f


reduce

user=> (map #(* (% :last-trade) (% :shares)) portfolio)(20325.00M 1456.00M 4380.00M)user=> (reduce + (map #(* (% :last-trade) (% :shares)) portfolio))26161.00Muser=>

(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


reduce

user=> (def input-string "Clojure is a fascinating language with unique capabilities and total integration with Java.")#'user/input-stringuser=> (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)))) {} (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=>

Optional initial value

f

45


Lazyfilter / remove

46

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=>

f?


Lazyfor: list comprehension

47

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=>


!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


Language Ownership

49


Who Owns the Java Language?

50

James Gosling Mark Reinhold


Not You51


Control of the Compiler

52

Operating System

JVM

Java Libraries

User Classes

ClojureRepl Input

Clojure Source Files

Java Compiler

Source Code

Evaluator


Who Owns Clojure?

53

Rich Hickey


Clojure: written in Clojure

54


Clojure Macros

55

Reader

Evaluator

Bytecode Generation

macro expansion

(and a b c d)

short circuit at first false/nil

and macro

(let [and__4422__auto__ a] (if and__4422__auto__ (and b c d) and__4422__auto__))

Recursively expanded


Macros # Special Forms

56

Code Forms

Macro Expansion

Bytecode Generation

def if let fn ….

(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#))))


Macros are Special Functions

57

• `(…) — Syntax Quote (allowing replacements)

• and# — Generate a new unique symbol

• ~x — Unquote x

• ~@next — Unquote next and splice in multiple values

(defmacro and ([] true) ([x] x) ([x & next] `(let [and# ~x] (if and# (and ~@next) and#))))


Macros are Special Functions

58

(and a b c d)

• 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)


Simplifying Boilerplate Code

59

(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*")))))

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();}

(defview root-index [env] :html [ :head [ :title [ "Cascade Blog" ] ] :body [ :h1 [ "Cascade Blog" ]

:ul { :class "recent-postings" } [ (template-for [posting (recent-postings env)]

:li [ (render-link env show-posting (posting :id) (posting :title)) ])

] ] ])


Embedded DSLs

60

:head

:html

:title

"Cascade Blog"

:h1

"Cascade Blog"

:ul

(template-for …)

:body


Macro Expansions

61

(defn list-items [coll] (template (format "%d items" (count coll)) :ul {:class :item-list} [ (template-for [item coll] :li [item])))

(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)))))))

Expand simple placeholder to

executable code

Extend Clojure language from within Clojure


!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


Wrap Up

63


Clojure

• 1.1 release: Dec 31 2009

• Simple, regular syntax

• Improves on Lisp: vectors, maps, sets

• Fully integrates with Java

• Impressive functional & concurrency support

• Many features not covered here

http://www.clojure.org

64


Stuart Halloway

Pragmatic Bookshelf

http://pragprog.com/titles/shcloj/programming-clojure

65


http://java.ociweb.com/mark/clojure/article.html

66


http://tapestryjava.blogspot.com


Object Oriented

68


Functional

69


Image Credits© 2007 Jon Fifehttp://flickr.com/photos/good-karma/577632972/

© 2003 A. Lipsonhttp://www.andrewlipson.com/escher/relativity.html

© 2007 Alan Chiahttp://flickr.com/photos/seven13avenue/2080281038/

© 2007 Woodley Wonderworkshttp://flickr.com/photos/wwworks/2222523486/

70

© Randall Munroehttp://xkcd.com/297/

© 2008 Manu Gómezhttp://www.flickr.com/photos/manugomi/2884678938/

© 2008 Marcin Wicharyhttp://www.flickr.com/photos/mwichary/2827326852/

© 2006 Marvin (PA)http://www.flickr.com/photos/mscolly/145052885/

© 2007 Casey Marshallhttp://www.flickr.com/photos/rsdio/497112391/

© 2009 Howard M. Lewis Shiphttp://www.flickr.com/photos/hlship/3603090614/

© 2009 Andrew Bairdhttp://www.flickr.com/photos/scruffy/3708615414/

© 2008 Miles Sabinhttp://www.flickr.com/photos/montpelier/2915114545/


Image Credits© 2007 John Kannenberghttp://www.flickr.com/photos/jkannenberg/541057337/

© 2006 scott ogilviehttp://www.flickr.com/photos/scottog/100582274/

71

© 2008 Ariel H.http://www.flickr.com/photos/fotosrotas/2730733412/

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