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© 2010 Howard Lewis Ship
Clojure: Functional Concurrency for the JVM
Howard M. Lewis ShipTWD [email protected]
1
© 2010 Howard Lewis Ship
Clojure: Why Functional Programming Matters
Howard M. Lewis ShipTWD [email protected]
2
© 2010 Howard Lewis Ship
Clojure: Towards the Essence of Programming
Howard M. Lewis ShipTWD [email protected]
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© 2010 Howard Lewis Ship
essence |ˈesəns|nounthe intrinsic nature or indispensable quality of something, esp. something abstract, that determines its character : conflict is the essence of drama.
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© 2010 Howard Lewis Ship
Mainstream Programming
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Operating System
Language
Libraries
Frameworks
Applications
© 2010 Howard Lewis Ship
Ceremony vs.
Essence6
© 2010 Howard Lewis Ship
Is your language ...
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© 2010 Howard Lewis Ship8
… holding you back?
© 2010 Howard Lewis Ship
Java: Data Encapsulated in Objects
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Stockticker: AAPL
lastTrade: 203.25open: 204.50shares: 100
Stockticker: MSFT
lastTrade: 29.12open: 29.08shares: 50
Stockticker: ORCL
lastTrade: 21.90open: 21.83shares: 200
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); }
© 2010 Howard Lewis Ship
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
© 2010 Howard Lewis Ship
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}]
© 2010 Howard Lewis Ship
Clojure: The Language
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(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)
© 2010 Howard Lewis Ship13
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!
© 2010 Howard Lewis Ship
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
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© 2010 Howard Lewis Ship
Java: Complex Structure
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int result = a * b - start + offset;
Higher precendence, then left to right: ((a * b) - start) + offset
*
a b
-
+
start
offset(+ (- (* a b) start)) offset)
© 2010 Howard Lewis Ship
Clojure: Form Is
Structure16
© 2010 Howard Lewis Ship
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
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Operating System
JVM
Java Libraries
User Classes
ClojureRepl Input
Clojure Source Files
Java Compiler
Source Code
Evaluator
© 2010 Howard Lewis Ship
REPL and Compilation
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© 2010 Howard Lewis Ship
Clojure Literals
user=> 4242user=> "A Clojure String""A Clojure String"user=> \space\spaceuser=> \A\Auser=> nilniluser=> :balance:balanceuser=> truetrueuser=> falsefalse
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© 2010 Howard Lewis Ship
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
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© 2010 Howard Lewis Ship
Java Interop: Function Calls
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(.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
© 2010 Howard Lewis Ship
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
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© 2010 Howard Lewis Ship
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
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© 2010 Howard Lewis Ship
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
© 2010 Howard Lewis Ship
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
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© 2010 Howard Lewis Ship
Functional Programming
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© 2010 Howard Lewis Ship27
10 X = 120 PRINT X30 X = X + 140 GOTO 20
My First Program
No it doesn't!
© 2010 Howard Lewis Ship
No Mutable
State28
© 2010 Howard Lewis Ship
No Side Effects
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© 2010 Howard Lewis Ship
Purity30
© 2010 Howard Lewis Ship
Simplicity Predictability
Testability31
© 2010 Howard Lewis Ship
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
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© 2010 Howard Lewis Ship
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);}
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Essence
© 2010 Howard Lewis Ship
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
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(defn require-extension [ext] (fn [file-name] (= ext (last (split-string file-name ".")))))
© 2010 Howard Lewis Ship
Composing Functions (filter #(not (.startsWith % ".")) names)
function as parameter to function
(filter (require-extension "gz") names)
function as return value
composing functions
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!Closure Oriented Programming"
© 2010 Howard Lewis Ship
Java: Iterative Steps
36
© 2010 Howard Lewis Ship
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;}
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Stockticker: AAPL
lastTrade: 203.25open: 204.50shares: 100
Stockticker: MSFT
lastTrade: 29.12open: 29.08shares: 50
Stockticker: ORCL
lastTrade: 21.90open: 21.83shares: 200
© 2010 Howard Lewis Ship
Clojure: Flow of Transformations
38
© 2010 Howard Lewis Ship
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
© 2010 Howard Lewis Ship
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
© 2010 Howard Lewis Ship
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 Ship42
© 2010 Howard Lewis Ship
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
© 2010 Howard Lewis Ship
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
© 2010 Howard Lewis Ship
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
© 2010 Howard Lewis Ship
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?
© 2010 Howard Lewis Ship
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=>
© 2010 Howard Lewis Ship
!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
Language Ownership
49
© 2010 Howard Lewis Ship
Who Owns the Java Language?
50
James Gosling Mark Reinhold
© 2010 Howard Lewis Ship
Not You51
© 2010 Howard Lewis Ship
Control of the Compiler
52
Operating System
JVM
Java Libraries
User Classes
ClojureRepl Input
Clojure Source Files
Java Compiler
Source Code
Evaluator
© 2010 Howard Lewis Ship
Who Owns Clojure?
53
Rich Hickey
© 2010 Howard Lewis Ship
Clojure: written in Clojure
54
© 2010 Howard Lewis Ship
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
© 2010 Howard Lewis Ship
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#))))
© 2010 Howard Lewis Ship
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#))))
© 2010 Howard Lewis Ship
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)
© 2010 Howard Lewis Ship
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)) ])
] ] ])
© 2010 Howard Lewis Ship
Embedded DSLs
60
:head
:html
:title
"Cascade Blog"
:h1
"Cascade Blog"
:ul
(template-for …)
:body
© 2010 Howard Lewis Ship
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
© 2010 Howard Lewis Ship
!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
Wrap Up
63
© 2010 Howard Lewis Ship
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
© 2010 Howard Lewis Ship
Stuart Halloway
Pragmatic Bookshelf
http://pragprog.com/titles/shcloj/programming-clojure
65
© 2010 Howard Lewis Ship
http://java.ociweb.com/mark/clojure/article.html
66
© 2010 Howard Lewis Ship67
http://tapestryjava.blogspot.com
© 2010 Howard Lewis Ship
Object Oriented
68
© 2010 Howard Lewis Ship
Functional
69
© 2010 Howard Lewis Ship
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/
© 2010 Howard Lewis Ship
Image Credits© 2007 John Kannenberghttp://www.flickr.com/photos/jkannenberg/541057337/
© 2006 scott ogilviehttp://www.flickr.com/photos/scottog/100582274/
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© 2008 Ariel H.http://www.flickr.com/photos/fotosrotas/2730733412/