© 2011 Howard M. Lewis Ship Clojure: Towards The Essence Of Programming Howard M. Lewis Ship
Jan 17, 2015
© 2011 Howard M. Lewis Ship
Clojure: Towards The Essence Of ProgrammingHoward M. Lewis Ship
© 2011 Howard M. 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.
© 2011 Howard M. Lewis Ship
Operating System
Language
Libraries
Frameworks
Applications
© 2011 Howard M. Lewis Ship© 2011 Howard M. Lewis Ship
Your code
© 2011 Howard M. Lewis Ship
Ceremony Vs. Essence
© 2011 Howard M. Lewis Ship
Is your language ...
© 2011 Howard M. Lewis Ship… holding you back?
© 2011 Howard M. Lewis Ship
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);}
© 2011 Howard M. Lewis Ship
{ }: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
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=> (defn value-at-open [stock] …)#'user/value-at-openuser=> (sort-by value-at-open portfolio)({:ticker "MSFT", :last-trade 29.12M, :open 29.08M, :shares 50} {:ticker "ORCL", :last-trade 21.90M, :open 21.83M, :shares 200} {:ticker "AAPL", :last-trade 203.25M, :open 204.50M, :shares 100})
© 2011 Howard M. Lewis Ship
Clojure: The Language
© 2011 Howard M. Lewis Ship
(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!
© 2011 Howard M. Lewis Ship
(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)
Panic Ensues
Ouch!
© 2011 Howard M. Lewis Ship
© 2011 Howard M. Lewis Ship
defn render-json env json-value
letresponse -> env :servlet-api :response
.setContentType response "application/json"
with-open writer .getWriter response
binding *out* writer
print-json json-value
© 2011 Howard M. Lewis Ship
public static boolean renderJSON(Environment env, JSONObject object) { HttpServletResponse response = env.getServletAPI().getResponse(); response.setContentType("application/json"); Writer writer = null; try { writer = response.getWriter(); printJSON(writer, object); } finally { if (writer != null) { writer.close(); } } return true;}
© 2011 Howard M. Lewis Ship
public static boolean renderJSON(Environment env, JSONObject object) { HttpServletResponse response = env.getServletAPI().getResponse(); response.setContentType("application/json"); Writer writer = null; try { writer = response.getWriter(); printJSON(writer, object); } finally { if (writer != null) { writer.close(); } } return true;}
(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)
24/234
24/375
© 2011 Howard M. Lewis Ship
public static float toFahrenheit(float celcius) { return 9 * celcius / 5 + 32;}
High Precedence
Low Precedence
*
9 celcius
/
+
5
32
(+ (/ (* 9 c) 5) 32)
© 2011 Howard M. Lewis Ship
public static toFahrenheit(F)F L0 LINENUMBER 5 L0 LDC 9.0 FLOAD 0 FMUL LDC 5.0 FDIV LDC 32.0 FADD FRETURN L1 LOCALVARIABLE celcius F L0 L1 0 MAXSTACK = 2 MAXLOCALS = 1
public static float toFahrenheit(float celcius) { return 9 * celcius / 5 + 32;}
$ hexdump bin/org/example/Conversions.class 0000000 ca fe ba be 00 00 00 31 00 17 07 00 02 01 00 170000010 6f 72 67 2f 65 78 61 6d 70 6c 65 2f 43 6f 6e 760000020 65 72 73 69 6f 6e 73 07 00 04 01 00 10 6a 61 760000030 61 2f 6c 61 6e 67 2f 4f 62 6a 65 63 74 01 00 060000040 3c 69 6e 69 74 3e 01 00 03 28 29 56 01 00 04 430000050 6f 64 65 0a 00 03 00 09 0c 00 05 00 06 01 00 0f0000060 4c 69 6e 65 4e 75 6d 62 65 72 54 61 62 6c 65 010000070 00 12 4c 6f 63 61 6c 56 61 72 69 61 62 6c 65 540000080 61 62 6c 65 01 00 04 74 68 69 73 01 00 19 4c 6f0000090 72 67 2f 65 78 61 6d 70 6c 65 2f 43 6f 6e 76 6500000a0 72 73 69 6f 6e 73 3b 01 00 0c 74 6f 46 61 68 7200000b0 65 6e 68 65 69 74 01 00 04 28 46 29 46 04 41 1000000c0 00 00 04 40 a0 00 00 04 42 00 00 00 01 00 07 6300000d0 65 6c 63 69 75 73 01 00 01 46 01 00 0a 53 6f 7500000e0 72 63 65 46 69 6c 65 01 00 10 43 6f 6e 76 65 7200000f0 73 69 6f 6e 73 2e 6a 61 76 61 00 21 00 01 00 030000100 00 00 00 00 00 02 00 01 00 05 00 06 00 01 00 070000110 00 00 00 2f 00 01 00 01 00 00 00 05 2a b7 00 080000120 b1 00 00 00 02 00 0a 00 00 00 06 00 01 00 00 000000130 03 00 0b 00 00 00 0c 00 01 00 00 00 05 00 0c 000000140 0d 00 00 00 09 00 0e 00 0f 00 01 00 07 00 00 000000150 35 00 02 00 01 00 00 00 0b 12 10 22 6a 12 11 6e0000160 12 12 62 ae 00 00 00 02 00 0a 00 00 00 06 00 010000170 00 00 00 05 00 0b 00 00 00 0c 00 01 00 00 00 0b0000180 00 13 00 14 00 00 00 01 00 15 00 00 00 02 00 160000190
© 2011 Howard M. Lewis Ship
Clojure: Form Is
Structure
© 2011 Howard M. Lewis Ship
(defn to-fahrenheit [celcius] (+ (/ (* 9 celcius) 5) 32))
© 2011 Howard M. Lewis Ship
(defn biggest "Find the maximum of two numbers" [x y] (if (> x y) x y))
(biggest 5 42)
'(1 2 "three") List of values
Function call
Function definition
© 2011 Howard M. Lewis Ship
Read Eval Print Loopuser=> (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
© 2011 Howard M. Lewis Ship23
Operating System
JVM
Java Libraries
User Classes
ClojureRepl Input
Clojure Source Files
Java Compiler
Source Code
Evaluator
© 2011 Howard M. Lewis Ship
Simple Literals• Strings
• Characters
• nil
• true
• false
• integers
• floating point numbers
• ratios
user=> "A Clojure String""A Clojure String"user=> \space\spaceuser=> \A\Auser=> nilniluser=> truetrueuser=> falsefalseuser=> 00user=> 2.52.5user=> 22/722/7
nil is Java null
© 2011 Howard M. Lewis Ship
Keywords•Literal values
•Singleton instances
•Especially useful as map keys
user=> (identical? :a-keyword :a-keyword)trueuser=> (str "a" "-" "string")"a-string"user=> (identical? "a-string" (str "a" "-" "string"))falseuser=> (keyword "a-keyword"):a-keyworduser=> (identical? :a-keyword (keyword "a-keyword"))true
© 2011 Howard M. Lewis Ship
Lists
2
3
lst
1user=> (def lst '(1 2 3))#=(var user/lst)user=> lst(1 2 3)user=> (first lst)1user=> (rest lst)(2 3)
© 2011 Howard M. Lewis Ship
Lists
2
3
lst
1user=> (conj lst 4)(4 1 2 3)user=> (cons 4 lst)(4 1 2 3)
4
conjoin: Add element to list
construct: new seq with new first element
© 2011 Howard M. Lewis Ship
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]
first, rest and others work on lists, vectors or
any seq
© 2011 Howard M. Lewis Ship
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=> (:first-name m)"Howard"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
Keywords act as a function that takes a map
© 2011 Howard M. Lewis Ship
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=> (conj s "Howard")#{"Howard" "Jim" "Molly" "Suzanne"}user=> (conj s "Scott")#{"Howard" "Jim" "Molly" "Suzanne" "Scott"}
© 2011 Howard M. Lewis Ship
Functional Programming
© 2011 Howard M. Lewis Ship
My First Program
10 X = 120 PRINT X30 X = X + 140 GOTO 20
"No it doesn't"-- Miss Dimascio
© 2011 Howard M. Lewis Ship
Immutability
© 2011 Howard M. Lewis Ship
function |ˈfə ng k sh ən|
nounA function, in a mathematical sense, expresses the idea that one quantity (the argument of the function, also known as the input) completely determines another quantity (the value, or the output).
© 2011 Howard M. Lewis Ship
Predictable
© 2011 Howard M. Lewis Ship
Isolation From Time
© 2011 Howard M. Lewis Ship
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")
(filter #(not (.startsWith % ".")) names)
#(…) anonymous function
% anonymous parameter
(filter (fn [name] (not (.startsWith name "."))) names)
Inline anoymous function Java Interop
© 2011 Howard M. Lewis Ship
(defn require-extension [ext] (fn [file-name] (= ext (last (split-string file-name ".")))))
(filter #(not (.startsWith % ".")) names)
(filter (require-extension "gz") names)
Function as parameter
Function as return value
❝Closure Oriented
Programming❞Composing functions
© 2011 Howard M. Lewis Ship
Java: Imperative Steps
© 2011 Howard M. Lewis Ship
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 List<Double> getOpens(List<Stock> portfolio) { List<Double> result = new ArrayList<Double>();
for (Stock stock : portfolio) { result.add(stock.getOpen()); }
return result;}
© 2011 Howard M. Lewis Ship
Clojure: Flow of Transformations
© 2011 Howard M. Lewis Ship
{ }: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
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=> (map :open portfolio)(204.50M 29.08M 21.83M)
© 2011 Howard M. Lewis Ship
Laziness Is a Virtue
© 2011 Howard M. Lewis Ship
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)
© 2011 Howard M. Lewis Ship
© 2011 Howard M. Lewis Ship
Lazyf
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=> (map #(assoc % :last-trade-value (last-trade-value %)) portfolio)({:last-trade-value 20325.00M, :ticker "AAPL", :last-trade 203.25M, :open 204.50M, :shares 100} {:last-trade-value 1456.00M, :ticker "MSFT", :last-trade 29.12M, :open 29.08M, :shares 50} {:last-trade-value 4380.00M, :ticker "ORCL", :last-trade 21.90M, :open 21.83M, :shares 200})
© 2011 Howard M. Lewis Ship
f
user=> (map last-trade-value portfolio)(20325.00M 1456.00M 4380.00M)user=> (reduce + (map last-trade-value portfolio))26161.00Muser=> (reduce + 0 [])0user=> (reduce + nil)0 Initial
Value
© 2011 Howard M. Lewis Ship
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}
f
© 2011 Howard M. Lewis Ship
Lazy
user=> (range 0 4)(0 1 2 3)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])
List Comprehension
© 2011 Howard M. Lewis ShipPaul Graham
❝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.❞
© 2011 Howard M. Lewis Ship
Language Ownership
© 2011 Howard M. Lewis Ship
Who Owns The Java Language?
Mark ReinholdBrian Goetz
© 2011 Howard M. Lewis Ship
Not You
© 2011 Howard M. Lewis Ship
Operating System
JVM
Java Libraries
User Classes
ClojureRepl Input
Clojure Source Files
Java Compiler
Source Code
Evaluator
© 2011 Howard M. Lewis Ship
Who Owns Clojure?
© 2011 Howard M. Lewis Ship
Clojure In Clojure
© 2011 Howard M. Lewis Ship
if (person.isPharaoh() && person.isDead() && buildPyramid(person)) { person.entomb();}
boolean isPharoah = person.isPharoah();boolean isDead = person.isDead();boolean pyramidComplete = buildPyramid(person);
if (isPharoah && isDead && pyramidComplete) { person.entomb();}
Short Circuiting Evaluation
Everyone gets a Pyramid!
Dead Pharoahs get a Pyramid
© 2011 Howard M. Lewis Ship
(if (all-true (.isPharaoh person) (.isDead person) (build-pyramid person)) (.entomb person))
public static boolean allTrue(boolean... inputs) { for (boolean input : inputs) { if (!input) return false; }
return true;}
Function invocation: evaluate all parameters
first
Java version of all-true
(defn all-true ([] true) ([x] x) ([x & more] (if x (apply all-true more) x)))
© 2011 Howard M. Lewis Ship
(if (and (.isPharaoh person) (.isDead person) (build-pyramid person)) (.entomb person))
and short-circuits, so it's not a function
And what exactly is if ?
user=> (doc and)-------------------------clojure.core/and([] [x] [x & next])Macro 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.nil
© 2011 Howard M. Lewis Ship
Caution: Head Exploding Zone
© 2011 Howard M. Lewis Ship
Literals
"Hello" 2.5 nil
Vectors
[ … ]
Lists
'(1 2 3)
Maps
{ … }Function
Calls,Special Forms,Macros(a b c)
Sets
#{ … }
Forms
© 2011 Howard M. Lewis Ship
(if test then else?)
Evaluates test. If not the singular values nil or false, evaluates and yields then, otherwise, evaluates and yields else. If else is not supplied it defaults to nil. …
If: Special Formuser=> (doc if)-------------------------ifSpecial Form Please see http://clojure.org/special_forms#ifnil
© 2011 Howard M. Lewis Ship
Clojure Macros
Reader
Evaluator
Bytecode Generation
Macro Expansion
© 2011 Howard M. Lewis Ship
(if (and (.isPharaoh person) (.isDead person) (build-pyramid person)) (.entomb person))
(if (if (.isPharaoh person) (if (.isDead person) (if (build-pyramid person) (.entomb person)))))
Macro Expansion
Approximate expansion of macro
© 2011 Howard M. Lewis Ship
(defmacro and ([] true) ([x] x) ([x & next] `(let [and# ~x] (if and# (and ~@next) and#))))
(if (and (.isPharaoh person) (.isDead person) (build-pyramid person)) (.entomb person))
(if (let [and_4422_auto (.isPharaoh person)] (if and_4422_auto (and (.isDead person) (build-pyramid person)) and_4422_auto)) (.entomb person))
Evaluate (.isPharaoh person)
only once
Macro Expansion
© 2011 Howard M. Lewis Ship
Bytecode Generation
Code Forms
Macro Expansion
def if let fn ….
© 2011 Howard M. Lewis Ship
Boilerplate
(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();}
© 2011 Howard M. Lewis Ship
Domain Specific Languages(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)) ])
] ] ])
:head
:html
:title
"Cascade Blog"
:h1
"Cascade Blog"
:ul
(template-for …)
:body
© 2011 Howard M. Lewis Ship
(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 w
ithin C
lojure
© 2011 Howard M. Lewis Ship
Wrap Up
© 2011 Howard M. 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.
© 2011 Howard M. Lewis ShipBrian Kernigan
❝Controlling complexity is the essence of computer
programming❞
© 2011 Howard M. Lewis Ship
Control is the Essence of
Programming
© 2011 Howard M. Lewis Ship
Evaluation
© 2011 Howard M. Lewis Ship
Language or Language Toolkit?
© 2011 Howard M. Lewis Ship
© 2011 Howard M. Lewis Ship
Clojure•1.2 release: 19 Aug 2010
•Simple, regular syntax
• Improves on Lisp: vectors, maps, sets
•Fully integrates with Java
• Impressive functional & concurrency support
•Most features not covered here
http://www.clojure.org
© 2011 Howard M. Lewis Ship
© 2011 Howard M. Lewis Ship
http://java.ociweb.com/mark/clojure/article.html
© 2011 Howard M. Lewis Ship
http://tapestryjava.blogspot.com
© 2011 Howard M. Lewis Ship© 2011 Howard M. Lewis Ship
© 2011 Howard M. Lewis Ship© 2011 Howard M. Lewis Ship
© 2011 Howard M. Lewis Ship
Image Credits© 2007 John Kannenberghttp://www.flickr.com/photos/jkannenberg/541057337/
© 2008 Alexandre Pizzerahttp://www.flickr.com/photos/alecss/2563917055
© 2008 Marcin Wicharyhttp://www.flickr.com/photos/mwichary/2827326852/
© 2008 Jonathan Ziapourhttp://www.flickr.com/photos/jonathanziapour/2613204502/
© Randall Munroehttp://xkcd.com/297/
© 2010 yuichi.sakurabahttp://www.flickr.com/photos/skrb/5107280055
© 2007 Jon Fifehttp://flickr.com/photos/good-karma/577632972/
© 2008 Howard M. Lewis Shiphttp://www.flickr.com/photos/hlship/3108306576
© 2006 scott ogilviehttp://www.flickr.com/photos/scottog/100582274/
© 2008 Ariel H.http://www.flickr.com/photos/fotosrotas/2730733412/
© 2008 Manu Gómezhttp://www.flickr.com/photos/manugomi/2884678938/
© 2011 Howard M. Lewis Ship
Image Credits© 2009 Howard M. Lewis Shiphttp://www.flickr.com/photos/hlship/3603090614/
© 2003 A. Lipsonhttp://www.andrewlipson.com/escher/relativity.html
© 2006 John Ryan Brubakerhttp://www.flickr.com/photos/subconscience/297682093/
© 2007 Alan Chiahttp://flickr.com/photos/seven13avenue/2080281038/
© 2003 Randall Munroehttp://xkcd.com/224/