Clojure: Towards The Essence Of Programming (What's Next? Conference, May 2011)

Clojure: Towards The Essence Of ProgrammingHoward 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.

Operating System

Language

Libraries

Frameworks

Applications

Your code

Ceremony Vs. Essence

Is your language ...

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);}

{ }: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})

Clojure: The Language

(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!

Panic Ensues

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

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;}

24/234

24/375

public static float toFahrenheit(float celcius) { return 9 * celcius / 5 + 32;}

High Precedence

Low Precedence

9 celcius

(+ (/ (* 9 c) 5) 32)

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

Clojure: Form Is

Structure

(defn to-fahrenheit [celcius] (+ (/ (* 9 celcius) 5) 32))

(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

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

Operating System

Java Libraries

User Classes

ClojureRepl Input

Clojure Source Files

Java Compiler

Source Code

Evaluator

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

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

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

1user=> (conj lst 4)(4 1 2 3)user=> (cons 4 lst)(4 1 2 3)

conjoin: Add element to list

construct: new seq with new first element

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

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

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"}

Functional Programming

My First Program

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

"No it doesn't"-- Miss Dimascio

Immutability

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).

Predictable

Isolation From Time

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

(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

Java: Imperative Steps

Stockticker: AAPL

Stockticker: MSFT

Stockticker: ORCL

public static List<Double> getOpens(List<Stock> portfolio) { List<Double> result = new ArrayList<Double>();

for (Stock stock : portfolio) { result.add(stock.getOpen()); }

return result;}

Clojure: Flow of Transformations

{ }: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)

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)

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})

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

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=> (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

❝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.❞

Language Ownership

Who Owns The Java Language?

Mark ReinholdBrian Goetz

Not You

Operating System

Java Libraries

User Classes

ClojureRepl Input

Clojure Source Files

Java Compiler

Source Code

Evaluator

Who Owns Clojure?

Clojure In Clojure

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

(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

Java version of all-true

(defn all-true ([] true) ([x] x) ([x & more] (if x (apply all-true more) x)))

(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

Caution: Head Exploding Zone

Literals

"Hello" 2.5 nil

Vectors

[ … ]

'(1 2 3)

{ … }Function

Calls,Special Forms,Macros(a b c)

#{ … }

(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

Clojure Macros

Reader

Evaluator

Bytecode Generation

Macro Expansion

(if (if (.isPharaoh person) (if (.isDead person) (if (build-pyramid person) (.entomb person)))))

Macro Expansion

Approximate expansion of macro

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

(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

Bytecode Generation

Code Forms

Macro Expansion

def if let fn ….

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();}

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)) ])

] ] ])

:title

"Cascade Blog"

(template-for …)

(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

Wrap Up

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

❝Controlling complexity is the essence of computer

programming❞

Control is the Essence of

Programming

Evaluation

Language or Language Toolkit?

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

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

http://tapestryjava.blogspot.com

Clojure: Towards The Essence Of Programming (What's Next? Conference, May 2011)

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