Clojure for OOP folks - Amazon S3s3-eu-west-1.amazonaws.com/presentations2013/30...A practical Lisp...

Clojure for OOP folks Stefan Tilkov | @stilkov | innoQ

http://xkcd.com/297/

Friday, May 17, 13

© 2012 innoQ Deutschland GmbH

OOP Thinkingmodel domains with classes & interfaces

encapsulate data in objects

prefer speci!c over generic solutions

explicitly provide for generic access

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27.04.10 14:17http://upload.wikimedia.org/wikipedia/en/1/1a/Clojure-glyph.svg

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Clojure 27.04.10 14:17http://upload.wikimedia.org/wikipedia/en/1/1a/Clojure-glyph.svg

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A practical Lisp variant for the JVM Functional programming

Dynamic Typing Full-featured macro system

Concurrent programming supportBi-directional Java interop

Immutable persistent data structures

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© 2012 innoQ Deutschland GmbHhttp://www.tbray.org/ongoing/When/200x/2008/09/25/-big/R0010774.jpg.html

Rich Hickey

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Data structuresNumbers 2 3 4 0.234

3/5 -2398989892820093093090292321

Strings "Hello" "World"

Characters \a \b \c

Keywords :first :last

Symbols a b c

Regexps #"Ch.*se"

Lists (a b c)((:first :last "Str" 3) (a b))

Vectors [2 4 6 9 23][2 4 6 [8 9] [10 11] 9 23]

Maps {:de "Deutschland", :fr "France"}

Sets #{"Bread" "Cheese" "Wine"}

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Syntax

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“You’ve just seen it” – Rich Hickey

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Syntax(def my-set #{:a :b :c :c :c}) ;; #{:a :b :c}(def v [2 4 6 9 23])(v 0) ;; 2(v 2) ;; 6

(def people {:pg "Phillip", :st "Stefan"})(people :st) ;; "Stefan"(:pg people) ;; "Phillip"(:xyz people) ;; nil(+ 2 2) ;; 4(+ 2 3 5 4) ;; 14(class (/ 4 3)) ;; clojure.lang.Ratio(* (/ 4 3) 3) ;; 4

(format "Hello, %s # %d" "world" 1)

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Syntax

; (a 2 3)(quote (a 2 3)) ;; (a 2 3)'(a 2 3) ;; (a 2 3)

; Evaluation (eval '(format "Hello, %s" "World"))(eval (read-string "(+ 2 2)"))

(format "Hello, %s # %d" "world" 1); "Hello, World # 1"

(apply format ["Hello, %s # %d" "world" 1])

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Functions(fn [x] (format "The value is %s\n" x));; user$eval__1706$fn__1707@390b755d

((fn [x] (format "The value is %s\n" x)) "Hello");; "The value is Hello"

(def testfn (fn [x] (format "The value is %s\n" x))) (testfn "Hello")

(defn testfn [x] (format "The value is %s\n" x))(testfn "Hello")

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Functions

(defn even [x] (= 0 (rem x 2))) (even 4) ;; true (def even-alias even)(even-alias 2) ;; true

(defn every-even? [l] (every? even l))(every-even? '(2 4 6 8 9)) ;; false(every? #(= 0 (rem % 2)) '(2 4 6 8 9)) ;; false

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Closures(defn make-counter [initial-value] (let [current-value (atom initial-value)] (fn [] (swap! current-value inc))))

(def counter1 (make-counter 0))(counter1) ;; 1(counter1) ;; 2

(def counter2 (make-counter 17))(counter1) ;; 3(counter2) ;; 18(counter1) ;; 4(counter2) ;; 19

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Recursion

(defn reduce-1 [f val coll] (if (empty? coll) val (reduce-1 f (f val (first coll)) (rest coll))))

(reduce-1 + 0 [1 2 3 4]) ;; 10(reduce-1 + 0 (range 5)) ;; 10(reduce-1 + 0 (range 50)) ;; 1225(reduce-1 + 0 (range 50000)) ;; java.lang.StackOverflowError

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(defn reduce-2 [f val coll] (if (empty? coll) val (recur f (f val (first coll)) (rest coll))))

(defn reduce-1 [f val coll] (if (empty? coll) val (reduce-1 f (f val (first coll)) (rest coll))))

Recursion

(reduce-2 + 0 [1 2 3 4]) ;; 10(reduce-2 + 0 (range 5)) ;; 10(reduce-2 + 0 (range 50)) ;; 1225(reduce-2 + 0 (range 50000)) ;; 1249975000

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Example(ns sample.grep "A simple complete Clojure program." (:use [clojure.contrib.io :only [read-lines]]) (:gen-class))

(defn numbered-lines [lines] (map vector (iterate inc 0) lines))

(defn grep-in-file [pattern file] {file (filter #(re-find pattern (second %)) (numbered-lines (read-lines file)))})

(defn grep-in-files [pattern files] (apply merge (map #(grep-in-file pattern %) files)))

(defn print-matches [matches] (doseq [[fname submatches] matches, [line-no, match] submatches] (println (str fname ":" line-no ":" match)))) (defn -main [pattern & files] (if (or (nil? pattern) (empty? files)) (println "Usage: grep <pattern> <file...>") (do (println (format "grep started with pattern %s and file(s) %s" pattern (apply str (interpose ", " files)))) (print-matches (grep-in-files (re-pattern pattern) files)) (println "Done."))))

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Lots of other cool stu" ‣ Persistent data structures

‣ Sequences

‣ Support for concurrent programming

‣ Destructuring

‣ List comprehensions

‣ Metadata

‣ Optiional type information

‣ Multimethods

‣ Pre & Post Conditions

‣ Records/Protocols

‣ Extensive core and contrib libraries

‣ …

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Syntax Idioms

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OOP Thinking

model domains with classes & interfaces

encapsulate data in objects

prefer speci!c over generic solutions

explicitly provide for generic access

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Namespaces

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... just like Java packages

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require: (re-)load libs:reload, :reload-all, :as

refer: import names:exclude [], :only [], :rename {…:…}

use: require + refer:exclude [], :only [], :rename {…:…}

ns: create namespace:require, :refer, :use, :gen-class

Handle var name clashesReduce dependencies

Dynamic reloadingNamespace aliases

Convenient REPL usage

Flexible handling in sourcesProvide encapsulation

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require: (re-)load libs:reload, :reload-all, :as, :refer

refer: import names:exclude [], :only [], :rename {…:…}

ns: create namespace:require, :refer, :use, :gen-class

Handle var name clashesReduce dependencies

Dynamic reloadingNamespace aliasesConvenient REPL usage

Flexible handling in sourcesProvide encapsulation

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(defn ...)(defmacro ...)(defmulti ...)(defmethod ...)

(defn- ...)(def ^:private ...)(def ^:dynamic ...)

(ns com.example.some-ns "Well-documented ns" (:use [com.example.n1 :only [xyz]]) (:require [com.example.ns2 :as n2]))

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Data

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Data structures vs. objectspublic class Point { private final double x; private final double y;

public Point(double x, double y) { this.x = x; this.y = y; }}

Point p1 = new Point(3, 4);

(def p1 [3 4])

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Data structures vs. objects(def p1 [3 4])

Immutable

Reusable

Compatible

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Data structures vs. objectsimport static java.lang.Math.sqrt;

public class Point { private final double x; private final double y;

public Point(double x, double y) { this.x = x; this.y = y; }

public double distanceTo(Point other) { double c1 = other.x - this.x; double c2 = other.y - this.y; return sqrt(c1 * c1 + c2 * c2); }}

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Data structures vs. objects(import-static java.lang.Math sqrt)

(defn distance [[x1 y1] [x2 y2]] (let [c1 (- x2 x1) c2 (- y2 y1)] (sqrt (+ (* c1 c1) (* c2 c2)))))

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Data structures vs. objects(defn rand-seq [limit] (repeatedly #(rand-int limit)))

(take 10 (partition 2 (rand-seq 10)))

in!nite randoms

pairs of random ints

10 random points

;((3 6) (6 1) (8 5) (0 7) (3 8) (0 6) (1 6) (7 6) (0 1) (8 9))

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Data structures vs. objects(defn circumference [vertices] (reduce + (map distance vertices (drop 1 (cycle vertices)))))

in!nite repetition

seq without !rstall

;((3 6) (6 1) (8 5) (0 7) (3 8) (0 6) (1 6) (7 6) (0 1) (8 9));((6 1) (8 5) (0 7) (3 8) (0 6) (1 6) (7 6) (0 1) (8 9) (3 6))

;58.06411369758525

...

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assocassoc-inbutlastconcatconjconscountcycledifferencedissocdistinctdistinct?drop-lastemptyempty?every?filterfirstflatten

group-byinterleaveinterposeintersectionintojoinlazy-catmapcatmergemerge-withnot-any?not-empty?not-every?nthpartitionpartition-allpartition-bypeekpop

popyprojectremovereplacerestrseqselectselect-keysshufflesomesplit-atsplit-withsubvectaketake-lasttake-nthtake-whileunionupdate-in

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app

request

response

objects object

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public interface Servlet { void init(ServletConfig servletConfig) throws ServletException; ServletConfig getServletConfig(); void service(ServletRequest servletRequest, ServletResponse servletResponse) throws ServletException, IOException; String getServletInfo(); void destroy();}

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public interface HttpServletRequest extends ServletRequest { public String getAuthType();

public Cookie[] getCookies();

public Enumeration<String> getHeaders(String name);

public Enumeration<String> getHeaderNames();

public String getMethod();

public String getQueryString();

public String getRemoteUser();

public HttpSession getSession(boolean create);

public boolean authenticate(HttpServletResponse response) throws IOException,ServletException; public void login(String username, String password) throws ServletException;...

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public interface HttpServletResponse extends ServletResponse {

public void addCookie(Cookie cookie);

public boolean containsHeader(String name); public void sendError(int sc, String msg) throws IOException;

public void sendRedirect(String location) throws IOException;

public void setDateHeader(String name, long date);

public void addDateHeader(String name, long date);

public void setHeader(String name, String value);

public void addHeader(String name, String value);

public void setStatus(int sc);

public int getStatus();...

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app

request

response

data function

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(defn hello-world-app [req] {:status 200 :headers {"Content-Type" "text/plain"} :body "Hello, World!"})

(hello-world-app {:uri "/foo" :request-method :get})> {...}

(run-jetty hello-world-app {:port 8080})

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Maps(def projects #{{:id "1", :kind :time-material, :description "Consulting for BigCo", :budget 25000, :team [:joe, :chuck, :james]} {:id "2", :kind :fixed-price, :description "Development for Startup", :budget 100000, :team [:john, :chuck, :james, :bill]} {:id "3", :kind :fixed-price, :description "Clojure Training", :budget 3000, :team [:joe, :john]}})

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Map access(defn all-members [projects] (reduce conj #{} (flatten (map :team projects))))

seq of vectors

seq of members with duplicates

set of all team members

;#{:chuck :joe :james :john :bill}(all-members projects)

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Map access & coupling(defn all-members [projects] (reduce conj #{} (flatten (map :team projects))))

#{{:id "2", :kind :fixed-price, :description "Development for Startup", :budget 100000, :team [:john, :chuck, :james, :bill]}}

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Map access & coupling(defn all-members [projects] (reduce conj #{} (flatten (map :team projects))))

#{{:id "2", :kind :fixed-price, :description "Development for Startup", :budget 100000, :team [:john, :chuck, :james, :bill]}}

:team

:team

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[{:kind "fixed-price", :team ["john" "chuck" "james" "bill"], :budget 100000, :id "2", :description "Development for Startup"}{:kind "fixed-price", :team ["joe" "john"], :budget 3000, :id "3", :description "Clojure Training"}{:kind "time-material", :team ["joe" "chuck" "james"], :budget 25000, :id "1", :description "Consulting for BigCo"}]

[{"kind":"fixed-price", "team":["john", "chuck", "james", "bill"], "budget":100000, "id":"2", "description":"Development for Startup"},{"kind":"fixed-price", "team":["joe", "john"], "budget":3000, "id":"3", "description":"Clojure Training"},{"kind":"time-material", "team":["joe", "chuck", "james"], "budget":25000, "id":"1", "description":"Consulting for BigCo"}]

(json-str)

(read-json)

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(defn ...)(defmacro ...)(defmulti ...)(defmethod ...)

(defn- ...)(def ^:private ...)

(ns com.example.some-ns "Well-documented ns" (:use [com.example.n1 :only [xyz]]) (:require [com.example.ns2 :as n2]))

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Implementation

Interface

Functionsw/ simple data

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Closures

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(defn make-id [prefix id] (join "-" [prefix (Long/toString id 16)]))

(prj-id);; "prj-1"(prj-id);; "prj-2"(prj-id);; "prj-3"

(defn make-project [map] (assoc map :id (prj-id)))

(defn id-generator ([prefix] (id-generator prefix 0)) ([prefix v] (let [cnt (atom v)] (fn [] (make-id prefix (swap! cnt inc))))))

(def prj-id (id-generator "prj"))

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Meet Miss Grant

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s/ar

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.asp

x?p=

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379

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(def fsm (make-fsm :idle :doorOpened {:idle [[unlock-door lock-panel] {:doorClosed :active}] :active [[] {:drawerOpened :waitingForLight :lightOn :waitingForDrawer}] :waitingForLight [[] {:lightOn :unlockedPanel}] :waitingForDrawer [[] {:drawerOpened :unlockedPanel}] :unlockedPanel [[unlock-panel lock-door] {:panelClosed :idle}]}))

(defn unlock-door [] (println "Unlocking door"))(defn lock-door [] (println "Locking door"))(defn unlock-panel [] (println "Unlocking panel"))(defn lock-panel [] (println "Locking panel"))

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(defn make-fsm "creates an fsm with initial state s0, a reset event, and a map of transitions. [state-transitions] must be a map of state->[[f1 f2 ...] {e0->s0, e1->s2, ...}]" [s0 reset-event state-transitions ] (let [s (atom s0)] (fn [evt] (if (= evt reset-event) (do (println "Reset event, returning to " s0) (swap! s (fn [_] s0))) (let [[actions transitions] (state-transitions @s)] (if-let [new-state (transitions evt)] (do (println "Event" evt "causes transition from" @s "to" new-state) (doseq [f actions] (f)) (swap! s (fn [_] new-state))) (println "Unexpected/unhandled event" evt "in state" @s)))))))

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(dorun (map fsm [:doorClosed :lightOn :drawOpened :panelClosed]))

(def fsm (make-fsm :idle :doorOpened {:idle [[unlock-door lock-panel] {:doorClosed :active}] :active [[] {:drawerOpened :waitingForLight :lightOn :waitingForDrawer}] :waitingForLight [[] {:lightOn :unlockedPanel}] :waitingForDrawer [[] {:drawerOpened :unlockedPanel}] :unlockedPanel [[unlock-panel lock-door] {:panelClosed :idle}]}))

;; Event :doorClosed causes transition from :idle to :active;; Unlocking door;; Locking panel;; Event :lightOn causes transition from :active to :waitingForDrawer;; Event :drawerOpened causes transition from :waitingForDrawer to :unlockedPanel;; Event :panelClosed causes transition from :unlockedPanel to :idle;; Unlocking panel;; Locking door;; Reset event, returning to :idle

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Map Function

Multimethod

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Method problems“Global” state

Coarse-grained re-use

Simple-minded dispatch

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Methods vs. Multimethods

Methods Multimethods

Dispatch Type customizable

on # of args 1 arbitrary

Hierarchybased on type

inheritancecustomizable

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Multimethods(def projects #{{:id "1", :kind :time-material, :description "Consulting for BigCo", :budget 25000, :team [:joe, :chuck, :james]} {:id "2", :kind :fixed-price, :description "Development for Startup", :budget 100000, :team [:john, :chuck, :james, :bill]} {:id "3", :kind :fixed-price, :description "Clojure Training", :budget 3000, :team [:joe, :john]}})

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Multimethods(defmulti expected-revenue :kind)

(defmethod expected-revenue :default [p] (:budget p)) (defmethod expected-revenue :fixed-price [p] (* 0.8 (:budget p))) (defn total-expected-revenue [projects] (reduce + (map expected-revenue projects)))

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Multimethods(defn make-rectangle [[p1 p2 p3 p4 :as vertices]] (let [a (distance p1 p2) b (distance p2 p3)] (assert (= a (distance p3 p4))) (assert (= b (distance p4 p1))) {:kind :rectangle, :vertices vertices, :a a, :b b}))

(defn make-circle [center r] {:kind :circle, :center center, :r r})

(defmulti area :kind)

(defmethod area :rectangle [{:keys [a b]}] (* a b)) (defmethod area :circle [{:keys [r]}] (* PI (pow r 2)))

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Multimethods

(defmulti circumference :kind :default :polygon)

(defmethod circumference :polygon [{:keys [vertices]}] (reduce + (map distance vertices (drop 1 (cycle vertices)))))

(defmethod circumference :rectangle [{:keys [a b]}] (* 2 (+ a b)))

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Multimethods(defmulti draw-shape (fn [shape canvas] [(:kind shape) (:type canvas)]))

(defmethod draw-shape :default [shape canvas] (str "Drawing " (:kind shape) " on " (:type canvas)))

(defmethod draw-shape [:circle :print-canvas] [shape canvas] "Printing a circle")

(defmethod draw-shape [:rectangle :display-canvas] [shape canvas] "Showing a rectangle")

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defrecord, de#ype

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Map

Record

Type

Function

Multimethod

Protocol

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defrecord

Supports map access

Flexible & extensible

Convenience functions

Better performance

Platform integration

Protocol support

No structural sharing

Code overhead

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de#ype

No generic overhead

Convenience functions

Best performance


Protocol support

No structural sharing

No map access

Static & !xed

Code overhead

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Protocols(defprotocol Shape (area [shape]) (circumference [shape]))

(defrecord Rectangle [vertices] Shape (area [shape] ...) (circumference [shape] ...)) (defrecord Circle [center r] Shape (area [shape] ...) (circumference [shape] ...))

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Protocols(defprotocol ShapeStorage (read-from [storage]) (write-to [storage shape]))

(extend-protocol ShapeStorage XmlStorage (read-from [storage] ...) (write-to [storage shape] ...) CouchDB (read-from [storage] ...) (write-to [storage shape] ...))

(extend-protocol ShapeStorage String (read-from [storage] ...) (write-to [storage shape] ...))

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Protocols

Performance

Grouping


Limited dispatch(single arg, type-based)

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Roadmap Recommendation1

Namespaces, Functions,Persistent Data Structures

2 Multimethods

3defrecorddefprotocol

4 de!ype

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Summary

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Functional programmingis di"erent (for a reason)

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Question your own knowledge

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Embrace data structures

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Thanks!

Q&A

Stefan [email protected]@stilkov

innoQ Deutschland GmbH

http://www.innoq.com

Krischerstr. 10040789 Monheim am RheinGermanyPhone: +49 2173 3366-0

innoQ Schweiz GmbH

[email protected]

Gewerbestr. 11CH-6330 ChamSwitzerlandPhone: +41 41 743 0116

Friday, May 17, 13

Clojure for OOP folks - Amazon S3s3-eu-west-1.amazonaws.com/presentations2013/30...A practical Lisp...

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