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Clojure for Non-Clojurians Takahiro Noda @tnoda_ Kansai Kansugata Dojo [2014-12-06 Sat]

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Outline Clojure? . . . The Good Parts . . . The Bad Parts . . . Applications . . .

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Clojure? Clojure

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Clojure? Clojure Java

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Hello World! Example (Java) $ curl -O http :// repo1.maven.org/maven2/org/clojure/ clojure /1.6.0/ clojure -1.6.0. zip $ unzip clojure -1.6.0. zip $ cd clojure -1.6.0/ $ java -cp clojure -1.6.0. jar clojure.main -e ’(println " Hello␣World!")’ Hello World!

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Nyampass Co., Ltd. Figure : http://nyampass.com/ They have been using Clojure to develop their mobile BaaS services.

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Non non biyori (2013)

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Hello World! (recap) Example (Java) $ java -cp clojure -1.6.0. jar clojure.main \ -e ’(println "Hello␣World!")’

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Sudden Jar ＿人人人人人人人人人人＿ ＞ clojure-1.6.0.jar　＜ ￣ＹＹＹＹＹＹＹＹＹＹ￣

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Java SE !"# javac $%%&'&()*+,-)''.'

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Clojure (Java) !"# javac $%%&'&()*+,-)''.' clojure-x.y.z.jar

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cloure-x.y.z.jar !"# javac $%%&'&()*+,-)''.' clojure-x.y.z.jar clojure.core ,%/*0-.(

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Syntax (Clojure) new ClassName(arg0 , arg1 , ...); ClassName.staticMethod(arg0 , arg1 , ...); obj.method(arg0 , arg1 , ...); Syntax (Java interop) (ClassName. arg0 arg1 ...) (ClassName/staticMethod arg0 arg1 ...) (. method obj arg0 arg1 ...) Syntax (Clojure) (func arg0 arg1 … )

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Type Inference Example (Java) ArrayList list = new ArrayList (); list.add (1); list.add (2); list.add (3);

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Type Inference Example (Java) ArrayList list = new ArrayList (); list.add (1); list.add (2); list.add (3); Example (Clojure) (let [alist (ArrayList .)] (.add alist 1) (.add alist 2) (.add alist 3))

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Macro Example (Plain Clojure) (let [alist (ArrayList .)] (.add alist 1) (.add alist 2) (.add alist 3))

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Macro Example (Plain Clojure) (let [alist (ArrayList .)] (.add alist 1) (.add alist 2) (.add alist 3)) Example (Macro) (doto (ArrayList .) (.add 1) (.add 2) (.add 3))

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clojure-x.y.z.jar !"# javac $%%&'&()*+,-)''.' clojure-x.y.z.jar clojure.core ,%/*0-.(

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clojure.core Java (JVM, Bootstrap classes) • Primitives, classes, methods, objects clojure.core • Immutable/Persistent data structures • Collections • Lazy sequences • Stream (Java) • List (Lisp)

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Built-in collection List • (1 2 3 4) Vector (Stack) • [1 2 3 4] Map • {:a 1, :b 2, :c 3} or {:a 1 :b 2 :c 3} Set • #{1 2 3}

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Clojure Programming !"##$%&'"( )*+,-. /$01$(%$ !"##$%&'"( )*+,-. /$01$(%$

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Clojure Programming !"##$%&'"( )*+,-. /$01$(%$ !"##$%&'"( )*+,-. /$01$(%$

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Conj Conjoin col and x (conj col x) Example (Built-in collections) • (conj [1 2 3 4] 5) • [1 2 3 4 5] • (conj {:a 1 :b 2 :c 3} [:d 4]) • {:a 1 :b 2 :c 3 :d 4} • (conj #{1 2 3} 4) • #{1 2 3 4} • (conj clojure.lang.PersistentQueue/EMPTY 1)

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Map-filter-reduce map (map inc [1 2]) ;=> (2 3) filter (filter #{1 2} [1 2 3 4]) ;=> (2) reduce (reduce + [1 2 3]) ;=> 6

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Clojure Programming !"##$%&'"( )*+,-. /$01$(%$ !"##$%&'"( )*+,-. /$01$(%$

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Checkpoint 1 • Java • clojure-x.y.z.jar • Syntax • Type inference • Macro • Immutable/Persistent data structures • Lazy sequences

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The Good Parts (1/6) • Dynamic development • Functional programming • Lisp • Runtime polymorphism • Concurrent programming • Hosted on JVM

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Dynamic Development • REPL • Dynamic compilation • Automatically compiled to JVM bytecode on the fly.

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REPL $ java -cp clojure -1.6.0. jar clojure.main

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The Good Parts (2/6) • Dynamic development • Functional programming • Lisp • Runtime polymorphism • Concurrent programming • Hosted on JVM

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Functional Programming • First-class functions • fn • defn = def + fn • Immutable data structures • Extensible absraction • seq • Constant-space recursive looping • recur

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The Good Parts (3/6) • Dynamic development • Functional programming • Lisp • Runtime polymorphism • Concurrent programming • Hosted on JVM

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Lisp • Macro

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Macro C, C++, Scala String or AST Clojure (Lisp) Sequence

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The Good Parts (4/6) • Dynamic development • Functional programming • Lisp • Runtime polymorphism • Concurrent programming • Hosted on JVM

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Runtime Polymorphism • Java Interface • Protocol • Type, Record, Protocol • Multimethod

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Implementing Java Interface (reify java.util.Comparator (compare [this x y] (- y x)) (equals [this x] (= this x))

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Multimethod (defmulti zoo (fn [x y] [(: diet x) (:diet y)])) (defmethod zoo [: carnivorous :herbivorous] [x y] :eat) (defmethod zoo [: herbivorous :carnivorous] [x y] : run-away) (defmethod zoo [: carnivorous :carnivorous] [x y] :fight) (defmethod zoo [: herbivorous :herbivorous] [x y] :mate)

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The Good Parts (5/6) • Dynamic development • Functional programming • Lisp • Runtime polymorphism • Concurrent programming • Hosted on JVM

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Concurrent programming • Multithreaded programming • Concurrent operations • JSR-166y (Fork/Join)

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Multithreaded programming Functions Callable, Runnable public interface Callable { V call (); } Collections Thread-safe Immutability Immutable variables

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Concurrent Operations • Atomic updates • atom • Asynchronous agents • agent

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Atomic updates Example (atom) (def a (atom 0)) (swap! a inc) (compare-and-set! a 1 10) (reset! a 20) Java java.util.concurrent.atomic

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Asynchronous agents Example (agent) (def a (agent 5000)) (send a inc) (senf-off a #( Thread/sleep %)) (await a b) Java • ExecutorService • Fixed-size thread pool (send) • Unbounded thread pool (send-off)

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Reducers (Fork/Join) Plain Old Clojure (->> s (map ->heptagonal) (filter triangular ?) (reduce +)) Reducer Version (->> s (map ->heptagonal) (r/filter triangular ?) (r/fold +)) http://tnoda-clojure.tumblr.com/post/60197800824/reducers

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The Good Parts (6/6) • Dynamic development • Functional programming • Lisp • Runtime polymorphism • Concurrent programming • Hosted on JVM

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Hosted on JVM • Type system • GC • Threads • JVM bytecode • Basic classes • java.util.*, Regular expressions • java.lang.Math • java.io.*

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Type System Combining static and dynamic type-checking Example (IFn) public interface IFn extends Callable , Runnable { public Object invoke (); public Object invoke(Object arg1); public Object invoke(Object arg1 , Object arg2); ; ... }

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Checkpoint 2 Dynamic development • REPL • dynamic compilation Functional programming • First-class functions • Immutable data structures • Extensible absraction • Constant-space recursive looping Lisp • Macro

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Checkpoint 2 Runtime polymorphism • Java Interface • Protocol • Multimethod Concurrent programming • Multithreaded programming • Concurrent operations • JSR-166y (Fork/Join) Hosted on JVM • Type system

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The Bad Parts • Arrays

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Type Hinting Example (Java) int x = a[i][j][k]; Example (Clojure) (aget a i j k) ; extremely slow (aget ^ints (aget ^objects (aget a i) j ) k)

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Remedy http://tnoda-clojure.tumblr.com/post/28996774133/arrays-as-funcions Macro (definline a [i j] (let [larray (gensym "larray")] ‘(let [~ larray (aget ~( with-meta ‘-a {:tag ’objects }) ~i)] (aget ~( with-meta larray {:tag ’longs }) ~j)))) (definline a= [i j x] (let [larray (gensym "larray")] ‘(let [~ larray (aget ~( with-meta ‘-a {:tag ’objects }) ~i)] (aset ~( with-meta larray {:tag ’longs }) ~j (long ~x))))) a, a=

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Checkpoint 3 Do not use mulitdimensional arrays in Clojure.

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Applications Middleware • Storm • TrapperKeeper • Datomic Products • Puppet Server • Netflix’s internal services

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Conclusion Clojure? Java The Good Parts Dynamic development, functional programming, lisp, runtime polymorphism concurrent programming, hosted on JVM The Bad Parts Multidimensinal arrays Applications Storm, Puppet Server