From Java To Clojure - Adieu Java -

Self-introduction /laʒenɔʁɛ̃k/ lagénorhynque (defprofile lagénorhynque :name "Kent OHASHI" :account @lagenorhynque :company "Opt, Inc." :languages [Clojure Haskell Python Scala English français Deutsch русский] :interests [programming language-learning mathematics])

Lisp × Java

Java as a language

Problems with Java as a language lack of functional programming (FP) support More FP! (OOP and static typing are not necessary) verbose syntax More simplicity! lack of exibility/extensibility More freedom!

JVM language comparison from my own point of view factor Java Groovy Scala Kotlin Clojure FP support × △ ◯ △ ◯ simplicity × ◯ △ ◯ ◎ exibility × ◯ ◯ ◯ ◎

Using Clojure, we can say goodbye to Java as a language!! Adieu, Java ! Now we have to say farewell to you, Java (;_;)/~~~

What is Clojure?

Origin of the name Clojure Clojure is pronounced exactly like closure, where the s/j has the zh sound as in azure, pleasure etc. The name was chosen to be unique. I wanted to involve c (c#), l (lisp) and j (java). Once I came up with Clojure, given the pun on closure, the available domains and vast emptiness of the googlespace, it was an easy decision. ― Rich Hickey, creator of Clojure cf. meaning and pronunciation of Clojure

Clojure /ˈkloʊʒɚ/ * NOT /ˈkloʊd͡ʒɚ/ element meaning /ˈkloʊʒɚ/ closure, functional programming C C#(.NET) as a platform, .NET language l Lisp dialect j Java as a platform, JVM language

1. Clojure as a FP language 2. Clojure as a Lisp dialect 3. Clojure as a JVM language

Clojure as a FP language

Immutable List, Vector, Map, Set, etc. user=> '(1 2 3) (1 2 3) user=> [1 2 3] [1 2 3] user=> {:a 1 :b 2 :c 3} {:a 1, :b 2, :c 3} user=> #{1 2 3} #{1 3 2}

Higher-order functions (filter, map, reduce, etc.) user=> (def xs [1 2 3]) #'user/xs user=> (filter odd? xs) (1 3) user=> (map #(* % %) xs) (1 4 9) user=> (reduce + 0 xs) 6 user=> (reduce + 0 (map #(* % %) (filter odd? xs))) 10 user=> (->> xs #_=> (filter odd?) #_=> (map #(* % %)) #_=> (reduce + 0)) 10

BTW, #( ) is #(* % %) ↓↓↓ (fn [x] (* x x)) a reader macro equivalent as above.

BTW, ->> is (->> a (f x) (g y) (h z)) ↓↓↓ (h z (g y (f x a))) a macro (a kind of threading macros) expanded as above.

Lazy sequences user=> (def nats (iterate inc 0)) #'user/nats user=> (take 10 nats) (0 1 2 3 4 5 6 7 8 9) user=> (take-while #(< % 10) nats) (0 1 2 3 4 5 6 7 8 9)

Clojure as a Lisp dialect

S-expressions (f a b c ...) f: function, macro, special form a, b, c, ...: arguments cf. Java f(a, b, c, ...)

Even a function/method de nition // Java public void greet(String name) { System.out.println("Bonjour, " + name + " !"); } is an S-expression. ;; Clojure (defn greet [name] (println (str "Bonjour, " name " !")))

Even namespace/package declaration and imports // Java package demo_app; import java.io.IOException; import java.util.ArrayList; import java.util.List; are S-expressions. ;; Clojure (ns demo-app.core (:import (java.io IOException) (java.util ArrayList List)))

first(≒ car), rest(≒ cdr), cons, ... user=> (def xs [1 2 3]) #'user/xs user=> (first xs) 1 user=> (rest xs) (2 3) user=> (cons 0 xs) (0 1 2 3)

S-expression code can be treated as data (code as data; ) homoiconicity user=> (first '(def xs [1 2 3])) def user=> (rest '(def xs [1 2 3])) (xs [1 2 3]) user=> (cons 'def '(xs [1 2 3])) (def xs [1 2 3]) user=> (eval (cons 'def '(xs [1 2 3]))) #'user/xs

Lisp macros powerful compile-time metaprogramming facility user=> (defmacro unless ; just a reimplementation of clojure.core/if-not macro #_=> ([test then] #_=> `(unless ~test ~then nil)) #_=> ([test then else] #_=> `(if (not ~test) #_=> ~then #_=> ~else))) #'user/unless user=> (unless (= 1 2) :ok) :ok user=> (macroexpand '(unless (= 1 2) :ok)) (if (clojure.core/not (= 1 2)) :ok nil)

BTW, defn used for function de nition user=> (macroexpand #_=> '(defn greet [name] #_=> (println (str "Bonjour, " name " !")))) (def greet (clojure.core/fn ([name] (println (str "Bonjour, " name " !"))))) is a macro composed of def, fn special forms.

Hard to handle a lot of parentheses? ⇒ Lisp-editing plugins make it very comfortable The Animated Guide to Paredit Parinfer - simpler Lisp editing

Clojure as a JVM language

Compiling to Java class les executable as a jar $ lein new app demo-app Generating a project called demo-app based on the 'app' template. $ cd demo-app/ $ lein uberjar Compiling demo-app.core Created /Users/lagenorhynchus/code/demo-app/target/uberjar/demo-app-0.1.0-SNAPSHOT.jar Created /Users/lagenorhynchus/code/demo-app/target/uberjar/demo-app-0.1.0-SNAPSHOT-standalone.jar $ java -jar target/uberjar/demo-app-0.1.0-SNAPSHOT-standalone.jar Hello, World!

Calling Java methods static methods // Java Integer.parseInt("123") ;; Clojure (Integer/parseInt "123") instance methods // Java "a b c".split("\\s") ;; Clojure (.split "a b c" "\\s")

destructive initialisation/setting // Java java.util.Map m = new java.util.HashMap<>(); m.put("a", 1); m.put("b", 2); m.put("c", 3); return m; ;; Clojure (doto (java.util.HashMap.) (.put "a" 1) (.put "b" 2) (.put "c" 3))

method chaining // Java new StringBuilder() .append("a") .append("b") .append("c") .toString() ;; Clojure (.. (StringBuilder.) (append "a") (append "b") (append "c") toString) ;; or (-> (StringBuilder.) (.append "a") (.append "b") (.append "c") .toString)

Interoperating with Java collection API Java collection → Clojure function user=> (def xs (doto (java.util.ArrayList.) #_=> (.add 1) #_=> (.add 2) #_=> (.add 3))) #'user/xs user=> (class xs) java.util.ArrayList user=> xs [1 2 3] user=> (map inc xs) (2 3 4)

Clojure collection → Java method user=> (def xs [1 2 3 4 5]) #'user/xs user=> (class xs) clojure.lang.PersistentVector user=> (instance? java.util.List xs) true user=> (.subList xs 1 4) [2 3 4]

cf. usage example of Google Sheets API (Java) // Java public Integer duplicateWorksheet(Sheets sheets, String spreadsheetId, Integer worksheetId, Strin List reqs = Arrays.asList( new Request().setDuplicateSheet( new DuplicateSheetRequest().setSourceSheetId(worksheetId) .setNewSheetName(worksheetName) .setInsertSheetIndex(1) ) ); return executeUpdate(sheets, spreadsheetId, worksheetId, reqs) .getReplies() .get(0) .getDuplicateSheet() .getProperties() .getSheetId(); } ;; Clojure (defn duplicate-worksheet [sheets spreadsheet-id worksheet-id worksheet-name] (let [reqs [(-> (Request.) (.setDuplicateSheet (-> (DuplicateSheetRequest.) (.setSourceSheetId worksheet-id) (.setNewSheetName worksheet-name) (.setInsertSheetIndex (int 1)))))]] (-> (execute-update sheets spreadsheet-id worksheet-id reqs) .getReplies first .getDuplicateSheet .getProperties .getSheetId)))

Furthermore, core.async transducers clojure.spec ClojureScript etc.

If you use Clojure, with the power of FP, Lisp and Java you can program more simply, more freely!!

Vive les S-expressions ! Long live S-expressions!

Further Reading : Clojure o cial site : Clojure build tool : Clojure build tool : Clojure online REPL : ClojureScript online REPL Chapter 7: Clojure Clojure Leiningen Boot Try Clojure Replumb REPL Seven Languages in Seven Weeks Programming Clojure (2nd edition)