Introduction to Scala @ TWJUG 2010/07

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Introduction Scala BrianHsu @ TWJUG

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關於我

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http://bone.twbbs.org.tw/blog/ 墳墓 (BrianHsu)

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其實我不懂 Scala

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Scala 非常難

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我只會簡單的部份但我用得很高興！

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特別是拿來當 Script Language

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工欲善其事必先利其器

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所有的武林高手都有趁手的兵刃

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程式員的兵器程式語言

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我的個性

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粗心、懶

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所以我的兵器在哪？

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動態語言

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3P 好好玩 Python/Perl/PHP

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後起之秀 Ruby

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因為粗心

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討厭執行期爆炸 def boom puts "Hello World" end puts " 一個小小的 typo" booom

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對不起，我是懦夫我不敢用這些語言

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靜態語言

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真男人就是要冠 C C 語言

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D 語言 Haskell

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我們最愛的 Java Programming Langauge

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我們最恨的 Java Programming Langauge

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因為懶

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public static void main (String [] args) { System.out.println(“Hello World”) }

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public static void main (String [] args) { System.out.println(“Hello World”) } 浪費生命！

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println ("Hello World")

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What is Scala?

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樓梯

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A Postfunctional Programming Language on Java Virtual Machine

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Compile to JVM .class Byte Code

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理論上 Full Java Interoperability

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What is Scala?

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A Scalable Language

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會隨 User 變形的程式語言

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A Scalable Language

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A Scalable Language

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Why Scala?

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三個願望一次滿足

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REPL Scripting Compiling

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REPL

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Scripting

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Compiling

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三個願望一次滿足

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Procedure Object Oriented Functional

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Procedure Programming

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Object Oriented Programming

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Functional Programming

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三個願望一次滿足

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Concise Static Typing Duck Typing

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Concise (Ruby)

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Concise (Scala)

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Static Typing (Ruby)

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Static Typing (Scala)

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真、 Duck Typing (Ruby)

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偽‧ Duck Typing (Scala)

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結論

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有 Scala 、無懦夫

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Scala vs Java

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Scala compared to Java Scala adds Scala removes + a pure object system - static members + operator overloading - primitive types + closures - break, continue + mixin composition with traits - special treatment of interfaces + existential types - wildcards + abstract types - raw types + pattern matching - enums Modeled in libraries: assert, enums, properties, events, actors, using, queries, …

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Scala cheat sheet (1): Definitions Scala method definitions: def fun(x: Int): Int = { result } def fun = result Scala variable definitions: var x: Int = expression val x: String = expression Java method definition: int fun(int x) { return result } (no parameterless methods) Java variable definitions: int x = expression final String x = expression

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Scala cheat sheet (2): Expressions Scala method calls: obj.meth(arg) obj meth arg Scala choice expressions: if (cond) expr1 else expr2 expr match { case pat 1 => expr 1 .... case pat n => expr n } Java method call: obj.meth(arg) (no operator overloading) Java choice expressions, stmts: cond ? expr1 : expr2 if (cond) return expr1; else return expr2; switch (expr) { case pat 1 : return expr 1 ; ... case pat n : return expr n ; } // statement only

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Scala cheat sheet (3): Objects and Classes Scala Class and Object class Sample(x: Int, val p: Int) { def instMeth(y: Int) = x + y } object Sample { def staticMeth(x: Int, y: Int) = x * y } Java Class with statics class Sample { private final int x; public final int p; Sample(int x, int p) { this.x = x; this.p = p; } int instMeth(int y) { return x + y; } static int staticMeth(int x, int y) { return x * y; } }

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Scala cheat sheet (4): Traits Scala Trait trait T { def abstractMth(x: String): Int def concreteMth(x: String) = x + field var field = “!” } Scala mixin composition: class C extends Super with T Java Interface interface T { int abstractMth(String x) } (no concrete methods) (no fields) Java extension + implementation: class C extends Super implements T

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Scala 的特異功能

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這次沒唬你萬物皆物件

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你可以這樣 1.toString

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我們都知道 toString 是 Method

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1+1 + 也是 Method ！

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1+2 = 1 .+ (2)

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向 static 說再見預設的 Singleton

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List.range(1, 5) = List(1,2,3,4,5)

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List 是 class range 是 class method

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錯

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List 是一個 Singleton 物件！

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range(1,2) 是一個 instance method

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只有一個唯一的 List 物件

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懶人的好幫手之一 Tuple

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簡單資料結構

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座標鍵值

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懶人的好幫手之二 case calss

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加強版的 swtich Pattern Matching

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加強版（幾乎沒在用）的 FOR expression

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建忘的救星 (2.8.0 才有喔！ ) Default / Named Arguments

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Function Programming Function Literal

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Function Programming Closure

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Function Programming High-Order Function

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Lazy

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可以有實作的 Interface Trait

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等等，這是孔明的陷阱！

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Scala 的限制與臭蟲 ● 限制 ● Cannot acess static protected Java field. – 對不起， Google Maps API 用到了。 ● 臭蟲 ● Static Java Inner class 造成 classfile borken / missing dependcy – Google Maps API 踩到地雷了 – Fixed in latest SVN trunk ● 解決方式 ● 用 Java 寫個 Wrapper 唄 Missing dependency

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Thanks!