Swift和Scala语法上的诸多相似之处

国外开发者最近发现，WWDC2014上苹果发布的新语言Swift，和古老的Scala语言在语法上存在众多的相似之处。

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本文以苹果官方教程 The Swift Programming Language 中的示例，比较Swift与Scala两种语言实现同一功能的代码。

Swift语言从语法上来看，几乎是Scala的一个分支，在以下功能上几乎是等同的：类型继承、闭包、元组（Tuple）、协议、扩展、泛型等。

不过Swift的运行环境和Scala的区别还是很大，这个概念才是Swift最重要的。Scala语言编译成JVM程序，使用垃圾收集机制，与Java无缝整合。但Swift最终编译到机器代码，使用引用计数机制，与Objective-C无缝整合。所以Swift和Scala在代码表象上的相似，应该并不太影响两种语言本质机理上的重大不一致。

语言基础

你好，世界。

// Swift
println("Hello, world!")
// Scala */
println("Hello, world!")

变量与常量

// Swift
var myVariable = 42
myVariable = 50
let myConstant = 42

// Scala
var myVariable = 42
myVariable = 50
val myConstant = 42

显式类型

// Swift
let explicitDouble: Double = 70
// Scala
val explicitDouble: Double = 70

强制类型转换

// Swift
let label = "The width is "
let width = 94
let widthLabel = label + String(width)

//Scala
val label = "The width is "
val width = 94
val widthLabel = label + width

字符串数据填充

// Swift
let apples = 3
let oranges = 5
let fruitSummary = "I have \(apples + oranges) " + 
                   "pieces of fruit."

// Scala
val apples = 3
val oranges = 5
val fruitSummary = s"I have ${apples + oranges} " +
                   " pieces of fruit."

整数半开区间运算符

// Swift
let names = ["Anna", "Alex", "Brian", "Jack"]
let count = names.count
for i in 0..count {
    println("Person \(i + 1) is called \(names[i])")
}
// Person 1 is called Anna
// Person 2 is called Alex
// Person 3 is called Brian
// Person 4 is called Jack

// Scala
val names = Array("Anna", "Alex", "Brian", "Jack")
val count = names.length
for (i <- 0 until count) {
    println(s"Person ${i + 1} is called ${names(i)}")
}
// Person 1 is called Anna
// Person 2 is called Alex
// Person 3 is called Brian
// Person 4 is called Jack

整数闭区间运算符

// Swift
for index in 1...5 {
    println("\(index) times 5 is \(index * 5)")
}
// 1 times 5 is 5
// 2 times 5 is 10
// 3 times 5 is 15
// 4 times 5 is 20
// 5 times 5 is 25

// Scala
for (index <- 1 to 5) {
    println(s"$index times 5 is ${index * 5}")
}
// 1 times 5 is 5
// 2 times 5 is 10
// 3 times 5 is 15
// 4 times 5 is 20
// 5 times 5 is 25

集合

数组

// Swift
var shoppingList = ["catfish", "water",
    "tulips", "blue paint"]
shoppingList[1] = "bottle of water"

// Scala
var shoppingList = Array("catfish",
    "water", "tulips", "blue paint")
shoppingList(1) = "bottle of water"

字典

// Swift
var occupations = [
    "Malcolm": "Captain",
    "Kaylee": "Mechanic",
]
occupations["Jayne"] = "Public Relations"

// Scala
var occupations = scala.collection.mutable.Map(
    "Malcolm" -> "Captain",
    "Kaylee" -> "Mechanic"
)
occupations("Jayne") = "Public Relations"

空集

// Swift
let emptyArray = String[]()
let emptyDictionary = Dictionary<String, Float>()
let emptyArrayNoType = []

// Scala
val emptyArray = Array[String]()
val emptyDictionary = Map[String, Float]()
val emptyArrayNoType = Array()

函数

函数定义

// Swift
func greet(name: String, day: String) -> String {
    return "Hello \(name), today is \(day)."
}
greet("Bob", "Tuesday")

// Scala
def greet(name: String, day: String): String = {
    return s"Hello $name, today is $day."
}
greet("Bob", "Tuesday")

元组（Tuple）返回值

// Swift
func getGasPrices() -> (Double, Double, Double) {
    return (3.59, 3.69, 3.79)
}

// Scala
def getGasPrices(): (Double, Double, Double) = {
    return (3.59, 3.69, 3.79)
}

可变数量参数

// Swift
func sumOf(numbers: Int...) -> Int {
    var sum = 0
    for number in numbers {
        sum += number
    }
    return sum
}
sumOf(42, 597, 12)

// Scala
def sumOf(numbers: Int*): Int = {
    var sum = 0
    for (number <- numbers) {
        sum += number
    }
    return sum
}
sumOf(42, 597, 12)

函数作为数据类型

// Swift
func makeIncrementer() -> (Int -> Int) {
    func addOne(number: Int) -> Int {
        return 1 + number
    }
    return addOne
}
var increment = makeIncrementer()
increment(7)

// Scala
def makeIncrementer(): Int => Int = {
    def addOne(number: Int): Int = {
        return 1 + number
    }
    return addOne
}
var increment = makeIncrementer()
increment(7)

集合迭代器（Map）

// Swift
var numbers = [20, 19, 7, 12]
numbers.map({ number in 3 * number })

// Scala
var numbers = Array(20, 19, 7, 12)
numbers.map( number => 3 * number )

排序

// Swift
sort([1, 5, 3, 12, 2]) { $0 > $1 }

// Scala
Array(1, 5, 3, 12, 2).sortWith(_ > _)

命名参数

// Swift
def area(width: Int, height: Int) -> Int {
    return width * height
}

area(width: 10, height: 10)

// Scala
def area(width: Int, height: Int): Int = {
    return width * height
}

area(width = 10, height = 10)

类

定义

// Swift
class Shape {
    var numberOfSides = 0
    func simpleDescription() -> String {
        return "A shape with \(numberOfSides) sides."
    }
}

// Scala
class Shape {
    var numberOfSides = 0
    def simpleDescription(): String = {
        return s"A shape with $numberOfSides sides."
    }
}

使用

// Swift
var shape = Shape()
shape.numberOfSides = 7
var shapeDescription = shape.simpleDescription()

// Scala
var shape = new Shape()
shape.numberOfSides = 7
var shapeDescription = shape.simpleDescription()

子类

// Swift
class NamedShape {
    var numberOfSides: Int = 0
    var name: String

    init(name: String) {
        self.name = name
    }

    func simpleDescription() -> String {
        return "A shape with \(numberOfSides) sides."
    }
}

class Square: NamedShape {
    var sideLength: Double

    init(sideLength: Double, name: String) {
        self.sideLength = sideLength
        super.init(name: name)
        numberOfSides = 4
    }

    func area() -> Double {
        return sideLength * sideLength
    }

    override func simpleDescription() -> String {
        return "A square with sides of length
                \(sideLength)."
    }
}

let test = Square(sideLength: 5.2)
test.area()
test.simpleDescription()

// Scala
class NamedShape(var name: String) {
    var numberOfSides: Int = 0

    def simpleDescription() =
        s"A shape with $numberOfSides sides."
}

class Square(var sideLength: Double, name: String)
    extends NamedShape(name) {
    numberOfSides = 4
    def area() = sideLength * sideLength

    override def simpleDescription() =
        s"A square with sides of length $sideLength."
}

val test = new Square(5.2, "my test square")
test.area()
test.simpleDescription()

检查实例所属的类

// Swift
var movieCount = 0
var songCount = 0

for item in library {
    if item is Movie {
        ++movieCount
    } else if item is Song {
        ++songCount
    }
}

// Scala
var movieCount = 0
var songCount = 0

for (item <- library) {
    if (item.isInstanceOf[Movie]) {
        movieCount += 1
    } else if (item.isInstanceOf[Song]) {
        songCount += 1
    }
}

基类转派生类（向下转换）

// Swift
for object in someObjects {
    let movie = object as Movie
    println("Movie: '\(movie.name)', dir. \(movie.director)")
}

// Scala
for (obj <- someObjects) {
    val movie = obj.asInstanceOf[Movie]
    println(s"Movie: '${movie.name}', dir. ${movie.director}")
}

协议

// Swift
protocol Nameable {
    func name() -> String
}

func f<T: Nameable>(x: T) {
    println("Name is " + x.name())
}

// Scala
trait Nameable {
    def name(): String
}

def f[T <: Nameable](x: T) = {
    println("Name is " + x.name())
}

扩展

// Swift
extension Double {
    var km: Double { return self * 1_000.0 }
    var m: Double { return self }
    var cm: Double { return self / 100.0 }
    var mm: Double { return self / 1_000.0 }
    var ft: Double { return self / 3.28084 }
}
let oneInch = 25.4.mm
println("One inch is \(oneInch) meters")
// prints "One inch is 0.0254 meters"
let threeFeet = 3.ft
println("Three feet is \(threeFeet) meters")
// prints "Three feet is 0.914399970739201 meters"

// Scala
object Extensions {
    implicit class DoubleUnit(d: Double) {
        def km: Double = { return d * 1000.0 }
        def m: Double = { return d }
        def cm: Double = { return d / 100.0 }
        def mm: Double = { return d / 1000.0 }
        def ft: Double = { return d / 3.28084 }
    }
}

import Extensions.DoubleUnit

val oneInch = 25.4.mm
println(s"One inch is $oneInch meters")
// prints "One inch is 0.0254 meters"
val threeFeet = 3.ft
println(s"Three feet is $threeFeet meters")
// prints "Three feet is 0.914399970739201 meters"

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原文地址：https://leverich.github.io/swiftislikescala/
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