SWIFT 类似 SCALA

各个编程语言都有类似的地方，我们只要精通一门语言，就可以对比着学习，要不了多久就可以精通另一门语言，在网上看到Swift语言和Scala语言的对比，我们就来看看他们之间的一些相似之处。

在WWDC 14上，苹果公司发布了一门新的编程语言Swift。这里有一个Swift和Scala的源代码对比，使用的是苹果公司在iTunes商店出版的《Swift编程语言》一书中的例子。

我猜测这两者是相关的，从语法上讲，Swift是Scala的一种方言。Swift从Scala继承了苹果列出的大部分标志性特性：类型推断、闭包、元组、协议、扩展、泛型、读取-执行-打印-循环（Read-Eval-Print-Loop）等等。

尽管它们的语法相似，Swift的运行时环境与Scala的截然不同，这可能是新语言最有趣的地方。Scala编译到JVM，使用垃圾收集，它的对象模型透明地与Java集成。Swift编译为本机代码，使用自动引用计数，其对象模型透明地与Objective-C集成。因此，这两种语言之间的相似性并没有延伸到表面之下。

Den Shabalin 提供了另一个优秀的 逐点比较.

p.s. 不要把这段代码看得太严重!大部分都是死记硬背，简化了苹果给出的快速例子。我给出了一些更符合Scala习惯用法的翻译。如果您发现任何令人难以忍受的问题，请随时提交GitHub问题/拉请求。

基础

Hello World

SWIFT

println("Hello, world!")

SCALA

println("Hello, world!")

变量和常量

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

字符串插值

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."

半开区间运算符 （Range Operator）

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

闭区间运算符（Inclusive Range Operator）

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/列表（Arrays）

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"

映射 Maps

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"

Empty Collections 空集合

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

Functions 函数

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")

Scala 惯用法 (IDIOMATIC SCALA)

def greet(name: String, day: String): String =
    s"Hello $name, today is $day."

greet("Bob", "Tuesday")

元组返回 Tuple Return

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)
}

Scala 惯用法 IDIOMATIC SCALA

def getGasPrices = (3.59, 3.69, 3.79)

Variable Number Of Arguments 可变参数

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)

Scala 惯用法 IDIOMATIC SCALA

Array(42, 597, 12).sum

Function Type 函数类型

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)

Scala 惯用法 IDIOMATIC SCALA

def makeIncrementer: Int => Int = 
    (number: Int) => 1 + number

var increment = makeIncrementer
increment(7)

映射 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 Sort

SWIFT

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

SCALA

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

Named Arguments 命名参数

SWIFT

func 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

Declaration 声明

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."
    }
}

Scala 惯用法 IDIOMATIC SCALA

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

Usage 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()

Subclass 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, name: "my test square")
test.area()
test.simpleDescription()

Scala 惯用法 IDIOMATIC 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

类型检测 Checking Type

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
    }
}

Pattern Match 模式匹配

SWIFT

var movieCount = 0
var songCount = 0

for item in library {
  switch item {
    case let movie as Movie:
      ++movieCount
      println("Movie: '\(movie.name)', dir. \(movie.director)")
    case let song as Song:
      ++songCount
      println("Song: '\(song.title)'")
  }
}

SCALA

var movieCount = 0
var songCount = 0

for (item <- library) {
  item match {
    case movie: Movie =>
      movieCount += 1
      println(s"Movie: '${movie.name}', dir. ${movie.director}")
    case song: Song =>
      songCount += 1
      println(s"Song: '${song.title}'")
  }
}

向下类型转换 Downcasting

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 Protocol

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())
}

类型扩展 Extensions

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"

Scala惯用法 IDIOMATIC SCALA

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