swift模式匹配

模式

什么是模式？

模式是用于匹配的规则，比如switch的case、捕捉错误的catch、if\guard\while\for语句的条件等

Swift中的模式有

• 通配符模式（Wildcard Pattern）
• 标识符模式（Identifier Pattern）
• 值绑定模式（Value-Binding Pattern）
• 元组模式（Tuple Pattern）
• 枚举Case模式（Enumeration Case Pattern）
• 可选模式（Optional Pattern）
• 类型转换模式（Type-Casting Pattern）
• 表达式模式（Expression Pattern）

通配符模式（Wildcard Pattern）

_ 匹配任何值

_? 匹配非nil值

enum Life { 
  case human(name: String, age: Int?) 
  case animal(name: String, age: Int?) 
}

func check(_ life: Life) {
  switch life { 
    case .human(let name, _):
        print("human", name) 
    case .animal(let name, _?):
        print("animal", name) 
    default:
        print("other") 
  }
}

check(.human(name: "Rose", age: 20)) // human Rose 
check(.human(name: "Jack", age: nil)) // human Jack 
check(.animal(name: "Dog", age: 5)) // animal Dog 
check(.animal(name: "Cat", age: nil)) // other

标识符模式（Identifier Pattern）

给对应的变量、常量名赋值

var age = 10 let name = "jack"

值绑定模式（Value-Binding Pattern）

let point = (3, 2) 
switch point { 
    case let (x, y):
    print("The point is at (\(x), \(y)).") 
}

元组模式（Tuple Pattern）

let points = [(0, 0), (1, 0), (2, 0)] 
for (x, _) in points { print(x) }

let name: String? = "jack" 
let age = 18 
let info: Any = [1, 2] 
switch (name, age, info) { 
  case (_?, _ , _ as String):
  print("case") 
  default:
  print("default") 
} // default

枚举Case模式（Enumeration Case Pattern）

if case语句等价于只有1个case的switch语句

let age = 2 // 原来的写法 
if age >= 0 && age <= 9 {
  print("[0, 9]") 
} // 枚举Case模式 
if case 0...9 = age {
  print("[0, 9]") 
} 
guard case 0...9 = age else { return } 
print("[0, 9]")

let ages: [Int?] = [2, 3, nil, 5] 
for case nil in ages {
  print("有nil值")
  break 
} // 有nil值

可选模式（Optional Pattern）

let age: Int? = 42 
if case .some(let x) = age { print(x) } 
if case let x? = age { print(x) }

let ages: [Int?] = [nil, 2, 3, nil, 5] 
for case let age? in ages { 
  print(age) 
} // 2 3 5

let ages: [Int?] = [nil, 2, 3, nil, 5] 
for item in ages {
  if let age = item {
    print(age)
  } 
} // 跟上面的for，效果是等价的

func check(_ num: Int?) {
  switch num { 
    case 2?: print("2") 
    case 4?: print("4") 
    case 6?: print("6") 
    case _?: print("other") 
    case _: print("nil") 
  }
} 
check(4) // 4 
check(8) // 
other check(nil) // nil

类型转换模式（Type-Casting Pattern）

let num: Any = 6 
switch num { 
  case is Int:
  // 编译器依然认为num是Any类型
  print("is Int", num) 
  //case let n as Int:
  // print("as Int", n + 1) 
  default:
  break 
}

class Animal { 
  func eat() { 
    print(type(of: self), "eat") 
  } 
} 
class Dog : Animal { 
  func run() { print(type(of: self), "run") } 
} 
class Cat : Animal { 
  func jump() { print(type(of: self), "jump") } 
} 
func check(_ animal: Animal) {
  switch animal { 
    case let dog as Dog:
    dog.eat()
    dog.run() 
    case is Cat:
    animal.eat() 
    default: break 
  }
} 
// Dog eat 
// Dog run 
check(Dog()) 
// Cat eat 
check(Cat())

表达式模式（Expression Pattern）

表达式模式用在case中

let point = (1, 2) 
switch point { 
  case (0, 0):
  print("(0, 0) is at the origin.") 
  case (-2...2, -2...2):
  print("(\(point.0), \(point.1)) is near the origin.") 
  default:
  print("The point is at (\(point.0), \(point.1)).") 
} // (1, 2) is near the origin.

自定义表达式模式

可以通过重载运算符，自定义表达式模式的匹配规则

struct Student { 
  var score = 0, name = "" 
  static func ~= (pattern: Int, value: Student) -> Bool { value.score >= pattern } 
  static func ~= (pattern: ClosedRange, value: Student) -> Bool { pattern.contains(value.score) } 
  static func ~= (pattern: Range, value: Student) -> Bool { pattern.contains(value.score) } 
}

var stu = Student(score: 75, name: "Jack") 
switch stu { 
  case 100: print(">= 100") 
  case 90: print(">= 90") 
  case 80..<90: print("[80, 90)") 
  case 60...79: print("[60, 79]") 
  case 0: print(">= 0") 
  default: break 
} // [60, 79]

if case 60 = stu { print(">= 60") } // >= 60

var info = (Student(score: 70, name: "Jack"), "及格") 
switch info { 
  case let (60, text): print(text) 
  default: break 
} // 及格

extension String { 
  static func ~= (pattern: (String) -> Bool, value: String) -> Bool { 
    pattern(value) 
  } 
}

func hasPrefix(_ prefix: String) -> ((String) -> Bool) { { $0.hasPrefix(prefix) } } 
func hasSuffix(_ suffix: String) -> ((String) -> Bool) { { $0.hasSuffix(suffix) } }

var str = "jack" 
switch str { 
  case hasPrefix("j"), hasSuffix("k"):
  print("以j开头，以k结尾") 
  default: break 
} // 以j开头，以k结尾

func isEven(_ i: Int) -> Bool { i % 2 == 0 } 
func isOdd(_ i: Int) -> Bool { i % 2 != 0 }

extension Int { 
  static func ~= (pattern: (Int) -> Bool, value: Int) -> Bool { 
    pattern(value) 
  } 
}


prefix operator ~> 
prefix operator ~>= 
prefix operator ~< 
prefix operator ~<= 
prefix func ~> (_ i: Int) -> ((Int) -> Bool) { { $0 > i } } 
prefix func ~>= (_ i: Int) -> ((Int) -> Bool) { { $0 >= i } } 
prefix func ~< (_ i: Int) -> ((Int) -> Bool) { { $0 < i } } 
prefix func ~<= (_ i: Int) -> ((Int) -> Bool) { { $0 <= i } }

var age = 9 
switch age { 
  case isEven:
  print("偶数") 
  case isOdd:
  print("奇数") 
  default:
  print("其他") 
}

var age = 9 switch age { 
  case ~>=0:
  print("1") 
  case ~>10:
  print("2") 
  default: break 
} // [0, 10]

where

可以使用where为模式匹配增加匹配条件

var data = (10, "Jack") 
switch data { 
  case let (age, _) where age > 10:
  print(data.1, "age>10") 
  case let (age, _) where age > 0:
  print(data.1, "age>0") 
  default: break 
}

var ages = [10, 20, 44, 23, 55] 
for age in ages where age > 30 { print(age) } // 44 55

protocol Stackable { associatedtype Element } 
protocol Container { associatedtype Stack : Stackable where Stack.Element : Equatable }

func equal(_ s1: S1, _ s2: S2) -> Bool where S1.Element == S2.Element, S1.Element : Hashable { 
    return false   
}

extension Container where Self.Stack.Element : Hashable { }