使用where语句之前必须在where之前有一个主语变量名

• 协议约束

//基类A继承了SomeProtocol协议才能添加扩展
extension SomeProtocol where Self: A {
    func showParamA() {
        print(self.a)
    }
}

• for...in...遍历

let arr = [1, 2, 4, 5, 6]
    
for (index, value) in arr.enumerated() where value > 4 {
    print(index, value)
}

• case 语句，相当于if判断

let arr = [1, 2, 4, 5, 6]
    
for (index, value) in arr.enumerated() where value > 4 {
   
    switch value {
    case let a where a < 6:
        print(index)
    default:
        print(value)
    }
}

• if let 和 guard中，在swift4.0版本以后，使用逗号代替where

let str : String? = "hello"
    
if let value  = str, value.count == 5 {
    print(value)
}
    
guard let value  = str, value.count == 5  else { return }
print(value)