map(_:)

let ranks = cards.map{ $0.Rank }

这是语法糖：

let ranks = cards.map({ (card: Card) -> Rank in
    return card.rank
})

应用的语法糖包括：

1. 尾随闭包语法

   let ranks = cards.map { (card: Card) -> Rank in
   return card.rank
   

   }

2. 参数类型推断

   let ranks = cards.map { card -> Rank in
   return card.rank
   

   }

3. 返回类型推断

   let ranks = cards.map { card in
   return card.rank
   

   }

4. 隐式返回值

   let ranks = cards.map { card in
   

   card.rank
   }

5. 匿名闭包参数

   let ranks = cards.map { $0.rank }
   

请查看有关闭包的语言指南部分（C＃称为
lambdas 的Swift名称）以获取更多信息，