比方说现在有一个需求

当x小于0 打印 "negative" 当x等于0 打印 "zero" 当x大于0 打印 "positive"

加入不使用if...else，用match

  fn main(){
      let x = 4;
      match x {
	  a if a < 0 => println!("negative"),
	  a if a ==0 => println!("zero"),
	  a  => println!("positive"),

      }
  }

除了这样写，好像还可以

  fn main(){
      let x = 4;
      match x {
	  a @ i32::MIN..=-1 => println!("negative"),
	  a @ 0 => println!("zero"),
	  a @ 1..=i32::MAX => println!("positive"),

      }
  }

但是我还是没搞清楚 @和 if在 match 这里用法的区别，

但我感觉@是不是只能是一个值或者离散值啊，if可以范围

而且好像还可以混用

  fn main(){
      let x = -1;
      match x {
	  a @ x if a < 0 => println!("negative"),
	  a if a ==0 => println!("zero"),
	  a  => println!("positive"),

      }
  }

有点迷糊