关于rust:rust学习使用-if-let-实现简单的控制流

if let 语法使您能够将 if 和 let 组合成一种不太简短的形式来解决与一种模式匹配的值，而疏忽其余模式。例如：该程序与 Option <u8> 值匹配，但仅想在值为 3 时执行代码：

fn main() {let some_u8_value = Some(0u8);
    match some_u8_value {Some(3) => println!("three"),
        _ => (println!("啥也不是")),
    }
}

咱们想对 Some(3) 匹配做点什么，但对其余 Some <u8> 值或 None 值什么都不做。为了满足 match 表达式，咱们只须要解决一个变体就增加_ =>()，然而须要增加的许多样板代码。
相同，咱们能够应用 if let 以更短的形式编写此代码并实现完全一致的匹配成果：

fn main() {let some_u8_value = Some(8u8);
    if let Some(3) = some_u8_value {println!("three");
    }

    if let Some(8) = some_u8_value {println!("匹配到了");
    }
}

D:\learn\cargo_learn>cargo run
   Compiling cargo_learn v0.1.0 (D:\learn\cargo_learn)
    Finished dev [unoptimized + debuginfo] target(s) in 0.27s
     Running `target\debug\cargo_learn.exe`
匹配到了

相应的如果咱们想要失去一个确切的后果，比方如果没有匹配到咱们须要的那个咱们心愿有返回后果，那么此时能够应用 else:

#[derive(Debug)]
enum UsState {
    Alabama,
    Alaska,
}

enum Coin {
    Penny,
    Quarter(UsState),
}

fn main() {
    let coin = Coin::Penny;
    let mut count = 0;
    if let Coin::Quarter(state) = coin {println!("State quarter from {:?}!", state);
    } else {
        count += 1;
        println!("没有匹配到任何后果，count 将主动 +1！！！")
    }
}


D:\learn\cargo_learn>cargo run
   Compiling cargo_learn v0.1.0 (D:\learn\cargo_learn)
warning: variable `count` is assigned to, but never used
  --> src\main.rs:14:9
   |
14 |     let mut count = 0;
   |         ^^^^^^^^^
   |
   = note: `#[warn(unused_variables)]` on by default
   = note: consider using `_count` instead

warning: value assigned to `count` is never read
  --> src\main.rs:18:9
   |
18 |         count += 1;
   |         ^^^^^
   |
   = note: `#[warn(unused_assignments)]` on by default
   = help: maybe it is overwritten before being read?

warning: variant is never constructed: `Alabama`
 --> src\main.rs:3:5
  |
3 |     Alabama,
  |     ^^^^^^^
  |
  = note: `#[warn(dead_code)]` on by default

warning: variant is never constructed: `Alaska`
 --> src\main.rs:4:5
  |
4 |     Alaska,
  |     ^^^^^^

warning: variant is never constructed: `Quarter`
 --> src\main.rs:9:5
  |
9 |     Quarter(UsState),
  |     ^^^^^^^^^^^^^^^^

warning: 5 warnings emitted

    Finished dev [unoptimized + debuginfo] target(s) in 0.51s
     Running `target\debug\cargo_learn.exe`
没有匹配到任何后果，count 将主动 +1！！！