解包选项和默认值

有多种方法可以解包 Option 并在其为 None 时回退到默认值。要选择满足我们需求的方法，我们需要考虑以下因素

• 我们需要急切求值还是惰性求值？
• 我们需要保持原始空值不变，还是对其进行修改？

or() 可链接，急切求值，保持空值不变

or() 可链接并急切求值其参数，如下例所示。请注意，由于 or 的参数是急切求值的，因此传递给 or 的变量会被移动。

#[derive(Debug)] 
enum Fruit { Apple, Orange, Banana, Kiwi, Lemon }

fn main() {
    let apple = Some(Fruit::Apple);
    let orange = Some(Fruit::Orange);
    let no_fruit: Option<Fruit> = None;

    let first_available_fruit = no_fruit.or(orange).or(apple);
    println!("first_available_fruit: {:?}", first_available_fruit);
    // first_available_fruit: Some(Orange)

    // `or` moves its argument.
    // In the example above, `or(orange)` returned a `Some`, so `or(apple)` was not invoked.
    // But the variable named `apple` has been moved regardless, and cannot be used anymore.
    // println!("Variable apple was moved, so this line won't compile: {:?}", apple);
    // TODO: uncomment the line above to see the compiler error
 }

or_else() 可链接，惰性求值，保持空值不变

另一种选择是使用 or_else，它也是可链接的，并且惰性求值，如下例所示

#[derive(Debug)] 
enum Fruit { Apple, Orange, Banana, Kiwi, Lemon }

fn main() {
    let no_fruit: Option<Fruit> = None;
    let get_kiwi_as_fallback = || {
        println!("Providing kiwi as fallback");
        Some(Fruit::Kiwi)
    };
    let get_lemon_as_fallback = || {
        println!("Providing lemon as fallback");
        Some(Fruit::Lemon)
    };

    let first_available_fruit = no_fruit
        .or_else(get_kiwi_as_fallback)
        .or_else(get_lemon_as_fallback);
    println!("first_available_fruit: {:?}", first_available_fruit);
    // Providing kiwi as fallback
    // first_available_fruit: Some(Kiwi)
}

get_or_insert() 急切求值，修改空值

为了确保 Option 包含一个值，我们可以使用 get_or_insert 用回退值对其进行修改，如下例所示。请注意，get_or_insert 急切求值其参数，因此变量 apple 被移动

#[derive(Debug)]
enum Fruit { Apple, Orange, Banana, Kiwi, Lemon }

fn main() {
    let mut my_fruit: Option<Fruit> = None;
    let apple = Fruit::Apple;
    let first_available_fruit = my_fruit.get_or_insert(apple);
    println!("first_available_fruit is: {:?}", first_available_fruit);
    println!("my_fruit is: {:?}", my_fruit);
    // first_available_fruit is: Apple
    // my_fruit is: Some(Apple)
    //println!("Variable named `apple` is moved: {:?}", apple);
    // TODO: uncomment the line above to see the compiler error
}

get_or_insert_with() 惰性求值，修改空值

我们可以传递一个闭包给 get_or_insert_with，而不是显式提供一个回退值，如下所示

#[derive(Debug)] 
enum Fruit { Apple, Orange, Banana, Kiwi, Lemon }

fn main() {
    let mut my_fruit: Option<Fruit> = None;
    let get_lemon_as_fallback = || {
        println!("Providing lemon as fallback");
        Fruit::Lemon
    };
    let first_available_fruit = my_fruit
        .get_or_insert_with(get_lemon_as_fallback);
    println!("first_available_fruit is: {:?}", first_available_fruit);
    println!("my_fruit is: {:?}", my_fruit);
    // Providing lemon as fallback
    // first_available_fruit is: Lemon
    // my_fruit is: Some(Lemon)

    // If the Option has a value, it is left unchanged, and the closure is not invoked
    let mut my_apple = Some(Fruit::Apple);
    let should_be_apple = my_apple.get_or_insert_with(get_lemon_as_fallback);
    println!("should_be_apple is: {:?}", should_be_apple);
    println!("my_apple is unchanged: {:?}", my_apple);
    // The output is a follows. Note that the closure `get_lemon_as_fallback` is not invoked
    // should_be_apple is: Apple
    // my_apple is unchanged: Some(Apple)
}

另请参阅

闭包，get_or_insert，get_or_insert_with，移动变量，or，or_else