环境

• Rust 1.56.1
• VSCode 1.61.2

概念

参考：https://doc.rust-lang.org/rust-by-example/error/option_unwrap/and_then.html

示例

main.rs

#![allow(dead_code)]

#[derive(Debug)]
enum Food {
    CordonBleu,
    Steak,
    Sushi,
}
#[derive(Debug)]
enum Day {
    Monday,
    Tuesday,
    Wednesday,
}

fn have_ingredients(food: Food) -> Option<Food> {
    match food {
        Food::Sushi => None,
        _ => Some(food),
    }
}

fn have_recipe(food: Food) -> Option<Food> {
    match food {
        Food::CordonBleu => None,
        _ => Some(food),
    }
}

fn cookable_v1(food: Food) -> Option<Food> {
    match have_recipe(food) {
        None => None,
        Some(food) => match have_ingredients(food) {
            None => None,
            Some(food) => Some(food),
        },
    }
}

fn cookable_v2(food: Food) -> Option<Food> {
    have_recipe(food).and_then(have_ingredients)
}

fn eat(food: Food, day: Day) {
    match cookable_v2(food) {
        Some(food) => println!("Yay! On {:?} we get to eat {:?}.", day, food),
        None => println!("Oh no. We don't get to eat on {:?}?", day),
    }
}

fn main() {
    let (cordon_bleu, steak, sushi) = (Food::CordonBleu, Food::Steak, Food::Sushi);

    eat(cordon_bleu, Day::Monday);
    eat(steak, Day::Tuesday);
    eat(sushi, Day::Wednesday);
}

总结

了解了 Rust 中 Option 的 and_then 方法，和其它语言中的 flatMap 类似。

附录