组合器：and_then

map() 被描述为一种可链接的方式来简化 match 语句。但是，在返回 Option<T> 的函数上使用 map() 会导致嵌套的 Option<Option<T>>。将多个调用链接在一起可能会变得混乱。这就是另一个名为 and_then() 的组合器（在某些语言中称为 flatmap）的用武之地。

and_then() 使用包装的值调用其函数输入并返回结果。如果 Option 是 None，则它返回 None。

在以下示例中，cookable_v3() 会生成一个 Option<Food>。使用 map() 而不是 and_then() 会得到一个 Option<Option<Food>>，这对 eat() 来说是无效类型。

#![allow(dead_code)]

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

// We don't have the ingredients to make Sushi.
fn have_ingredients(food: Food) -> Option<Food> {
    match food {
        Food::Sushi => None,
        _           => Some(food),
    }
}

// We have the recipe for everything except Cordon Bleu.
fn have_recipe(food: Food) -> Option<Food> {
    match food {
        Food::CordonBleu => None,
        _                => Some(food),
    }
}

// To make a dish, we need both the recipe and the ingredients.
// We can represent the logic with a chain of `match`es:
fn cookable_v1(food: Food) -> Option<Food> {
    match have_recipe(food) {
        None       => None,
        Some(food) => have_ingredients(food),
    }
}

// This can conveniently be rewritten more compactly with `and_then()`:
fn cookable_v3(food: Food) -> Option<Food> {
    have_recipe(food).and_then(have_ingredients)
}

// Otherwise we'd need to `flatten()` an `Option<Option<Food>>`
// to get an `Option<Food>`:
fn cookable_v2(food: Food) -> Option<Food> {
    have_recipe(food).map(have_ingredients).flatten()
}

fn eat(food: Food, day: Day) {
    match cookable_v3(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);
}

另请参阅

闭包、Option、Option::and_then() 和 Option::flatten()