返回Option时不需要一些吗？

我正在做“ Rust by Example”的更多组合器部分，并决定自己动手，看看用map代替多少努力and_then。

在尝试过程中，我遇到了一些非常奇怪的事情（对我来说，它甚至看起来像是编译器错误）。它看起来像我只需要返回一个Food类型时，返回类型应该是Option(Food)

我认为，cookable下面的功能应该可以简化为一行：

have_ingredients(food).map(|f| can_cook(f))

显然也可以是：

have_ingredients(food).and_then(can_cook)

尽管我看不到这两个函数之间的根本区别，但它们都返回Option<U>。

这样做时，我遇到一个奇怪的编译器错误，因此我按如下所示明确地破坏了匹配-看起来Food即使返回类型为，编译器也想返回Some(Food)。到底是怎么回事？？？

//! stack.rs
#[derive(Debug)]
enum Food {
    CordonBleu,
    Steak,
    Sushi,
}

#[derive(Debug)]
enum Day {
    Monday,
    Tuesday,
    Wednesday,
}

/// we don't have the ingredients for sushi
fn have_ingredients(food: Food) -> Option<Food> {
    match food {
        Food::Sushi => None,
        _ => Some(food),
    }
}

/// can cook anything but cordon blue
fn can_cook(food: Food) -> Option<Food> {
    match food {
        Food::CordonBlue => None,
        _ => Some(food),
    }
}

/// can be done using map
fn cookable(food: Food) -> Option<Food> {
    match have_ingredients(food).map(|f| can_cook(f)) {
        // Some(food) => food,  // Why is this correct???
        Some(food) => Some(food), // **error: mismatched types:
        None => None,
    }
}

fn eat(food: Food, day: Day) {
    match cookable(food) {
        Some(food) => println!("Yay! On {:?} we eat {:?}", day, food),
        None => println!("Oh no we didn'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);
}

这是上述程序的完整编译器错误：

 ch16_errors git:(master) ✗ rustc stack.rs
stack.rs:38:28: 38:32 error: mismatched types:
 expected `Food`,
    found `core::option::Option<Food>`
(expected enum `Food`,
    found enum `core::option::Option`) [E0308]
stack.rs:38         Some(food) => Some(food),
                                       ^~~~
stack.rs:38:28: 38:32 help: run `rustc --explain E0308` to see a detailed explanation
error: aborting due to previous error