Rust by Example

map() was described as a chainable way to simplify match statements. However, using map() on a function that returns an Option results in the nested Option>. Chaining multiple calls together can then become confusing. That's where another combinator called and_then(), known in some languages as flatmap, comes in.

and_then() calls its function input with the wrapped value and returns the result. If the Option is None, then it returns None instead.

In the following example, cookable_v2() results in an Option. Using map() instead of and_then() would have given an Option>, which is an invalid type for eat().

#![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 {

match food {

Food::Sushi => None,

_           => Some(food),

}

}

// We have the recipe for everything except Cordon Bleu.

fn have_recipe(food: Food) -> Option {

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 {

match have_recipe(food) {

None       => None,

Some(food) => match have_ingredients(food) {

None       => None,

Some(food) => Some(food),

},

}

}

// This can conveniently be rewritten more compactly with `and_then()`:

fn cookable_v2(food: Food) -> Option {

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);

}

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