Combinators: and_then - Rust By Example
Description: A description

# Combinators: `and_then`

`map()` was described as a chainable way to simplify `match` statements. However, using `map()` on a function that returns an `Option<T>` results in the nested `Option<Option<T>>`. 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<Food>`. Using `map()` instead of `and_then()` would have given an `Option<Option<Food>>`, 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<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 ingredients and the recipe.
    // We can represent the logic with a chain of `match`es:
    fn cookable_v1(food: Food) -> Option<Food> {
        match have_ingredients(food) {
            None       => None,
            Some(food) => match have_recipe(food) {
                None       => None,
                Some(food) => Some(food),
            },
        }
    }

    // This can conveniently be rewritten more compactly with `and_then()`:
    fn cookable_v2(food: Food) -> Option<Food> {
        have_ingredients(food).and_then(have_recipe)
    }

    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);
    }

### See also:

closures, `Option`, and `Option::and_then()`