Combinators: map - Rust By Example
Description: Rust by Example (RBE) is a collection of runnable examples that illustrate various Rust concepts and standard libraries.

# Combinators: `map`

`match` is a valid method for handling `Option`s. However, you may eventually find heavy usage tedious, especially with operations only valid with an input. In these cases, combinators can be used to manage control flow in a modular fashion.

`Option` has a built in method called `map()`, a combinator for the simple mapping of `Some -> Some` and `None -> None`. Multiple `map()` calls can be chained together for even more flexibility.

In the following example, `process()` replaces all functions previous to it while staying compact.

    #![allow(dead_code)]

    #[derive(Debug)] enum Food { Apple, Carrot, Potato }

    #[derive(Debug)] struct Peeled(Food);
    #[derive(Debug)] struct Chopped(Food);
    #[derive(Debug)] struct Cooked(Food);

    // Peeling food. If there isn't any, then return `None`.
    // Otherwise, return the peeled food.
    fn peel(food: Option<Food>) -> Option<Peeled> {
        match food {
            Some(food) => Some(Peeled(food)),
            None       => None,
        }
    }

    // Chopping food. If there isn't any, then return `None`.
    // Otherwise, return the chopped food.
    fn chop(peeled: Option<Peeled>) -> Option<Chopped> {
        match peeled {
            Some(Peeled(food)) => Some(Chopped(food)),
            None               => None,
        }
    }

    // Cooking food. Here, we showcase `map()` instead of `match` for case handling.
    fn cook(chopped: Option<Chopped>) -> Option<Cooked> {
        chopped.map(|Chopped(food)| Cooked(food))
    }

    // A function to peel, chop, and cook food all in sequence.
    // We chain multiple uses of `map()` to simplify the code.
    fn process(food: Option<Food>) -> Option<Cooked> {
        food.map(|f| Peeled(f))
            .map(|Peeled(f)| Chopped(f))
            .map(|Chopped(f)| Cooked(f))
    }

    // Check whether there's food or not before trying to eat it!
    fn eat(food: Option<Cooked>) {
        match food {
            Some(food) => println!("Mmm. I love {:?}", food),
            None       => println!("Oh no! It wasn't edible."),
        }
    }

    fn main() {
        let apple = Some(Food::Apple);
        let carrot = Some(Food::Carrot);
        let potato = None;

        let cooked_apple = cook(chop(peel(apple)));
        let cooked_carrot = cook(chop(peel(carrot)));
        // Let's try the simpler looking `process()` now.
        let cooked_potato = process(potato);

        eat(cooked_apple);
        eat(cooked_carrot);
        eat(cooked_potato);
    }

### See also:

closures, `Option`, `Option::map()`