Option & unwrap - Rust By Example
Description: A description

# `Option` & `unwrap`

In the last example, we showed that we can induce program failure at will. We told our program to `panic` if the princess received an inappropriate gift - a snake. But what if the princess expected a gift and didn't receive one? That case would be just as bad, so it needs to be handled!

We could test this against the null string (`""`) as we do with a snake. Since we're using Rust, let's instead have the compiler point out cases where there's no gift.

An `enum` called `Option<T>` in the `std` library is used when absence is a possibility. It manifests itself as one of two "options":

- `Some(T)`: An element of type `T` was found
- `None`: No element was found

These cases can either be explicitly handled via `match` or implicitly with `unwrap`. Implicit handling will either return the inner element or `panic`.

Note that it's possible to manually customize `panic` with expect, but `unwrap` otherwise leaves us with a less meaningful output than explicit handling. In the following example, explicit handling yields a more controlled result while retaining the option to `panic` if desired.

    // The commoner has seen it all, and can handle any gift well.
    // All gifts are handled explicitly using `match`.
    fn give_commoner(gift: Option<&str>) {
        // Specify a course of action for each case.
        match gift {
            Some("snake") => println!("Yuck! I'm putting this snake back in the forest."),
            Some(inner)   => println!("{}? How nice.", inner),
            None          => println!("No gift? Oh well."),
        }
    }

    // Our sheltered princess will `panic` at the sight of snakes.
    // All gifts are handled implicitly using `unwrap`.
    fn give_princess(gift: Option<&str>) {
        // `unwrap` returns a `panic` when it receives a `None`.
        let inside = gift.unwrap();
        if inside == "snake" { panic!("AAAaaaaa!!!!"); }

        println!("I love {}s!!!!!", inside);
    }

    fn main() {
        let food  = Some("cabbage");
        let snake = Some("snake");
        let void  = None;

        give_commoner(food);
        give_commoner(snake);
        give_commoner(void);

        let bird = Some("robin");
        let nothing = None;

        give_princess(bird);
        give_princess(nothing);
    }