Structs, Enums and Traits

Rust Option: Some and None

Rust has no null. A value that may be missing is an Option, and the compiler makes you handle None.

Intermediate
3 min read
1 runnable example

What you'll learn

Some(value) and None
match and unwrap_or
map on an Option

Run code →

find_user(id)

Some

id 1 → Some("Ada")
use the name

None

id 3 → None
handle the missing case

A value that may be missing

Rust has no null. A value that may be missing has the type Option<T>, which is either Some(value) or None. The compiler makes you handle the None case before you can use the value.

find_user returns Some for a known id and None otherwise, and match handles both. unwrap_or supplies a default when the value is missing. map changes the value inside Some and leaves None alone.

Key points

Option<T> is Some(value) or None.
unwrap_or gives a default.
map changes the value inside Some.

Runnable Rust example

Rust

Run code →

main.rs

fn find_user(id: u32) -> Option<&'static str> {
    match id {
        1 => Some("Ada"),
        2 => Some("Linus"),
        _ => None,
    }
}

fn main() {
    for id in [1, 3] {
        match find_user(id) {
            Some(name) => println!("user {id}: {name}"),
            None => println!("user {id}: not found"),
        }
    }

    let name = find_user(9).unwrap_or("guest");
    println!("{name}");

    let length = find_user(2).map(|name| name.len());
    println!("{:?}", length);
}

Get a Run button for this example Result of running the code

user 1: Ada
user 3: not found
guest
Some(5)

Practice

Open the example in the online compiler, change a value or add a line, and predict the output before you run it. Then break the code on purpose and read the compiler's message.

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