Option<T>

Learn the Option<T> type and its Some/None variants.

Option<T>

Rust has no null. The "there may or may not be a value" case is expressed with an enum from the standard library:

enum Option<T> {
    Some(T),
    None,
}

Option, Some and None are already in scope (the prelude); they're used without writing Option:::

fn bol(a: i32, b: i32) -> Option<i32> {
    if b == 0 { None } else { Some(a / b) }
}

Unpacking an Option

Ways of reaching the inner value:

let r = bol(10, 2);
match r {
    Some(n) => println!("sonuç {n}"),
    None => println!("sıfıra bölme"),
}
let n = bol(1, 0).unwrap_or(0);        // 0 if None
let k = bol(9, 3).map(|v| v * 10);     // Some(30)

unwrap() panics on None; use it only where the value is certain to exist.

Exercise Safe division

Write fn bol(a: i32, b: i32) -> Option<i32>: None if b == 0, otherwise Some(a / b). Print the results of bol(10, 2) and bol(1, 0) with {:?}, separated by a space.

Show example solution

fn bol(a: i32, b: i32) -> Option<i32> {
    if b == 0 {
        None
    } else {
        Some(a / b)
    }
}

fn main() {
    println!("{:?} {:?}", bol(10, 2), bol(1, 0));
}

Test yourself

Why doesn't <code>let x: Option<i32> = Some(5); let y = x + 1;</code> compile?
- Some is misspelled
- It compiles and gives 6
- Option<i32> isn't an i32; it must be unpacked first
- 5 is too small
Show answer

Option<i32> isn't an i32; it must be unpacked first — The compiler forces you to think about the None case; this eliminates null bugs at compile time.

No value

fn bol(a: i32, b: i32) -> Option<i32> {
    if b == 0 { ____ } else { Some(a / b) }
}

Show answer

fn bol(a: i32, b: i32) -> Option<i32> {
    if b == 0 { None } else { Some(a / b) }
}

Expression and result

bol(10, 2)

Some(5)
bol(1, 0)

None
bol(1, 0).unwrap_or(0)

0
bol(9, 3).map(|v| v * 10)

Some(30)

Next lesson: Option instead of null

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