Rust by Example

24 Option

Sometimes it's desirable to catch the failure of some parts of a program instead of calling panic!; this can be accomplished using the Option enum.

The Option<T> enum has two variants:

  • None, to indicate failure or lack of value, and
  • Some(value), a tuple struct that wraps a value with type T.
// An integer division that doesn't `panic!` fn checked_division(dividend: i32, divisor: i32) -> Option<i32> { if divisor == 0 { // Failure is represented as the `None` variant None } else { // Result is wrapped in a `Some` variant Some(dividend / divisor) } } // This function handles a division that may not succeed fn try_division(dividend: i32, divisor: i32) { // `Option` values can be pattern matched, just like other enums match checked_division(dividend, divisor) { None => println!("{} / {} failed!", dividend, divisor), Some(quotient) => { println!("{} / {} = {}", dividend, divisor, quotient) }, } } fn main() { try_division(4, 2); try_division(1, 0); // Binding `None` to a variable needs to be type annotated let none: Option<i32> = None; let _equivalent_none = None::<i32>; let optional_float = Some(0f32); // The `unwrap` method will extract the value wrapped in a `Some` variant, // or will `panic!` if called on a `None` variant println!("{:?} unwraps to {:?}", optional_float, optional_float.unwrap()); println!("{:?} unwraps to {:?}", none, none.unwrap()); }