Module nom::lib::std::option

Optional values.

Type Option represents an optional value: every Option is either Some and contains a value, or None, and does not. Option types are very common in Rust code, as they have a number of uses:

• Initial values
• Return values for functions that are not defined over their entire input range (partial functions)
• Return value for otherwise reporting simple errors, where None is returned on error
• Optional struct fields
• Struct fields that can be loaned or "taken"
• Optional function arguments
• Nullable pointers
• Swapping things out of difficult situations

Options are commonly paired with pattern matching to query the presence of a value and take action, always accounting for the None case.

fn divide(numerator: f64, denominator: f64) -> Option<f64> {
    if denominator == 0.0 {
        None
    } else {
        Some(numerator / denominator)
    }
}

// The return value of the function is an option
let result = divide(2.0, 3.0);

// Pattern match to retrieve the value
match result {
    // The division was valid
    Some(x) => println!("Result: {}", x),
    // The division was invalid
    None    => println!("Cannot divide by 0"),
}

Options and pointers ("nullable" pointers)

Rust's pointer types must always point to a valid location; there are no "null" pointers. Instead, Rust has optional pointers, like the optional owned box, Option<Box<T>>.

The following example uses Option to create an optional box of i32. Notice that in order to use the inner i32 value first, the check_optional function needs to use pattern matching to determine whether the box has a value (i.e., it is Some(...)) or not (None).

let optional = None;
check_optional(optional);

let optional = Some(Box::new(9000));
check_optional(optional);

fn check_optional(optional: Option<Box<i32>>) {
    match optional {
        Some(ref p) => println!("has value {}", p),
        None => println!("has no value"),
    }
}

This usage of Option to create safe nullable pointers is so common that Rust does special optimizations to make the representation of Option<Box<T>> a single pointer. Optional pointers in Rust are stored as efficiently as any other pointer type.

Examples

Basic pattern matching on Option:

let msg = Some("howdy");

// Take a reference to the contained string
if let Some(ref m) = msg {
    println!("{}", *m);
}

// Remove the contained string, destroying the Option
let unwrapped_msg = msg.unwrap_or("default message");

Initialize a result to None before a loop:

enum Kingdom { Plant(u32, &'static str), Animal(u32, &'static str) }

// A list of data to search through.
let all_the_big_things = [
    Kingdom::Plant(250, "redwood"),
    Kingdom::Plant(230, "noble fir"),
    Kingdom::Plant(229, "sugar pine"),
    Kingdom::Animal(25, "blue whale"),
    Kingdom::Animal(19, "fin whale"),
    Kingdom::Animal(15, "north pacific right whale"),
];

// We're going to search for the name of the biggest animal,
// but to start with we've just got `None`.
let mut name_of_biggest_animal = None;
let mut size_of_biggest_animal = 0;
for big_thing in &all_the_big_things {
    match *big_thing {
        Kingdom::Animal(size, name) if size > size_of_biggest_animal => {
            // Now we've found the name of some big animal
            size_of_biggest_animal = size;
            name_of_biggest_animal = Some(name);
        }
        Kingdom::Animal(..) | Kingdom::Plant(..) => ()
    }
}

match name_of_biggest_animal {
    Some(name) => println!("the biggest animal is {}", name),
    None => println!("there are no animals :("),
}

Structs

IntoIter	An iterator over the value in Some variant of an Option.
Iter	An iterator over a reference to the Some variant of an Option.
IterMut	An iterator over a mutable reference to the Some variant of an Option.
NoneError	Experimental The error type that results from applying the try operator (?) to a None value. If you wish to allow x? (where x is an Option<T>) to be converted into your error type, you can implement impl From<NoneError> for YourErrorType. In that case, x? within a function that returns Result<_, YourErrorType> will translate a None value into an Err result.

Enums

Option

The Option type. See the module level documentation for more.