Unpacking options and defaults

There is more than one way to unpack an Option and fall back on a default if it is None. To choose the one that meets our needs, we need to consider the following:

• do we need eager or lazy evaluation?
• do we need to keep the original empty value intact, or modify it in place?

or() is chainable, evaluates eagerly, keeps empty value intact

or()is chainable and eagerly evaluates its argument, as is shown in the following example. Note that because or's arguments are evaluated eagerly, the variable passed to or is moved.

#[derive(Debug)] 
enum Fruit { Apple, Orange, Banana, Kiwi, Lemon }

fn main() {
    let apple = Some(Fruit::Apple);
    let orange = Some(Fruit::Orange);
    let no_fruit: Option<Fruit> = None;

    let first_available_fruit = no_fruit.or(orange).or(apple);
    println!("first_available_fruit: {:?}", first_available_fruit);
    // first_available_fruit: Some(Orange)

    // `or` moves its argument.
    // In the example above, `or(orange)` returned a `Some`, so `or(apple)` was not invoked.
    // But the variable named `apple` has been moved regardless, and cannot be used anymore.
    // println!("Variable apple was moved, so this line won't compile: {:?}", apple);
    // TODO: uncomment the line above to see the compiler error
 }

or_else() is chainable, evaluates lazily, keeps empty value intact

Another alternative is to use or_else, which is also chainable, and evaluates lazily, as is shown in the following example:

#[derive(Debug)] 
enum Fruit { Apple, Orange, Banana, Kiwi, Lemon }

fn main() {
    let no_fruit: Option<Fruit> = None;
    let get_kiwi_as_fallback = || {
        println!("Providing kiwi as fallback");
        Some(Fruit::Kiwi)
    };
    let get_lemon_as_fallback = || {
        println!("Providing lemon as fallback");
        Some(Fruit::Lemon)
    };

    let first_available_fruit = no_fruit
        .or_else(get_kiwi_as_fallback)
        .or_else(get_lemon_as_fallback);
    println!("first_available_fruit: {:?}", first_available_fruit);
    // Providing kiwi as fallback
    // first_available_fruit: Some(Kiwi)
}

get_or_insert() evaluates eagerly, modifies empty value in place

To make sure that an Option contains a value, we can use get_or_insert to modify it in place with a fallback value, as is shown in the following example. Note that get_or_insert eagerly evaluates its parameter, so variable apple is moved:

#[derive(Debug)]
enum Fruit { Apple, Orange, Banana, Kiwi, Lemon }

fn main() {
    let mut my_fruit: Option<Fruit> = None;
    let apple = Fruit::Apple;
    let first_available_fruit = my_fruit.get_or_insert(apple);
    println!("first_available_fruit is: {:?}", first_available_fruit);
    println!("my_fruit is: {:?}", my_fruit);
    // first_available_fruit is: Apple
    // my_fruit is: Some(Apple)
    //println!("Variable named `apple` is moved: {:?}", apple);
    // TODO: uncomment the line above to see the compiler error
}

get_or_insert_with() evaluates lazily, modifies empty value in place

Instead of explicitly providing a value to fall back on, we can pass a closure to get_or_insert_with, as follows:

#[derive(Debug)] 
enum Fruit { Apple, Orange, Banana, Kiwi, Lemon }

fn main() {
    let mut my_fruit: Option<Fruit> = None;
    let get_lemon_as_fallback = || {
        println!("Providing lemon as fallback");
        Fruit::Lemon
    };
    let first_available_fruit = my_fruit
        .get_or_insert_with(get_lemon_as_fallback);
    println!("first_available_fruit is: {:?}", first_available_fruit);
    println!("my_fruit is: {:?}", my_fruit);
    // Providing lemon as fallback
    // first_available_fruit is: Lemon
    // my_fruit is: Some(Lemon)

    // If the Option has a value, it is left unchanged, and the closure is not invoked
    let mut my_apple = Some(Fruit::Apple);
    let should_be_apple = my_apple.get_or_insert_with(get_lemon_as_fallback);
    println!("should_be_apple is: {:?}", should_be_apple);
    println!("my_apple is unchanged: {:?}", my_apple);
    // The output is a follows. Note that the closure `get_lemon_as_fallback` is not invoked
    // should_be_apple is: Apple
    // my_apple is unchanged: Some(Apple)
}

See also:

closures, get_or_insert, get_or_insert_with, moved variables, or, or_else