src/error/option_unwrap/defaults.md

# 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.

```rust,editable
#[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:

```rust,editable
#[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:

```rust,editable
#[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:

```rust,editable
#[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`][closures], [`get_or_insert`][get_or_insert], [`get_or_insert_with`][get_or_insert_with], [`moved variables`][moved], [`or`][or], [`or_else`][or_else]

[closures]: https://doc.rust-lang.org/book/ch13-01-closures.html
[get_or_insert]: https://doc.rust-lang.org/core/option/enum.Option.html#method.get_or_insert
[get_or_insert_with]: https://doc.rust-lang.org/core/option/enum.Option.html#method.get_or_insert_with
[moved]: https://doc.rust-lang.org/book/ch04-02-references-and-borrowing.html
[or]: https://doc.rust-lang.org/core/option/enum.Option.html#method.or
[or_else]: https://doc.rust-lang.org/core/option/enum.Option.html#method.or_else