# Option.

# Definition.

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

# Usage.

```rust
fn maybe_icecream(hour: u16) -> Option<u16> {
    if hour > 10 { Some(5) }
    else { None }
}
```

# Subtypes.

## If-let.

```rust
let letter: Option<i32> = Some(12);
if let Some(i) = letter {
    // It was Some().
} else {
    // It was None.
}
```

## While-let.

```rust
let letter: Option<i32> = Some(0);
while let Some(i) = letter {
    if i > 9 {
        foo();
        letter = None;
    } else {
        bar()
        letter = Some(i + 1);
    }
}
```

## Match.

Use `ref` keyword to borrow value that doesn't implement `Copy` trait.

```rust
let y: Option<Point> = Some(Point { x: 100, y: 200 });
match y {
    Some(ref p) => println!("Co-ordinates are {},{} ", p.x, p.y),
    _           => panic!("no match!"),
}
```