使用選項與地圖然後

在處理陣列和向量時，map 操作是一個有用的工具，但它也可以用於以函式方式處理 Option 值。

fn main() {

    // We start with an Option value (Option<i32> in this case).
    let some_number = Some(9);

    // Let's do some consecutive calculations with our number.
    // The crucial point here is that we don't have to unwrap
    // the content of our Option type - instead, we're just
    // transforming its content. The result of the whole operation
    // will still be an Option<i32>. If the initial value of
    // 'some_number' was 'None' instead of 9, then the result
    //  would also be 'None'.
    let another_number = some_number
        .map(|n| n - 1) // => Some(8)
        .map(|n| n * n) // => Some(64)
        .and_then(|n| divide(n, 4)); // => Some(16)

    // In the last line above, we're doing a division using a helper
    // function (definition: see bottom).
    // 'and_then' is very similar to 'map', but allows us to pass a
    // function which returns an Option type itself. To ensure that we
    // don't end up with Option<Option<i32>>, 'and_then' flattens the
    // result (in other languages, 'and_then' is also known as 'flatmap').

    println!("{}", to_message(another_number));
    // => "16 is definitely a number!"

    // For the sake of completeness, let's check the result when
    // dividing by zero.
    let final_number = another_number
        .and_then(|n| divide(n, 0)); // => None

    println!("{}", to_message(final_number));
    // => "None!"
}

// Just a helper function for integer division. In case
// the divisor is zero, we'll get 'None' as result.
fn divide(number: i32, divisor: i32) -> Option<i32> {
    if divisor != 0  { Some(number/divisor) } else { None }
}

// Creates a message that tells us whether our
// Option<i32> contains a number or not. There are other
// ways to achieve the same result, but let's just use
// map again!
fn to_message(number: Option<i32>) -> String {
    number
        .map(|n| format!("{} is definitely a number!", n)) // => Some("...")
        .unwrap_or("None!".to_string()) // => "..."
}