Option

The Option type represents an optional value that can either be Some with a value or None. They are prevalent in Rust and serve various purposes. Options are often used with pattern matching (i.e., match expression). The documentation for Option provides extensive details on its usage.

Some(T) and None are two variants of the Option<T> enum.¹

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

We haven't discussed generics yet. For now, just note that the T in the enum represents any type.

Returning to our Loop

In the code snippet, we check if the pattern is a substring of the current line. If it is, we return a tuple with the line number (i+1)² and the line, enclosed within the Some variant of the Option enum. If not, we return the None variant.

    for (line_no, line) in
        poem.lines()
            .enumerate()
            .filter_map(|(i, line)| match line.contains(pattern) {
                true => Some((i + 1, line)),
                false => None,
            })
    {
        println!("{line_no}: {line}");
    }