fn strpos(a: &str, b: &str) -> StringPosition {
    b.find(a)
        .map_or_else(
            || StringPosition::NotFound,
            |pos: usize| StringPosition::Found(pos)
        )
}

enum StringPosition {
    Found(usize),
    NotFound,
}

fn main() {
    match strpos(&"World", &"Hello World") {
        StringPosition::Found(pos) => println!("substring found at position {}", pos),
        StringPosition::NotFound => println!("substring not found"),
    }
}