fn inc(x: Option<i32>) -> Option<i32> {
    match x {
        Some(v) => Some(v + 1),
        // 一般这里应该是None
        None => None,
    }
}

fn main() {
    let s = Some(5);
    println!("{:?}", s);

    // 自动推断
    let op: Option<_> = Some(2);
    println!("{:?}", op);

    match op {
        Some(x) => println!("{}", x),
        None => println!("none"),
    }

    // Some(3)
    // None
    println!("{:?}", inc(op));
    println!("{:?}", inc(None));

    if let Some(v) = inc(op) {
        println!("v:{}", v);
    } else {
        println!("none");
    }


    if let Some(v) = inc(None) {
        println!("v:{}", v);
    } else {
        println!("none");
    }
}