fn main() {
    println!("{}", make_adder(1)(2));
    println!("{}", make_adder(2)(3));
    let indent = "xxx".to_owned();
    let f = make_str_adder(&indent);
    println!("add = {}", f("_abc_"));
}

fn make_adder(a: i32) -> Box<Fn(i32) -> i32> {
    if a == 1 {
        Box::new(move |b| a + b)
    } else {
        Box::new(move |b| a * b)
    }
}

fn make_str_adder<'a>(a: &'a str) -> Box<Fn(&str) -> String + 'a> {
    if a.chars().all(|c| c.is_ascii_whitespace()) {
        Box::new(move |l| String::from(l))
    } else {
        Box::new(move |l| {
            let mut r = String::new();
            r.push_str(a);
            r.push_str(l);
            r
        })
    }
}