fn c<T>(s: String) -> Box<Fn(Box<Fn(String) -> T>) -> T> {
    Box::new(move |k: Box<Fn(String) -> T>| k(s.clone()))
}

fn d<T>(k: &'static Fn(String) -> T) -> Box<Fn(i32) -> T> {
     Box::new(move |i: i32| k(i.to_string()))
}

fn s<T>(k: &'static Fn(String) -> T) -> Box<Fn(String) -> T> {
    Box::new(move |st: String| k(st))
}

fn sprintf<T>(f: &Fn(&Fn(String) -> String) -> T) -> T {
    f(&|x| format!("integer: {}", x))
}

fn append<T, U, V>(
    f: &Fn(&Fn(String) -> T) -> V,
    g: &Fn(&Fn(String) -> U) -> T,
    h: &Fn(String) -> U,
) -> V {
    f(&(move |s1: String| {
        g(&(move |s2: String| {
            let v = vec![s1.clone(), s2.clone()];
            let v = <[String]>::concat(&v);
            h(v)
        }))
    }))
}

fn main() {
    //let result: &Fn(i32) -> String = sprintf(append(&d, &c, &String::from("foo")));
    // println!("--> {}", result(10));
}