iface to_str {
    fn to_str() -> ~str;
}

impl of to_str for int {
    fn to_str() -> ~str { int::str(self) }
}

impl <T: to_str> of to_str for ~[T] {
    fn to_str() -> ~str {
        ~"[" + str::connect(vec::map(self, |e| e.to_str() ), ~", ") + ~"]"
    }
}

fn main() {
    assert 1.to_str() == ~"1";
    assert (~[2, 3, 4]).to_str() == ~"[2, 3, 4]";

    fn indirect<T: to_str>(x: T) -> ~str {
        x.to_str() + ~"!"
    }
    assert indirect(~[10, 20]) == ~"[10, 20]!";

    fn indirect2<T: to_str>(x: T) -> ~str {
        indirect(x)
    }
    assert indirect2(~[1]) == ~"[1]!";
}