iface to_str {
    fn to_str() -> str;
}
impl of to_str for int {
    fn to_str() -> str { int::str(self) }
}
impl of to_str for str {
    fn to_str() -> str { self }
}
impl of to_str for () {
    fn to_str() -> str { "()" }
}

iface map<T> {
    fn map<U>(f: block(T) -> U) -> [U];
}
impl <T> of map<T> for [T] {
    fn map<U>(f: block(T) -> U) -> [U] {
        let r = [];
        for x in self { r += [f(x)]; }
        r
    }
}

fn foo<U, T: map<U>>(x: T) -> [str] {
    x.map({|_e| "hi" })
}
fn bar<U: to_str, T: map<U>>(x: T) -> [str] {
    x.map({|_e| _e.to_str() })
}

fn main() {
    assert foo([1]) == ["hi"];
    assert bar::<int, [int]>([4, 5]) == ["4", "5"];
    assert bar::<str, [str]>(["x", "y"]) == ["x", "y"];
    assert bar::<(), [()]>([()]) == ["()"];
}