fn-box.rs

#![feature(default_type_params)]

trait FnBox<A,R = ()> {
    fn call_box(self: Box<Self>, arg: A) -> R;
}

impl<A,R,F> FnBox<A,R> for F
    where F : FnOnce(A) -> R
{
    fn call_box(self: Box<F>, arg: A) -> R {
        (*self)(arg)
    }
}

fn do_it<F:FnOnce(int) -> int>(f: F) -> int {
    do_it_boxed(box f)
}

fn do_it_boxed(b: Box<FnBox<int,int>>) -> int {
    b.call_box(22)
}

fn main() {
    assert_eq!(do_it(|x| x * 44), 968);
}