That sounds nice, I spent a good chunk of today hacking on a FRP implementation similar to Elm and the ability to not clone + a few other things around closures kept painting me into a corner each time. I even went as far to drop down to std::mem::transmute_copy, but when I hit a Box<Fn(A) -> B> the type has already been erased and I can't recover it.

It seems like if you have no references in your closure you should be able to pass them around as a value-type(with generics, of course). From what I can tell without that there's a whole range of FP patterns that just aren't representable in Rust because of it.