Rust by Example

8.2.5 As output parameters

Closures as input parameters are possible, so returning closures as output parameters should also be possible. However, returning closure types are problematic because Rust currently only supports returning concrete (non-generic) types. Anonymous closure types are, by definition, unknown and so returning a closure is only possible by making it concrete. This can be done via boxing.

The valid traits for returns are slightly different than before:

  • Fn: normal
  • FnMut: normal
  • FnOnce: There are some unusual things at play here, so the FnBox type is currently needed, and is unstable. This is expected to change in the future.

Beyond this, the move keyword must be used, which signals that all captures occur by value. This is required because any captures by reference would be dropped as soon as the function exited, leaving invalid references in the closure.

fn create_fn() -> Box<Fn()> {
    let text = "Fn".to_owned();

    Box::new(move || println!("This is a: {}", text))
}

fn create_fnmut() -> Box<FnMut()> {
    let text = "FnMut".to_owned();

    Box::new(move || println!("This is a: {}", text))
}

fn main() {
    let fn_plain = create_fn();
    let mut fn_mut = create_fnmut();

    fn_plain();
    fn_mut();
}

See also:

Boxing, Fn, FnMut, and Generics.