Function thunk 

pub fn thunk<R, F, Fut>(f: F) -> BoxThunk<R>
where
    F: FnOnce() -> Fut + Send + 'static,
    Fut: Future<Output = Result<R, Error>> + Send + 'static,
    R: Send + 'static,

Box a thunk into a BoxThunk so heterogeneous thunks can share a Vec.