Function from_fn 

pub fn from_fn<St, F>(state: St, f: Covar<F>) -> FromFn<St, F>
where
    F: for<'all> FnMutHKAOpt<'all, &'all mut St>,

Creates a lender from a state and a closure F: FnMut(&mut St) -> Option<T>.

Note that functions passed to this function must be built using the covar! or covar_mut! macros, which also check for covariance of the returned type.

Examples

let mut lender = lender::from_fn(0,
    covar_mut!(for<'all>
        |state: &'all mut i32|
            -> Option<&'all mut i32> {
    if *state < 10 {
        *state += 1;
        Some(state)
    } else {
        None
    }
}));
assert_eq!(lender.next(), Some(&mut 1));