Function from_fallible_fn 

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

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

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_fallible_fn::<_, Error, _>(
    0,
    covar_mut!(for<'all>
        |state: &'all mut i32|
            -> Result<Option<&'all mut i32>, Error> {
        if *state < 3 {
            *state += 1;
            Ok(Some(state))
        } else {
            Ok(None)
        }
    })
);
assert_eq!(lender.next().unwrap(), Some(&mut 1));