Function from_fn

pub fn from_fn<I, E, F>(fun: F) -> FromFn<F>
where
    F: FnMut() -> Result<Option<I>, E>,

Creates an iterator from a fallible function generating values.

let mut count = 0;
let counter = from_fn(move || {
    // Increment our count. This is why we started at zero.
    count += 1;

    // Check to see if we've finished counting or not.
    if count < 6 {
        Ok(Some(count))
    } else if count < 7 {
        Ok(None)
    } else {
        Err(())
    }
});
assert_eq!(&counter.collect::<Vec<_>>().unwrap(), &[1, 2, 3, 4, 5]);