generics

fn mean<'a, T>(data: impl ExactSizeIterator<Item = &'a T>) -> Result<f64, &'static str>
where
    T: 'a + std::iter::Sum<&'a T> + Into<f64>,
{
    let len = data.len() as f64;
    if len < 1 {
        return Err("Invalid divisor");
    }
    Ok((data.sum::<T>()).into() / len)
}