extern crate num_traits;

fn generic_scalar_product<T>(v: &[T], w: &[T]) -> Result<T,String>
where
    T: num_traits::Zero + std::ops::Mul<Output = T> + Copy
{
    let length = v.len();

    if length != w.len() {
        return Err("The error message".to_string());
    }

    // We initialise `sum` to a "zero" of type T
    // using the `zero` method provided by the `Zero` trait
    let mut sum = T::zero();
    for index in 0..length {
        sum = sum + v[index] * w[index];
    }
    Ok(sum)
}

fn main() {
    // Unsigned integers
    let x: Vec<u32> = vec![1, 1, 1];
    let y: Vec<u32> = vec![1, 0, 1];
    assert_eq!(generic_scalar_product(&x, &y).unwrap(), 2);
}