Optional model state and #[derive(...)] for common traits

In addition of using ndarray instead of Vec<f64>, we also slightly modify the model struct.

To make our RidgeEstimator struct more ergonomic, we derive a few useful traits: Debug, Clone, and Default:

• The Debug trait allows us to print the struct for inspection using println!("{:?}", ...), which is helpful during development.
• Clone lets us duplicate the struct, which is often needed in data processing pipelines.
• Default enables us to create a default value using RidgeEstimator::default(), which internally calls the new() method we define.

#![allow(unused)]
fn main() {
#[derive(Debug, Clone, Default)]
pub struct RidgeEstimator {
    pub beta: Option<f64>,
}
}

The line

#![allow(unused)]
fn main() {
beta: Option<f64>
}

means beta can be either:

• Some(value): if the model is trained
• None: if the model has not been fitted yet

This way, we can explicitly model the fact that the estimator may not be fitted yet (i.e., no coefficients computed). When we initialize the model, we set beta to None as follows:

#![allow(unused)]
fn main() {
impl RidgeEstimator {
    /// Creates a new, unfitted Ridge estimator.
    ///
    /// # Returns
    /// A `RidgeEstimator` with `beta` set to `None`.
    pub fn new() -> Self {
        Self { beta: None }
    }
}
}