What are generics?

Generics let you write code that works with many types, not just one.

Instead of writing:

#![allow(unused)]
fn main() {
struct RidgeEstimator {
    beta: f64,
}
}

You can write:

#![allow(unused)]
fn main() {
use num_traits::Float;

struct RidgeEstimator<F> {
    beta: F,
}
}

Here, F is a type parameter: it could be f32, f64, or another type. In Rust, generic types have no behavior by default.

#![allow(unused)]
fn main() {
fn sum(xs: &[F]) -> F {
    xs.iter().sum() // This will not compile
}
}

Trait bounds

To fix that, we must tell the compiler which traits F should implement.

For example:

#![allow(unused)]
fn main() {
use num_traits::Float;
use std::iter::Sum;

impl<F: Float + Sum> RidgeModel<F> for GenRidgeEstimator<F> {
    ...
}
}

This means:

• F must implement Float (it must behave like a floating point number: support powi, abs, etc.)
• F must implement Sum (so we can sum an iterator of F)

This allows code like:

#![allow(unused)]
fn main() {
let mean = xs.iter().copied().sum::<F>() / F::from(xs.len()).unwrap();
}

Using generic bounds allows the estimator to work with f32, f64, or any numeric type implementing Float. The compiler can generate specialized code for each concrete type.