Tuple Structs & Newtypes

Newtype pattern

struct Price(f64);
struct Qty(f64);

fn notional(p: Price, q: Qty) -> f64 {
    p.0 * q.0
}

fn main() {
    println!("{:.2}", notional(Price(190.5), Qty(10.0)));
}

1905.00

Calling notional(qty, price) becomes a type error if you swap arguments — a cheap safety win.

Unit structs

struct ExchangeOpen;

Useful as markers/traits later.

When to bother

For teaching and for public APIs of risk systems — yes. For a 10-line script — maybe overkill. Learn the pattern now.