Newtype Pattern

Newtype pattern ใน Rust ใช้สำหรับ type safety, abstraction และ implementing traits บน external types

1. Basic Newtype

Newtype คือ tuple struct ที่มี single field ให้ type ใหม่ที่แยกจาก underlying type

Creating Newtypes

// Newtype = tuple struct with single field
struct Meters(f64);
struct Kilometers(f64);
struct Miles(f64);

impl Meters {
    fn new(value: f64) -> Self {
        Meters(value)
    }

    fn value(&self) -> f64 {
        self.0
    }
}

// Conversion traits
impl From<Kilometers> for Meters {
    fn from(km: Kilometers) -> Self {
        Meters(km.0 * 1000.0)
    }
}

impl From<Miles> for Meters {
    fn from(mi: Miles) -> Self {
        Meters(mi.0 * 1609.344)
    }
}

fn main() {
    let m = Meters::new(1000.0);
    let km = Kilometers(5.0);
    let mi = Miles(3.0);

    println!("{} meters", m.value());
    println!("{} km = {} meters", km.0, Meters::from(km).value());
    println!("{} miles = {} meters", mi.0, Meters::from(mi).value());

    // Type safety: can't mix types
    // let total = m + km; // Compile error!
}