A useful method of unit conversions can be examined by implementing Add
with a phantom type parameter. The Addtrait is examined below:
// This construction would impose: `Self + RHS = Output`
// where RHS defaults to Self if not specified in the implementation.
pub trait Add<RHS = Self> {
type Output;
fn add(self, rhs: RHS) -> Self::Output;
}
// `Output` must be `T<U>` so that `T<U> + T<U> = T<U>`.
impl<U> Add for T<U> {
type Output = T<U>;
...
}
The whole implementation:
use std::ops::Add;
use std::marker::PhantomData;
/// Create void enumerations to define unit types.
#[derive(Debug, Clone, Copy)]
enum Inch {}
#[derive(Debug, Clone, Copy)]
enum Mm {}
/// `Length` is a type with phantom type parameter `Unit`.
///
/// `f64` already implements the `Clone` and `Copy` traits.
#[derive(Debug, Clone, Copy)]
struct Length<Unit>(f64,PhantomData<Unit>);
/// The `Add` trait defines the behavior of the `+` operator.
impl<Unit> Add for Length<Unit> {
type Output = Length<Unit>;
// add() returns a new `Length` struct containing the sum.
fn add(self, rhs: Length<Unit>) -> Length<Unit> {
// `+` calls the `Add` implementation for `f64`.
Length(self.0 + rhs.0, PhantomData)
}
}
fn main() {
// Specifies `one_foot` to have phantom type parameter `Inch`.
let one_foot: Length<Inch> = Length(12.0, PhantomData);
// `one_meter` has phantom type parameter `Mm`.
let one_meter: Length<Mm> = Length(1000.0, PhantomData);
// `+` calls the `add()` method we implemented for `Length<Unit>`.
//
// Since `Length` implements `Copy`, `add()` does not consume
// `one_foot` and `one_meter` but copies them into `self` and `rhs`.
let two_feet = one_foot + one_foot;
let two_meters = one_meter + one_meter;
// Addition works.
println!("one foot + one_foot = {:?}", two_feet);
println!("one meter + one_meter = {:?}", two_meters);
// Nonsensical operations fail as they should:
// Compile-time Error: type mismatch.
//let one_feter = one_foot + one_meter;
}