Learning how to define and use a trait

#1

Can someone tell me what is wrong with the following code which defines a way to compute the distance between two points? I think it's close to working.

fn main() {
    struct Point {
        x: f64,
        y: f64,
    }

    trait Distance<T> {
        fn distanceTo(self: &Self, other: &Self) -> T {}
    }

    impl Distance<f64> for Point {
        fn distanceTo(self: &Point, other: &Point) -> f64 {
            let dx = self.x - other.x;
            let dy = self.y - other.y;
            return (dx.powf(2.0) + dy.powf(2.0)).sqrt();
        }
    }

    let p1 = Point { x: 3.0, y: 4.0 };
    let p2 = Point { x: 6.0, y: 8.0 };
    let d = p1.distanceTo(&p2);
    println!("distance is {}", d);
}

(Playground)

Errors:

   Compiling playground v0.0.1 (/playground)
error[E0308]: mismatched types
 --> src/main.rs:8:55
  |
7 |     trait Distance<T> {
  |                    - this type parameter
8 |         fn distanceTo(self: &Self, other: &Self) -> T {}
  |                                                       ^^ expected type parameter `T`, found `()`
  |
  = note: expected type parameter `T`
                  found unit type `()`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0308`.
error: could not compile `playground`

To learn more, run the command again with --verbose.
#2

Very close. Here's the change:

-        fn distanceTo(self: &Self, other: &Self) -> T {}
+        fn distanceTo(self: &Self, other: &Self) -> T;

Trait functions can have default implementations, and when it saw the { ... }, the compiler looked for such a default implementation. To just declare it, use ;.

2 Likes
#3

Replace the -> {} in the trait definition with a semicolon. The trait definition need not define a default implementation.

2 Likes