Equality Operator/Trait == for Result<T, E>

Question

I was wondering, if I have two variables x and y of type Result<T, E>, how can I overload the equality operator == for it? So that one can easily check x == y. Here is some sample code, where I tried just that:

enum ErrorKind {
    OutOfRange,
    InvalidInput,
}

type MyResult = Result<i32, ErrorKind>;

impl PartialEq for MyResult {
    fn eq(&self, other: &Self) -> bool {
        match (self, other) {
            (Ok(x), Ok(y)) => x == y,
            (Err(x), Err(y)) => x == y,
            _ => false,
        }
    }
}

fn func(x: i32) -> MyResult {
    if x < 0 {
        return Err(ErrorKind::OutOfRange);
    } else {
        return Ok(x);
    }
}

fn main() {
    let x = func(-1);
    let y = func(15);
    let z = func(15);

    println!("x == z --> {}", x == z);
    println!("y == z --> {}", y == z);
}

Unfortunately, this gives error[E0117]: only traits defined in the current crate can be implemented for arbitrary types.

I also tried impl MyResult { ... } (without the PartialEq) but that gives error[E0116]: cannot define inherent 'impl' for a type outside of the crate where the type is defined.

---

Is it possible to somehow overload/define operator == for Result<T, E> (generic) or Result<i32, ErrorKind> (specific specialisation)?

Answer 1

The type Result<T, E> already implements PartialEq, so you just need to derive that trait for ErrorKind. The Result type will implement it as a consequence.

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