Monoid implementation in Rust

monoid.rs

trait Empty {
    fn empty() -> Self;
}

trait Semigroup {
    fn append(&self, other: Self) -> Self;
}

trait Monoid : Semigroup+Empty {}

impl Empty for i32 {
    fn empty() -> i32 { 0 }
}

impl Semigroup for i32 {
    fn append(&self, other: i32) -> i32 { self + other }
}

impl Monoid for i32 {}

fn sum<T: Semigroup>(a: T, b: T) -> T { a.append(b) }

fn main() {
    println!("1 + 1 = {}", sum(1, 1));
}