CODEWITHSUNDEEP
rusttraits
Matteo Monti
Matteo Monti

Reputation: 8970

Is it theoretically possible to specialise trait behaviour?

Consider the following scenario:

I know that, in Rust, we have no negative trait bounds. My intuition is that we should probably be able to use opt-in built-in to achieve this (although, I did try unsuccessfully).

I am wondering if it is at all possible, no matter the complexity of the code, to achieve what I would like to do on stable Rust. If not, is it a design choice? Is it inherently bad to want to do something like this? It sounds like such a common problem to me that I'd be very surprised to find out it's a deprecated practice.

Upvotes: 2

Views: 94

Answers (1)

starblue
starblue

Reputation: 56822

It is possible on nightly with the still unstable specialization feature and a default implementation (see also the tracking issue for specialization):

#![feature(specialization)]

trait DoIt {
    fn do_it(&self);
}

impl<T> DoIt for T
where
    T: CanDo,
{
    default fn do_it(&self) {
        self.do_impl()
    }
}

impl<T> DoIt for T
where
    T: CanDoQuickly,
{
    fn do_it(&self) {
        self.do_quickly_impl()
    }
}

trait CanDo {
    fn do_impl(&self) {
        println!("slowly");
    }
}

trait CanDoQuickly: CanDo {
    fn do_quickly_impl(&self) {
        println!("quickly");
    }
}

struct S1;
impl CanDo for S1 {}
impl CanDoQuickly for S1 {}

struct S2;
impl CanDo for S2 {}

fn main() {
    S1.do_it();
    S2.do_it();
}

Upvotes: 2

Related Questions