Std::ops with generic type and built in lhs

[vron]

Hi,

I am struggling to define e.g. std::ops::Add for my generic type ( Custom say) where i want to be able to add e.g. a f64 both from the left and the right.

I.e I would like:

let c: Custom<T> = ...;
let a = 1.0;
return a + c + a;

to work - but cannot see how to implement that. See for example Rust Playground for a simplified example of what does not work.

Any pointer appreciated.

/V

[H2CO3]

a + b is syntactic sugar for A::add(a, b), so b + a always calls the add() method on B. Thus, you'd have to blanket-impl Add<Custom<T>> for T, but that isn't allowed due to coherence.

@vron

where i want to be able to add e.g. a f64 both from the left and the right.

I.e I would like:

let c: Custom<T> = ...;
let a = 1.0;
return a + c + a;

Note that this doesn't require the above impl! Addition associates to the left, so if <A as Add<C>>::Output = A, then this will work with the above unilateral impl and impl Add<Custom<T>> for Custom<T>: Playground.

[vron]

Right,

Thanks and I see you point with a + c + a actually working - bad specification by me.

But that means then that it's impossible for me to get plain a + c to work? (as well as e.g 2.0*c etc.)

[H2CO3]

In a generic setting, it's indeed not possible. You can spell out some concrete types instead.

[vron]

Thanks