Call a generic function with a bunch of (predefined) types

help
1

Say we have the following:

trait Trait {}

struct A;
impl Trait for A {}

struct B;
impl Trait for B {}

Is there a good way of writing a helper function that will call a generic function which takes a type T: Trait and passes A and B as the generic parameter? i.e.:

fn helper<Args>(f: /* ??? */, args: Args) {
    f::<A>(args);
    f::<B>(args);
}

fn do_thing<T: Trait>() { }

helper(do_thing);
2

No, not directly, as Rust doesn't have higher-order types.

However, you can define your own trait with a generic method. Demo.

3

We don't have types or bounds that are higher-ranked over type generics so far, and even if we did it wouldn't apply to do_thing without larger language changes, as T would be unconstrained in the implementation of the Fn traits. (T would need to appear in the arguments.)

Also your code is inconsistent since do_thing doesn't take Args. (Rust also has no stable mechanism for "any number of args the closure may take", if that's what you were going for.)

4

Thanks! This ends up being pretty verbose but can probably be improved with a macro.