How to combine multiple traits as a "trait tuple"?

Question

I write a lib like:

struct Foo<A: AppleTrait, B: BananaTrait, C: CarrotTrait> {...}

impl<A: AppleTrait, B: BananaTrait, C: CarrotTrait> Foo<A,B,C> {...}

struct Bar<A: AppleTrait, B: BananaTrait, C: CarrotTrait> {...}

impl<A: AppleTrait, B: BananaTrait, C: CarrotTrait> Bar<A,B,C> {...}

... and many more...

Notice that, things like <A: AppleTrait, B: BananaTrait, C: CarrotTrait> always come together, and has appeared many many times. So I want to eliminate such a burden.

Thus I hope I can do something like:

define_the_trait_alias ABCTrait = (A: AppleTrait, B: BananaTrait, C: CarrotTrait); // How to do this?

struct Foo<ABC: ABCTrait> {...}

impl<ABC: ABCTrait> Foo<ABC> {...}

Thanks!

Answer 1

Perhaps you can use associated types rather than generics. For example:

trait AbcTrait {
    type Apple: AppleTrait;
    type Banana: BananaTrait;
    type Carrot: CarrotTrait;
}

struct Foo<Abc: AbcTrait> {
    abc: Abc,
}
impl<Abc: AbcTrait> Foo<Abc> {}

struct Bar<Abc: AbcTrait> {
    abc: Abc,
}
impl<Abc: AbcTrait> Bar<Abc> {}

When defining a concrete implementation of the trait, you get to choose which concrete type implements which associated type requested by the trait:

impl AbcTrait for Salad {
    type Apple = GrannySmith;
    type Banana = Cavendish;
    type Carrot = LunarWhite;
}