Vec of Vecs, where the inner Vec to only contain one type, but different types in the outer Vec-CodePudding

I'm trying to hint at an inner Vec to only contain one type of enum discriminant. I can use a trait, or something if that can work here too.

Is this possible, or is there another way of doing this?

Asking because I just want no accidental wrong type, even though the code can be checked during run-time, I'd rather have it at compile time.

enum Kind {
    Apple(Apple),    
    Banana(f32),
}

/*
This could also be two different types:
struct Apple { v: f32 };
struct Banana { v: u8 }; 
*/

struct Outer {
    inners: Vec<Inner<Kind::?>>,  // I want to specify that the Inner should be a specific discriminant.
    // Maybe an option of a T? where: T: Apple | Banana?
}

struct Inner<T> {
   // I want all these to either be `Kind::Apple` or `Kind::Banana`, or the same type.
   items: Vec<T> 
}

The other option I can think of is just expanding into the struct, and not use an enum.

struct Outer {
    apples: Vec<Apple>,
    bananas: Vec<Banana>,
}

Edit: I forgot to mention there are values in the enum and the struct.

CodePudding user response：

No, this is not possible. Kind represents a domain of values, and Kind::Apple and Kind::Banana together are the entire domain.

Kind is a type. The variants of Kind are values of the Kind type; they aren't themselves types.

It's a bit like asking if you could have a Vec of i32 values, but specify at the type level that the i32s in one Vec must all be the same value, like a Vec<0_i32>, which doesn't make sense either. You might as well have a Vec<()>, since you can infer that the value is 0.

If the enum variants have a payload, you could store just the payload. For example:

enum Kind {
    Apple(Apple),
    Banana(Banana),
}

struct Apple { /* ... */ }
struct Banana { /* ... */ }

Now you could have a Vec<Apple> and a Vec<Banana>, but you'd just be storing values of those types, and the "kind" is inferred by which vector you get the payload from.