如何创建一个接受泛型类型作为关联类型的枚举

Question

I'm trying to create an enumeration that accepts a generic type as associated value.
The compiler complains:

Reference to generic type 'GenericItem' requires arguments in <...>

The scheme is pretty simple:

struct GenericItem <Item:FormattableAsStringWithPrecision> {
    let value: Item
}

enum Enumeration {
    case Generic(values: [GenericItem])
}

I can't understand how to make this possible.

Answer 1

You need to add the Generic type to the enum too, the types can be inferred from the initialiser so you do not need to pass it as a generic type argument.

Below is an example of how you might do it.

struct GenericItem<T: CustomDebugStringConvertible> {
  let value: T
}

enum Enumeration<T: CustomDebugStringConvertible> {
  case Generic(value: [GenericItem<T>])
}

let someValue = Enumeration.Generic(value: [ GenericItem(value: "") ])

edit: I changed the FormattableAsStringWithPrecision to CustomDebugStringConvertible as I assumed it was one of your own custom protocols which can be easily swapped out, but the same logic would still apply for any protocol.