You’re asking for GADTs:
https://en.wikipedia.org/wiki/Generalized_algebraic_data_type
<https://en.wikipedia.org/wiki/Generalized_algebraic_data_type>
This feature is difficult to implement correctly and efficiently and I doubt
Swift will ever support it, but you never know…
Slava
> On Mar 14, 2017, at 12:44 PM, Rex Fenley via swift-evolution
> <swift-evolution@swift.org> wrote:
>
> I've run into a problem recently when using generics with enums where I'm
> required to specify the type for a generic that isn't used for the enum case
> that's constructed.
>
> Example:
>
> protocol Mapping {
> associatedtype Destination
> }
>
> enum EvictionPolicy {
> case append
> case replace
> }
>
> enum Map<T: Mapping, U: Mapping> where U.Destination: Collection {
> case object(T)
> case collection(U, EvictionPolicy)
> }
>
> class IntMapping: Mapping {
> typealias Destination = Int
> }
>
> class IntsMapping: Mapping {
> typealias Destination = [Int]
> }
>
> // Must specify types for generics that are not in use
> let o = Map<IntMapping, IntsMapping>.object(IntMapping())
> let c = Map<IntMapping, IntsMapping>.collection(IntsMapping(),
> EvictionPolicy.append)
>
> What I'd like to be able to do is the following
>
> enum Map<T: Mapping> {
> case object(T)
> case collection(T, EvictionPolicy) where T.Destination: Collection
> }
>
> Could this be added to the language?
>
> --
> Rex Fenley | IOS DEVELOPER
>
>
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