[swift-evolution] where clauses on enum cases

[Slava Pestov]

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 at 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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