[swift-evolution] Covariance and Contravariance

[John McCall]

> On Dec 8, 2015, at 11:47 PM, Simon Pilkington via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Hi,
> 
> Is providing Covariance and Contravariance[1] of generics going to be part of the work on generics for Swift 3? I am sure this topic has come up within the core team and I was wondering what their opinion on the topic was.
> 
> I can see this as beneficial as it would allow the compiler - in conjunction with type inference - to retain more type information and hence allow code be more type safe. For example -
> 
> class ConcreteClass<GenType : GenericType> {
>     ...
>     
>     func getFunction() -> GenType {
>         ...
>     }
>     
>     func putFunction(input: GenType) -> Bool {
>         return ...
>     }
>     
> }
> 
> protocol GenericType {
>     ...
> }
> 
> class GenericType1 : GenericType {
>     ...
> }
> 
> class GenericType2 : GenericType {
>     ...
> }
> 
> let array = [ConcreteClass<GenericType2>(...), ConcreteClass<GenericType1>(...)]
> 
> let x : GenericType = array[0].getFunction() // this would compile as array would be of type ConcreteClass<types that extend GenericType>
>                       // currently array is of type AnyObject so this line doesn’t compile
> array[0].putFunction(…) // this would still not compile as it would break type guarantees
> 
> As a downside I can see it as making generics more complex and difficult to understand. On balance I think probably the benefit in improved type safety is worth it but I was interested in what others thought.

One challenge here is that subtyping in Swift doesn’t mean equivalence of representation.  For example, Int is a subtype of Int?, but the later requires extra space to store in memory.  So while it would make sense to allow, say, [Int] to be a subtype of [Int?], the actual conversion at runtime wouldn’t be trivial — we’d either need to eagerly apply the element-wise transform, or Array would need some ability to apply it lazily.  Both come with fairly serious performance costs.

John.
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