[swift-evolution] Casting Bug Swift

[Radek Pietruszewski]

Hm. Obviously, you have far more insight into current implementation and performance difficulties, but purely from a programmer's perspective, going from [Foo] to AnyCollection<Foo> and from 'xs as? [Bar]' to 'xs.mapAsType(Bar.self)' seems like a step back.

Regarding the latter, wouldn't it be better to somehow generalize covariant collection casts? Perhaps a special protocol different collection types can conform to to define their cast implementation?

Sent from my iPhone

> On 25 Feb 2016, at 20:50, Joe Groff <jgroff at apple.com> wrote:
> 
> 
>> On Feb 25, 2016, at 11:45 AM, Radek Pietruszewski <radexpl at gmail.com> wrote:
>> 
>> Interesting. Two Qs:
>> 
>> - when you say "moving in the direction of encouraging the use of abstract AnyCollection values", do you mean in general? To encourage people to use AnyCollection<T> instead of [T] in their code?
> 
> Yeah. In Cocoa, the NSArray and NSDictionary class clusters serve the purpose not only of standard collections but as abstract interfaces for ordered and keyed collections, after all.
> 
>> - how could covariant collection casting look like in this world?
> 
> A no-language-support-needed approach might be to give the CollectionType protocol a `mapToType(T.self)` method which can be given specialized implementations for cases that can be efficiently implemented (such as mapping an array of class references to another class type), and/or fall back to wrapping with a lazy adapter in cases like going from a collection of ConcreteType to a collection of ProtocolType.
> 
> -Joe