[swift-evolution] [Pitch] CRTP in Swift

[David Sweeris]

> On Feb 23, 2017, at 10:54 AM, Adrian Zubarev via swift-evolution <swift-evolution at swift.org> wrote:
> 
> First of all, writing CRTP is already allowed in Swift, however it doesn’t work at all. 
> 
> For example one would assume that the output of the following code sample might look like this 1 2 3 4:
> 
> print(1)
> 
> class Template<T> {}
> 
> class RealThing : Template<RealThing> {
>     override init() { print(2) }
>     func foo() { print(3) }
> }
> 
> let instance = RealThing()
> instance.foo()
> 
> print(4)
> However the result is only 1 and your application will simply stuck at that point without crashing or throwing any bad runtime error.
> 
> https://bugs.swift.org/browse/SR–2549 <https://bugs.swift.org/browse/SR-2549>
> I discovered an API in UIKit which uses that pattern, obviously it’s written in Objective-C but it is imported in Swift and it actually works, because it’s not pure swift.
> 
> // Generic superclass
> open class NSLayoutAnchor<AnchorType : AnyObject> : NSObject { ... }
> 
> // Non-generic subclasses with the `Sub : Super<Sub>` pattern
> open class NSLayoutXAxisAnchor : NSLayoutAnchor<NSLayoutXAxisAnchor> {}
> open class NSLayoutYAxisAnchor : NSLayoutAnchor<NSLayoutYAxisAnchor> {}
> open class NSLayoutDimension : NSLayoutAnchor<NSLayoutDimension> { ... }
> My questions are:
> 
> Will we ever allow this in Swift?
> Does CRTP bring any technical improvement to Swifts generic system?
> Is it something for Swift 4 to consider?
> Does it need a proposal or should we only see it as a bug?

For those who might not know what the acronym stands for: https://en.wikipedia.org/wiki/Curiously_recurring_template_pattern

I can’t think of any non-technical reasons to prohibit it, so to me, this not being supported is either a bug or a technical limitation of type system. Actually, I’d say the specific behavior you’re describing is a bug regardless, since the compiler should throw an error if it comes across code that it can see will get the type system into an infinite loop.

- Dave Sweeris
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