[swift-evolution] Generic types—covariance/contravariance

Braeden Profile jhaezhyr12 at gmail.com
Thu Dec 8 18:45:50 CST 2016

Has the core team or the community considered the possibility of implementing covariant/contravariant generic types?  It would really be appreciated.

I know with Array, vague-ifying or specific-ifying the type ([Int] to [Any]) has help from the compiler—and we can use `map` if all else fails—but that only lessens the impact of the missing functionality.  This is my exact use case here, using SceneKit to identify the first-hit Controller object:

class ControllerNode<Controller: AnyObject>: SCNNode
	let controller: Controller

// Ordered front to back, returns the first Controller object.
for hit in hitTest
	// Determine if this node is part of a controller.
	let ancestrySequence = sequence(first: hit.node, next: { $0.parent })
	let lastControllerNode: ControllerNode<AnyObject>? = ancestrySequence.reduce(nil)
		{ ($1 as? ControllerNode) ?? $0 }
	if let cabinet = lastControllerNode?.controller as? CabinetController
		{ return cabinet }
	if let wall = lastControllerNode?.controller as? WallController
		{ return wall }

This compiles, but unfortunately, this will never work.  The `reduce` algorithm always ends up trying to convert things like `ControllerNode<WallController> as ControllerNode<AnyObject>`, which—unintuitively—always fails.  Without compiler help, so would things like `myIntArray as [Any]` or `Optional<Boy>(Boy()) as Optional<Human>`.

If Swift is supposed to welcome generic programming, this would be a great thing to have.
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