[swift-evolution] [Proposal] Revamp the playground quicklook APIs

[Ben Rimmington]

Something like this was attempted (and reverted) for Swift 3:

<https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160808/026088.html>

<https://github.com/apple/swift-evolution/commit/e2610e3fa91b437e06e768aaef6820d755489717>

<https://github.com/apple/swift-xcode-playground-support/commit/0f42ade5a6302cf953a3ed32a892f23e8e150c62>

Is it now possible to `import PlaygroundSupport` outside of a playground?

-- Ben

> On 9 Jan 2018, at 23:19, Connor Wakamo wrote:
> 
> Good afternoon,
> 
> In preparation for ABI stability, I’ve reviewed the API exposed by the standard library for providing customized “quick looks” in playgrounds. This is exposed as the PlaygroundQuickLook enum and the CustomPlaygroundQuickLookable protocol. The PlaygroundQuickLook has a handful of issues:
> 
> 	- It hard-codes the list of supported types in the standard library, meaning that PlaygroundLogger/IDEs cannot gain support for new types without standard library changes (including swift-evolution review)
> 	- The cases of the enum are poorly typed: there are cases like `.view` and `.color` which take NS/UIView or NS/UIColor instances, respectively, but since they’re in the standard library, they have to be typed as taking `Any` instead
> 	- The names of some of these enum cases do not seem to match Swift naming conventions)
> 
> To that end, I am proposing the following:
> 
> 	- Deprecate PlaygroundQuickLook and CustomPlaygroundQuickLookable in Swift 4.1 (including in the Swift 3 compatibility mode)
> 	- Remove PlaygroundQuickLook and CustomPlaygroundQuickLookable in Swift 5 to avoid including them in the stable ABI (this affects the compatibility modes, too)
> 	- Introduce a new protocol, CustomPlaygroundRepresentable, in the PlaygroundSupport library in Swift 4.1:
> 
> 		protocol CustomPlaygroundRepresentable {
> 			/// Returns an alternate object or value which should stand in for the receiver in playground logging, or nil if the receiver’s default representation is preferred.
> 			var playgroundRepresentation: Any? { get }
> 		}
> 
> 	- Update the PlaygroundLogger library in Swift 4.1 to support both CustomPlaygroundRepresentable and PlaygroundQuickLook/CustomPlaygroundQuickLookable
> 	- Provide a compatibility shim library which preserves PlaygroundQuickLook and CustomPlaygroundQuickLookable as deprecated in Swift 3/4 and unavailable in Swift 5, but only in playgrounds (including in the auxiliary source files stored inside a playground)
> 
> I’ve put a full proposal below. Please let me know what you think of this proposal; I’d like to get some feedback before taking this through the review process, but I’ll need to get that quickly so I can get it under review soon as this is targeted at Swift 4.1.
> 
> Thanks,
> Connor