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

Chris Lattner clattner at nondot.org
Fri Jan 12 01:51:03 CST 2018

On Jan 11, 2018, at 11:22 AM, Connor Wakamo <cwakamo at apple.com> wrote:
>>> I don’t think we can change this to return `Any` instead of `Any?`. I think there are potentially cases where a developer might want to selectively opt-in to this behavior.
>> Which cases?  How important are they?
> I can think of a couple of cases where this could be useful.
> The first is an enum. Extending Riley’s example from earlier in the thread:
> 	enum MyUnion {
> 		case string(String)
> 		case image(UIImage)
> 		case intPair(Int, Int)
> 		case none
> 	}
> This enum might want to present the string and image cases as strings and images, but treat the intPair and none cases the “default” way for the enum. This is probably not the most compelling example as there is a workaround — return a second enum or other structured type from playgroundRepresentation — but that feels not great.

IMO, this is sugaring something that doesn’t need to be sugared.  There are simple solutions to this problem without complicating the design of your feature.  The few people who care about this can write it out long hand.

> The second case, and the one I’m more worried about, is subclassing. If, for instance, you have the following:
> 	class FooView: UIView, CustomPlaygroundRepresentable {
> 		var playgroundRepresentation: Any {
> 			return “A string describing this view"
> 		}
> 	}
> 	class BarView: FooView {
> 		override var playgroundRepresentation: Any {
> 			// BarView instances wanted to be logged as themselves, but there’s no way to express that
> 			return ???
> 		}
> 	}
> There’s nothing that BarView can do to ensure it gets logged like a view because FooView declared a conformance to CustomPlaygroundRepresentable.

I really don’t understand this.  FooView declares that it conforms, and it provides a “playgroundRepresentation” member.  Cool for FooView.

BarView comes around and overrides FooView’s implementation.  They don’t have to conform, because it *isa* FooView, so of course it conforms.  If it wants to customize its presentation, it overrides its  “playgroundRepresentation” method and it… just works.  If it conditionally wants the FooView representation for some reason, it can even call “super.playgroundRepresentation”.  What’s the problem?  This seems ideal to me.

>>> I also don’t think that `Optional` would get a conditional conformance to this. I’m not proposing that any standard library or corelibs types gain conformances to this protocol. Instead, it’s up to a playground logger (such as PlaygroundLogger in swift-xcode-playground-support <https://github.com/apple/swift-xcode-playground-support>) to recognize these types and handle them accordingly. The playground logger would look through the `Optional` so that this would effectively be true, but ideally the log data generated by a logger would indicate that it was wrapped by `Optional.some`.
>> Why not?  I understand that that is how the old algorithm worked, but it contained a lot of special case hacks due to the state of Swift 1 :-).  This is a chance to dissolve those away.
> It’s a feature that Optional (and other standard library/corelibs/OS types) don’t conform to CustomPlaygroundRepresentable. In my mind, it’s the role of the PlaygroundLogger library to understand the types for which it wishes to generate an opaque representation instead of the standard/fallback structured representation. So Optional, String, Int, UIColor, NSView, etc. don’t themselves conform to CustomPlaygroundRepresentable — they’re not customizing their presentation in a playground.

IMO, this was the right Swift 1 attitude: "make it work at all costs" but this is not the right approach for Swift over the long term, and not the right answer for Swift 5 in particular.

Swift is still very young and immature in some ways, but it it is intentionally design for extensibility and to be used in surprising and delightful ways.  It isn’t an accident of history that Int and Bool are defined in the standard library instead of being burned into the compiler.

IMO, every time you choose to privilege “well known” types in the standard library with special code in Xcode (or some other high level system loosely integrated with swift.org) you are introducing technical debt into the Swift ecosystem.  This is because you are violating the basic principles on which Swift was established, which is that users can [re]define primitives to build their own abstractions and carve out new domains beyond what was originally envisioned when you’re writing the UI code.

> Semi-related to this proposal, I’m working on a rewrite of the PlaygroundLogger library (currently at <https://github.com/cwakamo/swift-xcode-playground-support/tree/runtime-framework-and-improved-logger <https://github.com/cwakamo/swift-xcode-playground-support/tree/runtime-framework-and-improved-logger>>) which makes it so that the only special standard library behavior it depends on is Mirror — it no longer relies on the Swift runtime (via PlaygroundQuickLook(reflecting:)) to figure out what to log, instead opting to check protocol conformances itself. So if this proposal is accepted into Swift, concurrent with that we’ll have a new PlaygroundLogger implementation which gets rid of as many hacks as possible.


I hope that someday when we have a better API than Mirror (e.g. that supports mutation) that allows reflecting on stored properties, methods, and all of the other things that Swift needs to eventually support that you’ll switch to it. I understand that  the glorious future isn’t very helpful to you today though.

>>> `CustomPlaygroundLoggable` would be a little clunkier to implement than `CustomPlaygroundRepresentable` is, as in the common case folks would have to write `return .custom(…)`. It’s possible that the clarity and additional flexibility this grants outweighs that cost; I’m not sure, and would love feedback on that.
>> I just don’t understand the usecase for “conditional customizing” at all.  By way of example, we don’t have the ability to do that with CustomStringConvertible.   What is different about this case?
> I think the big difference with CustomStringConvertible is that it’s possible for a conformance to reimplement the default behavior on its own. For instance, if I have:
> 	enum Foo {
> 		case one(Any)
> 		case two
> 	}
> As noted above, recovering the default behavior with CustomPlaygroundRepresentable is not always possible if the return type is `Any`. That very well might be an acceptable trade-off to keep the API simple.

Why can’t an implementation just return “.two” instead of nil?

Is the problem the “one” case?  If that is the problem, then it might be better to take a completely different approach where you embrace the fact that you have structured data producing 2D results that need to be displayed.  Each result could either return an atomic result or a result that wraps some other recursive 2D presentation.

Fundamentally though, playground presentation is solving several different problems:

1. Types with no special behavior can always be represented as strings, which is handled by base protoocols.
2. Some types want custom string representations to show up in playgrounds, but not in “print” and string interpolation.
3. Some types want to provide a 2D “quicklook” style presentation in addition and beyond the string representation.
4. Fewer types want to provide an animated 2d representation, that is either “live” or precomputed.  

I’d suggest that this protocol only tackle problems 2 and 3, but you should cleanly distinguish between them, probably with a bespoke enum or struct that models these capabilities.

>>> I do like the `playgroundDescription` name for the property, but am a little hesitant to use the name `CustomPlaygroundConvertible` because conforming types can’t be converted to playgrounds. I can’t come up with an appropriate word in `CustomPlaygroundThingConvertible` to use in place of `Thing`, though. (If we end up pivoting to the enum I described above then something like `CustomPlaygroundLoggable` would be more appropriate.)
>> I would strongly recommend aligning with the state of the art in CustomStringConvertible (which has been extensively discussed) and ignore the precedent in the existing playground logging stuff (which hasn’t).
> That’s very reasonable. I’ll update the proposal to use CustomPlaygroundConvertible (unless I or someone else can come up with a really good “Thing” for a name like CustomPlaygroundThingConvertible, as that would even better match CustomStringConvertible/CustomDebugStringConvertible).

Thank you!


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