[swift-evolution] [Pitch] Property reflection

[Goffredo Marocchi]

You made me think about JNI and now I am sad... You monster :P.

Sent from my iPhone

> On 27 May 2016, at 08:54, L. Mihalkovic via swift-evolution <swift-evolution at swift.org> wrote:
> 
> It might be good to learn from the mistakes java made with its reflection API because it might help avoid facing the impedence mismatch java faced a couple years ago when they tried to model the language on top of reflection. 
> The point is I think that starting in a small corner and trying to walk your way up is like the quantum mechanical way of trying to capture a lion (put the cage on the table and wait for the lion to show up inside), which in this case means do something somewhere, repeat, and hope it collectively makes sense when it's all done.
> IMO the design does not emerge spontaneously from the syntax, it is the syntax that is created because it serves some higher order principles.
> 
>> On May 27, 2016, at 3:25 AM, Austin Zheng via swift-evolution <swift-evolution at swift.org> wrote:
>> 
>> Hi swift-evolution,
>> 
>> For those who are interested I'd like to present a pre-pre-proposal for reflection upon a type's properties and solicit feedback. 
>> 
>> First of all, some caveats: this is only a very small piece of what reflection in Swift might look like one day, and it's certainly not the only possible design for such a feature. Reflection comes in many different forms, and "no reflection" is also an option. Deciding what sort of reflection capabilities Swift should support is a prerequisite to stabilizing the runtime API, which I imagine has resilience consequences. I'm not really interested in defending this specific proposal per se, as I am looking for a jumping-off point to explore designs in this space.
>> 
>> Anyways, here is a gist outlining the public API to the feature: 
>> 
>> https://gist.github.com/austinzheng/699d47f50899b88645f56964c0b7109a
>> 
>> A couple of notes regarding the proposal:
>> 
>> The API names need improvement. Suggestions welcome.
>> 
>> It's opt-in: types have to conform to a special protocol for the compiler to generate whatever hooks, metadata, and support code is necessary. Once a type conforms, the interface to the reflection features naturally present themselves as protocol methods. It would be great to allow an extension to retroactively enable reflection on a type vended by another module, although I have no idea how feasible that is.
>> 
>> It uses "views": there are four types of views, two of each in the following categories: typed vs untyped, get-only versus get-set. A view is a struct representing a property on an instance of a type (or maybe a metatype, for type properties). It allows you to get information about that property (like its name) and try getting and setting its values.
>> 
>> (You can get a get-only view to a property, and then try and upgrade it later to a get-set view, if the underlying property is get-set. If you don't care about setting, though, you can just work exclusively with get-only views.)
>> 
>> It supports both typed and untyped access. You can ask for a property view specifically for (e.g.) a `String` property, and if you get one you can be assured that your getting and setting operations will be type safe. You can also ask for an "untyped" property view that exposes the value as an Any, and allows you to try (and possibly fail, with a thrown error) to set the value.
>> 
>> The requirements part of it is composable. For example, you can imagine a future "FullyReflectable" protocol that simply inherits from "PropertyReflectable", "MethodReflectable", and other reflectable protocols. Or maybe a library requires reflection access to types that it needs to work with, and it can create its own protocols that inherit from "PropertyReflectable" and naturally enforce reflection support on the necessary types.
>> 
>> It looks a bit cumbersome, but there's room for refinement. Users won't necessarily see all the types, though, and the interface is pretty straightforward:
>> 
>> ```
>> myPerson.typedReadWriteProperty<Int>("age")?.set(30)
>> 
>> try myPerson.allNamedProperties["age"]?.set(30)
>> ```
>> 
>> I'm not yet sure how it should interact with access control (my inclination is that it would only expose the properties you'd be able to directly access), or property behaviors (I think get-set behavior is fundamental to properties, although "behavior metadata" on the views might be useful).
>> 
>> I'd also have to figure out how it would operate with generic types or existentials.
>> 
>> Anyways, thanks for reading all the way to the end, and any feedback, criticism, or alternative proposals would be greatly appreciated.
>> 
>> Best,
>> Austin
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