[swift-evolution] [Pitch] Property reflection

Fri May 27 11:54:01 CDT 2016

I think there is a consensus forming that we probably don't yet know enough about Swift's future capabilities to nail down specific designs yet. (Not that I want to stop people from talking about them if they wish.)

However, now that there is a conversation topic about reflection, I also wanted to ask the following questions:

1. What sorts of features are important, and what would you use them for?

For me, the list looks like the following:

- Describe the structure of a type. (We have that right now, with introspection using "Mirror".)

- Find all the properties on a type. Get and set their values, and get information about them (like their dynamic type).
- Find all the methods on a type. Be able to get a reference to and invoke a method on an instance of a type.
- Find all the subscripts and initializers on a type?

- Get a list of all concrete types opting in to extended reflection.
- Get a list of all concrete types visible to the caller that conform to some protocol(s), possibly with additional requirements on associated types.
- Get a list of all protocols.

- Create a reference to a type using a string, and perform certain actions using that type. (It would be interesting if you could say "try parsing this type into a concrete type that conforms to X, Y, and Z protocols", and if successful you get a metatype to use or something you can pass to a generic function.
- Create a reference to a method or property using a string, and use it as above.

- Reify methods and properties marked with the 'dynamic' keyword in such a way that where they are dispatched to can be controlled at runtime. For example, maybe a dynamic method of a class might use some default implementation, or if some condition is met forward invocations of it to a dynamic method on another class instead.

2. What sorts of priorities should inform Swift's reflection capabilities?

For me, the option to perform both "statically typed" and runtime type checked operations, when feasible, is a big one. I think something like variadic generics could allow for typesafe reflection on methods and functions at runtime - rather than having to call a performSelector: like method, it would be possible to get a full-fledged value of function type and use it just like any other method. This is something few languages have AFAIK.

The reflection machinery should cooperate with the rest of the language. If there must be a way to, for example, access private members of an instance through reflection, it should not be unconditional and should be carefully considered such that there are certain invariants that are still honored. Like Laurent said earlier, bad things can happen if you use reflection to subvert access control.

I think reflection should be opt-in in most cases. Reflection opting-in should be composable, inheritable, and retroactive, which covers the most common cases in which reflection would be useful: working with Cocoa/Cocoa Touch, for example.

Best,
Austin

> On May 27, 2016, at 7:37 AM, plx via swift-evolution <swift-evolution at swift.org> wrote:
> 
> I think this proposal should “simmer" until we know what property behaviors will look like (or find out they are never happening, etc.). 
> 
> The interaction with “property behaviors” seems likely to be quite subtle, even for something simple like `lazy`.
> 
> For sake of argument, let’s say that a `lazy` declaration like so:
> 
>   class SomeClass { 
>     lazy var foo: Foo = Foo(bar: self.prepareBar())
>   }
> 
> …gets implicitly-expanded into e.g. something like the below:
> 
>   class SomeClass {
>  
>     var foo: Foo {
>       guard let foo = _foo else {
>         _foo = Foo(bar: self.prepareBar())
>         return _foo
>       }
>       return foo
>     }
> 
>     private var _foo: Foo? = nil
> 
>   }
> 
> …which immediately exposes a few lazy-specific questions:
> 
> - should `foo` be exposed via reflection? (IMHO: yes)
> - should `_foo` be exposed via reflection? (IMHO: probably not, but not certain)
> - should `foo`’s lazy-ness be exposable? (IMHO: yes, but how exactly?)
> 
> …as well as a few more-general questions:
> 
> - should computed properties, in general, be exposed to reflection? (IMHO: probably not, but there are some exceptions…)
> - if user-specified property behaviors get special exposure, how should that work?
> - how should reflection work for enums/enums-with-payloads? 
> 
> Finally, I worry a lot less about the details of getter/setter pairs than I worry about being able to use reflection for construction/initialization.
> 
> I don’t have any actual proposal on that front, but it seems like it should be factored into any reflection design.
> 
>> On May 26, 2016, at 8:25 PM, Austin Zheng via swift-evolution <swift-evolution at swift.org <mailto: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 <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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> 
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