[swift-evolution] [Pitch] Property reflection

[Matthew Johnson]

> On May 27, 2016, at 9:50 AM, L Mihalkovic <laurent.mihalkovic at gmail.com> wrote:
> 
>> 
>> On May 27, 2016, at 4:05 PM, Matthew Johnson via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>> 
>> 
>> Sent from my iPad
>> 
>> On May 26, 2016, at 9:44 PM, Austin Zheng <austinzheng at gmail.com <mailto:austinzheng at gmail.com>> wrote:
>> 
>>> Thanks, as always, for the thoughtful feedback. (inline)
>>> 
>>> On Thu, May 26, 2016 at 7:20 PM, Matthew Johnson <matthew at anandabits.com <mailto:matthew at anandabits.com>> wrote:
>>> 
>>> 
>>> These names are a good place to start but I agree that it would be nice to improve them.  I will give it some thought.  One comment for now - you use both `get` / `set` and `read` / `write`.  It’s probably better to pick one.
>>> 
>>> Yes, that's a good start. I'll sleep on it.
>>>  
>>> 
>>>> 
>>>> 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.
>>> 
>>> What do you think of using the `deriving` syntax for this (assuming we go in that direction for Equatable, Hashable, and other synthesized conformances).
>>> 
>>> 'deriving' is pretty much what I had in mind. If Equatable gets a magical attribute, keyword, or whatever, I'd like to use it for this feature as well.
>>>  
>>> 
>>>> 
>>>> 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.
>>> 
>>> How did you arrive at `get` and `set` methods?
>>> 
>>> One big advantage methods have over properties is that you can (as of right now) get a reference to a method as a value of function type, but not a property.
>> 
>> True, but as with 'throws' this is a temporary limitation.  I don't think we should design our reflection API around a temporary limitation.  IIRC the intended plan for getting first class access to properties is a lens.  If we model it as a property we will eventually get first class access to it as a lens (which means we play nice with lenses even though we don't know what they will look like yet).
>> 
>>>  
>>> 
>>> I am wondering how this might relate to lenses.  If we’re going to introduce those it feels like the property value should be introduced as a lens.  I’m unsure of exactly what that would look like but I it is definitely worth thinking about.
>>> 
>>> I think it's worth consideration. Maybe views into properties can eventually conform to some sort of lens protocol or interface, allowing them to be composed and used as such.
>> 
>> I think Joe Groff has talked about lenses more than anyone else.  Maybe he has some thoughts on this.  But as I noted above, maybe exposing access as a property is the best first step.  It's easy enough to wrap in a closure if you need to pass a function around before property references / lenses are introduced.
>> 
>>>  
>>> 
>>> Another option if we don’t go with a lens is a simple property (`var value { get }` and `var value { get set }`).  IIRC we are going to have throwing computed properties eventually so you could still throw from the setter.
>>> 
>>>> ```
>>>> 
>>>> 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), 
>>> 
>>> This one is tricky.  I am generally be opposed to any way to get around access control.  But people are going to implement things like serialization using this which may require access to private properties.  I think we want to try to understand the consequences of different options and when in doubt decide in favor caution.
>>> 
>>> My personal preference would be to honor access control, and consider ser/de separately (especially since there are so many other possible considerations for that feature). Access control in Swift isn't just another safety feature, it's also a boundary for things like the optimizer.
>>> 
>>> To be honest, I expect the first big use of a feature like this to be turning JSON or XML received from a network call into model objects for an app. There are quite a few Objective-C libraries that use KVC to implement this functionality.
>> 
>> As several have noted, this is what makes things tricky.  People will want to write a library packaged as a module that can serialize private properties from types in a different module.  We need to at least consider that use case. 
> 
> 
> or perhaps simply read what the people who have a lot of experience the topic have to say about it, and hopefully learn from the errors others have described. Serialization has long been a source of serious security issues in Java. Swift could do without these
> 
> -1 

That’s a good point.  Talking about the issues involved and stating the reasons for our decision is giving the use case consideration.  I’m not advocating either way.  I’m just saying that we need to make a conscious decision with a good and clearly stated rationale.

-Matthew

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