[swift-evolution] [Review #2] SE-0161: Smart KeyPaths: Better Key-Value Coding for Swift

[Joe Groff]

> On Apr 7, 2017, at 11:48 AM, John McCall <rjmccall at apple.com> wrote:
> 
>> On Apr 7, 2017, at 1:40 PM, Joe Groff <jgroff at apple.com> wrote:
>>> On Apr 7, 2017, at 10:20 AM, John McCall via swift-evolution <swift-evolution at swift.org> wrote:
>>> 
>>>> 
>>>> On Apr 7, 2017, at 12:48 AM, Douglas Gregor <dgregor at apple.com> wrote:
>>>> 
>>>> 
>>>>> On Apr 6, 2017, at 9:46 PM, John McCall <rjmccall at apple.com> wrote:
>>>>> 
>>>>>> On Apr 7, 2017, at 12:27 AM, Rick Mann <rmann at latencyzero.com> wrote:
>>>>>>> On Apr 6, 2017, at 20:37 , John McCall <rjmccall at apple.com> wrote:
>>>>>>> 
>>>>>>>> On Apr 6, 2017, at 9:28 PM, Rick Mann via swift-evolution <swift-evolution at swift.org> wrote:
>>>>>>>> I tend to dislike the backslash as well, but can't suggest a good alternative.
>>>>>>>> 
>>>>>>>> Does any of this allow for operations within the key path? e.g. Department.employees. at sum.salary?
>>>>>>> 
>>>>>>> You can express things like this in the feature as proposed using subscripts:
>>>>>>> 
>>>>>>> extension Collection {
>>>>>>> subscript<T: Integer>(summing path: KeyPath<Element, T>) -> T {
>>>>>>> var sum: T = 0
>>>>>>> for let elt in self {
>>>>>>> sum += elt[keyPath: path]
>>>>>>> }
>>>>>>> return sum
>>>>>>> }
>>>>>>> }
>>>>>> 
>>>>>> I'm just remembering how AppKit/Cocoa lets you do things like this in a very expressive way. Your proposal seems a bit cumbersome. Maybe when we have custom annotations, they can be extended to use within key paths.
>>>>> 
>>>>> I'm not seriously endorsing this exact spelling.  It would be much better to be able to write something like:
>>>>> \Department.employees.sum(of: \.salary)
>>>>> However, since "sum" would presumably be a method on Collection, I think this would have to be a future extension to the proposal, and the overall thing might have to be a function rather than a key path because it would no longer have identity.
>>>> 
>>>> Also, less clever but potentially easier to reason about:
>>>> 
>>>> 	extension Array where Element == Employee {
>>>> 	  var sumOfSalary: Double {
>>>> 		return // ...
>>>> 	  }
>>>> 	}
>>>> 
>>>> If you can express it in a computed property, you can refer to it via a key path:
>>>> 
>>>> 	\Department.employees.sumOfSalary
>>> 
>>> Yeah, you can, but that's definitely an expressivity hit.
>> 
>> True, but there are some benefits to requiring a subscript/property rather than an arbitrary closure, particularly that it gives the operation a stable identity and structure so the key path can still be equated/hashed and (eventually) iterated through.
> 
> Right, I think if you add a method to the chain, the result definitely has to be a function rather than a key path.  The idea is that you basically decompose:
> 
>  \Base.A.B.C
> 
> into
>  ([inout]? Base, parameters(A)..., parameters(B)..., parameters(C)...) -> result(C)
> 
> except that if all of the components A, B, and C are just properties or applied subscripts you can make it a KeyPath<Base,C> instead, which can then contextually devolve into a function.

It seems to me that method references (non-mutating ones, at least) could still be treated as read-only key path components. There's not much more than syntax as a difference between a nonmutating method and get-only property or subscript. The method decl is still something we can ascribe identity to.

-Joe