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

[John McCall]

> 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.

John.