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

[Joe Groff]

> 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.

-Joe