> On Apr 7, 2017, at 10:20 AM, John McCall via swift-evolution
> <swift-evolution@swift.org> wrote:
>
>>
>> On Apr 7, 2017, at 12:48 AM, Douglas Gregor <dgre...@apple.com> wrote:
>>
>>
>>> On Apr 6, 2017, at 9:46 PM, John McCall <rjmcc...@apple.com> wrote:
>>>
>>>> On Apr 7, 2017, at 12:27 AM, Rick Mann <rm...@latencyzero.com> wrote:
>>>>> On Apr 6, 2017, at 20:37 , John McCall <rjmcc...@apple.com> wrote:
>>>>>
>>>>>> On Apr 6, 2017, at 9:28 PM, Rick Mann via swift-evolution
>>>>>> <swift-evolution@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.@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
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