> On Apr 7, 2017, at 11:48 AM, John McCall <rjmcc...@apple.com> wrote:
>
>> On Apr 7, 2017, at 1:40 PM, Joe Groff <jgr...@apple.com> wrote:
>>> 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.
>
> 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
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