[swift-evolution] Smart KeyPaths

[Matthew Johnson]

Sent from my iPad

> On Mar 30, 2017, at 12:35 AM, David Hart via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 
> 
> 
> 
> Sent from my iPhone
> On 30 Mar 2017, at 01:13, Michael J LeHew Jr via swift-evolution <swift-evolution at swift.org> wrote:
> 
>> Thanks for the feedback everyone!  We have pushed a changed a bit ago to the proposal reflecting these desires.
>> 
>> https://github.com/apple/swift-evolution/pull/644/files
>> 
>> -Michael
> 
> I'm not a fan of the new syntax for creating key paths. To me, it feels like they've been demoted to second class citizens of the language simply because of how more verbose it now is. The new syntax is also too confusingly similar to string key paths: I had to look closely at the code to see the difference. Is there no symbol we can use to make it ambiguous? Ideas:
> 
> Person::friend.lastName
> Person/friend.lastName
> Person#friend.lastName
> 
> I'm a fan of the first one as it has similarities to names pacing in C++.

I'm a big fan of the last one.  I argued for it earlier as the best syntax to use if we deviated from the initial proposal.  I like it for several reasons:

- # suggests compiler magic is at work which is the case here.
- #friend.lastName works nicely as a shorthand in contexts expecting a key path with a fixed Root
- # would work for unbound methods solving the no arguments case.  IMO all unbound members should be accessed using the same syntax.
- # enables the possibility of mixing property access and method calls in the path as a future enhancement

The arguments supporting this approach are pretty strong to me.  I agree with David that the #keyPath syntax makes it feel more like a second class citizen, not just because of the verbosity but also because it is directly borrowed from an Objective-C interop feature.  This is a very powerful feature that deserves to be a first class syntactic citizen every bit as much as unbound methods do.

> 
> David.
> 
>>>> On Mar 29, 2017, at 2:49 PM, Douglas Gregor <dgregor at apple.com> wrote:
>>>> 
>>>> 
>>>> On Mar 17, 2017, at 10:04 AM, Michael LeHew via swift-evolution <swift-evolution at swift.org> wrote:
>>>> 
>>>> Hi friendly swift-evolution folks,
>>>> 
>>>> The Foundation and Swift team  would like for you to consider the following proposal:
>>> 
>>> 
>>> The Swift core team discussed this proposal draft and had a little bit of pre-review feedback.
>>> 
>>>> Access and Mutation Through KeyPaths
>>>> To get or set values for a given root and key path we effectively add the following subscripts to all Swift types. 
>>>> 
>>>> Swift
>>>> extension Any {
>>>>     subscript(path: AnyKeyPath) -> Any? { get }
>>>>     subscript<Root: Self>(path: PartialKeyPath<Root>) -> Any { get }
>>>>     subscript<Root: Self, Value>(path: KeyPath<Root, Value>) -> Value { get }
>>>>     subscript<Root: Self, Value>(path: WritableKeyPath<Root, Value>) -> Value { set, get }
>>>> }
>>> 
>>> Swift doesn’t currently have the ability to extend Any, so this is (currently) pseudocode for compiler magic that one day we might be able to place. Additionally, the “Root: Self” constraint isn’t something we support in the generics system. A small note indicating that this is pseudo-code meant to get the point across (rather than real code to drop into the standard library/Foundation) would be appreciated.
>>> 
>>> More importantly, this adds an unlabeled subscript to every type, which raises concerns about introducing ambiguities—even if not hard ambiguities that prevent code from compiling (e.g., from a Dictionary<AnyKeyPath, …>)---they can still show up in code completion, diagnostics, etc.
>>> 
>>> The core team would prefer that this subscript distinguish itself more, e.g., by labeling the first parameter “keyPath” (or some better name, if there is one). Syntactically, that would look like:
>>> 
>>> 	person[keyPath: theKeyPathIHave]
>>> 
>>>> Referencing Key Paths
>>>> 
>>>> Forming a KeyPath borrows from the same syntax used to reference methods and initializers,Type.instanceMethod only now working for properties and collections. Optionals are handled via optional-chaining. Multiply dotted expressions are allowed as well, and work just as if they were composed via the appending methods on KeyPath.
>>>> 
>>> The core team was concerned about the use of the Type.instanceProperty syntax for a few reasons:
>>> 
>>> 	* It doesn’t work for forming keypaths to class/static properties (or is ambiguous with the existing meaning(, so we would need another syntax to deal with that case
>>> 	* It’s quite subtle, even more so that the existing Type.instanceMethod syntax for currying instance methods
>>> 
>>>> There is no change or interaction with the #keyPath() syntax introduced in Swift 3. 
>>>> 
>>> The core team felt that extending the #keyPath syntax was a better syntactic direction to produce key-paths.
>>> 
>>> 	- Doug
>>> 
>> 
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