[swift-evolution] Smart KeyPaths

[Karim Nassar]

> Date: Fri, 31 Mar 2017 04:33:43 -0700
> From: Brent Royal-Gordon <brent at architechies.com>
> To: Haravikk <swift-evolution at haravikk.me>
> Cc: Haravikk via swift-evolution <swift-evolution at swift.org>
> Subject: Re: [swift-evolution] Smart KeyPaths
> Message-ID: <ADB64B59-D79C-41CC-9465-DC4697CB25BB at architechies.com>
> Content-Type: text/plain; charset="utf-8"
> 
>> On Mar 31, 2017, at 3:07 AM, Haravikk via swift-evolution <swift-evolution at swift.org> wrote:
>> 
>> Is it actually in-use or just reserved? Not sure I've ever needed it in the debugger.
> 
> Pop into the REPL for a minute:
> 
> 	$ swift
> 	Welcome to Apple Swift version 3.1 (swiftlang-802.0.41 clang-802.0.36). Type :help for assistance.
> 	  1> "hello"
> 	$R0: String = "hello"
> 	  2> $R0
> 	$R1: String = "hello"
> 
> You may not have ever noticed it was there, but it was.
> 
>> What about @? It has a certain appropriateness in how it reads for a path like: @Person.friend.lastName
> 
> We're already using @ for attributes; I don't think we want to overload its meaning.
> 
>> Another alternative might be something like an unnamed compiler directive, for example: #(Person.friend.lastName)
>> If you consider the statement to mean "expand this".
> 
> The unnamed compiler directive seems like *really* valuable real estate, arguably much more so than unresolved-member-on-KeyPath-type is. I think it'd be foolish to assign it to anything before we have a macro system designed.


I agree that #() is too valuable to burn at this point. 

So… Crazy Thought: Is KeyPath enough of a term-of-art beyond ObjC that we want to carry it’s semantics directly into Swift even when it conflicts with established ObjC inter-op? In other words, if we didn’t already know ObjC & KVO/KVC, does “KeyPath” mean what we mean by it in Swift?

If the issue is the conflict with the current semantics of #keyPath(), what about something like:

```
   let fooProp: Property<Foo, String> = #property(bar.bast)
```

And in the inferred/ambiguous case:

```
    let fooProp = #property(bar.bast, of: Foo)
```

It perhaps moves too far from the KeyPath nomenclature, but it reads rather nicely (IMHO).

—Karim