[swift-evolution] Smart KeyPaths

[Charles Srstka]

> On Mar 20, 2017, at 9:23 AM, Christopher Kornher via swift-evolution <swift-evolution at swift.org> wrote:
> 
>> On Mar 20, 2017, at 5:12 AM, David Hart via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>> 
>> 
>>> On 20 Mar 2017, at 10:39, Jonathan Hull via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> 
>>> +1.  This is my favorite solution so far. 
>>> 
>>> With ‘Person.keypath.name' it is obvious that we are creating a key path.  There is no ambiguity for the reader.  With autocomplete it will be very little extra typing anyway…
>> 
>> But that adds a lot of verbosity. They disregarded #keyPath because it was too verbose.
> 
> The syntax in the original proposal is terse and elegant, and will probably be fine when a developer who is experienced with Swift and a particular codebase is first writing the code. Using “key path” or “keypaths” of perhaps a shorter term or even a single leading character (`#` ?) will make this feature more discoverable, tool-friendly and its usages more maintainable.
> 
> An extra term or character does add verbosity. How much is subjective, but I would not call it “a lot”. It does not add any nesting or code complexity. KVO code is usually a small fraction of most Objective-C projects (in my experience, at least) and it is probably safe to assume that the usage of this feature in Swift will be similar.
> 
> Verbosity vs clarity is often a tradeoff and I think that on balance, for a feature like this a little extra verbosity is worth it. Swift does not have the most terse syntax possible. `++` was removed, for example.
> 
> Just because an assignment is already implicitly typed in Swift does not mean that the ambiguity has to keep increasing without end for implementation of all new features, especially for ones that are not used very frequently.

+1 to all of this.

Particularly the point that KVO code typically is a small portion of the overall code; this not only makes the added verbosity not that egregious—certainly less so than “try” or “override”—but it also underscores the fact that as a relatively uncommon feature, it’s not what a reader of the code is going to be expecting to see. This latter point is why I feel that without some kind of additional syntax—even if it’s just one character—key paths will frequently get mistaken for property accesses if they are implemented this way.

Charles

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