[swift-evolution] [Pitch] Improving KeyPath

[Joe Groff]

> On Aug 23, 2017, at 11:18 PM, Logan Shire via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Hey folks! 
> 
> Recently I’ve been working on a small library which leverages the Swift 4 Codable protocol
> and KeyPaths to provide a Swift-y interface to CoreData. (It maps back and forth between
> native, immutable Swift structs and NSManagedObjects). In doing so I found a couple of 
> frustrating limitations to the KeyPath API. Firstly, KeyPath does not provide the name of the 
> property on the type it indexes. For example, if I have a struct:
> 
> 
> struct Person {
>    let firstName: String
>    let lastName: String
> }
> 
> let keyPath = \Person.firstName
> 
> 
> But once I have a keyPath, I can’t actually figure out what property it accesses.
> So, I wind up having to make a wrapper:
> 
> 
> struct Attribute {
>    let keyPath: AnyKeyPath
>    let propertyName: String
> }
> 
> let firstNameAttribute = Attribute(keyPath: \Person.firstName, propertyName: “firstName”)
> 
> 
> This forces me to write out the property name myself as a string which is very error prone.
> All I want is to be able to access:
> 
> 
> keyPath.propertyName // “firstName”
> 
> 
> It would also be nice if we provided the full path as a string as well:
> 
> 
> keyPath.fullPath // “Person.firstName"
> 
> 
> Also, if I want to get all of the attributes from a given Swift type, my options are to try to hack
> something together with Mirrors, or forcing the type to declare a function / computed property
> returning an array of all of its key path / property name pairings. I would really like to be able to 
> retrieve a type-erased array of any type’s key paths with:
> 
> 
> let person = Person(firstName: “John”, lastName: “Doe”)
> let keyPaths = Person.keyPaths
> let firstNameKeyPath = keyPaths.first { $0.propertyName = “firstName” } as! KeyPath<Person, String>
> let firstName = person[keypath: firstNameKeyPath] // “John"
> 
> 
> And finally, without straying too far into Objective-C land, it would be nice if we could initialize key paths
> with a throwing initializer.
> 
> 
> let keyPath = try Person.keyPath(“firstName”) // KeyPath<Person, String> type erased to AnyKeyPath
> let keyPath = AnyKeyPath(“Person.firstName”)
> 
> 
> Let me know what you think about any / all of these suggestions!

These would all be great additional features to eventually add to key paths. I think reflection mechanisms centered on key paths like what you describe would be a superior replacement for most of what Mirror attempts to provide.

-Joe