[swift-evolution] [Pitch] Improving KeyPath

[Logan Shire]

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!


Thanks,
Logan