[swift-evolution] Smart KeyPaths

Michael LeHew lehewjr at apple.com
Fri Mar 17 12:04:20 CDT 2017

Hi friendly swift-evolution folks,

The Foundation and Swift team  would like for you to consider the following proposal:

Many thanks,

Smart KeyPaths: Better Key-Value Coding for Swift
Proposal: SE-NNNN
Authors: David Smith <https://github.com/Catfish-Man>, Michael LeHew <https://github.com/mlehew>, Joe Groff <https://github.com/jckarter>
Review Manager: TBD
Status: Awaiting Review
Associated PRs:
#644 <https://github.com/apple/swift-evolution/pull/644>
We propose a family of concrete Key Path types that represent uninvoked references to properties that can be composed to form paths through many values and directly get/set their underlying values.

We Can Do Better than String

On Darwin platforms Swift's existing #keyPath() syntax provides a convenient way to safely refer to properties. Unfortunately, once validated, the expression becomes a String which has a number of important limitations:

Loss of type information (requiring awkward Any APIs)
Unnecessarily slow to parse
Only applicable to NSObjects
Limited to Darwin platforms
Use/Mention Distinctions

While methods can be referred to without invoking them (let x = foo.bar instead of  let x = foo.bar()), this is not currently possible for properties and subscripts.

Making indirect references to a properties' concrete types also lets us expose metadata about the property, and in the future additional behaviors.

More Expressive KeyPaths

We would also like to support being able to use Key Paths to access into collections, which is not currently possible.

Proposed solution
We propose introducing a new expression akin to Type.method, but for properties and subscripts. These property reference expressions produce KeyPath objects, rather than Strings. KeyPaths are a family of generic classes (structs and protocols here would be ideal, but requires generalized existentials) which encapsulate a property reference or chain of property references, including the type, mutability, property name(s), and ability to set/get values.

Here's a sample of it in use:

struct Person {
    var name: String
    var friends: [Person]
    var bestFriend: Person?

var han = Person(name: "Han Solo", friends: [])
var luke = Person(name: "Luke Skywalker", friends: [han])

let firstFriendsNameKeyPath = Person.friends[0].name

let firstFriend = luke[path] // han

// or equivalently, with type inferred from context
let firstFriendName = luke[.friends[0].name]

// rename Luke's first friend
luke[firstFriendsNameKeyPath] = "A Disreputable Smuggler"

let bestFriendsName = luke[.bestFriend]?.name  // nil, if he is the last jedi
Detailed design
Core KeyPath Types

KeyPaths are a hierarchy of progressively more specific classes, based on whether we have prior knowledge of the path through the object graph we wish to traverse. 

Unknown Path / Unknown Root Type

AnyKeyPath is fully type-erased, referring to 'any route' through an object/value graph for 'any root'. Because of type-erasure many operations can fail at runtime and are thus nillable. 

class AnyKeyPath: CustomDebugStringConvertible, Hashable {
    // MARK - Composition
    // Returns nil if path.rootType != self.valueType
    func appending(path: AnyKeyPath) -> AnyKeyPath?
    // MARK - Runtime Information        
    class var rootType: Any.Type
    class var valueType: Any.Type
    static func == (lhs: AnyKeyPath, rhs: AnyKeyPath) -> Bool
    var hashValue: Int
Unknown Path / Known Root Type

If we know a little more type information (what kind of thing the key path is relative to), then we can use PartialKeyPath <Root>, which refers to an 'any route' from a known root:

class PartialKeyPath<Root>: AnyKeyPath {
    // MARK - Composition
    // Returns nil if Value != self.valueType
    func appending(path: AnyKeyPath) -> PartialKeyPath<Root>?
    func appending<Value, AppendedValue>(path: KeyPath<Value, AppendedValue>) -> KeyPath<Root, AppendedValue>?
    func appending<Value, AppendedValue>(path: ReferenceKeyPath<Value, AppendedValue>) -> ReferenceKeyPath<Root, AppendedValue>?
Known Path / Known Root Type

When we know both what the path is relative to and what it refers to, we can use KeyPath. Thanks to the knowledge of the Root and Value types, all of the failable operations lose their Optional. 

public class KeyPath<Root, Value>: PartialKeyPath<Root> {
    // MARK - Composition
    func appending<AppendedValue>(path: KeyPath<Value, AppendedValue>) -> KeyPath<Root, AppendedValue>
    func appending<AppendedValue>(path: WritableKeyPath<Value, AppendedValue>) -> Self
    func appending<AppendedValue>(path: ReferenceWritableKeyPath<Value, AppendedValue>) -> ReferenceWritableKeyPath<Root, AppendedValue>
Value/Reference Mutation Semantics Mutation

Finally, we have a pair of subclasses encapsulating value/reference mutation semantics. These have to be distinct because mutating a copy of a value is not very useful, so we need to mutate an inout value.

class WritableKeyPath<Root, Value>: KeyPath<Root, Value> {
    // MARK - Composition
    func appending<AppendedPathValue>(path: WritableKeyPath<Value, AppendedPathValue>) -> WritableKeyPath<Root, AppendedPathValue>

class ReferenceWritableKeyPath<Root, Value>: WritableKeyPath<Root, Value> {
    override func appending<AppendedPathValue>(path: WritableKeyPath<Value, AppendedPathValue>) -> ReferenceWritableKeyPath<Root, AppendedPathValue>
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. 

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 }
This allows for code like

person[.name] // Self.type is inferred
which is both appealingly readable, and doesn't require read-modify-write copies (subscripts access self inout). Conflicts with existing subscripts are avoided by using generic subscripts to specifically only accept key paths with a Root of the type in question.

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.

There is no change or interaction with the #keyPath() syntax introduced in Swift 3. 


The performance of interacting with a property via KeyPaths should be close to the cost of calling the property directly.

Source compatibility
This change is additive and there should no affect on existing source. 

Effect on ABI stability
This feature adds the following requirements to ABI stability: 

mechanism to access key paths of public properties
We think a protocol-based design would be preferable once the language has sufficient support for generalized existentials to make that ergonomic. By keeping the class hierarchy closed and the concrete implementations private to the implementation it should be tractable to provide compatibility with an open protocol-based design in the future.

Effect on API resilience
This should not significantly impact API resilience, as it merely provides a new mechanism for operating on existing APIs.

Alternatives considered
More Features

Various drafts of this proposal have included additional features (decomposable key paths, prefix comparisons, support for custom KeyPath subclasses, creating a KeyPath from a String at runtime, KeyPaths conforming to Codable, bound key paths as a concrete type, etc.). We anticipate approaching these enhancements additively once the core KeyPath functionality is in place. 


We also explored many different spellings, each with different strengths. We have chosen the current syntax due to the balance with existing function type references.

Current	#keyPath	Lisp-style
Person.friends[0].name	#keyPath(Person, .friends[0].name)	`Person.friend.name
luke[.friends[0].name]	#keyPath(luke, .friends[0].name)	luke`.friends[0].name
luke.friends[0][.name]	#keyPath(luke.friends[0], .name)	luke.friends[0]`.name
While the crispness is very appealing, the spelling of the 'escape' character was hard to agree upon (along with the fact that it requires parentheses to reduce ambiguity).  #keyPath was very specific, but verbose especially when composing multiple key paths together.
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