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Dynamic property iteration using key paths

Richard Wei, Dan Zheng

Last updated: March 2019


KeyPathIterable is being incubated in the 'tensorflow' branch of apple/swift and released as part of the Swift for TensorFlow toolchains, which you can play with. The authors will propose this feature through Swift Evolution in 2019. Updates will be posted on the initial pitch thread.

Background and motivation

The ability to iterate over the properties of a type is a powerful reflection technique. It enables algorithms that can abstract over types with arbitrary properties: examples include property-based default implementations of equality/comparison/hash functions.

Many dynamic languages offer custom property iteration as an accessible top-level feature (e.g. dir in Python and Object.values in JavaScript).

Static types can complicate custom property iteration: since a type’s properties may have different types, representing a unified property type may require type erasure. Statically-typed languages typically achieve custom property iteration via metaprogramming or runtime reflection.

Here’s a non-comprehensive list of different approaches to custom property iteration:

A motivating use case for custom property iteration is machine learning optimization. Machine learning optimizers update structs of parameters using some algorithm (e.g. stochastic gradient descent).

Parameters are represented as struct stored properties and may have different types: they may be floating-point numbers (e.g. Float or Double), or aggregates like collections (e.g. [Float]) or other nested structs of parameters.

How can we define generic optimizers that work with arbitrary parameters?

It is not possible to represent a "heterogenous collection of parameters" without losing flexibility or type information. Instead, stored property iteration is necessary:

func update(layer: inout Parameters, gradients: Parameters) {
    // Pseudocode `` syntax for iterating over stored properties.
    // Detailed design below.
    for (inout θ, dθ) in zip(parameters, gradients) {
        θ -= learningRate * dθ


In Swift, custom property iteration is implemented using key paths and the KeyPathIterable protocol.

Key paths are a statically-typed mechanism for referring to the properties (and other members) of a type. The KeyPathIterable protocol represents types whose values provide custom key paths to properties or elements. It has two requirements, similar to CaseIterable:

/// A type whose values provides custom key paths to properties or elements.
public protocol KeyPathIterable {
    /// A type that can represent a collection of all key paths of this type.
    associatedtype AllKeyPaths: Collection
        where AllKeyPaths.Element == PartialKeyPath<Self>

   /// A collection of all custom key paths of this value.
    var allKeyPaths: AllKeyPaths { get }

The compiler can automatically provide an implementation of the KeyPathIterable requirements for any struct type, based on its stored properties: AllKeyPaths is implemented as [PartialKeyPath<Self>] and allKeyPaths returns a collection of key paths to all stored properties.

KeyPathIterable defines some default methods for accessing nested key paths and filtering key paths based on value type and mutability. These default methods are all computed based on allKeyPaths.

Key paths to top-level members Key paths to recursively all nested members
Non-writable, untyped
allKeyPaths recursivelyAllKeyPaths
Non-writable, typed
[KeyPath<Self, T>]
allKeyPaths(to: T.Type) recursivelyAllKeyPaths(to: T.Type)
Writable, typed
[WritableKeyPath<Self, T>]
allWritableKeyPaths(to: T.Type) recursivelyAllWritableKeyPaths(to: T.Type)

Additionally, conformances to KeyPathIterable for Array and Dictionary are provided in the standard library: Array.allKeyPaths returns key paths to all elements and Dictionary.allKeyPaths returns key paths to all values. These enables recursivelyAllKeyPaths to recurse through the elements/values of these collections.

extension Array: KeyPathIterable {
    public typealias AllKeyPaths = [PartialKeyPath<Array>]
    public var allKeyPaths: [PartialKeyPath<Array>] {
        return { \Array[$0] }

extension Dictionary: KeyPathIterable {
    public typealias AllKeyPaths = [PartialKeyPath<Dictionary>]
    public var allKeyPaths: [PartialKeyPath<Dictionary>] {
        return { \Dictionary[$0]! }

Basic example

Here is a contrived example demonstrating basic KeyPathIterable functionality.

struct Point: KeyPathIterable {
    var x, y: Float
struct Foo: KeyPathIterable {
    let int: Int
    let float: Float
    var points: [Point]

var foo = Foo(int: 0, float: 0, points: [Point2D(x: 1, y: 2)])
// [\, \Foo.float, \Foo.points]
print(foo.allKeyPaths(to: Float.self))
// [\Foo.float]
print(foo.recursivelyAllKeyPaths(to: Float.self))
// [\Foo.float, \Foo.points[0].x, \Foo.points[0].y]
print(foo.recursivelyAllWritableKeyPaths(to: Float.self))
// [\Foo.points[0].x, \Foo.points[0].y]

for kp in foo.recursivelyAllWritableKeyPaths(to: Float.self) {
    foo[keyPath: kp] += 1
// Foo(int: 0, float: 0, points: [Point2D(x: 2, y: 3)])


KeyPathIterable can be used to define a default Hashable implementation without using derived conformances. This is adapted from a tweet by Joe Groff:

// These implementations will be used for types that conform to
// both `KeyPathIterable` and `Hashable`.
extension Hashable where Self: KeyPathIterable {
    static func == (lhs: Self, rhs: Self) -> Bool {
        guard lhs.allKeyPaths.elementsEqual(rhs.allKeyPaths) else {
            return false
        for kp in lhs.allKeyPaths {
            guard let lhsValue = lhs[keyPath: kp] as? AnyHashable,
                  let rhsValue = rhs[keyPath: kp] as? AnyHashable,
                  lhsValue == rhsValue
            else {
                return false
        return true

    func hash(into hasher: inout Hasher) {
        for kp in allKeyPaths {
            guard let value = self[keyPath: kp] as? AnyHashable else {
            value.hash(into: &hasher)

struct Person: KeyPathIterable, Hashable {
    var firstName: String
    var age: Int
    var friends: [Person]
let johnSmith = Person(firstName: "John", age: 30, friends: [])
let johnDoe = Person(firstName: "John", age: 30, friends: [])
let jane = Person(firstName: "Jane", age: 27, friends: [johnSmith])

print(johnSmith == johnDoe) // true
print(johnSmith.hashValue == johnDoe.hashValue) // true

print(johnSmith == jane) // false
print(johnSmith.hashValue == jane.hashValue) // false

KeyPathIterable can also be used to implement machine learning optimizers. Optimizers update structs of parameters by iterating over recursively all writable key paths to parameters.

class SGD<Parameter: BinaryFloatingPoint> {
    let learningRate: Parameter

    init(learningRate: Parameter = 0.01) {
        precondition(learningRate >= 0, "Learning rate must be non-negative")
        self.learningRate = learningRate

    func update<Parameters: KeyPathIterable>(
        _ parameters: inout Parameters,
        with gradient: Parameters
    ) {
        // Iterate over recursively all writable key paths to the
        // parameter type.
        for kp in parameters.recursivelyAllWritableKeyPaths(to: Parameter.self) {
            parameters[keyPath: kp] -= learningRate * gradient[keyPath: kp]

struct DenseLayer: KeyPathIterable {
    // Parameters.
    var weight, bias: Float
    // Non-parameter.
    var activation: (Float) -> Float = { $0 }

    init(weight: Float, bias: Float) {
        self.weight = weight
        self.bias = bias

struct MyMLModel: KeyPathIterable {
    // Parameters.
    var layers: [DenseLayer]
    // Non-parameters.
    var isTraining: Bool = false

    init(layers: [DenseLayer]) {
        self.layers = layers

let dense = DenseLayer(weight: 1, bias: 1)
var model = MyMLModel(layers: [dense, dense])
let gradient = model
let optimizer = SGD<Float>()
optimizer.update(&model, with: gradient)

// ▿ MyMLModel
//   ▿ layers: 2 elements
//     ▿ DenseLayer
//       - weight: 0.99
//       - bias: 0.99
//       - activation: (Function)
//     ▿ DenseLayer
//       - weight: 0.99
//       - bias: 0.99
//       - activation: (Function)
//   - isTraining: false

Extended design

We explored an extended modular design for custom property iteration that uses two protocols: StoredPropertyIterable and CustomKeyPathIterable.

  • StoredPropertyIterable models the static layout of structs. It provides a static computed property allStoredProperties that represents a collection of key paths to all stored properties in the type.

  • CustomKeyPathIterable models both static and dynamic structures. It provides an instance computed property allKeyPaths that can represent stored properties, or dynamic properties like elements.


The authors would like to thank Dominik Grewe and Dimitrios Vytiniotis for their key-path-related ideas, which contributed to the design of the KeyPathIterable protocol.