KeyedSet

A hybrid of Set and Dictionary. Mostly a Set, but with Dictionary-like subscripting.

Imagine that we want a set of Person instances, unique by social security number, and we wish to perform lookups by social security number.

let belle = Person(ssn: "111-11-2222", name: "Isabella")
let greg = Person(ssn: "9876-54-321", name: "Greg")
let bob = Person(ssn: "123-45-6789", name: "Bob")
var people: KeyedSet<Person> = [belle, greg]
// A KeyedSet is a set, with all the usual SetAlgebra operations.
people.update(with: bob)
// But it provides dictionary-style lookup by key.
if let belle = people["111-11-2222"] {
  print(belle.name)
}

A KeyedSet is far more Set than Dictionary. The only Dictionary operation it supports is subscripting. In all other respects, KeyedSet is an ordinary Set. (Of course, Set is a value type, so KeyedSet is not a subtype of Set. More like a fraternal twin.)

Keyed

In order to be placed into a KeyedSet, the element type must implement the Keyed protocol:

protocol Keyed: Hashable {
  associatedtype KeyedSetKey: Hashable
  static var keyedSetKeyPath: KeyPath<Self, Key> { get }
}

// `Keyed` provides `KeyedSet`-compliant default
// implementations of `Hashable` and `Equatable`.
struct Person: Keyed {
  // This is how our first example knew which attribute
  // to use for key lookup.
  static let keyedSetKeyPath = \Person.ssn
  let ssn: String
  let name: String
}

This protocol tells KeyedSet which attribute of the element type is to be used for dictionary subscript operations. Let us call this attribute the key attribute. No two elements having the same key attribute may be present in the set at the same time. Otherwise, an unrecoverable runtime exception will eventually occur.

It is up to you to enforce this rule, but the easiest way to do so is to ensure that the element type is both Hashable and Equatable by the key attribute. If the element type conforms to the Keyed protocol, default implementations of Hashable and Equatable which follow this rule are provided.

Dictionary Operations

As mentioned earlier, a KeyedSet is a set. The only dictionary operation it supports is subscripting. Subscripting is type-safe, and the type of the key used in dictionary subscripting is declared by the element type's Keyed implementation.

Subscripting is both readable and writable. When writing, it is important that the keys match. Saying something like people["Bob"] = Person(name: "Fred") will cause a runtime exception (assuming name is the key attribute).

In general, it is best not to use writable subscripting directly. Instead, use the standard set operations, e.g., people.update(with: Person(name: "Fred")). Writable subscripting exists to permit in-place attribute mutations, e.g., people["Bob"]?.age = 44.

Dictionary Operations on Sequences

This framework adds a subscript to all sequences whose element type conforms to Keyed. This subscript allows read-only (but pretty inefficient) key lookup operations.

let orders = [Order(identifier: identifier), Order(), Order()]
print(orders[key: identifier]!)

This subscript finds the first element with a conforming key.