HashableByKeyPath framework release 1.0.0

Today I have released the 1.0.0 version of a Swift package that aids with adding Equatable and Hashable conformance by using KeyPaths.

The package is available on GitHub.

I created the Swift Playground that sparked this concept in December 2018, so this concept has been rattling around in my brain for a couple of years. The API has changed a lot since the original concept, but the core has stayed the same: a protocol that requires a single function to be implemented that uses KeyPaths to synthesise Equatable and/or Hashable conformance.

One of the mistakes I've made more times than I care to admit is comparing the wrong properties or objects when adding Equatable conformance, e.g.:

struct Foo: Equatable {
    static func == (lhs: Foo, rhs: Foo) -> Bool {
        return lhs.bar1 == rhs.bar1 && lhs.bar2 == rhs.bar1 && rhs.bar3 == rhs.bar3
    }

    var bar1: String
    var bar2: String
    var bar3: Int
}

These can be easy to type and hard to spot (even with better layout). The EquatableByKeyPath protocol fixes this issue:

struct Foo: EquatableByKeyPath {
    static func addEquatableKeyPaths<Consumer: EquatableKeyPathConsumer>(to consumer: inout Consumer) where Consumer.Root == Self {
        consumer.addEquatableKeyPath(\.bar1)
        consumer.addEquatableKeyPath(\.bar2)
        consumer.addEquatableKeyPath(\.bar3)
    }

    var bar1: String
    var bar2: String
    var bar3: Int
}

Adding the addEquatableKeyPaths(to:) function will synthesise Equatable conformance. Since you provide every KeyPath once it's not possible to compare the wrong properties or objects.

Another mistake I've made a lot is not keeping my Equatable and Hashable conformance in sync. e.g. I add a new property but only add it to == and not hash(into:). According to Hashable, this is bad:

Hashing a value means feeding its essential components into a hash function, represented by the Hasher type. Essential components are those that contribute to the type’s implementation of Equatable. Two instances that are equal must feed the same values to Hasher in hash(into:), in the same order.

Source: https://developer.apple.com/documentation/swift/hashable

The HashableByKeyPath protocol fixes this issue:

struct Foo: HashableByKeyPath {
    static func addHashableKeyPaths<Consumer: HashableKeyPathConsumer>(to consumer: inout Consumer) where Consumer.Root == Self {
        consumer.addHashableKeyPath(\.bar1)
        consumer.addHashableKeyPath(\.bar2)
        consumer.addHashableKeyPath(\.bar3)
    }

    var bar1: String
    var bar2: String
    var bar3: Int
}

Adding the addHashableKeyPaths(to:) function will synthesise Hashable, EquatableByKeyPath, and Equatable conformance. Since you provide every KeyPath once and only have 1 functions it's not possible to compare the wrong properties or objects or have your Equatable and Hashable conformances out of sync.

I will admit that this can't replace every Equatable and Hashable conformance out there, but I've not had a scenario yet I had to manually implement == or hash(into:).