EqualKeyPath.swift

// MARK: - EqualKeyPath

struct EqualKeyPath<T, A> where A: Equatable {
    init(on: T.Type, for k1: KeyPath<T, A>) {
        self.k1 = k1
    }

    let k1: KeyPath<T, A>
    func equalFor(_ a: T, _ b: T) -> Bool {
        a[keyPath: k1] == b[keyPath: k1]
    }
}

// MARK: - EqualKeyPath2

struct EqualKeyPath2<T, A, B> where A: Equatable, B: Equatable {
    let k1: KeyPath<T, A>
    let k2: KeyPath<T, B>
    func equalFor(_ a: T, _ b: T) -> Bool {
        let bools = [
            a[keyPath: k1] == b[keyPath: k1],
            a[keyPath: k2] == b[keyPath: k2],
        ]
        return bools.allSatisfy { $0 }
    }
}

// MARK: - EqualKeyPath3

struct EqualKeyPath3<T, A, B, C> where
    A: Equatable,
    B: Equatable,
    C: Equatable
{
    init(on: T.Type,
         _ k1: KeyPath<T, A>,
         _ k2: KeyPath<T, B>,
         _ k3: KeyPath<T, B>)
    {
        self.k1 = k1
        self.k2 = k2
        self.k3 = k3
    }

    let k1: KeyPath<T, A>
    let k2: KeyPath<T, B>
    let k3: KeyPath<T, B>
    func equalFor(_ a: T, _ b: T) -> Bool {
        let bools = [
            a[keyPath: k1] == b[keyPath: k1],
            a[keyPath: k2] == b[keyPath: k2],
            a[keyPath: k3] == b[keyPath: k3],
        ]
        return bools.allSatisfy { $0 }
    }
}