Versioned: monad-like structure for holding version number along with any value

Versioned.swift

public struct Versioned<T> {
    public init(_ value: T) {
        self.value = value
        self.version = VersionGenerator.generateVersion()
    }
    
    public init(_ value: T, version: VersionGenerator.VersionType) {
        self.value = value
        self.version = version
    }
    
    public let value: T
    public let version: VersionGenerator.VersionType
}

public func ==<T: Equatable>(a: Versioned<T>, b: Versioned<T>) -> Bool {
    return a.version == b.version && a == b
}

public struct VersionGenerator {
    public typealias VersionType = UInt64
    
    private static func generateVersion() -> VersionType {
        return ++VersionGenerator.currentVersion
    }
    private static var currentVersion: VersionType = 0
}

Example usage:

let array = Versioned([1, 2, 3])
let indexOf3 = Versioned<Int>(2, version: array.version)

if array.version == indexOf3.version {
    // Index matches array!
    println(array.value[indexOf3.value])
}