An example of Extension based on RxSwift in Swift.

ExtensionCompatibleSample.swift

public struct Extension<Base> {
    public let base: Base
    public init(_ base: Base) {
        self.base = base
    }
}

public protocol ExtensionCompatible {
    associatedtype Compatible
    var ex: Extension<Compatible> { get set }
    static var ex: Extension<Compatible>.Type { get set }
}

public extension ExtensionCompatible {
    public var ex: Extension<Self> {
        get {
            return Extension(self)
        }
        set {
            // this enables using Extension to "mutate" base object
        }
    }
    public static var ex: Extension<Self>.Type {
        get {
            return Extension<Self>.self
        }
        set {
            // this enables using Extension to "mutate" base type
        }
    }
}


/// Int Extension

extension Int: ExtensionCompatible { }

public extension Extension where Base == Int {
    public var cube: Int {
        return base * base * base
    }
    public static var almostMax: Int {
        return Int.max - 1
    }
}

4.ex.cube           // 64
Int.ex.almostMax    // 9223372036854775806


/// String Extension

extension String: ExtensionCompatible { }

public extension Extension where Base == String {
    public var reversed: String {
        return String(base.characters.reversed())
    }
    public static func greet(to name: String) -> String {
        return "Hi, \(name)."
    }
}

"hello".ex.reversed             // olleh
String.ex.greet(to: "Michael")  // Hi, Michael.