Intersection Types in Kotlin

I recently found myself wanting intersection types in Kotlin. Specifically, I was looking to write a client for WebDriver that depended on its WebDriver and HasInputDevices interfaces but not on a concrete implementation like RemoteWebDriver.

They are not natively supported yet (see KT-13108: Denotable union and intersection types). However, you can get something very like them using Kotlin’s by delegation:

// Third party code - I can't change
interface WebDriver
interface HasInputDevices
class RemoteWebDriver : WebDriver, HasInputDevices
class EventFiringWebDriver : WebDriver, HasInputDevices

// My code
interface CompositeWebDriver : WebDriver, HasInputDevices

class CompositeRemoteWebDriver(
  delegate: RemoteWebDriver = RemoteWebDriver()
) : CompositeWebDriver,
  WebDriver by delegate, 
  HasInputDevices by delegate

class CompositeEventFiringWebDriver(
  delegate: EventFiringWebDriver = EventFiringWebDriver()
) : CompositeWebDriver,
  WebDriver by delegate, 
  HasInputDevices by delegate

class WebDriverClient(driver: CompositeWebDriver) {
  // code depending on methods in WebDriver & HasInputDevices
}

// Client can now depend on either implementation
val client1 = WebDriverClient(CompositeRemoteWebDriver())
val client2 = WebDriverClient(CompositeEventFiringWebDriver())

It’s still a bit painful; obviously there’s a reasonable amount of repetition in the declaration of each implementation, and adding a new interface to CompositeWebDriver’s supertypes means also adding it as an extra supertype to every implementation with a by delegate declaration. Still, it decouples the client code from the actual third party implementation without having to implement every method in each interface.