How to specify "own type" as return type in Kotlin

Question

Is there a way to specify the return type of a function to be the type of the called object?

e.g.

trait Foo {     fun bar(): <??> /* what to put here? */ {         return this     } }  class FooClassA : Foo {     fun a() {} }  class FooClassB : Foo {     fun b() {} }  // this is the desired effect: val a = FooClassA().bar() // should be of type FooClassA a.a()                     // so this would work  val b = FooClassB().bar() // should be of type FooClassB b.b()                     // so this would work 

In effect, this would be roughly equivalent to instancetype in Objective-C or Self in Swift.

Answer 1

There's no language feature supporting this, but you can always use recursive generics (which is the pattern many libraries use):

// Define a recursive generic parameter Me trait Foo<Me: Foo<Me>> {     fun bar(): Me {         // Here we have to cast, because the compiler does not know that Me is the same as this class         return this as Me     } }  // In subclasses, pass itself to the superclass as an argument: class FooClassA : Foo<FooClassA> {     fun a() {} }  class FooClassB : Foo<FooClassB> {     fun b() {} } 

Answer 2

You can return something's own type with extension functions.

interface ExampleInterface // Everything that implements ExampleInterface will have this method. fun <T : ExampleInterface> T.doSomething(): T {     return this }  class ClassA : ExampleInterface {     fun classASpecificMethod() {} }  class ClassB : ExampleInterface {     fun classBSpecificMethod() {} }  fun example() {     // doSomething() returns ClassA!     ClassA().doSomething().classASpecificMethod()     // doSomething() returns ClassB!     ClassB().doSomething().classBSpecificMethod() }