Programming Kotlin

Extending Functions

Functions are objects in Kotlin, and you can inject methods into functions, like you can inject methods into classes. In Java 8, the functional interface Function<T, R> has an andThen() method to combine two functions, and we can use this to compose operations—see Functional Programming in Java [Sub14]. Kotlin’s Function doesn’t have that method, but we can inject an andThen() method into Kotlin functions like this, for example:

fluency/extendfunctions.kts

fun <T, R, U> ((T) -> R).andThen(next: (R) -> U): (T) -> U =
  { input: T -> next(this(input)) }

The extension function signature says that andThen() is added to a function that takes a parametrized type T and returns a result of type R. The parameter passed to ...